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
Bone deformationContracturesSubluxationsDislocationsRing prophylaxisBlocking screwsGuidance screwsLatencyRate and rhythmTensor fascia lataTardy regenerateBackground
All limb lengthening operations and the majority of deformity correction procedures have potential to create secondary deformities in long bones, as well as contractures, subluxations, and dislocations of adjacent joints. This seems paradoxical; after all, how could an operation designed to correct a deformity cause one?
Certain biological tissues do not readily lengthen, whereas others do. For instance, nerves and blood vessels can be easily stretched with slow, steady traction, applied over a period of days, weeks, or even months. Indeed, this is how nerves are repaired in cases of traumatic segmental loss. When, for example, such a loss (a couple of inches) occurs in the anterior forearm, the surgeon bends the elbow sufficiently to approximate the nerve stumps and sutures them together. After preliminary healing of the suture line, the patient’s elbow is gradually extended, at the rate of a couple of degrees each week, elongating the repaired nerve.
There are, however, structures of the limb that do not readily elongate, resisting traction. Tough bands of fascia are particularly hard to stretch, as are tendons. Muscle tissue, although somewhat softer, also resists elongation, both actively (by contraction) and passively, primarily through resistance of the perimysium, the membrane that surrounds individual muscle bundles [1].
In general, undesirable bone deformities during elongation are a consequence of resistance from longitudinal fascial bands within the limb , whereas secondary joint problems are the result of resistance to elongation by musculotendinous structures.
In both situations, knowledge of surrounding anatomy allows the surgeon to predict likely problems and to establish prophylactic strategies to prevent these problems from occurring.
As a principle, techniques designed to prevent the bone from undesirable angulation (or translation) during lengthening are incorporated in the surgical procedure, while methods of preventing joint contractures, subluxations, and dislocations are part of the postoperative regimen.
Operative Strategies to Prevent Bone Deformities During Lengthening
Bone Deformities During Lengthening
Whenever a surgeon performs limb elongation , the bone has a tendency to deform with its apex opposite to the thickest muscles or densest fascia. For this reason, experienced surgeons apply hardware in strategic locations to prevent such angulation during elongation.
At every bony level, whenever fragments are moving with respect to each other, typical patterns of deformity occur. The surgeon must be ever vigilant, with the aim of preventing such deformities from occurring if possible and dealing with the problems as they happen.
Tibial Deformation
When lengthening a tibia, for example, the power of the calf musculature has a tendency to produce knee flexion contractures (gastrocnemius), ankle equinus (gastrocnemius/soleus), and antecurvatum (apex forward) of the osteotomy site. Simultaneously, resistant anterior compartment components, and especially the interosseous membrane, can cause the deformity apex of a lengthening tibia to point medially. The combination of valgus and antecurvatum forces acting upon a tibia will result in deformation of the elongating bone with its apex anteromedial. Moreover, in view of the observation that thick fascial tissue contributes to angular deformities of long bones during lengthening, it is no surprise that the apex of the deformity of any elongating tibia is opposite to the attachment of the interosseous membrane on the posterolateral edge of the bone .
Femoral Deformation
In the thigh, the hamstrings and linea aspera generally cause the femur to angulate apex anteriorly during lengthening. The proximal femur is particularly prone to anterior angulation, as a product of flexion produced by the iliopsoas at the hip. In the coronal plane, the proximal femur tends to angulate into varus, due to the combined action of the hip abductors attached to the greater trochanter of the femur and the adductors inserting along the distal shaft. This combination of deformities results in a proximal femoral angulation with its apex directed anterolaterally.
An osteotomy of the distal femur may angulate toward either valgus (because of tension from the iliotibial band) or varus (the result of adductor muscle pull) during distraction.
The proximal humerus tends to angulate into varus (with an anterolaterally directed apex) because of the abductor strength of the rotator cuff and the action of the medial upper arm muscles. Distally, the humerus usually angulates with the apex posteromedial .
Forearm Deformation
In the forearm, the ulna can angulate apex anterolateral, while a distal radial osteotomy may angulate apex medially. With forearm applications, however, muscle tension is likely to be insufficient to overcome the intrinsic resistance to deformation offered by an intramedullary nail.
Preventing Deformities (External Fixator Principles)
Ring Prophylaxis
Because bone deformities during elongation occur with every method of limb lengthening surgery, specialized techniques specific for each type of apparatus have evolved.
Ilizarov and co-workers were, obviously, the first surgeons to observe such problems. To mitigate these issues, they created a tension wire fixator technique called “ring prophylaxis.” Based on experience and anatomic considerations, Ilizarov surgeons, when applying a circular frame for a simple longitudinal lengthening, do not mount the rings parallel to each other and perpendicular to the long axis of the bone, as is often shown in illustrations. Instead, they tilt the ring nearest the anticipated level of deformity in a manner that parallels such a deformity if it actually exists at the time of surgery .
In other words, as osteotomies of the upper tibia typically angulate during lengthening with the apex of the evolving deformity pointing anteromedially, the proximal ring of the configuration is tilted higher on the anteromedial corner and lower on the posterolateral corner. Since, in this position, the ring is no longer perpendicular to the longitudinal connecting rods of the frame, there must necessarily be hinges between the ring and the four longitudinal lengthening rods. Moreover, the rotation axes of each of these four hinges must be parallel with each other and perpendicular to the plane of the anticipated deformity.
Once lengthening begins, the patient is instructed to lengthen the short posterior and lateral distracting rods at a greater rate than the anterior and medial ones. In this manner, the hinged ring is gradually tilted downward in the anteromedial corner and upward in the posterolateral corner as elongation proceeds. Thus, the ring and its associated tension wires counteract the evolving deformity by correcting for it as it occurs .
Such ring prophylaxis is a characteristic feature of a properly applied, circular, tensioned wire, Ilizarov external skeletal fixator. The flexibility of Ilizarov’s tension wires make such ring prophylaxis necessary. Without this prophylaxis, bones deform predictably during lengthening (Fig. 6.1).
Fig. 6.1
Ring prophylaxis with the Ilizarov method. The frame configuration anticipates the likely deformity during elongation, in this case the apex of deformity, and is tilted as though the deformity already exists (left). As the deformity evolves , the frame is gradually squared off (middle) to correct changes as they occur. At conclusion of the lengthening process, the frame is squared off, with the rings parallel and the deformity prevented (right). Copyright 2016 NuVasive
Strategic Pin Placement
With introduction of stiff half-pins as a substitute for flexible tensioned wires in many locations of an Ilizarov frame configuration, a technique developed by the author while working at Rancho Los Amigos Medical Center, one would think that the necessity for ring prophylaxis would be mitigated. Indeed, the “Rancho technique,” as it is known, reduces the tendency for bone deformation during elongation, but does not eliminate it completely. However, the use of half-pins in many anatomic locations allows the surgeon to insert a pin on both sides of an osteotomy in the plane of an anticipated deformity during elongation. In this manner, for a deformity to occur during elongation, the bone must push directly into a transcutaneous implant placed to prevent that deformity. This is usually sufficient prophylaxis, if local neurovascular anatomy allows such pin placement.
The Waypoint Method
With the introduction of hexapod circular external fixation (i.e., the Taylor Spatial Frame), connection of the fixator to bone can be accomplished with either tensioned wires, half-pins, or a combination of both. Therefore, one would think that either ring prophylaxis or strategic pin placement would be required to prevent deformation during lengthening, depending upon the mounting components used to secure the frame to the bone.
As it turns out, however, the computer program used to create the prescription for daily strut length changes can also generate a modified prescription to deal with evolving deformities as they occur during limb elongation. This “waypoint” method of dealing with secondary bone deformities is similar to interim recalculation of a ship’s planned route to account for changing seas and shifting winds (Fig. 6.2).
Fig. 6.2
Deformity prevention with the waypoint method . The frame is mounted orthogonal to the bone with rings parallel to each other (left). As the deformity evolves, the angulation of the deformity is measured and entered into a computer containing the case parameters (middle). At the end of elongation, the rings are nonparallel but the deformity is corrected (right). Copyright 2016 NuVasive
Corrections involve reentering the “deformity” parameters that define the evolving angulation and translation problem while maintaining the mounting parameters, existing strut lengths, and other data that went into the original prescription.
Needless to say, waypoint corrections can also be made with classic Ilizarov-type ring fixators and those with various modifications. Typically, however, such corrections require exchanging hinges placed at the apex of the evolving deformity for the longitudinal rods and the attachment of twisted plates and a distraction strut on the opposite side of the configuration; this exchange requires approximately 2 h of office time.
Biomechanical Axis Considerations
The objective of any lower extremity limb lengthening or deformity correction procedure is to maintain, restore, or achieve a natural biomechanical axis. While the details of deformity correction are beyond the scope of this publication, it is worthwhile to consider the effects of lengthening a bone along its own anatomic axis. After all, that is exactly what intramedullary lengthening devices do: they lengthen the marrow canal and surrounding cortex along the device.
Tibia
When elongating an already normally aligned tibia, the anatomic axis (a line following the center of the bone) and the biomechanical axis (a line passing through the middle of the femoral head, the middle of the knee joint, and the middle of the ankle joint) are nearly identical. Therefore, lengthening a tibia, whether using external or internal fixation means, typically maintains both the anatomical and biomechanical axes.
Femur
The femur , however, presents a far more challenging problem. In the coronal plane, the femur slants inward approximately 7° from vertical. Thus, from the anterior view, the anatomic axis of the bone makes a “V” with the biomechanical axis.
In the sagittal plane, when viewed laterally, the femur has a curving anterior bow, resulting in a curved anatomic axis. The biomechanical axis, however, goes straight down to the floor from the center of the femoral head.
Since it is nearly impossible to reproduce the curve of the femur with either external or intramedullary bone lengthening, elongation of the bone has different consequences when comparing external skeletal fixation to intramedullary lengthening devices.
With the external fixator, an osteotomy at the apex of the curve results in a straight segment between the curved fragments.
With an intramedullary lengthening device, an osteotomy at the apex of the curve tends to convert the femur into two half curves meeting at the osteotomy site once the implant is fully inserted. This has the effect of shifting the knee anteriorly at the end of lengthening, in the lateral view.
Sagittal Plane Malalignment
Shifting the mechanical axis of the femur anteriorly probably has little, if any, long-term effect, because, in general, the body is reasonably tolerant of joint deformities in the plane of function of that joint. Since the knee flexes and extends in a plane parallel to walking forward, angulation of the distal femur or upper tibia is better tolerated in the sagittal (laterally viewed) plane than in any other plane. The reason for this seemingly strange observation is that a joint typically uses only a small portion of its total range of motion in day-to-day functioning. Walking on level ground, for instance, involves only 30–40° of motion, whereas the total range of motion at the knee joint is approximately 135°. Therefore, reduced motion of the knee joint is well tolerated in Western culture, especially if the loss is at the extreme of flexion. In Eastern cultures, however, where food is eaten while kneeling or sitting cross-legged on the ground, and excretion is accomplished by squatting, loss of flexion can be a significant problem.
Angular malalignment of the distal femur or proximal tibia in an apex-posterior direction has the effect of increasing apparent knee extension and decreasing flexion. Since, as noted above, knee flexion loss is better tolerated than knee extension loss, a patient with such deformity, if not too great, compensates during gait by preventing full extension of the knee during midstance, assuming the presence of normal muscular control.
Angular malalignment of the distal femur or proximal tibia in an apex-anterior direction effectively reduces knee extension; however, as the calf normally contacts the thigh at full flexion, additional gain in flexion will not be realized. The lack of full extension during midstance causes a flexed-knee gait pattern, which has the effect of shortening the limb and resulting in an uneven gait pattern .
Frontal Plane Malalignment
Malalignment of the distal femur or proximal tibia in the coronal (frontal) plane is far more devastating. Here, varus or valgus angulation typically leads to gradual erosion and osteoarthroses of the weight-bearing cartilage of either the medial or lateral compartment of the knee, respectively. Moreover, as the cartilage and underlying bones erode away, the joint gradually assumes a progressive varus or valgus tendency. This stretches the joint capsule and ligaments opposite the narrowing, a cause of significant pain with activity. Indeed, some authorities consider joint instability and concomitant capsular ligament stretching to be the principal reason for pain in erosive osteoarthroses, overshadowing pain caused by bone-on-bone contact.
Because the femur slants inward when viewed anteriorly, the anatomic axis of the bone (its centerline) deviates approximately 7° from the biomechanical axis—a line from the center of the ball of the hip joint to the center of the ankle. This line also passes through the center of the knee or may be slightly medial.
Femoral Valgization During Elongation
Lengthening the femur along its anatomic axis, whether with intramedullary or external devices, pushes the distal end of the femur progressively medial, thereby increasing the valgus thrust of the knee during weight bearing. The concern, of course, is the potential for lateral compartment osteoarthroses of the knee, as the outer side of the knee bears a disproportionate share of the weight-bearing load.
It is estimated that for each centimeter of femur elongation, the valgus attitude of the knee increases by 1° (Fig. 6.3).
Fig. 6.3
Lengthening the femur along its central anatomical axis pushes the knee joint medially, toward the opposite knee, causing valgization of that knee and lateralization of the limb’s biomechanical axis. Copyright 2016 NuVasive
Correction with External Fixators
Ilizarov recognized the significance of increasing valgization during femoral elongation and compensates for this tendency in a unique manner. Because of the modularity of the Ilizarov apparatus, an Ilizarov femoral lengthening frame contains hinges that angulate the elongating regenerate into valgus at the upper end and then into a corresponding varus at the lower end. The net effect of these two complementary angles is to create a zigzag-looking femur with the regenerate new bone parallel to the biomechanical axis, while the upper and lower ends of the bone retain their original angular relation with the biomechanical axis. Hence, valgization of the knee is avoided.
During the 1970s, femoral lengthening with the monolateral external Wagner® device was a popular method of dealing with a unilateral short femur, whether caused by a traumatic growth arrest or a variant of proximal femoral focal deficiency called “congenital short thigh” [2]. The device lengthened the femur, after open osteotomy, at a rate of 1 mm/day in a single step (a parent turned a knob at the end of the fixator for elongation). Regenerate new bone rarely formed with this protocol, except in very young children. Once the desired femoral length was achieved, the surgeon replaced the fixator with a sturdy internal plate and inserted an autogenous bone graft in the resulting distraction gap.
Because Wagner’s fixator was mounted parallel to the femoral shaft, it elongated the bone along its anatomic axis (Fig. 6.4).
Fig. 6.4
The Wagner® fixator typically lengthened the femur along its anatomical axis. Copyright 2016 NuVasive
Follow-up research, performed years later, showed that many patients subjected to this procedure developed knee joint problems in later life [3].
Valgus Prophylaxis with Intramedullary Lengthening Nails
Of necessity, an intramedullary lengthening nail elongates a femur along the axis of the implant—the centerline of the marrow canal. Thus, as with the Wagner external fixator, the knee and distal femur is pushed medially during lengthening.
To overcome this problem, Baumgart, the developer of the Fitbone® self-lengthening nail, devised a distally based (“retrograde”) nail insertion strategy to correct for the anticipated valgus angulation at the time of nail insertion. He calls this method “reverse planning,” because Baumgart originally used paper cutouts of the femur and tibia to trace the path of the angular correction needed to restore the knee to its proper relationship with the biomechanical axis of the limb [4].
His method works with any intramedullary lengthening nail system and should be considered in any femoral lengthening over 3 cm (Fig. 6.5).
Fig. 6.5
The Baumgart reverse planning method for preventing valgization of the knee during intramedullary femoral lengthening. With paper cutouts or a computer program, transect the femur image at the planned level (a). Place the center of the femoral head along the upward extension of the biomechanical axis at the planned final length (b). Next, slide the shaft fragment down the biomechanical axis back to the osteotomy level, indicating the offset needed to correctly lengthen the bone (c–arrow). The completed elongation will have a zigzag in the shaft, but the alignment will be perfect (d). Copyright 2016 NuVasive
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