THE USE OF STRUCTURAL ALLOGRAFT TO COMPENSATE FOR BONE LOSS IN ARTHRODESIS OF THE FOOT AND ANKLE
January 1st, 2002
Steven K. Neufeld, MD, Jaime Uribe, MD, and Mark S. Myerson, MD
SYNOPSIS
Structural cadaveric allograft for surgical procedures of the foot and ankle is a useful adjunct in the treatment of various foot arthrodeses. This paper will review the possible uses of structural allografts and will outline common structural allograft techniques in foot and ankle surgery.
INTRODUCTION
Bone grafting has been used to treat conditions including delayed union and nonunion of fractures. It is also used to treat osseous defects from trauma, infection, and tumors and to augment arthrodeses. Bone remains the second most common transplanted tissue or organ, blood being the first. Bone grafts may be cortical, cancellous, or corticocancellous. Cortical bone exhibits superior structural properties, whereas cancellous bone demonstrates superior osteogenic properties. If structural strength is required, cortical or corticocancellous bone grafts are used. Traditionally, corticocancellous bone graft has been autograft harvested from the iliac crest.27,28 Although this graft can successfully span large defects of the foot or ankle, the potential complications at the harvest site include pain, bleeding, infection, nerve injury, and fracture.3,4,16 In addition, when a large amount of structural bone graft is necessary, the amount of autograft available may not be sufficient. For these reasons, many surgeons have turned to alternative sources of graft material.
Structural allograft offer several potential advantages as compared with autograft. Unlike autograft, structural allograft prevents the morbidity associated with the harvest of autogenous graft and markedly reduce the time of surgery. Unlike iliac crest autograft, allograft is available in unlimited supply and shape, for they are harvested from cadavers and have prolonged storage capacity. Allograft is generally available as particulate, intercalary, or osteochondral transplant material.
The use of structural cadaveric allograft for surgical procedures in the foot and ankle is a useful adjunct in the treatment of various foot arthrodeses. Many studies suggest similar or even better results when substituting allograft for autograft.15,27-29 The use of allograft is well documented in the treatment of benign and malignant bone tumors,19 wrist arthrodesis,2,9 large defects in fractures, spinal arthrodesis,14,25 and in foot and ankle surgery (Myerson, M. S., Neufeld, S. K., and Uribe, J., manuscript in preparation).10,15,29
This paper will review the rationale for using structural allografts and the possible uses of structural allograft in foot and ankle surgery. Surgical techniques and examples will be described.
BONE AND TISSUE BANKING
The availability, safety, and efficacy of structural allogenic bone depend largely on the methods used for banking these tissues.18 The goals of bone banking are to preserve the physical integrity of the allograft and its inductive proteins, reduce immunogenicity, and ensure sterility. Guidelines for the procurement, processing, and clinical use of bone have been established by the American Association of Tissue Banks (AATB, McLean, VA). Freezing at –70C effectively decreases immunogenicity and maintains sterility, but decreases both the tensile and the compressive strength of the allograft by approximately 10%.5 Ethylene oxide sterilization is also effective, but may destroy the inductive proteins of the bone. With this method, the ethylene oxide is removed and the bone is preserved by freeze-drying. Freeze-drying decreases torsional strength by about 50% and compressive strength by 10%.34 In general, bone subjected to freeze-drying, deproteinization, or freezing incorporates more slowly than fresh autograft.11,23 Decalcified allograft undergoes repair and incorporation at a faster rate than allograft that is frozen without decalcification, although decalcified allograft is mechanically weaker.32
BIOLOGY OF ALLOGRAFT INCORPORATION
A successful bone graft will eventually become incorporated into the host’s native bone. When rigid fixation is used, massive frozen allografts are incorporated well and are clinically successful.26,33,35 The host replaces the donor bone tissue by depositing new bone, and the speed of incorporation depends on the size, structure, position, fixation, and genetic composition of the graft. It is well known that allograft provides the form and matrix of bone tissue, but no viable cells. Allograft is more slowly incorporated into the host than autograft. Fresh allograft induces an immune response that may delay the osteoinductive phase of bone graft incorporation by eliciting antibody production and cell-mediated immunity. Ultimately, the donor tissue is destroyed.8,21 When allograft preparation has not destroyed the bone morphogenic proteins within the grafts, osteoinduction as well as osteoconduction is possible. An osteoinductive graft may incorporate more rapidly than one that is merely osteoconductive. Incorporation of any bone transplant requires callus formation at the bone graft-host junction and internal repair of the allograft.30 Furthermore, if successful host-graft union occurs by 1 year, the differences between an allograft and an autograft gradually diminish. Long-term studies show no statistically significant difference in the morphology of repair between autograft and allograft.20,36
In addition to creeping substitution, which involves the invasion of the allograft by osteoclasts followed by a blood vessel bud and new osteoblasts,8 structural allograft may be incorporated by a process of serial stress fractures resulting in graft remodeling. If the graft is subjected to excessive strain, microcracks may develop, followed by local remodeling. This process may occur in other parts of the graft and can become apparent by complaints of pain, often after very minor trauma. Even though radiographs may be normal in such cases, the patient should be on crutches or other support until the pain is gone to prevent a catastrophic failure of the graft.21 During our period of study, despite the use of very large structural allograft (up to 4.2 cm in length) we have not diagnosed a stress fracture through the graft. It is possible that this occurs only with larger grafts in the lower extremity.
RISKS ASSOCIATED WITH THE USE OF FEMORAL HEAD ALLOGRAFTS
In addition to understanding the methods of processing and storing bone allograft, it is important to screen for and protect against bacterial, fungal, or viral pathogens. Measures taken should include a detailed patient history and screening tests for hepatitis, HIV, and syphilis. One concern with the use of structural allograft is the possible transmission of disease or malignancy. This risk is small. In an analysis of 1,146 femoral head allografts,42 there was one case of a low-grade chondrosarcoma and two lymphocytic lymphomas. A review of 303 allografts provided by the United States Navy Tissue Bank40 revealed one case in which a contaminated allograft was potentially responsible for clinical infection. There are a few sporadic case reports of viral transmission through allografts, and the risks may be related to the type of allograft used.1,12 For example, the risk of viral transmission through processed, freeze-dried allograft chips is practically zero, whereas the risk of transmission through transplantation of a frozen, unprocessed femoral head is similar to the risk of transmission of a disease through transfusion of a unit of blood.38 Procedures designed to ensure the supply of safe allogenic bone of good quality for clinical use are well established and include donor selection, tissue procurement, preservation, and storage. Screening is undertaken to exclude donors with potentially serious transmissible diseases, malignancy, and systemic disorders.17,37,39
Infections from structural allograft result from a combination of host factors and possible contamination of the graft. Host factors include systemic disease, debilitation, and multiple surgeries. Most of this information comes from studies using structural allograft in total hip arthroplasty, total knee arthroplasty, spine fusion, or malignancy reconstruction.38 Most infections occur within the first postoperative month and are related to wound problems. Pin tract infection, skin slough, or skin necrosis are the most frequent risk factors in many series.13,24 These reports show an infection rate of 10% to 15% with the use of massive allografts. There was a 0% infection rate in a study of 113 patients requiring smaller allografts in the form of cancellous or corticocancellous chips.41 In a series from our institution (Myerson, M. S., Neufeld, S. K., and Uribe, J., manuscript in preparation), there was a 2.7% incidence of deep infection using a structural allograft in 78 patients. To our knowledge, there are no other studies reporting infection with the use of structural allograft in the foot and ankle.
The risk of HIV is even less. In the worst-case scenario (e.g., a large frozen allograft with peripheral blood and marrow still remaining in the graft), the risk of the transmission of HIV is approximately 1 in 1 million if the bone is obtained from a tissue bank accredited by the American Association of Tissue Banks.7
SELECTION AND PREPARATION OF THE ALLOGRAFT
An ideal graft should be strong, potentially viable, sterile, storable, affordable, and capable of being shaped during surgery. The eventual choice, however, depends on the type of structural functions desired and the size and shape of the various donor bones (e.g., femoral head, chips, diaphysis). With these criteria in mind, fresh-frozen cadaveric femoral head and neck allograft is the preferred source for structural allograft in the foot and ankle. A femoral head allograft can be used in arthrodesis of the foot and ankle when bone loss or the need for large angular correction necessitates the use of more autograft than can be harvested from the patient. During the operation, the sterility of the allograft should be protected. Avoidance of prolonged exposure to air, saline, and heat can prevent destruction of cells and inductive factors.5 Heating the host bed with power burrs or coating bleeding surfaces with bone wax also destroys viable host cells and can impair subsequent incorporation of the graft.11 For these reasons, we rarely use a burr in the host bone or bone wax on the interface surfaces. Contouring the allograft with a saw or burr does not have the same drawbacks.
Specimens are thawed in normal saline for 10 minutes before use, and a decision is then made regarding which portion of the femoral specimen will be used. This decision is determined by the location, size, and shape of the arthrodesis or osteotomy. Whenever possible, the thicker cortical calcar region of the femoral neck should be used for greater structural support. The size of the graft is determined after the patient undergoes resection and debridement of all sclerotic and avascular bone, leaving bleeding margins to accept the allograft. A ruler is used to measure the exact size of graft needed. A smooth lamina spreader can be placed in the arthrodesis site and withdrawn once the graft is impacted in place (Fig. 1). Care must be taken not to crush the patient’s bone during distraction.
The challenge for the surgeon is to form a three-dimensional mental image of the size, depth, and height of the defect. Once this image is formed, the femoral head must be customized and shaped using reciprocating saws, burrs, rongeurs, and osteotomes (Fig. 2). In general, we use a femoral head allograft and fashion the transplanted graft from a portion of the head that would maximize the dimensions of the osteotomy or arthrodesis site. If the graft does not replicate the defect accurately, unwanted rotational and angular deformities can occur in the joint being fused and/or in adjacent joints.
Frequently, the surgeon is operating in an area of decreased blood supply, poor skin coverage, or residual organisms from previous infections. It is important to have good skin coverage present or obtainable at the time of surgery, and there should be no chronic or active infection. If the blood supply is compromised, the graft should be protected longer than usual until healing is complete. It is impossible to know exactly when bone incorporation has occurred because it is not clearly evident radiographically due to the exact apposition of the graft with the host bone. Alternatively, we use clinical parameters such as warmth, swelling, and pain, and in complex cases a computerized tomogram may be obtained. Below we outline some of the more common procedures for which we have used structural allografts in the foot and ankle.
STRUCTURAL ALLOGRAFT PROCEDURES FOR THE FOOT AND ANKLE
Distraction Subtalar Joint Bone-Block Arthrodesis
Arthrodesis of the subtalar joint may require bone grafting in situations where posttraumatic arthrosis is combined with severe malalignment and bone loss. Typically, this occurs after intra-articular calcaneal fractures with severe joint depression, followed by a decrease of the talar declination angle, loss of height of the hindfoot, and impingement of the anterior ankle. Additionally, bone graft may be necessary when the subtalar arthrodesis is part of a triple or pantalar fusion for severe structural deformities. In bone-block arthrodesis, the size of the structural allograft is determined by the amount of distraction required to correct the talar declination angle. With a lamina spreader in place, intraoperative radiographs or fluoroscopy can determine the proper position of the talus. The posterior height of the subtalar space can also be measured intraoperatively with a ruler (Fig. 3). We use a vertical incision for this procedure, because any distraction in the height of the hindfoot will increase the incidence of wound complications if an incision is made in a plane contrary to the distraction. A transverse incision will tend to gape once height is restored (Myerson, M. S., Neufeld, S. K., and Uribe, J., manuscript in preparation).
In a patient where the calcaneus fracture has healed in varus, an osteotomy through the original primary fracture line (running proximal, dorsal and lateral to distal plantar and medial) can be made at the time of the subtalar distraction arthrodesis. In rare cases, the varus deformity cannot be corrected simultaneously with the heel height and the varus cannot be corrected through the subtalar joint fusion. For these patients, an osteotomy of the calcaneus in addition to the distraction with allograft is required. This will enable correction of the heel back into valgus while distracting the subtalar joint. A lamina spreader is placed in the medial part of the subtalar joint to overdistract the contracted soft tissues and correct the varus deformity. The structural allograft then needs to be shaped into a trapezoidal block; therefore, the graft is harvested from either the central portion of the head or from the calcar. This block needs to be configured in two planes because the block is higher medially and lower laterally, and also declines from posterior to anterior to provide correction.
Once the block is in proper position, one guide pin is placed into the talar body and another pin is placed into the talar neck. Two fully threaded large (6.5-mm) cancellous screws are required for rigid internal fixation. Partially threaded screws may be used, although excessive compression across the arthrodesis is avoided to prevent crushing of the graft (Fig. 4 A-C). Postoperatively, the patient remains nonweightbearing for 6 weeks in a below-the-knee cast followed by 6 weeks of partial weightbearing in a cast or walking boot.
Although a cortical or corticocancellous structural graft is strong when first implanted, the incorporation process inevitably weakens it, and a fatigue fracture through the graft may occur many months or years after implantation.6 Therefore, plates, screws, and other rigid internal fixation devices are used to augment the strength of the graft during incorporation. In addition, when a bone graft has both a cortical and a cancellous surface, exposing the cancellous bone to the surrounding soft tissues facilitates vascularization and enhances incorporation of the graft.
Ankle Joint Arthrodesis
Arthrodesis of the tibiotalar joint may require a structural graft when end-stage arthritis exists with combined marked structural deformity. Structural allograft frequently can be used in posttraumatic ankle joint arthritis with severe valgus deformity or deformed ankle joints associated with rheumatoid arthritis. Comminuted fractures of the distal tibia with subsequent avascular necrosis and resorption of the plafond will require structural allograft to restore bone stock to the arthrodesis site (Fig. 5). Additionally, in unsuccessful total ankle arthroplasties that require fusions, a structural allograft can be used to fill the void left by the prosthesis (Fig. 6 A, B). The important surgical goal is to restore the height of the tibiotalar joint with the structural allograft. This will maintain fibular length and prevent impingement of the fibula on the calcaneus. If the fibula is too long, it may impede subtalar motion.
Fixation of the structural allograft to the host bone needs to be rigid with at least three 6.5- or 7.3-mm partially threaded cannulated cancellous screws. The first screw is placed from the medial aspect of the distal tibia proximal to the medial malleolus into the body of the talus. The second and most important screw is placed from the posterior lateral aspect of the tibia (lateral to the Achilles tendon avoiding the sural nerve) into the neck of the talus. The surgeon must remember to make the skin incision for this screw more proximal than the anticipated tibial entry point. This is typically 6 to 7 cm above the tibiotalar joint line. The third screw can be placed from the fibula or from the anterolateral tibia into the talar body.
Tibiocalcaneal Arthrodesis
Structural allografts can be used in these complex hindfoot reconstructions. Frequently, there is bone loss from avascular necrosis of the talus or tibia or severe deformity from prior trauma or infection. Two options exist with a necrotic talar body. The first option involves removing the necrotic bone, leaving the viable head, and directly fusing the tibia to the calcaneus. The disadvantage of this procedure is the loss of hindfoot height. The advantages include the ability to avoid using an allograft therefore limiting the healing to only one arthrodesis surface. Although the latter is feasible, obtaining flush apposing bone surfaces for a tibiocalcaneal fusion is difficult due to the inherent differences in the joint architecture. The second option, which maintains limb height, is to use a large allograft to correct the bone loss. A lateral approach is used with removal of the remaining fibula. Once any remaining articular surfaces are denuded, the fibula may be morselized and used to augment the allograft (Fig. 7 A-C). Positioning of the arthrodesis is difficult, and the posterior aspect of the tibia should be aligned with the posterior facet of the calcaneus. Guide pins are then inserted to maintain position and check alignment, after which sizing of the graft is possible. The graft is usually a large triangular or trapezoidal shape that is wedged into the anterior aspect of the articulation to prevent any dorsiflexion of the hindfoot and subsequent calcaneus deformity.
Lateral Column Lengthening
Two procedures exist to achieve lengthening of the lateral column. The first is through a distraction calcaneocuboid arthrodesis, and the second is through a calcaneal neck lengthening osteotomy. In both of these procedures, structural allograft has been used successfully (Myerson, M. S., Neufeld, S. K., and Uribe, J., manuscript in preparation) (Fig. 8 A-D). The calcaneal neck lengthening osteotomy is made distal to the medial facet. This will allow adequate correction and prevent penetration into the articular surface. The entry point for the osteotomy is approximately 1 to 1.5 cm proximal to the calcaneal cuboid joint and aimed from proximal lateral to distal medial. Therefore, this osteotomy is not parallel to the calcaneocuboid joint or perpendicular to the lateral border of the foot, but should end between the anterior and middle facets of the subtalar joint.29 The structural allograft is fashioned as a trapezoidal wedge with the lateral wall measuring approximately 1 to 1.3 cm and the medial wall about 5 mm smaller. The graft is contoured wider dorsally to assist in restoration of an arch. While distraction arthrodesis of the calcaneocuboid joint is a common procedure in some institutions, the current authors perform lateral column lengthening with an osteotomy and not an arthrodesis.
Charcot Foot Reconstruction
Most Charcot foot reconstructions involve resection of bony deformities to re-create a plantigrade, braceable foot. There may be situations, however, when an unstable midfoot or hindfoot deformity will need structural graft to achieve this goal. In some of these patients, structural allograft may be used. When the tarsometatarsal joint is involved, there is often collapse of the arch combined with abduction of the midfoot, resulting in a rockerbottom deformity. Often the deformity is quite severe and is refractory to bracing or custom-molded shoes. Patients who do not respond to brace or shoe modifications frequently require surgical reconstruction to prevent development of nonhealing pressure ulcers. In these cases, an interpositional structural allograft can provide the angular correction needed to obtain the stable, plantigrade foot. Furthermore, the deformity involving the subtalar or transverse tarsal joints can result in severe valgus or varus foot malposition.
Hallux Metatarsophalangeal Joint Arthrodesis
Arthrodesis of the first metatarsophalangeal (MP) joint is one of the most common locations where a structural allograft is useful. After an unsuccessful first MP joint prostheses, it is common to have substantial bone loss requiring extensive debridement and bone grafting,22 and it is often difficult to obtain an autograft large enough to fill the defect. If bone loss is not clinically severe, arthrodesis of the hallux MP joint may be performed as an in situ position that leaves the hallux shorter.31 This procedure may be easier to perform, but if the first metatarsal is already short and if lesser metatarsalgia is present, an in situ arthrodesis of the hallux MP joint may not improve clinical function. In these situations, restoration of length of the hallux with interposition allograft may be used as part of the salvage procedure. This will improve the weightbearing support of the first metatarsal and the alleviate pain underlying the lesser metatarsal heads.
It is important to debride the host site to healthy, bleeding tissue, because a healthy vascular bed is essential for graft incorporation. Fixation must be stable, and postoperative nonweightbearing is often delayed until radiographs demonstrate consolidation at the host-graft interface. The graft should be harvested from the margin of the neck and the curved femoral head and contoured to fit into 15º of dorsiflexion of the hallux with respect to the metatarsal shaft (Fig. 9 A-C).
SUMMARY
Allografts have several important advantages over other forms of bone graft augmentation. In addition to preventing the morbidity of autogenous bone graft harvesting, the quantity of allograft bone is essentially unlimited and is therefore valuable for use in treating very large defects that exceed the yield of the iliac crest. An allograft is particularly helpful in obese patients or patients with systemic disease because the allograft has the potential to reduce blood loss and anesthesia time. Regional anesthesia with either an ankle or a spinal block may be safer for the rheumatoid patient with cervical instability. In addition, the use of an allograft makes it possible to do many foot and ankle fusions as outpatient procedures, whereas it is commonly necessary to hospitalize patients who have bone graft harvested from the iliac crest. Like the findings of other authors, our experience suggests that structural allograft is an attractive alternative in reconstructive surgery and in arthrodesis of the foot and ankle and is a valuable part of the surgeon’s armamentarium.
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