Principles of Internal Fixation







Additional videos related to the subject of this chapter are available from the Medizinische Hochschule Hannover collection. The following videos are included with this chapter and may be viewed at expertconsult.inkling.com :



  • 8-1.

    Reduction technique—surgical strategy.


  • 8-2.

    Intraoperative three-dimensional imaging (ISO-C).




Introduction


Conservative management of fractures has limitations. Traction, splintage, and casting are limited in their capacity to restore form and function. The fundamental purpose of the skeleton is to provide structure to the body and create attachment points for muscles, tendons, and ligaments, thereby enabling joints to move. When the form of the skeleton is disrupted, the function of the skeleton is affected. In the mid-1900s, the functional limitations of conservative management became known as “fracture disease.” Fracture disease consisted of skin ulceration, muscle atrophy, joint stiffness, and disuse osteopenia. The recognition of this problem propelled a search for solutions.


A battle ensued regarding the optimal treatment of fractures. Early proponents of internal fixation struggled to create generalizability of technique and therefore outcome. Complications of internal fixation—infection, wound healing problems, fixation failure—seemed more egregious than the limitations of conservative management. It was not until the late 1950s that a small group of surgeons leaned into the problem with a systematic approach that included documentation, education, and research. They established a culture that prioritized the adherence to fundamental basic principles of operative fracture care. They partnered with industry to set standards for implant quality and instrument design. They were selective in the surgeons who were allowed use of the equipment, requiring a priori study of the basic principles under the tutelage of a limited group of experts. They exhibited a precise documentation of case variables in an effort to determine what affected outcome. They followed a path of purposeful, repeated application of a method with a view toward perfecting the craft and a willingness to evolve patterns of thought. A paradigm shift ensued. Internal fixation of fractures became the standard rather than the exception. These principles, although modified, are still in use today on a much broader scope. Although fracture care has improved, failure to acknowledge and follow these basic principles is still leading to treatment disasters.




Plan of Attack and How to Use This Chapter


This purpose of this chapter is twofold: (1) to provide a review of the basic principles of fracture care with an emphasis on the “why” and (2) to assist in the application of these principles to particular cases with a focus on the “how.” Although we will concentrate on the general rather than the specific, these general principles should prepare the surgeon to treat specific fractures all over the body. While the individual fracture chapters will make assumptions about your preexisting level of knowledge, this one will not do so. I will make an attempt to introduce the basic language of internal fixation and try to focus on the big ideas rather than the details and exceptions (which can be found in the individual fracture chapters later in the book). The basis of what will be presented represents information that was studied and systematically promoted by the Arbeitsgemeinschaft für Osteosynthesefragen (AO) group over the last 50 years in osteosynthesis manuals and courses. Whereas this is the basis, elaboration based on other material and personal experience will be included when deemed beneficial.


The system of fracture care can be simplified into a summary flowchart ( Fig. 8-1 ). The system can be thought of as an exercise in preoperative planning or proactive failure analysis. Every solution should include a conception of how to obviate failure in all its possible manifestations. In this chapter, we will walk through the different steps in the flowchart and spend time understanding how each step relates to prevention of failure in fracture care. Each step is actually a labyrinth within itself and could provide for a lifetime of study, but this is not required and will be resisted in order to establish a simple applicable framework for preoperative and intraoperative decision making. For the system to provide the desired result of successful fracture care, the steps must remain in context, as this provides limits. To clarify, the steps are separate but interdependent and must all be respected for a winning outcome. Within each section, there will be clinical examples to aid in the application of the basic principles. At the end of each section, you will note summary statements to ensure the important points are fleshed out from many different perspectives (it is often said that we do not truly understand something until we hear it in a particular way).




Figure 8-1


Summary flowchart of the system of fracture care. The process moves from an evaluation of the fracture pattern toward a decision regarding desired construct stability. Once this choice is made, further decision points required include surgical exposure, reduction method, and fixation choice. Each decision box will be covered in detail during this chapter with the exception of external fixation (which is covered in Chapter 7 ). Although we will separately cover the individual parts, the system must be considered as an integrated whole for successful fracture management. EF, External fixation; IMN, intramedullary nail.


When you feel superior to the material, be warned and remember this quotation from one of the greatest thinkers of our time: “If you can’t explain it simply, you don’t understand it well enough.” Reflect on the errors in your practice. Associate your failures with disregard for the basic principles of fracture care. Commit yourself to this language and these principles. See the improvement in patient care.




Fracture Pattern


Five things should be gleaned from every injury radiograph. These can be subsumed under the heading of fracture pattern but extend far beyond that simple title. Observational skills vary greatly among those treating fractures. Interestingly, one of the things thought to be associated with expertise is pattern recognition. When you see an expert digest an injury radiograph, you realize how much information is available in the most limited of studies. Seeing these five things should help improve pattern recognition.


Fracture Pattern = Law of Conservation of Energy


First, the fracture pattern is the radiographic representation of the Law of Conservation of Energy. The total amount of energy in an isolated system remains constant over time. To clarify, there is a significant amount of energy associated with the fractures that we take care of. This energy is conserved through the accident, but changes forms. The representation of the energy is evident by viewing the injury films and the injured part. Specific types of forces create specific types of fractures. Figure 8-2 is a common picture of fracture mechanics. Without reading the figure legend, draw which fracture pattern is expected from each force. Chapter 5, “Biomechanics of Fracture,” provides more facts, laws, and specifics. In this chapter, we will focus more on why.




Figure 8-2


Characteristic fracture patterns in long bones occur with pure loading modes. Compression creates diaphyseal failure that is typically oblique to the long bone axis. Tension creates a failure mode that is perpendicular to the axis of loading (e.g., transverse fracture pattern). Bending loads are a combination of compression and tension loads. A transverse pattern is noted on the tension side and transitions into an oblique fracture on the compressive side. A butterfly fragment, when present, is noted on the compression side. Torsion creates spiral fracture patterns.


A force is an influence on a structure (e.g., skeleton) that tends to produce motion or deformation. There are a limited number of forces that the skeleton sees. These include compression, tension, and shear ( Fig. 8-3 ). Of note, commonly described forces of bending and torsion are more specific types that can be subsumed under the main categories of compression, tension, and shear. A compressive force is perpendicular and inward relative to the surface of an object. A tensile force is perpendicular and outward relative to the surface of an object. A bending force occurs in many forms (e.g., two-, three-, and four-point, and cantilever) and can be “simplified” into tensile and compressive components. The surfaces or molecules in an object subjected to bending are seeing a tensile or compressive force (this fact can be conceptually useful when you are considering which side of an injury is likely to have the purest fracture interdigitation or least damaged soft tissue). A shear force acts parallel or tangential to the surface of an object. A torsion force is a specific type of shear that consists of a twisting or rotation around an axis in an object.




Figure 8-3


Three fundamental force components acting on bone are compression, tension, and shear. Bending and torsion are subsumed under these three fundamental forces.


Why does it matter whether you know that a torsional force leads to a spiral fracture pattern? Just associating the deforming force with the pattern provides little. Understanding the character of that pattern provides a great deal. Simple patterns (transverse, oblique, spiral) are typically thought to be lower energy than their butterfly, comminuted, and segmental counterparts. This lower energy designation assists with predicting the complication profile and expected outcome when counseling the patient preoperatively.


More importantly, associating the fracture pattern with the amount of absorbed energy helps provide your margin and guide your decision making. Surgery is a controlled form of energy transfer. Energy transfer is cumulative. If you start with a fracture pattern that signifies a large amount of energy, you should realize that the energy imparted via your surgical intervention must be limited and/or delayed. This is not a license for percutaneous malreductions, but rather a warning shot that signifies potential danger for early invasive intervention. Highly comminuted diaphyseal and metaphyseal fractures demand an atraumatic surgical technique (irrespective of the chosen implant). It is well known at this point that the biologic cost of restoring every single piece to anatomic alignment is not worth the benefit.


Fracture Pattern Reveals the Intrinsic Stability of the Bone after Reduction


Second, the fracture pattern predicts the intrinsic stability of the bone after reduction. This has utility both in deciding whether or not a fracture can be successfully treated conservatively and in understanding the ultimate stability of the construct. This in turn determines the safety of physiologic loading. The specialized vocabulary is increasing, so it is important to unpack new words as we proceed. We should begin with stability, construct, and physiologic load.


Stability has many definitions. As it relates to fracture care principles, stability is defined as the amount of motion between fracture fragments when a construct is placed under physiologic load. A construct is a structure that is built by a combination of implant and bone. A physiologic load is typically felt to be functional aftercare or motion of a joint rather than weight-bearing. To bring this together, the fracture pattern as noted on the injury film clarifies how stable the bone would be on its own after being reduced but prior to being fixed (i.e., intrinsic stability). Certain fracture patterns are clearly length-unstable even after acceptable reductions (e.g., comminuted pattern). If length restoration and maintenance is important in the care of that fracture, then operative techniques become necessary (i.e., it cannot be treated conservatively). When operative treatment is chosen for an intrinsically unstable pattern, then the type of instability should be clearly defined and the method of fixation should rationally follow. During the treatment period, the implant will be loaded almost exclusively until it can be protected by fracture healing, which creates some intrinsic stability. As fracture care is a race between fracture healing and implant failure, this issue deserves more detailed attention and will be covered many more times in this chapter.


Fracture Pattern Characterizes the Unbalanced Forces That Create Displacement and Subsequent Deformity


Third, the fracture pattern on injury films characterizes the unbalanced forces in the equation. Newton’s third law states that for an object to remain at rest, there must be an equal and opposite reaction for every action. When there is an unbalanced force, an object is not at rest. Think of this in terms of fracture treatment. The surgeon desires to restore anatomy via reduction and to maintain that reduction until fracture healing with an implant. The goal, therefore, is to characterize unbalanced forces and then balance them. Structures obey the laws of nature; therefore, the desires of the surgeon must correspond with basic physics. Internal forces and external loads act on fracture fixation constructs. Any fixation construct has a limited number of load cycles prior to failure. By considering these forces and loads, it is possible to design a fixation construct that minimizes failure potential. To reiterate, unbalanced forces create displacement and subsequent deformity and these forces must be characterized and the plan for fracture treatment must include specific resistance to them. When this is unclear, the unbalanced forces win. This is why malunion and nonunion radiographs typically resemble the injury films with hardware or implant/bone junction failure ( Fig. 8-4 ).




Figure 8-4


A, Anterior-posterior (AP) injury radiograph of subtrochanteric fracture with varus coronal plane displacement. B, AP failure radiograph once again revealing varus displacement. If you want to know what your failure films will most likely look like, take the injury films and draw in broken hardware or intact hardware with implant/bone junction failure.


Fracture Pattern Predicts Expected Soft Tissue Damage


Fourth, the fracture pattern on the injury films predicts the expected soft tissue injury, both from a general and a specific sense. From a general sense, high-energy fracture patterns are typically associated with high-energy soft tissue patterns. As previously noted, high-energy soft tissue patterns forebode danger when early invasive surgical approaches are chosen. This is why multiple historical publications have shown a higher rate of wound healing problems and delayed union and nonunion with complex fracture patterns. High-energy radiographs portend more vascular compromise to fracture fragments and skin, thereby naturally leading to longer healing times and more complications.


From a specific sense, this pattern recognition becomes even more valuable, especially in areas of the body where fractures and ligamentous injuries are often combined. Let us step up the level of discussion to complex knee injuries. Bicondylar tibial plateau fractures have recently been shown to have variable medial plateau injury patterns. One of the most common medial plateau injury patterns consists of the anteromedial plateau remaining attached to the tibial diaphysis and the posteromedial plateau being separated ( Fig. 8-5 ). The posteromedial plateau is a functional correlate for the posteromedial corner of the knee. The posteromedial corner of the knee is the secondary stabilizer against anterior translation of the tibia (the primary stabilizer being the anterior cruciate ligament [ACL]). It logically follows, that when tibial eminence fractures are also present in this fracture pattern (and leave the ACL dysfunctional), the instability pattern is different (the primary and secondary stabilizers now being gone). Look closely at which relationships are maintained on the lateral image. This finding is available on the injury films via pattern recognition and may guide surgical decision making, if noticed.




Figure 8-5


Anteroposterior ( A ) and lateral ( B ) views of a bicondylar tibial plateau fracture in which the medial-sided injury consists of a posteromedial fragment. Anterior translation of the tibial shaft (which is connected to the anteromedial fragment) is noted on the lateral radiograph. Primary and secondary stabilizers of the shaft against anterior translation are absent secondary to tibial eminence injuries and a dysfunctional posteromedial corner injury.


Now consider medial plateau fracture-dislocations in which the lateral plateau maintains continuity to the tibial diaphysis. While they may fall within the same category, there are broad differentiations. Look closely at Figure 8-6 . Both are medial plateau fractures. The lateral plateau maintains continuity to the tibial diaphysis in both patterns. This is where the similarities end. The first pattern exhibits lateral condylar widening, centrolateral articular impaction, shortening, and a variable medial plateau fracture pattern. The second pattern exhibits medial plateau articular impaction and varus hinge instability with avulsion of the lateral capsule, lateral collateral ligament, and biceps femoris. These injuries are treated differently. Fracture pattern recognition allows for the prediction of expected soft tissue damage. This can be the difference between success and failure in operative treatment.




Figure 8-6


Medial plateau fractures with varying injury patterns, which indicate different soft tissue injuries. A, Medial tibial plateau fracture-dislocation revealing lateral condylar widening, centrolateral articular impaction, and shortening. B, Medial plateau fracture revealing medial articular impaction and varus hinge instability with avulsion of the lateral capsule, lateral collateral ligament, and biceps femoris.


Fracture Pattern Defines Expected Mode of Healing


Fifth and most important of all, fracture pattern on injury films defines the expected mode of healing. Ignoring this leads to disastrous consequences for the patient and the surgeon. This important point will be elaborated on throughout many other portions of this chapter, as it must be considered throughout the flowchart diagram (see Fig. 8-1 ). After all, the goal of operative fracture care is restoration of function through reduction, fixation, and healing. Without healing, it is impossible to reach this goal.


This labyrinth of fracture healing reaches very deep, and Chapter 4 in this volume will provide many details about “Biology and Enhancement of Skeletal Repair.” Let us step away from the details and look at the basic principles of fracture healing through a few examples. Look closely at Figure 8-7 . The articular fracture patterns can be ignored at this point (we will cover them in more detail in another section, but suffice it to say that every articular fracture pattern should be anatomically reduced, compressed when possible, and heal via primary bone healing). Both injury films reveal supracondylar femur fractures. The metaphyseal fracture patterns are very different. One is a simple oblique fracture pattern, whereas the other is complex (comminuted). How does this affect your operative decision making? To adequately answer this question, we need to cover more vocabulary and get further along the flowchart. Refer back to this question after you finish the “Desired Stability” section of the chapter.




Figure 8-7


Supracondylar femur fractures with varying metaphyseal fracture patterns. A, Simple metaphyseal fracture pattern. B, Complex metaphyseal fracture pattern.


Speaking of Fracture Patterns








    • “That pilon fracture pattern appears extremely complex. It is surprising that the surgeon used an extensile approach this early in the injury period. Adding that much additional energy to the injury that early likely factored into the current wound healing complications that we are seeing.”



    • “That tibial fracture pattern is very comminuted. I know that if I choose operative treatment, my implant will be load bearing rather than load sharing. I should be careful with the soft tissue to ensure early healing, which will provide implant protection.”



    • “The vertical medial malleolar fracture with medial gutter impaction is the result of compression rather than tension. I wonder why the surgeon attempted to use a tension band construct for fixation. It doesn’t seem that it logically balances the unbalanced forces.”



    • “This fracture occurred secondary to bending based on the radiographic pattern. Notice how the soft tissue damage reflects the position of the fulcrum. I should be more careful with the soft tissue on that side of the injury. It appears crushed.”






Soft Tissue Pattern


You have likely heard the expressions that a fracture is a soft tissue injury with a broken bone inside or that operative fracture care is more like gardening than carpentry work. The underlying message in these expressions is that the soft tissue injury must take precedence over the osseous injury. You cannot effectively and consistently treat fractures while ignoring soft tissue injuries. The most drastic complications of fracture care are typically defined by the soft tissue envelope rather than the fracture itself. So how do you prioritize the soft tissue in fracture care? Consider four ways.


Recognize the Severity of the Soft Tissue Injury Preoperatively


We previously covered the idea that the fracture pattern predicts the expected soft tissue injury. This is a solid general principle to follow, but does have exceptions (e.g., the transverse fracture with a crush impact injury). It is always necessary to closely evaluate the soft tissue envelope in addition to spending time dissecting the injury films. Soft tissue injury takes different forms: contusions, abrasions, blisters, lacerations, avulsions, degloving (closed and open), and crush. These are all different manifestations of energy transfer. Each affects surgical decision making, both with respect to timing and placement of operative approaches. Open fracture management will be covered in detail in Chapter 17 of this volume. Let us focus on the most commonly used closed fracture classification system.


Fractures with Soft Tissue Injuries is a classic publication from 1984 in which editors Tscherne and Gotzen defined a soft tissue classification of closed fractures that is still referenced today. The key point of the classification scheme is that increased energy levels are represented by higher grades of injury. These grades of injury provide an understanding of prognosis and guide decision making. Grade 0 closed fractures represent injuries that are caused by indirect violence and reveal negligible soft tissue damage. The corresponding fracture pattern is typically of simple configuration (e.g., torsion fractures in skiers). These injuries can be treated in many ways and the margin for error is high. Grade I closed fractures represent soft tissue injuries created by fragment pressure from within the soft tissue envelope. The fracture pattern itself is typically mild to moderately severe (e.g., pronation abduction fracture-dislocations of the ankle in which the fractured margin of the medial malleolus creates an abrasion or contusion on the medial skin of the ankle). This soft tissue injury must be respected with early reduction of the displacement to limit further soft tissue damage. Any surgical approach in the area of damaged tissue must be done with extra care. A delay in definitive surgical treatment in the injured region may be necessary. Grade II closed fractures represent soft tissue injuries created by direct external pressure or violence. Deep, contaminated abrasions with local skin or muscle contusion are often associated with moderate to severe fracture patterns (e.g., segmental tibial shaft fractures caused by bumper injuries). Impending compartment syndrome must be ruled out or emergently treated if present. These injuries have a high propensity for soft tissue complications and must be treated with the utmost respect. Grade III closed fractures round out the closed fracture classification scheme. The skin is extensively contused or crushed, the muscle damage may be severe, and the fracture configuration is severe (e.g., multifragmentary or comminuted tibial shaft fractures caused by crushing mechanism). Closed fractures that are associated with major vascular injuries, subcutaneous avulsions and degloving, and established compartment syndrome are included in this grade III category. Treatment of these injuries is challenging and may lead to the need for soft tissue coverage procedures. Recognition of this at the beginning is important in setting realistic expectations preoperatively.


Modify Surgical Plans Based on Soft Tissue Injury Pattern


Surgical plans are created based on the fracture pattern as recognized through the injury films. Plans should incorporate the desired surgical incision; but a surgical incision is a means to an end in fracture care. The desired ends include visualization of the fracture, preparation of the bone ends, reduction of the fracture, and fixation of the fracture. These ends need to be accomplished in the absence of wound healing complications. Unfortunately, the desired approach to optimize visualization, reduction, and fixation may not be safely possible. This is where modification of the surgical plan based on the soft tissue pattern becomes necessary. Decisions need to be made by balancing desires with requirements. Every choice comes with a compromise. Choosing where to make incisions requires a familiarity with the zones of blood supply to the skin. Moving away from ideal mechanical locations to stabilize a fracture requires a familiarity with methods to empower a fracture fixation construct. Let us take time to cover these issues next.


Familiarize Yourself with the Concept of Angiosomes


One way to optimize care is by familiarizing yourself with the concept of angiosomes. An angiosome is a composite block of tissue including deep tissue and overlying skin supplied by a named source artery ( Fig. 8-8 ). It is likely that this concept will be covered in detail in Chapter 18, “Soft Tissue Reconstruction.” Comprehensive articles are available in plastic surgery journals that orthopaedic surgeons may not often read. Rather than focusing on comprehensive details, let us review a specific example and see how knowledge of angiosomes may affect surgical decision making.




Figure 8-8


Angiosome of the anterior tibial artery. The diagram reveals the typical vascular anatomy of the lower limb. The picture reveals the vascular territory that is supplied by the anterior tibial artery.

(Source: From Attinger C: Vascular anatomy of the foot and ankle, Oper Tech Plast Reconstr Surg 4:183, 1997.)


Tibial pilon fractures are complex injuries to treat, primarily because of soft tissue complications. It is an accepted fact that potential soft tissue complications drive surgical decision making. Some of the early results of immediate internal fixation were disastrous. Wound healing complications and infection led to unacceptable outcomes such as amputation. Some surgeons have chosen to avoid soft tissue complications by limiting surgical incisions. The compromise with this approach is limited access to the articular surface for reduction and the necessity of prolonged external fixator frame duration. Others have moved toward staging surgical treatment (e.g., starting with external fixation to realign the limb while waiting for soft tissue recovery prior to proceeding with definitive care). This staging has allowed for safer surgical incisions with the benefit of more direct access to the articular surface for reduction. When the decision is made to proceed with definitive internal fixation, care must be taken to choose the optimal surgical approach. The optimal surgical approach is based on the reduction strategy and the mechanics of instability (i.e., consider where you need to be to see, clean, reduce, and stabilize the fracture). This optimal surgical approach should take into account the angiosomes of the ankle and presence of vascular compromise. This is necessary because soft tissue complications occur even in the presence of staged treatment. It has been shown that a large percentage of tibial pilon fractures are associated with irregular arterial flow. It is logical that making incisions in areas of compromised arterial flow can lead to soft tissue healing problems. Understanding the angiosomes should assist in limiting these complications. In the leg and ankle, large cutaneous vessels arise primarily from the deep fascia around the perimeter of muscles. Most tissues are crossed by two or more angiosomes, receiving supply from each. Junctional zones between angiosomes are the danger areas. The primary junctional zone in the ankle is around the medial face of the tibia. The skin in this area is supplied almost exclusively by the anterior tibial artery ( Fig. 8-9 ). When this artery is compromised, it follows that surgical incisions in this region can be problematic. It just so happens that this is the most likely area for surgical incision breakdown in tibial pilon fracture treatment ( Fig. 8-10 ). Recognizing anterior tibial artery compromise preoperatively could logically mitigate some of these complications.




Figure 8-9


The arterial blood supply to the distal tibial metaphysis is shown in an axial section on the left . The arrows represent standard anteromedial and anterolateral surgical approaches to this area. The diagram to the right represents the arterial blood supply to the skin in this same region. Overlapping areas are noted, except along the anteromedial surface of the tibia. This is a critical junctional zone that is supplied only by the anterior tibial artery. It logically follows that surgical approaches in this area in the face of an anterior tibial artery injury would be more dangerous. PA, Anterior peroneal artery; PP, posterior peroneal artery; TA, anterior tibial artery; TP, posterior tibial artery.

(Source: From Heim U: The pilon tibial fracture: classification, surgical techniques, results, in which it was modified from Aubry P, Fievé J (1984) Vascularisation osseuse et cutanée du quart inférieur de jambe, Rev Chir Orthop 70:596, 1995.)



Figure 8-10


Tibial pilon fracture wound healing complication associated with anteromedial approach. Note the location of the healing problem is in the junctional area that is provided blood supply by the anterior tibial artery.


Empower Fracture Fixation Constructs


There are times when the soft tissue pattern drives the placement of fixation to less than ideal mechanical locations. Let us consider a different scenario. Potential soft tissue compromise comes in different forms. Sometimes it is a direct result of the injury. Other times it is just a consequence of normal anatomy. Consider the patella or the olecranon. Both locations are subcutaneous. Hardware prominence is a documented issue at both sites. Most implants are designed to serve as tension bands in these locations. The concept of a tension band will receive more attention under the “Fixation” heading in the flowchart, but let us start with the basics. A tension band is a torque converter applied to the tension surface of an eccentrically loaded bone. To clarify, the implant (whether a wire or plate or suture) must be applied to the tension surface of the bone (the one that sees stretching). The tension surface of the olecranon or the patella is the subcutaneous surface (dorsal for the olecranon, anterior for the patella). Subcutaneous implants are associated with prominence and irritation of the overlying skin. It is tempting to move the implant from the tension surface to one that has more soft tissue coverage (e.g., the medial or lateral surface of the patella or olecranon). Doing so satisfies the desire to limit implant prominence but comes at a cost. The implant is no longer in the correct mechanical position to serve as a tension band. Either the construct must be empowered or the postoperative protocol must be modified (so that the implant sees less load until some healing occurs and it is protected). Failure to do so may lead to construct failure ( Fig. 8-11 ).




Figure 8-11


Patella fracture treated with implants placed away from the tension surface. The patella is loaded both in tension (as the quadriceps contract and pull the proximal piece away from the remainder) and in bending (as the fulcrum of the trochlea causes apex anterior bending forces). Implants placed away from the tension surface are mechanically challenged to resist bending.


Speaking of the Soft Tissue Pattern








    • “Fracture blisters are already developing around the ankle. That is a sign that the soft tissue is not ready for an extensile approach and additional surgical trauma.”



    • “There is blanching of the skin secondary to fragment displacement in this tongue-type calcaneus fracture. That requires immediate attention and possibly even a very limited surgical exposure, despite the fact that delayed surgical treatment (when soft tissue swelling decreases) is standard.”



    • “No matter how atraumatic my soft tissue dissection in that area, it is a watershed zone that is already compromised. I better consider alternative surgical exposures instead.”



    • The best mechanical position for that implant is clear based on the injury films. Unfortunately, a surgical approach to that area does not allow for joint visualization and I want to anatomically reduce the articular surface. I think I will compromise the mechanical position of the implant to allow for the benefit of better articular exposure. Next I need to consider how to empower that implant to prevent mechanical failure.”






Area Involved


Refer to the flowchart and review the concepts of “Fracture Pattern” and “Soft Tissue Pattern” as they relate to proactive failure analysis in the system of fracture care. Now we will move to the heading of “Area Involved.” To understand this heading, it is necessary to review the aims of the AO method ( Fig. 8-12 ).




Figure 8-12


The aims of the Arbeitsgemeinschaft für Osteosynthesefragen (AO) method.

(Source: From Müller ME, Allgöwer M, Schneider R, Willenegger H: Manual of internal fixation: techniques recommended by the AO-ASIF group, Berlin/Heidelberg/New York, 1991, Springer-Verlag, p 1.)


Two important aims of the AO method include (1) the anatomic reduction of the fracture fragments and (2) preservation of the blood supply to the fracture fragments and the soft tissue by means of atraumatic surgery. We should strive to meet these aims while realizing the two create a conflict. We have previously discussed the idea that the quality of a reduction is inversely related to the ability of the surgeon to maintain the blood supply to fracture fragments and soft tissue. To clarify, it is not difficult to enact an anatomic reduction if you remove all the soft tissue from the fracture fragments through a poorly executed extensile exposure; unfortunately, this leaves the fracture fragments avascular and creates healing challenges. Similarly, it is not hard to maintain nearly all the blood supply to fracture fragments and enact a malreduction; unfortunately, the fracture will heal in a nonanatomic position. Neither of these is acceptable. It follows that the compromise should be to enact the quality of reduction required for each specific injury as atraumatically as possible. This means that there is a hierarchy of reduction mandates that should be understood. Thankfully, this hierarchy can be divided into the well-defined segments of the bone, specifically the articular surface (epiphysis), the metaphysis, and the diaphysis. Let us cover each one individually and see how the location of the fracture (i.e., area involved) aids us in making decisions about fracture care.


Articular Surface


The articular surface (epiphysis) mandates an anatomic reduction of all articular fragments. As a general rule, open approaches are generally required to enact an anatomic articular reduction. It is accepted that more damage to the blood supply is likely to occur with the open approach, but the desire is still to limit that as much as possible. The articular cartilage has three functions: (1) to distribute forces evenly, (2) to provide a near-frictionless motion surface, and (3) to serve as a shock absorber during loading. When the articular surface is displaced, it cannot optimally serve these functions. Displacement occurs in two primary forms: (1) articular incongruence and (2) articular malalignment. Articular incongruence is defined as the inability of the joint surfaces to coincide when superimposed. Articular malalignment is defined as an incorrect relationship between the articular surface and the axis of the limb (i.e., rather than the ankle joint surface being perpendicular to the weight-bearing axis and parallel to the floor with loading, it is crooked).


Displacement leads to two primary dysfunctions: (1) point loading and (2) joint instability. One of the few mathematical formulas that is useful in the operating room is Stress = Force/Area. When joint stress is kept at a reasonable level, the articular cartilage remains healthy. To maintain joint stress at a reasonable level, it is important to distribute the joint forces over large areas. This occurs in an anatomically reduced joint with balanced forces. When the area for force distribution is limited (e.g., a malreduced articular fracture that creates point loading), the stress increases and joint degradation occurs. A simple analogy is watching a lady walk on soft ground with two different types of shoes: a stiletto and a flat. The stiletto concentrates her body weight into a smaller area, increasing the stress and causing her heel to sink into the soft ground. In contrast, the flat would distribute her body weight over a larger area, decreasing the stress and allowing her to walk without sinking. The same thing is occurring at the articular surface level, but instead of sinking, the cartilage in the point-loaded area just degenerates.


Joint instability is defined as the potential for subluxation or dislocation with functional loading. Joint instability occurs from both articular incongruence and joint malalignment. Both cause shear forces and lead to cartilage degeneration. Subtle findings can often be noted on intraoperative radiographs. When the joint space on radiograph is not congruent after reduction, a search for malalignment and/or instability should ensue. If this instability remains, then cartilage loading will continue to be nonanatomic, and the risk for posttraumatic arthrosis should logically increase.


Metaphysis and Diaphysis


The metaphysis and diaphysis can be taken together as they follow similar principles. The metaphysis and diaphysis do not require anatomic restoration of all fracture fragments in order to function appropriately; rather, they require the restoration of the relationships between the joint surface and the weight-bearing axis of the limb (alternatively the restoration of the relationship between the joint above and the joint below the fracture). Because all the fracture fragments do not require perfect anatomic reduction, it is less common to proceed with open extensile approaches to the metaphysis and diaphysis. This will be emphasized and clarified again in the “Reduction” heading of the flowchart.


Once again, the parts of the flowchart (and therefore the fracture fixation system) are interrelated and require some redundancy in thought. While we have attempted to limit explanation of details and exceptions in favor of simplicity, this is an area where recognizing a few caveats will benefit understanding. Three caveats to not establishing a perfect anatomic reduction of all fragments of a metaphyseal or dia­physeal fracture are (1) when there is simple pattern metaphyseal/diaphyseal extension of an articular fracture, (2) when the benefits of construct stability provided by an anatomic fracture reduction outweigh the vascular compromise created by increased soft tissue dissection, and (3) when the strain theory is not respected. The first two caveats can be simply explained with examples. The third will be covered under the “Desired Stability” heading, as it is typically harder to understand and apply.


First, when there is simple pattern metaphyseal/diaphyseal extension of an articular fracture, this must be anatomically reduced ( Fig. 8-13 ). We previously accepted the statement that the articular surface requires an anatomic reduction in order to function appropriately. It should become clear that it is impossible to anatomically reduce the articular surface in this fracture without also anatomically reducing the meta-diaphyseal extension (the exception being when the cortical extension can bend, allowing an anatomic reduction of the articular surface with a near anatomic reduction of the dia­physeal extension). The articular surface always takes priority and drives the fracture fixation choices.




Figure 8-13


Simple pattern diaphyseal extension of a segment of the articular surface of the tibial plafond. Without an anatomic reduction of the diaphyseal extension, the articular surface cannot be placed back into appropriate position with respect to the articular surface of the fibula (which is not fractured) and the Chaput or anterolateral segment of the tibia.


Second, when the benefits of anatomic fracture reduction outweigh the vascular compromise created by increased soft tissue dissection, it is necessary to proceed with a more extensile approach to achieve that anatomic reduction. Try to imagine a situation when this is the case ( Fig. 8-14 ). In this osteoporotic, interprosthetic fracture with limited joint motion above and below the fracture, construct stability is clearly an issue. Choosing a load-bearing construct (through inexact fracture reduction) may work, but the advantages of anatomically reducing the fracture and getting the bone to share the load should be obvious. This is a judgment call and care must be taken to limit soft tissue dissection and perform atraumatic reduction techniques despite the choice to proceed with a more extensile approach. Dead bone does not heal, even when it is sharing load.




Figure 8-14


Anterior-posterior (AP) radiograph of an interprosthetic femur fracture. Choosing bridge plating would create a load-bearing implant in bone of poor quality. Choosing an anatomic reduction with independent lag screws and neutralization plating empowers the fixation construct and creates some implant protection through load sharing.


Speaking of the Area Involved








    • “That is a multifragmentary diaphyseal femur fracture. The benefits of an anatomic reduction of every fracture fragment are far outweighed by the soft tissue damage that will be created in order to achieve a precise anatomic reduction.”



    • “That is a complex articular fracture pattern. The benefits of an anatomic reduction of every articular fragment should outweigh the danger of an extensile approach, especially if I respect the soft tissue envelope.”



    • “It appears that there is diaphyseal extension of that articular fragment. I better consider how I am going to anatomically reduce that in my preoperative planning. Which surgical approach will allow visualization of both the articular surface and the diaphyseal extension?”



    • “This fracture pattern has a complex articular and a complex metadiaphyseal pattern. It is interesting that there are different reduction mandates for those, considering they are similarly complex. It is not just about fracture pattern. The area involved really matters.”






Desired Stability


Once again refer to Figure 8-1 to orient yourself to the big picture of the system of fracture care. We are moving to the “Desired Stability” heading. This labyrinth is rich and extremely important to grasp. A misunderstanding here will lead to many treatment failures. First, let us define some new vocabulary. As previously noted, stability has many different definitions; but, when it is used in fracture care, the definition is distinct and should be consistent. Stability in fracture care refers to the degree of immobility between fracture fragments when the fracture fixation construct is subjected to physiologic load. Another word used more commonly in everyday language to describe this concept is stiffness . The stiffness of a material is the resistance of that material to deformation. Consider objects likely in front of you currently: a phone and a pen. You might define your pen as more flexible or less stiff than your phone. When you subject them to three-point bending in your hands, there is a clear difference. This is a simple way to conceptualize what we mean when we refer to the stability of a fracture fixation construct. It is important to stop again and define another term. I use the term fracture fixation construct carefully. In orthopaedic fracture care, a construct is defined as a surgeon-built structure that consists of the combination of implant and bone. We must be careful at this point to step back and acknowledge that not all fractures are treated with implants. Conservative care is still acceptable in many situations. In those situations, the construct would consist of the combination of an external stabilizing device (e.g., cast, splint, brace) and bone.


Spectrum of Stability


Stability is a spectrum, just like stiffness ( Fig. 8-15 ). On one end of the spectrum is absolute stability. Absolute stability is defined as the absence of motion between fracture fragments under physiologic load (i.e., stiff). On the other end of the spectrum is instability. Instability is defined as an excessive amount of motion between fracture fragments under physiologic load (i.e., flimsy). In between absolute stability and instability is relative stability. Relative stability is defined as controlled motion between fracture fragments under physiologic load (i.e., flexible). The decision between absolute and relative stability is made prior to the surgical intervention, but occasionally must be altered when the desired reduction goals are not being met with the original plan. The decision matters; in fact, it is one of the most important decisions you make in fracture treatment. It matters for two primary reasons: (1) It determines the type and success of fracture healing. (2) It defines the point in time that functional recovery begins.




Figure 8-15


Spectrum of stability is noted with constructs used in tibia fracture treatment. Instability is noted at the far left with a complex tibia fracture with no support. With loading, this fracture would displace and not return to its original position. Relative stability is noted in the middle images with casting, external fixation, intramedullary rodding, and bridge plating. With loading, these constructs should produce micromotion to induce callus formation for secondary healing. Absolute stability is noted with neutralization plating and independent lag screws on the far right . With loading, no motion should occur. This construct relies on primary bone healing as no induction of callus formation through motion can occur.


Let us begin with the type and success of fracture healing. Chapter 4, “Biology and Enhancement of Skeletal Repair,” will provide extensive detail regarding the fracture healing process. Please refer to that chapter to focus on details. Here, we are going to maintain a broad perspective. There are two successful types of fracture healing. Neither of them works well in an area of compromised blood supply. Stated another way, biology is paramount. With biology appropriately prioritized, it is also important to recognize that healing depends on the mechanical environment. Understanding how to manipulate mechanics is necessary. The first type of healing is known as primary bone healing (also called direct bone healing ). This type results from internal remodeling of the bone. It necessitates osteon-to-osteon reduction and functions poorly if at all in the presence of gaps. It requires absolute stability (no motion between fracture fragments under physiologic load). Stated another way, absolute stability leads to primary bone healing and requires an anatomic reduction with compression of adjacent fragments. Absolute stability does not lead to primary bone healing when the reduction is poor. Absolute stability does not lead to primary bone healing when significant gaps are present. Absolute stability does not lead to primary bone healing when adjacent fragments are moving with respect to each other (in fact this defies the definition). Absolute stability fails in these scenarios.


The second type of healing is known as secondary bone healing (also called indirect bone healing ). It is the body’s more normal response to injury (i.e., it is what occurs when bone heals in the absence of surgical intervention). It is evidenced by callus formation and requires relative stability (controlled motion between fracture fragments under physiologic load). Stated another way, relative stability leads to secondary bone healing and requires a flexible fixation construct that maintains the reduction but allows motion between fracture fragments. Relative stability does not lead to secondary bone healing when extreme gaps are present. Relative stability does not lead to secondary bone healing when no motion is occurring between fracture fragments (i.e., stiff construct). Relative stability may not lead to secondary bone healing when excessive motion is occurring between fracture fragments (i.e., flimsy construct). If healing does occur in this situation, it will be in an unacceptable alignment with a loss of the reduction ( Fig. 8-16 ).




Figure 8-16


Relative stability requires the golden mean of flexibility to achieve healing while preventing plastic deformation. Both too little and too much flexibility lead to problems.


The second reason that the choice of stability matters is that it defines the time point when functional recovery can begin. Remember the goal of fracture treatment (and the AO method) is rapid recovery of the injured limb through an appropriate reduction, stable fixation, preservation of the blood supply, and early active pain-free mobilization. Both absolute stability and relative stability can accomplish this goal, but instability cannot. As you move toward the extremes of construct flexibility, it becomes harder to mobilize joints. The addition of immobilization after fracture fixation is less than ideal for achieving an early functional recovery.


Absolute Stability


Now let us focus more on absolute stability by discussing the indications and requirements. You should recognize the clear overlap between the different headings on the flowchart at this point, especially the ones that we have already covered (fracture pattern, soft tissue pattern, and area involved). Absolute stability is indicated for all intraarticular fractures and some metaphyseal and diaphyseal fractures. Regardless of the complexity of an intraarticular fracture pattern, absolute stability is indicated. Anatomic restoration of all articular fragments is required. This is required because the goal is to reestablish congruence between the two opposing surfaces of the joint in order to distribute forces over the largest area possible. This minimizes joint stress and improves articular cartilage health. Healing of articular fragments with hyaline cartilage occurs best in the presence of anatomic reduction and compression. Rarely, intercalary osteochondral fragments will be missing (e.g., severe open fractures with joint surface loss). This is the only exception to the rule. In this rare scenario, compression of the peripheral articular cartilage fragments may constrain the joint and alter the ability to achieve congruence with the opposing articular surface (i.e., the other side of the joint). Once again, this is a rare and very complicated situation. It requires the maintenance of overall joint surface width and/or depth, and eliminates the ability to achieve compression between the peripheral joint surface fragments. Absolute stability is indicated for some metaphyseal and diaphyseal fractures, but only when the fracture pattern is simple; even then, absolute stability is not always indicated. For example, simple metaphyseal and diaphyseal fracture patterns can be successfully treated with relative stability. The best example of this scenario is the use of an intramedullary rod to treat a simple pattern tibial or femoral shaft fracture. By the nature of its mechanics, the intramedullary rod creates relative stability (controlled motion between fracture fragments under physiologic load). Another example of this scenario is the use of a plate to bridge simple pattern tibial or femoral shaft fractures. Although this is not considered ideal, it can be successful with a clear understanding of construct flexibility. When is it reasonable to treat simple pattern metaphyseal and diaphyseal fracture patterns with absolute stability? Remember the two scenarios previously mentioned. First, when there is simple pattern metaphyseal/diaphyseal extension of an articular fracture, this must be anatomically reduced. It must be anatomically reduced and treated with absolute stability in order to ensure the articular surface reduction is anatomic and remains anatomic until healing. Second, when the benefits of anatomic fracture reduction outweigh the vascular compromise created by increased soft tissue dissection, it is necessary to proceed with a more extensile approach to achieve that anatomic reduction. Consider the interprosthetic fracture with the stiff joint above and below the fracture. The decision can be made to optimize the mechanical environment by anatomically reducing the fracture and compressing it to achieve absolute stability. Here the decision is made to protect the metal by creating a more load-sharing environment. Once again, the biology must be maintained through biologically friendly exposures, atraumatic reduction techniques, and tissue-sparing implant placement. It should be clear that the requirements for absolute stability include an anatomic reduction, interfragmentary compression, and biologic techniques.


Let us spend more time with what has been called the hallmark of absolute stability, namely compression. Compression is the act of pressing surfaces together. The primary purpose of compression is to create friction between the opposing surfaces via fracture interdigitation. Remember the definition of absolute stability is the absence of motion between fracture fragments under physiologic load. As a fracture is subjected to different types of load, the amount of friction between the surfaces of the fragments acts to prevent motion between the surfaces. This friction provides intrinsic stability to the reduced bone and thereby protects the implants by unloading them. This allows the implant to serve more efficiently in a load-sharing environment and win the race between fracture healing and implant failure. An implant in this scenario is less dependent on early fracture healing to protect it from reaching its load cycle limit and failing.


Compression can be divided into different categories. Let us first consider the difference between axial compression and transaxial compression. In this setting, axial is defined as along the axis of the limb. Ideally, compression is applied perpendicular to the orientation of the fracture. If applied in any direction other than perpendicular, some of the compression is lost to shear (force along the surface of the fracture rather than perpendicular to the surface of the fracture). A simple way to envision this is to consider the concept of a vector. A vector is a geometric entity that has both a magnitude and a direction. A resultant vector can be broken down into its component vectors. The magnitude in this scenario is the amount of force that is being created. The direction is the orientation of application of that force. Look closely at Figure 8-17 . The force of compression is in a less than ideal direction. Because of the orientation of the fracture surfaces, a substantial amount of shear is introduced. When shear is created, it is both inefficient for compression and harmful to the reduction of the fracture (i.e., may displace an anatomically reduced fracture). Following this vector concept, axial compression would be best used when the fracture is nearly perpendicular to the long axis of the bone. The fracture pattern that would be most perpendicular to the long axis of the bone would be a transversely oriented fracture.




Figure 8-17


Intraoperative radiographs with attempted axial compression of an oblique fracture pattern with the articulated tensioning device. Because the compression was not perpendicular to the fracture line itself, shear was created. Shear medially translated the distal fragment along the obliquity of the fracture line. Look closely at the position of the most medial callus on the “before” and “after” images.


Many different tools and implant design modifications have been created to assist with axial compression. These tools are designed to be used primarily with plate osteosynthesis. They were created primarily to achieve preload. Preload is defined as tensioning an implant and reciprocally compressing the bone or fracture surfaces, before the patient actively subjects the implant to load. A logical way to approach this is through an abbreviated review of plate hole design. By understanding how and why the design of plate holes has changed over time, it allows you to reason through methods of axial compression. More time will be spent describing the differences in plate holes later under the “Fixation” heading in the flowchart.


The earliest plate holes were round and slightly larger than the outer diameter of the screw shaft but smaller than the head of the screw. This required screws to be placed perpendicular to the orientation of the hole in order to fit through and seat into the hole. At this point in history, any compression that could be achieved across a fracture needed to be done outside of the plate itself. For example, compression could be achieved by loading the limb manually or by placing a clamp along the axis of the limb, which does not work very well if you think of clamp application for a transversely oriented fracture. Special plates were designed that contained a compression screw device at the end of the plate (e.g., the Danis coapteur). Alternatively, devices were created that could temporarily attach to a plate in order to enact compression, and then be removed (e.g., the articulated tensioning device). Alternative options included using the universal distractor in compression or using a Verbrugge clamp attached to a single hole in the plate and a screw outside of the plate ( Fig. 8-18 ). As every choice necessarily comes with a compromise, design continued to evolve. The compromises made with each of the previously listed devices were increased surgical exposure, equipment, and surgical time. This led to the development of a modified plate hole that allowed for compression with the plate–screw relationship alone (i.e., no longer requiring an additional device). It seems that two plate holes were being simultaneously designed to function in this manner. The first was present on the Bagby plate. The second was present on the AO plate, and became known as the dynamic compression unit (DCU). It was found on a plate termed the dynamic compression plate (DCP). It consisted of an oblong hole, which was the combination of an inclined and transverse cylinder ( Fig. 8-19 ). The plate was first attached to one side of the fracture with a screw. A screw hole on the other side of the fracture was then drilled eccentrically in the plate hole (i.e., toward the side of the hole furthest away from the fracture). As the screw head engaged the plate hole, it began to move horizontally down the transverse cylinder. This movement created a compressive force between the fracture ends by moving one relative to the other. The advantage of this plate hole design modification is that compression no longer required additional devices, exposure, or time. The disadvantage was that the compression that could be achieved was limited compared to the previously used devices. Remember that these devices and design modifications were most optimally used in transverse fracture patterns that were perpendicular to the long axis of the bone (i.e., axial compression); but fracture patterns vary and compression must be more generalizable to different patterns. These devices and plate hole modifications can be used in oblique fracture pattern variants, assuming the plate can be attached such that it creates an axilla to prevent shear from creating deformity. Review Figure 8-20 without reading the figure legend and apply the concept of a vector. Both fracture patterns are oblique. Both see the same compressive vector. Both see shear as the direction of compression is not perpendicular to the fracture surface. Which one works? It should be becoming clear that both the orientation of the fracture line and the possible position of the implant (i.e., what anatomic surface the implant can be placed onto safely) will determine the type of compression that you choose.








Figure 8-18


Examples of compression tools and devices. A–C, Articulated tensioning device. Note the same concept previously covered in Figure 8-17 . Compression along an obliquity can be detrimental to reduction if the axilla created by the plate and bone is not in the position to capture the spike of the other fragment. In this example, the plate should have been attached to the other fragment first, such that compression into the axilla could occur. D–F, Universal distractor. This device consists of a spindle rod, a carriage, and nuts that allow for either compression or distraction through attachment to Schanz pins on each side of the fracture. After distraction, the fracture ends can be aligned, followed by compression across the fracture through reversing the force created by the universal distractor into compression (i.e., moving the other nut against the carriage, which is connected to the Schanz pin). G–H, Push–pull concept using the lamina spreader and a Verbrugge clamp. The plate is attached to one side of the fracture. A lamina spreader is used to distract across the fracture site via an independent screw placed outside of the plate. Once realignment of the bone ends has occurred with Weber clamp guidance, the lamina spreader is traded for a Verbrugge clamp, which then compresses the fracture using the same independent screw.

(Redrawn from Rüedi TP, Buckley RE, Morgan CG: AO principles of fracture management, vol 1, ed 2, expanded edition, Thieme, 2007, Switzerland, Figure 3.1.1-7a-b, p 170; Figure 3.1.1-4 a-c, p 175; Figure 3.2.17a-c, p 241.)



Figure 8-19


The modified screw hole known as the dynamic compression unit (DCU) allowed for eccentric placement of a screw into a plate to create a compressive effect without requiring an external device. This concept is based on a carpenter’s principle, but was technically improved to limit parasitized forces.



Figure 8-20


Oblique fracture patterns subjected to compression with plate application. A, Attaching the plate to the side that creates an obtuse angle creates an axilla such that compression prevents shear by trapping the fragment. B, Attaching the plate to the side that creates an acute angle fails to create an axilla such that compression leads to shear along the surface of the obliquity. This is the reason that both the fracture orientation and the surface of bone that normally accepts a plate are important in defining the mechanics of your fixation device.


Transaxial compression differs from axial compression in that the direction of the compressive force applied is across or more perpendicular to the long axis of the limb. The simplest form of transaxial compression is the lag screw. This is somewhat of a misnomer, as any type of screw can accomplish the lag principle. It is first and foremost a technique. That stated, some screws have been designed specifically to lag. The lag principle states that a screw thread must not engage the near cortex but must engage the far cortex. To simplify, a hole is drilled in the near cortex and the medullary bone until reaching the fracture site that is larger than the outer diameter of the screw. The remaining medullary bone and the far cortex are drilled to create a hole that is smaller than the outer diameter of the screw (typically the same diameter as the core of the screw). This creates a glide hole in which the screw has no purchase and a thread hole in which the screw gains optimal purchase. As the head of the screw impacts the near cortex (or plate that it is placed through), it acts as a torque converter, converting the torque energy created by the operator’s rotating hand into compression at the fracture site ( Fig. 8-21 ). Careful preoperative planning allows the screw to be placed in an atraumatic fashion with minimal soft tissue/vascular compromise. Of note, the screw should be placed near the center of the fragment in order to prevent propagation of the fracture line into the drill hole (this may need to be modified if more than one lag screw is being used). Additionally, it should be placed perpendicular to the fracture itself such that it creates pure compression through the direction of the vector.






Figure 8-21


Lag screw placement by technique. A, The fracture is anatomically reduced and held in compression via a pointed reduction clamp. A small fragment lag screw is then placed by technique. The glide hole (which is approximately the size of the outer diameter of the screw) is first drilled. The drill guide is then inserted into the glide hole. The far cortex hole (which is approximately the size of the inner diameter of the screw) is then drilled. B, Lag screw placement by design. The screw is partially threaded. Only the screw threads contact the bone in the drilled hole, thereby creating the same effect as though a glide hole had been drilled.

(Source: A, Redrawn from Heim D, Luria S, Mosheiff R, Weil Y: Forearm shaft 22-A2.1: lag screw and plate fixation, AO Foundation, AO Surgery Reference. Available at: www2.aofoundation.org/wps/portal/surgery?showPage=redfix&bone=Radius&segment=Shaft&classification=22-A2.1&treatment=&method=Lag%20screw%20and%20plate%20fixation&implantstype=&approach=&redfix_url=1325866239919&Language=en ; B, Redrawn from Rüedi TP, Buckley RE, Morgan CG: AO principles of fracture management, vol 1, ed 2, expanded edition, Switzerland, 2007, Thieme, Figure 3.2.1-4, p 160.)


Rarely lag screws can be placed as the sole fixation. This is indicated only when the fracture sees minimal load and the fracture length is at least twice the diameter of the bone at the fracture center. In this rare scenario, the lag screws must be carefully positioned such that more than one can be placed. Even in this rare scenario, it is important to consider the stability afforded by the screws alone. The lag screw is compromised by the limited lever arm with which it works. This lever arm is often too small to resist functional loads of bending and shear. In addition, it provides no factor of safety. If the lag screw loosens, there is little else to prevent displacement of the fracture fragments. To complicate things further, the obliquity of the fracture line is rarely parallel to the long axis of the bone; hence, the perfect position of the lag screw is rarely perpendicular to the long axis of the bone. As the bone is loaded axially, shearing occurs along the obliquity. Screws placed perpendicular to the long axis of the bone are in a better orientation to resist shear (but rarely in the correct orientation to provide pure compression across a fracture). Because of these reasons, the lag screw should almost always be protected by a plate (the mechanical function of which is termed a neutralization plate or protection plate ). This will be covered further under the “Fixation” heading of the flowchart.


Another form of transaxial compression occurs when a plate is first attached to the acute angle of an oblique fracture. In this scenario, attempting to create compression along the axis of the bone will lead to uncontrolled shear and displacement of the reduction. This is commonly necessary in the proximal femur but can be used in other areas of the skeleton as well. Review Figure 8-17 once again. In this example, it was necessary to attach the plate first to the acute angle of the fracture (because of the design of the implant itself). Attempting to create compression along the axis of the bone with an articulated tensioning device led to shear, which created shortening and medial translation of the distal segment along the plane of the fracture. Recognizing this problem led to the decision to place a conventional screw through the plate distal to the fracture. This conventional screw application created compression across the axis of the fracture (transaxial) by pulling the distal fragment toward the plate, thereby compressing the distal side of the oblique fracture to the proximal side.


To review, compression can be divided into different categories. We have just distinguished axial compression from transaxial compression and described the different tools and techniques used with each. Now let us differentiate between static compression and dynamic compression. Static compression is that which is applied at the time of the surgical intervention. Once applied, it remains virtually unaltered (in reality, it decreases over time as the law of entropy states that all systems in the universe move from a position of order to one of disorder). Examples of static compression include all the axial and transaxial compression scenarios that we have previously discussed. Dynamic compression differs from static compression in that postoperative functional use of the limb leads to periodic partial loading and unloading. Stated another way, the fracture fragments are not only compressed by the preload of the implant (static), but also subjected to additional compression, which results from harnessing forces generated at the level of the fracture when the skeleton comes under physiologic load. This is not to be confused with the DCU or DCP, both of which were designed to produce static compression. An example of dynamic compression is the tension band concept. This will be covered in more detail under the “Fixation” heading.


Summary of Absolute Stability


In summary, absolute stability is the absence of motion between fracture fragments when subjected to physiologic load. It is always indicated for articular fractures, regardless of the fracture pattern. It is almost never indicated for complex metaphyseal and diaphyseal fracture patterns (as the compromise to the blood supply required to enact the anatomic reduction of every small fragment outweighs the advantage of anatomically reducing each piece). It is occasionally indicated for simple pattern metaphyseal and diaphyseal fracture patterns. It requires the perfect restoration of all loaded fracture fragments back into anatomic position. It achieves load sharing through compression of fracture surfaces that interdigitate and increase friction at the fracture site. The compression can be achieved through axial or transaxial means. It leads to primary bone healing when done correctly. This necessitates biologically friendly surgical approaches, reduction techniques, and implant placement. That can be difficult.


Speaking of Absolute Stability








    • “I would like to achieve absolute stability of this tibial shaft fracture, but I am concerned that complexity of the fracture pattern will lead me to damage the blood supply to the fragments too much while trying to obtain an anatomic reduction.”



    • “This oblique metaphyseal tibial fracture orientation is from proximal lateral to distal medial. I am planning on using a medial plate. If I attach it distally first, I should be able to compress with the plate without creating displacement through shear.”



    • “That ulnar shaft fracture orientation is transverse. I would love to use a lag screw, but I don’t see how that would be possible. I better choose an axial compression technique instead.”



    • “I have anatomically reduced this humeral shaft fracture and compressed the fracture with independent lag screws. I better protect these lag screws with a plate, since they have difficulty resisting loading all by themselves.”




Relative Stability


At this point, let us shift our attention away from absolute stability and toward relative stability. Once again this is a decision that is made prior to surgery. Relative stability is defined as controlled motion of fracture fragments under physiologic loading conditions. Indications for relative stability include most metaphyseal and diaphyseal fractures (excluding the two previously covered caveats). Epiphyseal or articular fractures are never appropriate for relative stability. Complex fracture patterns are ideal for relative stability. Simple fracture patterns are amenable to relative stability, but less than ideal for relative stability. Think for a moment why this would be the case. Remember why choosing the correct type of stability matters. It matters for two primary reasons: (1) It determines the type and success of fracture healing, and (2) it defines the time point that functional recovery can begin. Let us focus on determining how the type (not location) of fracture pattern and the choice of stability are intrinsically linked. This teeters on two important concepts: (1) strain and (2) stress concentration versus stress distribution. Let us start with the one that is the most challenging to grasp, namely strain.


Strain Theory of Perren


In 1977, Perren and Cordey penned a manuscript in German that first described an interpretation of mechanical influences on tissue differentiation. This became known as the Strain Theory of Perren. In 1980, a second manuscript by the same authors was published in English. Within this manuscript, Perren wrote: “These thoughts about the mechanical influences on tissue differentiation are not intended as conclusive evidence since precise data are still not available, but we hope that they will stimulate thought and provide a basis for discussion.” More than 30 years later, these thoughts are still stimulating discussion and research on cell mechanotransduction. As importantly, this theory is still being manipulated in operative theaters all around the world in an attempt to more consistently achieve fracture healing. Let us consider how to apply this theory in a practical manner in surgery.


In physics, strain is a magnitude of deformation. It is the change in the dimension of a deformed objected during loading divided by its original dimension. When translated to fracture care, it is equal to the change in length between fracture fragments during loading divided by the original (prior to loading) length between fracture fragments.


<SPAN role=presentation tabIndex=0 id=MathJax-Element-1-Frame class=MathJax style="POSITION: relative" data-mathml='Strain=ΔLength/Length’>Strain=𝛥Length/LengthStrain=ΔLength/Length
Strain = Δ Length/Length


Most of you reading this chapter are not mechanical engineers or physics wizards. In light of that, just for a moment, I would like to trade perfect accuracy of mechanical terms for understanding. Stated the most useful way in terms of fracture care, strain is the motion that occurs between fracture fragments during loading divided by the resting distance between the same fracture fragments after fixation.


<SPAN role=presentation tabIndex=0 id=MathJax-Element-2-Frame class=MathJax style="POSITION: relative" data-mathml='Strain=Magnitude of Displacement betweenFragments during LoadingTotal Resting Distance betweenFragments after Stabilization’>Strain=Magnitude of Displacement betweenFragments during LoadingTotal Resting Distance betweenFragments after StabilizationStrain=Magnitude of Displacement betweenFragments during LoadingTotal Resting Distance betweenFragments after Stabilization
Strain = Magnitude of Displacement between Fragments during Loading Total Resting Distance between Fragments after Stabilization


This is a formula that we all can work with. You really only need to remember this formula and one detail to be able to manipulate strain to your advantage in the operating room. The detail is that a low strain environment leads to bone formation (i.e., healing). You already know that primary bone healing occurs in the absence of motion (absolute stability) and secondary bone healing occurs with controlled motion (relative stability). Let us take some time to consider three different scenarios to see how this works.


In scenario 1 ( Fig. 8-22, A ), we have a complex metadiaphyseal fracture pattern. We know that a complex metadiaphyseal fracture pattern is an indication for relative stability and that relative stability provides controlled motion of fracture fragments under physiologic load. We know that restoring the relationship between the joint surface and the diaphysis is all that is necessary. Stated another way, reducing every single fracture fragment anatomically would be both unnecessary and counterproductive (i.e., it would require excessive soft tissue stripping and thereby lead to avascular fragments). Restoring length, alignment, and rotation rather than the perfect anatomic restoration of every fragment is preferred. We know that this can be accomplished with many different types of implants. Let us refer to the formula:


<SPAN role=presentation tabIndex=0 id=MathJax-Element-3-Frame class=MathJax style="POSITION: relative" data-mathml='Strain=Magnitude of Displacement betweenFragments during LoadingTotal Resting Distance betweenFragments after Stabilization’>Strain=Magnitude of Displacement betweenFragments during LoadingTotal Resting Distance betweenFragments after StabilizationStrain=Magnitude of Displacement betweenFragments during LoadingTotal Resting Distance betweenFragments after Stabilization
Strain = Magnitude of Displacement between Fragments during Loading Total Resting Distance between Fragments after Stabilization



Figure 8-22


Fracture pattern affects strain. This must be considered when creating a fixation montage. The type of stability that will be present should be decided at the beginning in the preoperative plan and is based partly on the fracture pattern as noted. A, Complex metaphyseal fracture pattern noted in a supracondylar femur fracture. B, Simple articular split noted in medial femoral condyle fracture. C, Simple metaphyseal pattern in supracondylar femur fracture.


The total resting distance between all fracture fragments is a large number (as it always is with comminuted, multifragmentary fractures because of the cumulative distance between so many different fragments). When a large number is in the denominator of a fraction, then the overall value is likely to be a low number (because it is impossible to create so much motion that the numerator will be high enough to make the overall value high); therefore, the strain is likely to be low. Low strain leads to bone healing. It is hard to lose in this scenario. This is one of the easiest fracture patterns to treat successfully, despite the fact that it is broken into many pieces.


In scenario 2 (see Fig. 8-22, B ), we have a simple articular fracture pattern. We know that any type of articular fracture pattern is an indication for absolute stability and that absolute stability is defined as no motion between fracture fragments under physiologic load. Anatomic restoration of the fracture fragments is required. Interfragmentary compression is important. Let us refer to the formula:


<SPAN role=presentation tabIndex=0 id=MathJax-Element-4-Frame class=MathJax style="POSITION: relative" data-mathml='Strain=Magnitude of Displacement betweenFragments during LoadingTotal Resting Distance betweenFragments after Stabilization’>Strain=Magnitude of Displacement betweenFragments during LoadingTotal Resting Distance betweenFragments after StabilizationStrain=Magnitude of Displacement betweenFragments during LoadingTotal Resting Distance betweenFragments after Stabilization
Strain = Magnitude of Displacement between Fragments during Loading Total Resting Distance between Fragments after Stabilization


The total resting distance between fracture fragments is going to be very low (as the two fragments are compressed together in anatomic position). That means the denominator is small. Reaching a low strain in this scenario requires virtually no motion between fragments. Thankfully, that is what absolute stability provides. Using absolute stability for the treatment of simple fracture patterns requires the surgical skill to anatomically reduce the fracture with a biologically friendly technique. Assuming you possess it that day, it is hard to lose in this scenario.


In scenario 3 ( Fig. 8-22, C ), we have a simple metaphyseal fracture pattern. We know that metaphyseal fracture patterns do not have to be anatomically reduced like articular fractures. All that is required from a reduction standpoint is the restoration of the relationship between the articular surface and the diaphysis. We are left with a choice. Do we choose absolute stability and anatomically reduce the simple metaphyseal fracture pattern? If we make that choice, we know that we can reach a low strain environment and achieve primary bone healing just as we did in scenario 2. Or, do we choose relative stability instead, as perfect restoration of all fragments is not required? Certainly that is a temptation because it would allow us to do less soft tissue dissection and a more biologically friendly surgical approach. This is where it gets interesting. Let us refer to the formula:


<SPAN role=presentation tabIndex=0 id=MathJax-Element-5-Frame class=MathJax style="POSITION: relative" data-mathml='Strain=Magnitude of Displacement betweenFragments during LoadingTotal Resting Distance betweenFragments after Stabilization’>Strain=Magnitude of Displacement betweenFragments during LoadingTotal Resting Distance betweenFragments after StabilizationStrain=Magnitude of Displacement betweenFragments during LoadingTotal Resting Distance betweenFragments after Stabilization
Strain = Magnitude of Displacement between Fragments during Loading Total Resting Distance between Fragments after Stabilization

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Jun 11, 2019 | Posted by in ORTHOPEDIC | Comments Off on Principles of Internal Fixation
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