Malunions and Nonunions about the Knee

Introduction

The diagnosis and treatment of a malunion or nonunion about the knee should be approached systematically. A malunion is defined as a fully healed fracture resulting in a deformity of the bone. To identify a malunion about the knee requires the accurate measurement of limb alignment and joint orientation. This includes the alignment of the femur and tibia, the articular congruency of the distal femur and proximal tibia, the stability and motion of the knee joint, and length of the individual segments. The location of the malunion is crucial to its clinical significance, as a malunion close to a joint can more significantly alter the mechanical axis of the bone, the joint, and, thus, the entire extremity. This alteration of the mechanical axis or joint alignment can result in eccentric wear on the articular surface, leading to arthritic changes, pain, and decreased function.

The definition of nonunions is not clearly delineated; the definition has historically relied on time as the relevant criterion. The U.S. Food and Drug Administration defines a nonunion as a fracture that has not healed after 9 months and has not shown signs of progressive healing for 3 consecutive months. Muller defined 8 months of nonoperative treatment as the cutoff for the diagnosis of nonunion in tibial fractures. See Chapter 25 for details regarding diagnosis and overall treatment of nonunions.

The temporally dependent definitions of nonunion are not detailed enough for individual treatment because they are not specific to the bone affected (femur, tibia, humerus, etc.), the region of the bone (metaphyseal, diaphyseal), or the pattern of the underlying fracture (transverse, comminuted, segmental bone loss), among other factors. Currently, the most useful definition of nonunion is a fracture that has no reasonable likelihood of achieving union without surgical intervention. Similarly, a delayed union is defined as a fracture that shows slower progression to healing than anticipated for that level in that specific bone and is at risk of continuing to a nonunion without intervention.

Treatment of nonunions about the knee rely on understanding the underlying cause of the nonunion, while treatment of a malunion requires understanding deformity planning and correction principles. The goal of this chapter is to describe the principles and techniques of treatment of posttraumatic nonunions and malunions of the distal femur, proximal tibia, and knee joint.

General Principles

Basic Biomechanics

A biomechanical analysis should be performed for surgical planning purposes. The hip and ankle joint should be always included in the deformity analysis of the knee and deformities about the knee. Paley outlined the normal values and limits for the deformities ( Fig. 63-1 ). Six parameters should be assessed in every deformity to guide surgical treatment:

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  Frontal alignment (varus/valgus)
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  Knee joint line angulation (varus/valgus/subluxation, joint line incongruity)
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  Sagittal alignment (recurvatum/procurvatum)
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  Soft tissue contractures or laxity (flexion contracture/extension contracture, anteroposterior or mediolateral laxity)
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  Patellofemoral tracking (maltracking/dislocation)
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  Rotational deformities (internal/external rotation)

The combination of soft tissue contractures, osseous deformities, and arthritic changes should be assessed. This allows the possibility of unloading a painful anterior, posterior, medial, or lateral compartment while correcting the overall deformity. For example, in patients with posterior and posterolateral knee instability with a varus thrust and posteromedial chondral destruction, a three-dimensional (3-D) combination of a valgus with a flexion osteotomy may provide very effective treatment since all components of joint pathology are addressed in one procedure.

Frontal Alignment and Knee Joint Line Angulation

Frontal alignment should be based on standardized bilateral long-leg, standing three-joint anterior-posterior (AP) radiographs or a standing 3-D radiograph ( Fig. 63-2 ). A supine computed tomography (CT) scanogram is less helpful as the mechanical forces across the joints cannot be measured without weight-bearing. The deformity analysis of the frontal plane should include the mechanical axes of the femur and tibia, overall alignment (varus or valgus), mechanical axis deviation (MAD) of the entire limb, the lateral distal femoral angle (LDFA), medial proximal tibial angle (MPTA), and the joint line convergence angle (JLCA). The normal mechanical lateral distal femoral angle (mLDFA) is 88 degrees (range, 86 to 90 degrees), whereas the normal mechanical medial proximal tibia angle (mMPTA) is 87 degrees (range, 85 to 89 degrees). Using the point where the weight-bearing line (Mikulicz line) crosses the tibia plateau width, the knee joint stress can be calculated, with increasing deformity resulting in greater forces on the compartment closer to the axis.

When the mechanical axis is medial to the normal axis, the leg is in varus, increasing the stress on the medial compartment. On the contrary, when the mechanical axis is lateral to the normal axis, the leg is in valgus, increasing the forces on the lateral patellofemoral compartment. The individual components of the deformity must be measured to determine the extent from each component: metaphyseal bone deformity, diaphyseal bone deformity, articular surface incongruity, and joint capsule incongruence.

Sagittal Alignment and Soft Tissue Contractures

Sagittal alignment should be based on standardized long-leg, standing (three-joint) lateral radiographs or a 3-D EOS radiograph (EOS Imaging, Paris, France) (see Fig. 63-2 ). The deformity analysis of the sagittal plane should include the anatomic axis of the femur, which includes a 10-degree procurvatum bow, the anatomic axis of the tibia, contracture or hyperextension of the knee joint, and the posterior proximal tibia angle (PPTA) and the posterior distal femoral angle (PDFA). In the sagittal plane, the anatomic posterior proximal tibia angle (aPPTA; 81 degrees [range, 77–84 degrees]) reflects the tibial slope, and the anatomic posterior distal femur angle (aPDFA; 83 degrees [range, 79–87 degrees]) reflects the flexion and extension positioning of the distal femur.

The position of the patella should also be evaluated for patella baja or alta using the Insall-Salvati ratio, which defines a 1 : 1 ratio between the patella tendon and the patella as normal, with a range from 0.8 to 1.2. Alternatively, in the presence of deformity of the patella or tibial tubercle because of fracture or apophysitis, the Blackburne-Peel ratio can be used. The Blackburne-Peel ratio compares the length of the articular surface of the patella and the distance of the inferior point of the articular patella surface from the tibial plateau.

Changes of the tibial slope cause significant pathologic forces on the meniscus and articular surfaces and should be corrected when possible. An increased tibial slope, or decreased PPTA, can cause posterior subluxation of the femur on the tibia, while the opposite can cause anterior subluxation of the femur. Fractures of the tibial and femoral shafts cause diaphyseal deformities resulting in recurvatum or procurvatum. Metaphyseal fractures can result in abnormal posterior tibial and femoral angles in addition to fixed flexion or extension deformities.

Soft tissue contractures can result from a number of causes after trauma: lack of physical therapy after surgery; prolonged immobilization of fractures; formation of heterotopic bone or myositis ossificans; significant soft tissue loss at the time of injury; or wound healing complications. Anterior, posterior, or rotary subluxation of the knee can be the result of ligamentous injury, such as the anterior cruciate ligament, posterior cruciate ligament, or posterolateral corner, knee joint dislocations, tendinous or muscular injuries, or a combination of these.

Poor management initially of tibial fractures can result in Achilles tendon contractures, and neurologic injury can result in spasticity of the gastrocsoleus complex, both of which can lead to anterior tibial subluxation. Contracture or spasticity of the hamstring muscles results in a knee flexion contracture, which is often secondary to pain or poor patient compliance with physical therapy. Range of motion of the extremity will occur via the path of least resistance, which can potentially be a fracture site if adjacent to the joint and the fracture is not well stabilized. In the presence of an arthrofibrotic knee, an adjacent distal femur fracture will have less resistance than the arthrofibrotic knee, and motion will occur through the fracture site resulting in a nonunion. To stabilize fractures adjacent to the joint, it is imperative to neutralize the lever arms for both fracture segments, regardless of the proximity to the knee joint or small size. This can be accomplished by spanning stabilization across the knee joint temporarily during fracture healing ( Fig. 63-3 ).

Patella-Femoral Tracking and Rotational Deformities

The patella-femoral joint should be evaluated in its tracking, stability, and range of motion. In addition to the considerations of the patella in the sagittal plane as stated earlier, rotational deformities of nonunions or malunions and the effect on patella tracking should be evaluated. Intraarticular femur fractures can cause a diastasis or incongruence of the trochlear groove causing pain, subluxation, or dislocations of the patella. Rotational deformity of the femur can cause external or internal rotation of the distal femur and uneven loading of a patellar facet can result in chondromalacia or arthritis. Severe deformities can cause patella subluxation or dislocation. Proximal or metaphyseal tibial fractures can result in deformity of the tibial tubercle and maltracking of the patella. Treatment of primary femoral shaft fractures often results in rotational deformity of greater than 10 degrees. Even the use of computer navigation for the primary treatment of femoral shaft fractures results in a significant rotational deformity compared to the unaffected extremity.

Analysis of the patella-femoral joint is best accomplished via weight-bearing axial view and is helpful to identify patients with osteoarthritic changes and patellar maltracking. Furthermore, in the sagittal plane, the patella height (baja or alta) reflects an important factor. After osteotomy, the biomechanical changes of the patella-femoral joint should be taken into account.

Nonunion Classification

Nonunions can be classified according to their radiographic appearance or according to their mobility. Radiographic classification divides nonunions into atrophic, hypertrophic, or synovial pseudarthrosis. Mobility divides nonunions into stiff, partially mobile, and flail. The mobility classification was initially the idea of Ilizarov and was modified by Paley ( Fig. 63-4 ). Stiff hypertrophic nonunions have a maximum of 5 degrees of angulation in one plane. Partially mobile nonunions have between 5 and 20 degrees of angulation in any one plane. Mobile pseudarthroses have greater than 20 degrees’ angulation in any plane. The rationale of the mobility classification is that it correlates with the tissue between the bone ends. The stiffer the nonunion, the stiffer the tissue between the bone ends. Stiff nonunions have fibrocartilage between the bone ends; partially mobile nonunions have dense fibrous tissue; and flail nonunions have loose connective tissue or synovial tissue. For these reasons, the mobility and the radiographic classifications correlate well: stiff with hypertrophic, partially mobile with atrophic, and flail with synovial pseudoarthrosis.

Intraarticular Versus Extraarticular Osteotomies

Extraarticular osteotomies of the femur and tibia are used for realignment of the hip, knee, and ankle. The closer each osteotomy is to an adjacent joint, the greater the reorientation with angular correction. Extraarticular realignment and reorientation can redistribute forces on these major weight-bearing joints. The resultant pain reduction and decreased wear rate increase the longevity of these joints. Extraarticular osteotomies do not address problems of joint incongruity, which can be a result of proximal tibia and distal femur intraarticular fractures. Joint incongruity and associated instability, subluxation, and impingement lead to rapid degeneration of the hip, knee, and ankle of the affected leg. Intraarticular osteotomies are not a common treatment consideration for the hip, knee, or ankle. Intraarticular osteotomy of the medial proximal tibia is perhaps the only such osteotomy that is well recognized. Intraarticular osteotomy of the distal femur has been previously described for posttraumatic and sequelae of neonatal sepsis and septic arthropathy.

Intraarticular malunions are caused by tibial plateau fractures with incongruity of one plateau relative to the other. On the medial side, the plateau is most commonly tilted, whereas on the lateral side, there is most commonly a segmental depression or diastasis with widening. For hemi-plateau depressions, the plateau may be osteotomized from a small incision and reoriented with an opening wedge correction ( Figs. 63-5 and 63-6 ). On the lateral side, the plateau can be narrowed by resection of the defect or the depressed segment can be elevated and bone grafted, or both ( Fig. 63-7 ).

Paley reported a case series of nine intraarticular osteotomies performed to treat tibial plateau fracture malunions. Follow-up was between 28 and 108 months. The original fractures were classified according to Schatzker classification with one each of type I and II, two type IV, two type V, and three patients with type VI fractures. All patients had alignment and knee stability restored to within normal limits. No patients had pain in follow-up. Knee range of motion at follow-up was an average of 105 degrees and not significantly different from preoperative range of motion.

Limb Length Discrepancy

It is crucial to realize the increased likelihood of a limb length discrepancy with malunions and nonunions, prior, during, and after treatment. Malunions result in an overall limb length discrepancy because even if the overall length of the femur or tibia has been maintained, the malunion causes a deviation of one of the segments from the normal mechanical axis. Nonunions can result in a limb length discrepancy due to loss of bone at the site of nonunion site. Additionally, both conditions can sustain additional bone loss as a result of initial or subsequent surgical treatment. Injuries about the knee that occur in children or adolescents can affect the adjacent physes, resulting in a complete or partial growth arrest at the two physes that contribute most to limb length. The younger the age at the time of growth arrest, the greater the limb length discrepancy.

The treatment of limb length discrepancy has significantly improved recently, specifically for adults. The advent of U.S. Food and Drug Administration (FDA)-approved internal lengthening intramedullary nails has revolutionized the correction of limb length discrepancy, such as the Internal Skeletal Kinetic Distractor (ISKD; Orthofix, Lewisville, Texas) and the Precice intramedullary nail (Ellipse Technologies, Irvine, California). The safety and efficacy of these internal lengthening devices have limited the indications for external fixation. External fixators are now reserved most commonly for children and adolescents, in whom the internal devices are not possible because of the size of the intramedullary canal. The internal devices use the same underlying principle of distraction osteogenesis, usually at the rate of 1.0 mm per day in the femur and 0.75 mm per day in the tibia. The decrease of muscular scarring by eliminating the need for half-pins or wires facilitates physical therapy during lengthening, decreasing muscle and skin scarring, hastening recovery after the procedure, and avoiding pin site infections and clothing difficulty. Limb lengthening can be combined with deformity correction if the osteotomy can be planned at or near the center of rotation of angulation (CORA) and an optimal location for lengthening ( Fig. 63-8 ).