Surgical Principles of Intramedullary Lengthening

and Mark T. Dahl2



(1)
Department of Orthopedic Surgery, University of California – Irvine, Orange, CA, USA

(2)
Limb Length and Deformity Correction Clinics, Gillette Children’s Specialty Healthcare and University of Minnesota, St. Paul / Minneapolis, MN, USA

 



Keywords
Lengthening nailsTrauma nailsGuide wirePreoperative planningDeformity correctionPiriformis fossaFixator-assisted nailingVentingDepth plungingFat embolism



General Principles



Similarity to Intramedullary Trauma Nails


An appealing aspect of intramedullary lengthening nails is that the surgical technique for insertion is readily familiar to orthopedic surgeons who use intramedullary nails to treat long bone fractures . Indeed, the portals of entry at the ends of long bones are the same for trauma and intramedullary lengthening nails. Likewise, familiar contraindications for certain locations—particularly, avoiding piriformis fossa entry in pediatric cases—apply to both trauma and intramedullary lengthening implants.

Nevertheless, there are distinct differences between the practical surgical use of trauma nails and intramedullary lengthening nails.


Differences from Intramedullary Trauma Nails


Unlike fracture cases, a bone to be lengthened with an intramedullary elongating nail is intact prior to commencing the operation. As a result, reaming the bone marrow cavity of an intact bone can potentially push marrow fat into the venous circulation ahead of the reamer. Subsequently, this marrow fat travels via the inferior vena cava into the heart and thereafter into the lungs, occluding the pulmonary circulation. This fat embolism can be fatal. A surprising number of patients have died on the operating table from fat emboli caused by reaming a closed medullary canal in surgical procedures for intramedullary shortening, as well as that in total hip and knee replacement.

Also, trauma nails, without internal mechanisms, are stronger than lengthening nails.


Guide Wire Issues


Trauma nails are hollow, whereas intramedullary lengthening nails are not. A hollow nail permits the use of a guide wire throughout the entire insertion process, thereby simplifying nail insertion once the fracture fragments have been reduced to near-anatomic position and the guide wire crosses the fracture line.

While guide wire s are used with intramedullary lengthening nails during the initial bone reaming phases of the surgical procedure, a surgeon must remove the guide wire before inserting the nail. This may result in displacement of the transected adjacent bone ends, suddenly making a seemingly straightforward operation technically challenging. We will describe techniques to prevent loss of alignment during nail insertion.


Short or Long Nails


Another area of ongoing discourse among surgeons with experience in intramedullary bone lengthening deals with the question: What is better for intramedullary lengthening, a short nail or long nail? In brief, a short nail can be inserted without concern for the anterior bow of the femur, a matter discussed in more detail below. The short nail, used with either a proximal or distal femoral osteotomy (with antegrade or retrograde nail insertion, respectively), does not reach into the bone far enough to risk anterior cortex penetration where the bone curves away from a straight-line entry axis.

However, a very proximal or distal osteotomy, performed at or near the metaphyseal/diaphyseal junction, results in substantial space around the nail, enough to allow angulation during lengthening unless blocking screws are used.

A longer nail , on the other hand, permits osteotomy in a region where the nail fits fairly snugly in the bone, meaning that angular displacements are nearly impossible for purely mechanical reasons.

In view of the paucity of published experience with large numbers of intramedullary lengthening nails—after all, the technique is quite new—there is no consensus on this matter yet.


Preoperative Planning


Detailed analysis of the nature and cause of the limb length discrepancy is essential when contemplating operative limb lengthening. Full-length x-ray images, preferably with a lift under the short limb to level the pelvis, will help determine which bone or bones require lengthening (Fig. 5.1). Furthermore, accurate measurement of the width, cortical thickness, and medullary canal diameter of the involved bone is essential. A standardized marker, placed on the skin at the same distance from the x-ray plate as the bone, will permit computation to account for image magnification.

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Fig. 5.1
Four cases demonstrating apparently identical shortening, requiring the same lift. (a) Supra-pelvic deformity. (b) Short femur. (c) Short tibia and fibula. (d) Shortening of both femur and tibiofibula. Copyright 2016 Zeeca Publishing Co

Careful examination of the patient should include assessment of the range of joint motion in the involved and contralateral limb. It is at this point that the surgeon must decide what surgical releases should accompany the operative lengthening. While it is possible to delay surgical releases in the hope that intensive physical therapy and a certain amount of patient stoicism will be sufficient to prevent contractures, subluxations, and dislocations, return trips to the operating room can be kept to a minimum by anticipating likely deformities and prophylactically releasing appropriate myofascial structures. This matter will be discussed in greater detail in the next chapter .


Assessing Deformities


With Ilizarov’s circular external fixation method , limb lengthening is often combined with deformity correction because the hinges, translation rails, rotation constructs allow almost infinity spatial modification of bone fragments with respect to each other. Likewise, with circular hexapod fixators, the six oblique struts, used in conjunction with a properly configured computer program, will do the same.

Intramedullary lengthening nails presently available have none of these adjustable features, but they may be forthcoming as innovative surgeons and engineers come up with ingenious mechanisms to accomplish the seemingly impossible.

As mentioned elsewhere in this monograph, small angular and rotational corrections, perhaps up to 10 degrees of angulation and 20 degrees of rotation, are possible through the osteotomy site at the time of implant surgery. Greater changes risk injury to the periosteum (specifically, strangulation of microcirculation) that generates the new bone in the widening distraction zone. For this reason, surgeons performing limb elongation surgery must be conversant in the language of deformity correction, which includes an understanding of normal anatomical joint and axis alignment values in both the coronal and sagittal plane, familiarity with methods of measuring both the anatomical and biomechanical axes of bones, and appreciating the difference between them (Figs. 5.2 and 5.3). Additionally, limb lengthening surgeons should appreciate the fact that many deformities are not oriented in the radiographic anteroposterior and lateral projection planes but are instead obliquely oriented .

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Fig. 5.2
Normal angles, frontal plane , for preoperative planning. Fibulae and patellae omitted for clarity. Left: anatomic axes. Right: biomechanical axis (After Paley and Herzenberg)


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Fig. 5.3
Normal angles , for preoperative planning . Fibulae and patellae omitted for clarity. Left: femoral angles. Right: biomechanical axis (After Paley and Herzenberg)


Hidden Deformities


Certain deformities are obvious, evident to anyone looking at a limb. Others, however, are subtler, revealed after measuring the aforementioned angles and axes . Some hidden deformities are compensatory, especially those that develop in a growing child. For example, a partial medial growth arrest of the proximal tibial growth plate will cause the bone to gradually grow into a progressively more varus attitude. The evident bowing of the limb may mask a compensatory valgus overgrowth of the distal femoral physeal plate. Correcting the tibial varus deformation will unmask the femoral valgus because, after correction of the tibia, the limb will stick out to the side, making ambulation impossible (Fig. 5.4).

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Fig. 5.4
Correcting obvious deformities (left) of shortening and tibia varus may unmask hidden deformities including (1) distal femur valgus, (2) internal tibial torsion, (3) heel valgus, and (4) foot pronation


Full-Length X-ray Studies


Full-length standing x-ray films are essential for proper assessment of lower limb length discrepancies and deformities. Reproducible positioning helps follow-up x-ray examinations as well. For this reason, the “patella forward” position is commonly used for this purpose. The short limb should be elevated on stacked 1.0 cm blocks until the pelvis is level both clinically and radiographically (Fig. 5.5).

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Fig. 5.5
Centering patella will help standardize serial x-ray studies. Note magnification marker (red arrow). Courtesy of Janet D. Conway, M.D. Used with permission from the Rubin Institute for Advanced Orthopedics, Sinai Hospital of Baltimore


Clinical Illustration: Failure to Identify Hidden Deformity


This case illustrates a femoral deformity in a 14-year-old boy who suffered a partial growth arrest following osteomyelitis of the distal femur. The valgus malalignment and shortening of the femur was corrected with an intramedullary lengthening nail. The surgery and elongation proceeded uneventfully. Final full-length x-ray images demonstrated an unrecognized compensatory varus deformity of the upper tibia, subsequently treated with a hemi-epiphyseodesis (Fig. 5.6).

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Fig. 5.6
Unmasking a hidden deformity. (a) Pre-op AP x-ray image. (b) Post-op AP x-ray image. Green arrow points to unrecognized compensatory upper tibial varus deformity


Level of the Osteotomy


When elongating a limb with external fixation, osteotomy can be performed anywhere along the length of the bone, provided there exists sufficient bone to safely insert wires or pins into both fragments. When the planned lengthening includes a deformity correction, the location of the osteotomy is dictated by the geometry of the deformity and the anticipated correction. For straight lengthening, when possible, the osteotomy should be performed at the metaphyseal-diaphyseal junction (where the bone begins to flare out as it approaches the adjacent joint). According to Professor Ilizarov, the regenerate develops best in this region .


The Osteotomy Formula


When elongating a limb with an intramedullary lengthening nail, certain recommendations prevail. Such implants contain a nested telescopic rod that extends in response to an externally applied device. The outer tube thus has a larger diameter than the inner rod. Given that stiffness is proportional to the fourth power of the diameter, it stands to reason that the outer tube is stiffer than the inner rod, even though the former is hollow and the latter is solid. Therefore, osteotomy should not be performed at the level of the protruding portion of the nail. Thus, the first consideration in planning the level for osteotomy is to measure the length of the protruding portion of the nail (3.0 cm in the most commonly used PRECICE® nail) and, as a first step, record that length .

Likewise, to place the osteotomy in a manner that assures that the entire widening distraction zone remains along the outer tube of the implant throughout the elongation process, the contemplated length of distraction must be added to the length of the protruding portion of the nail to set the level for osteotomy.

Moreover, to guarantee that the distraction zone does not end near the junction of the inner and outer components of the implant, which would constitute a potential stress riser, the level of the osteotomy should be 4.0–5.0 cm (~2 in.) along the wider component of the implant, away from the telescoping junction.

Therefore, the “osteotomy formula” reads as follows:

Protruding portion (3.0 cm typically) + 5.0 cm (or 4.0 cm) + lengthening

When using a typical PRECICE® nail, therefore, the formula can be shortened to:

8.0 cm + lengthening

Thus, if 7.5 cm of lengthening is planned, the osteotomy level should be 15.5 cm up from the tip of a fully collapsed nail toward the threaded end. In this manner, the entire regenerate zone will be along the thicker portion of the nail, with an additional 5-cm safety margin (Fig. 5.7).

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Fig. 5.7
The osteotomy formula helps select an osteotomy level to best prevent nail breakage. The planned lengthening (6.0 cm in this case) is added to the length of the protruding portion of the end of the nail (3.0 cm), which is combined with a “safe zone” at the exterior tube’s lower end (usually 4.0–5.0 cm). Add these values and measure upward from the nail’s tip. Mark the osteotomy level on the skin after checking with fluoroscopy. Copyright 2016 NuVasive

In certain unusual circumstances, it may be necessary to shorten the safety zone to 3.0 or 4.0 cm rather than 5.0 cm. This might occur, for instance, with a very short bone, such as that observed when lengthening the limb of a patient with achondroplastic dwarfism.


Fixator-Assisted Nailing


Because intramedullary lengthening nails lack a hollow center, guide wires cannot help maintain alignment of fragments until the implant crosses the osteotomy site. Likewise, oftentimes the lengthening nail is employed to not only elongate a bone but to also acutely correct small angular and/or rotational deformities. Moreover, risk of malrotation of the fragments with respect to each other looms large in the minds of many lengthening surgeons, particularly in femoral lengthening cases.

The tibia’s rotation after osteotomy is usually stabilized by performing the fibular osteotomy at a different—typically lower—level on the limb, so external fixation isn’t used unless precise control of deformity correction necessitates such a device.

For these reasons, temporary (intraoperative) external skeletal fixation serves to ensure proper final position of the fragments at the end of surgery, especially in femur lengthenings (Fig. 5.8).

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Fig. 5.8
Fixator-assisted nailing. (a) Distal external fixation half-pin located behind the nail path (red arrow). Note distal femoral osteotomy vent holes (green arrow). (b) Proximal external fixation half-pin located behind narrower path of telescoping portion of nail (yellow arrow). (c) Temporary monolateral external fixator used to secure fragments in final alignment prior to nail


Periosteal Elevation


Because intramedullary lengthening involves reaming the marrow thereby destroying endosteal blood supply, formation of the regenerate in the widening distraction region depends exclusively on periosteal blood supply. Therefore, careful elevation of the periosteum from the bone’s surface must precede any drilling or osteotomy. Maximum preservation of the periosteum requires approaching the operative region through a small incision, typically the width of an osteotome, made in line with the limb’s longitudinal axis.

Approach the bone with a minimal amount of soft tissue dissection. Over the crest of the tibia, for example, a single incision down to the bone will do. Elevate the periosteum with a small elevator as far around the bone as possible on both sides, and extend the elevation for a centimeter up and down the bone. The elevator should be used to protect the periosteum during venting and osteotomy (Fig. 5.9).

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Fig. 5.9
Protection of the periosteum is critically important because it is the only osteogenic tissue remaining after reaming the marrow canal


Venting


To reduce the likelihood of fat embolism , long bone canals must be “vented” prior to reaming. The vent holes, drilled across the involved bone, allow marrow fat to escape into the surrounding soft tissues during reaming. Since a common technique during osteotomy is predrilling at the level where the osteotomy will be performed, these predrilled holes may serve as vent holes for intramedullary reaming. Venting must be performed after first elevating the periosteum to prevent injury to this structure, so important to new bone formation in the distraction zone (Fig. 5.10).

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Fig. 5.10
Multiple drill holes (left, following periosteal elevation) allow marrow contents to vent out of the bone during reaming of the medullary canal, reducing the risk of fat embolism . The holes also simplify osteotomy of the bone (right). Copyright 2016 NuVasive

Furthermore, as the vented marrow tissue includes not only fat droplets but also precursor bone cells, reamings that collect around the vent holes can enhance healing at the osteotomy site during lengthening.

Slow reaming, gradually increasing reamer sizes in 0.5 mm steps, will help reduce the incidence of fat embolism syndrome .


Depth Plunging


Over-drilling the far cortex, referred to as “depth plunging ,” can potentially damage neurovascular structures on the opposite side of bone (Fig. 5.11). The matter is discussed at length in the Chap. 11.

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Fig. 5.11
Depth plunging can damage structures on the far side of a bone. See Chap. 11 for discussion of preventive measures. Copyright 2016 NuVasive


Compartment Syndrome


Certain authorities have recognized that the added volume of such bone marrow reamings can, in the anterolateral tibial compartment, produce a “compartment syndrome” (a serious form of pressure-induced muscle necrosis). For this reason, it has been suggested that venting should be avoided on the lateral side of the tibia.

Furthermore, as reaming continues beyond the level of the osteotomy site (where the vent holes are located), additional vent holes at the distal end of the bone may be necessary.


Thermal Injury While Drilling Vent Holes


The heat generated by a drill bit spinning in cortical bone has been the subject of considerable research. Temperatures exceeding 55 °C will likely cause necrosis of osteocytes in the lacunae around the drill bit [1]. The radius of this zone of injury is proportional to the heat generated by drilling. Ordinarily, when drilling is part of an internal fixation procedure, the dead bone surrounding a drill hole will, at least initially, hold a screw as securely as do viable bone. With time, however, loosening may occur.

With external skeletal fixation, dead bone surrounding a drill hole subsequently used to secure transcutaneous pins remains in contact with the microflora of the pinhole, a potential nidus of osteomyelitis taking a unique form: the ring sequestrum. In this situation, a zone of separation occurs between the living and dead bone around the hole. Local stresses cause the living bone to convert into strain-tolerant granulation tissue, separating it from the nonviable bone immediately around the hole. Thus, does the burnt bone take the shape of a ring or cylinder surrounding the pin or wire? When the implant is removed, the ring is often left behind, creating a persistent nonviable surface for microorganisms to inhabit.

The matter of heat generated during reaming for an intramedullary lengthening nail is discussed elsewhere in this monograph, but this section deals with heat generated by a drill bit during venting .

The quality of regenerate bone formation in a widening distraction gap depends on many factors described in previous chapters. The new-forming bone grows from both sides of the gap, so it stands to reason that viable bone cells are essential for rapid regenerate formation and consolidation. Hence, thermal injury caused by drilling vent holes will likely degrade the regenerate.

Clearly, a sharp drill bit, a stop-start drilling pattern, irrigation of the drill bit, and a measure of patience will reduce thermal bone damage [2]. The issue of depth plunging, a source of potential damage to structures deep to bone being drilled, is considered in the chapter containing the anatomy cross sections .


Osteotomy Technique


To reduce the likelihood of displacement, experienced surgeons will create an incomplete osteotomy at the level of transection. Subsequently, the surgeon inserts the nail up to, but not across, the osteotomy site. The surgeon next completes the osteotomy with a gentle tap on the chisel or osteotome, placed subperiosteal, taking care not to displace the adjacent bone fragments with respect to each other. The nail is then immediately advanced across the osteotomy site, thereby aligning the fragments (Figs. 5.12 and 5.13).

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Fig. 5.12
Advance the nail up to the vented osteotomy site after reaming the canal for nail placement. Complete the osteotomy using the percutaneous technique while protecting the periosteum. Copyright 2016 Zeeca Publishing Co


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Fig. 5.13
Immediately after completion of osteotomy, advance the nail across the osteotomy site to prevent displacement. Copyright 2016 Zeeca Publishing Co


Transverse Locking Screws : Proximal Guide


Proximal locking of intramedullary lengthening nails is performed in the same manner as the technique developed for trauma nails. The insertion guide always includes holes that line up with the holes in the nail when the guide is securely fixed to the implant. However, mismatching is possible; the guide could, conceivably, be attached backward or to the wrong device. Therefore, the surgeon should make sure that drill sleeves, drill bits, and perhaps even the screws line up precisely with the locking screw holes of the implant when passed through the holes on the guide handle. This is accomplished by preassembling the proximal guide to the nail and checking that the implant’s holes line up with the drill sleeves.


Testing the Implant


Intramedullary lengthening nails incorporate complicated mechanisms that must be tested before the patient leaves the operating room. One or two millimeters of separation should be enough to visually confirm that the device is functioning properly. Observation of a widening of the osteotomy gap or a change within the implant itself can satisfy the need for confirmation. With the PRECICE® nail, for instance, widening of the internal leadscrew gap confirms lengthening.


Preliminary Osteotomy Gap


While some surgeons, to save operating time, leave the bone ends distracted a couple of millimeters after this separation test, the classic Ilizarov strategy calls for reduction of the osteotomy back to a non-displaced fracture. That is, the bone ends should be restored to their anatomic position before the patient is sent to the recovery room, if the device permits retraction of the nail. This author believes that delayed consolidation of the regenerate described by many authors using intramedullary lengthening implants can be traced to the initial unreduced gap at the osteotomy site.


A Spare Implant


Because of the mechanical complexity of intramedullary lengthening nails, the implants cannot be autoclaved for sterilization. Instead, gamma rays are used to kill all potential microbes on or in the device. Therefore, if a surgeon or assistant fails to make a diving grab of an intramedullary lengthening nail before it hits the floor, having a spare nail of the same size nearby will seem like a wise strategy indeed!


Correcting Deformities with Intramedullary Lengthening Nails


At present, the ability to make substantial deformity corrections with lengthening intramedullary nails is rather limited. Too much manipulation of the osteotomy site risks retarding regenerate new bone formation in the distraction zone—a region whose intramedullary blood supply has already been compromised by reaming for the nail. Even simple corrections of rotational malalignment with the nail in place are appealing, but excessive rotation at the osteotomy site, beyond 15–20°, may slow maturation of new bone formation.

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Nov 5, 2018 | Posted by in RHEUMATOLOGY | Comments Off on Surgical Principles of Intramedullary Lengthening
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