B o n e Graft a n d Total Hip A r t h r o p l a s t y A Review

M i c h a e l H. H u o , M D , * G a r y E. F r i e d l a e n d e r , a n d E d u a r d o A. S a l v a t i , MD:[:

MD,t

The success of total hip arthroplasty has been well documented. Aseptic loosening remains the major long-term problem that can lead to significant bone loss and structural deficits. Bone graft has been used with increasing frequency to reconstruct these difficult cases. In this review, the authors detail the biology, biomechanics, and banking of bone grafts. A summary of currently available data on the clinical result of autograft and allografi in reconstructive hip surgery is presented. Key words: bone graft, graft biology, biomechanics, total hip arthroplasty. Abstract:

elated with the loosening process, can pose a significant challenge to the surgeon during revision THA. Similar situations may be encountered in severely dysplastic hips during primary THA. Solid bony support of prosthetic components is a prerequisite to stable fixation and successful clinical outcome. Bone grafting has become an integral adjunct to prosthetic reconstructions of the hip joint w h e n there is substantial bone loss. In this article, we present the principles of bone graft biology, currently r e c o m m e n d e d guidelines for bone banking, and the reported clinical results of using autograft and allograft in reconstructive hip surgery.

Total hip arthroplasty (THA) has become one of the most successful reconstructive procedures in orthopaedic surgery today. The clinical results in properly performed THAs in older patients have been excellent, even after I 0 - 1 5 years. 48"57'85 The results in young, active, or obese patients, and in patients with osteonecrosis or hip dysplasia, are less successful. 1'~3,~6 Revision THAs are being performed with increasing frequency. Failure of revision THA, necessitating rerevision, has been reported to be as frequent as 5 - 9 % at 2 - 5 years follow-up. 1~'46 Longterm f o l l o w - u p studies of c e m e n t e d revision THA and patients with multiple revisions, has not been encouraging. 47"66The incidence of radiographic loosening in patients after revision hip surgery has been reported to be as high as 45% w i t h i n the first 5

Graft Biology

y e a r s . 46

Bone loss in the pelvis or the proximal femur, assoBone and cartilage transplantations currently offer opportunities to reconstruct major skeletal deficits following ablative surgery for tumors, traumatic injuries, failed joint arthroplasties, and n u m e r o u s less extensive skeletal defects. 2 ~.53 Nevertheless, the precise mechanisms that control the biologic events of o s -

From the *Waterbury Hospital Joint Replacement Center, Waterbury, Connecticut; i-Department of Orthopaedics and Rehabilitation, Yale University School of Medicine, New Haven, Connecticut; ~Cornell University Medical College, and The Hospitalfor Special Surgery, New York, New York. Reprint requests: Michael H. Huo, MD, 1201 West Main Street, Waterbury, CT 06708.

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seous incorporation are not completely understood. Similarly, the surgical principles known to maximize the efficacy of bone grafts, as well as the circumstances that detract from their full potential benefit, may not be fully appreciated. Bone graft incorporation represents a predictable sequence of events that begins with intact bone homeostasis, progresses through events associated with fracture repair, and concludes with complex interactions between graft and host, leading to remodeling of the skeletal structure to fulfill the physiologic and mechanical demands of normal functional activity.

osteogenic cells. New bone formation takes place on the surfaces of preexisting cancellous trabeculae. In time, the graft is almost completely replaced by viable host new bone. l o Vascular invasion into cortical graft is slower. The incorporation process is never as complete as for cancellous grafts. An admixture of viable and preexisting acellular bone remains, even after years in situ. This circumstance, however, is usually compatible with satisfactory biologic and biomechanical function of the graft.

Autograft

Incorporation of vascularized grafts with an intact nutrient blood supply is characterized by minimal graft necrosis and a more rapid rate of repairY" 83 TissUe repair at either end of the vascularized graft is analogous to normal fracture healing. The advantages of this grafting technique include the short repair time, and that the graft does not depend on vascular and cellular contributions from the recipient bed. This is especially beneficial when the recipient site is significantly compromised, such as deficient soft tissue, pseudoarthrosis, infection, and prior irradiation. The most widely used donor sites include fibula, ribs, and the iliac crest, s2" 83 Limitations include poor host-site vasculature, potential donor-site morbidity, lengthy operative time, and surgical expertise. The use of immediately vascularized grafts is currently limited to fresh autografts since allograft transplantation is accompanied by an intense cellular immune response that precludes vascular patency.

The initial events in the incorporation of nonvascularized autografts are similar for cancellous and cortical grafts. The process is characterized at the onset by hemorrhage, and is followed by a nonspecific inflammatory response evoked by cell necrosis. Fibrovascular tissue derived from the host invades the graft site, and revascularization of cancellous bone occurs through the marrow spaces, as the new blood supply to cortical bone initially enters through preexisting vascular channels. This process in cortical bone is accompanied by osteoclastic resorption and widening of these Volkmann and haversian canals. It is crucial for the surgeon to appreciate that a transient but prolonged increase in porosity and decrease in mechanical strength of a cortical graft is a normal and inevitable consequence of the incorporation process. This weakness can be adequately addressed by the judicious use of internal fixation. Osteoinduction occurs when graft-derived factors actively stimulate osteogenic activity from the recipient bed. Surviving cellular elements in the graft may also contribute to this process. These signals emanate from the graft matrix in the form of molecules termed bone morphogenetic protein, 81 a low molecular weight, noncollagenous protein that has been extracted from the cortex, dentin, and various bone tumors. Bone morphogenetic protein is capable of stimulating DNA synthesis and undifferentiated cell replication, but does not appear to directly influence differentiated osteoblasts. Osteoconduction is a passive process characterized by recipient site-derived neovascularization, bone resorption, and new bone formation. This occurs within the framework or scaffold passively provided by the graft and its structural characteristics. Cancellous grafts differ from cortical grafts in the rate and completeness of repair. The intrinsic porosity of cancellous tissue allows for more rapid vascular invasion, which is a prerequisite to recruitment of

Vascularized Autograft

AIIograft The basic sequence of biologic events occurring during incorporation of bone allografts is similar to autogenous grafts. However, quantitative and temporal aspects of repair vary depending upon the type of graft, preservation technique, and immunologic disparity between donor and host. 23 Freeze-dried allografts are incorporated more slowly than deep-frozen allografts. 23" 41 However, both types of allografts have proven reliable in clinical practice. 54 Immunocompetent cells may play a significant role in regulating normal bone biology. 64 Therefore, the relationship between the immune system and bone remodeling may be of considerable importance to bone graft incorporation. The precise biologic significance of immune response evoked by bone allografts has not yet been clearly defined. Immunologic activity, however, may account for many of the clini-

Bone Graft and THA

cal failures associated with these reconstructive tissues. Animal studies have demonstrated the presence of cell-mediated and humoral immune responses following transplantation of fresh allogeneic bone.24, 34, 62 Frozen bone allografts are also capable of inducing sensitization, although measured responses are diminished. Similarly, freeze-drying causes more profound decrease in immunogenicity, often to the point of being undetectable by routine methods of bioassay. Limited studies in humans have also demonstrated development of HLA-specific antibodies in response to fresh and preserved bone allografts.25, 49 As observed in animal models, responses are reduced following freezing and more so, freezedrying. Sensitization, however, has not been conclusively shown to be associated with poor clinical resuits. 6 I Graft rejection, in general, is a function of T cellmediated responses. Investigations are now focused on characterization of alloantigen presentation, processing, and the activation of T cell subsets through mediators such as lymphokines. 42 Better understanding of the interactions between the immune system and the bone remodeling cycle may further improve the clinical success of bone transplantation. Another major issue pertaining to allograft is the effect of preservation techniques on bone biomechanics. Structural integrity of the bone graft is of param o u n t importance in reconstructive hip surgery. Both freezing and freeze-drying are commonly used methods for long-term preservation of bone allografts. Freezing does not alter the biomechanical properties of bone when tested in compression, torsion, or bending. Freeze-drying causes little change in compression, however a 50-60% reduction of torsional and bending strength has been described with this preservation technique. Irradiation has been used clinically to secondarily sterilize allografts. Irradiation with greater than 3 megarads has been shown to compromise the torsional strength by 10- 50 %. Furthermore, combination of freeze- drying and irradiation greater than 3 megarads, decrease the biomechanical strength of the allograft even more (30-70% reduction in torsion and bending). 65 Age, sex, and physiologic characteristics of the donor are also variables of potential significance to the initial biomechanical properties of the graft and its subsequent behavior during the incorporation process.

Bone Banking The goals of a bone bank are to assure a timely supply of safe bone allografts with predictable biologic and biomechanical properties suitable for their

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intended clinical application. In support of these requirements, bone banking methodology must address issues pertaining to donor selection, tissue recovery, preservation technique, long-term preservation, and record keeping.

Donor Selection Femoral head allografts are the most commonly used allograft material in reconstructive hip surgery. They can be readily obtained from appropriate donors during primary THA who do not need their heads as grafts, and stored until needed in another patient. Segmental osteochondral allografts are obtained postmortem. Appropriate authorization for donation must be obtained either antemortem from the individual, or more usually through permission granted by the next-of-kin following death. The Uniform Anatomic Gift Act of 1969 provides the legal foundation for expediting the required consent. 7° Both living and cadaveric donors must be screened extensively for potentially harmful transmissible diseases using present and past medical histories, and an autopsy whenever possible. Laboratory tests for routine bacterial, viral, and fungal cultures, as well as hepatitis and venereal diseases, must be routinely performed to confirm the satisfactory nature of the tissue for human transplantation. Other contraindications to donation include presence of malignancy, collagen and metabolic bone disease, toxic substance, or the chronic use of corticosteroids. The issue of Human Immunodeficiency Virus (HIV) transmission through bone allograft transplantation requires particular emphasis. HIV may be transmitted by blood transfusions or organ transplantation, emphasizing the need to eliminate potential donors with high-risk factors for this pathogen.5, 35 The first case of known HIV transmission through bone allograft implantation in the United States was reported by the Center for Disease Control in I988. The report described a woman developing AIDS 3{- years after receiving a frozen femoral head allograft for spinal fusion. The donor was later discovered to have AIDS, most probably acquired through intravenous drug abuse. Another case was recently reported by Simonds and associates. 75 Fifty-eight tissues and organs for transplantation were obtained from a 22-year-old man who died from a gun shot in 1985. He had no k n o w n risk factors for HIV infection. Two serum samples tested negative for HIV at the time of tissue recovery. However, HIV antigen was discovered in the donor's cryopreserved spleen cells later by these

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investigators. Forty-one of the 48 identified recipients were tested for HIV infection. All four recipients of organ (2 kidneys, heart, liver), and three of four recipients of fresh-frozen bone (2 femoral heads, patella, femoral shaft) became infected with HIV. None of the 2 5 patients who received freeze-dried, ethanol-treated bone grafts became infected. The most commonly employed approach used for detection of HIV infection is an ELISA screening test followed by the confirmatory Western Blot assay.31, 67 The sensitivity and specificity of the commercially available ELISA test kits range from 93-99%. It has long been known that an unpredictable interval exists between infection and the presence of detectable antibodies. This period is generally no longer than 3-6 months, but has been reported infrequently to exceed 1-2 years. Therefore, a repeat test is desirable 3 months following donation from living donors in order to minimize initially false negative results due to the latency period between infection and detectable antibody production. At present, the most practical and efficacious screening guidelines include a careful clinical history to identify donors in high-risk categories, and the use of commercially available screening tests for antibody detection at the time of donation and 3 months later in the case of living donors. Enzymatic amplification of viral DNA and peripheral lymphocyte cultures are more sophisticated tests that can identify infection earlier. 67 However, they are costly and quality control has been difficult. Nonetheless, assays for the virus (antigen) rather than antibody are being developed, which should be at least as sensitive, and probably more specific. These assays will eliminate false negative results associated with the latency period required for antibody production.

vious reports. 23 It is our preference to first wrap long bones in a plastic bag, then in two layers of sterile towel, followed by a final outer plastic bag. Femoral heads can be stored in double glass jars or wrapped in a manner similar to long bones.TS Regardless of the storage technique, the packages should be indelibly labeled with an identification code for the donor, date of tissue recovery, and type and size of allograft. Other essential information that should be recorded include: age, sex, pertinent medical history, blood type, and culture results. Tissue typing is not being done routinely for bone allografts since the biologic significance of histoincompatibility in humans has not yet been clarified.

Long-term Preservation There are numerous approaches for the long-term preservation of bone. Freezing and freeze-drying are the most common techniques. Freeze-drying equipment is relatively expensive, the process is time consuming, and the methodology requires close monitoring. This approach, however, has the advantage of indefinite storage time at room temperature, provided the evacuated containers remain intact. Freeze-drying is not compatible with cartilage preservation. Furthermore, it alters the biomechanical properties of the allograft.65 Freezing is relatively simple, inexpensive, and compatible with cyropreservation of articular cartilage using dimethyl sulfoxide or glycerol. It has been shown that proteolytic enzymatic activity within the graft can be eliminated by temperatures in the - 7 0 ° C to -80°C range. 22 Freezer equipment must be monitored and should have back-up systems in case of power failure.

Tissue Recovery Complications Tissue recovery from living donors is performed under sterile conditions, otherwise methods of secondary sterilization must be used. In the case of cadaveric donors, tissues may be recovered in a nonsterile environment. Nonsterile tissues require a method of secondary sterilization. Ethylene oxide and irradiation are two of the most commonly used methods. These additional processes may alter both the biologic potential and structural parameters of the grafts. Whenever possible, allografts should be retrieved and packaged in a sterile fashion, similar to customary operating room protocol. Surgical techniques for removal of segmental osteochondral allograft have been described in pre-

Similar to other tissues and implants used in reconstructive surgery, one must understand the biologic nature and temporal sequence of graft incorporation, as well as factors that may detract from their clinical success. Any biological or physical event that adversely influences graft-derived or recipient-derived contributions may interfere with graft incorporation. Autogenous bone grafts present the greatest biologic potential and are the least likely vector for transmissible diseases. These tissues, however, are obtained at a cost to the patient. Potential morbidity includes: (1) an additional incision; (2) increased

Bone Graft and THA • bleeding and prolonged operative time; (3) possible fracture of weakened donor site bone; (4) another site for infection; (5) pelvic instability from disruption of the posterior ligaments or sacral iliac joint; and (6) added discomfort after surgery. 17" 52, 68 Cortical grafts undergo a period of increased porosity due to resorption prior to regaining bone mass and normal biomechanical strength. Preservation techniques may also affect graft biology and biomechanics. Consequently, internal fixation and postoperative rehabilitation should be compatible with this transient weakness. The experience at Massachusetts General Hospital Orthopedic Oncology Service indicated an allograft fracture rate of 16% in patients receiving segmental osteochondral allografts. 2 The incidence of bacterial infections in bone allograft recipients has been reported to be 6.9% as compared to 3.0% for autografts, although most infections associated with these procedures are not directly transmitted by the graft. 51.77 Infection of the allograft is virtually incompatible with a successful result, and usually requires removal of the graft. The recipient bed contributes significantly to the incorporation process, providing most of the cells and blood supply. If vascularity is compromised by irradiation or tissue necrosis, the success rate diminishes. Biologic characteristics of the host bone are also important. Osteopenic bone from immobilization, bone affected with neoplastic or metabolic diseases, and bones subjected to prolonged systemic pharmacologic agents, such as corticosteroids or chemotherapeutic drugs, may all detract from the incorporation process.

Bone Graft in Reconstructive Surgery of the Hip A major challenge in reconstructive hip surgery today is the restoration of deficient bone stock, particularly in hip dysplasia, and following protracted and severe prosthetic loosening. Bone grafting has become an important technique in restoring bone stock to support prosthetic fixation.

Autograft Autogenous bone graft has been particularly useful in reconstructing dysplastic and protrusio acetabuli during primary THA. 32, 37, 56 The resected formal head has been the most commonly used graft mate-

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rial. It is autogeneous, sufficiently large in size, biomechanically sound, and easily contoured to adequately fit the host ilium. Harris and colleagues initially reported a high rate of graft incorporation and clinical improvement in 27 hips using femoral head autograft to supplement deficient acetabuli.37 The grafts were fixed to the host ilium by bolts. Gerber and Harris later reported a 10% incidence of cup failure in 47 THAs using autogenous graft to augment lateral acetabular deficiency and followed for a minimum of 5 years (mean, 7.1 years).32 No graft migrated and only five grafts showed some degree of resorption, but radiographic loosening of the acetabular component was evident in an additional 15% of the cases. Recently, Mulroy and Harris reported increased incidence of acetabular component failure at an average of 11.8 years of follow-up in the same patients. 6° Twenty percent of the hips required revision surgery, while an additional 26% showed definite radiographical loosening of the cups. McCollum, Nunley, and Harrelson reported uniformly good results in 32 patients who had bone graft and THA for acetabular protrusio at a mean followup of 4.7 years. 56 Femoral head autograft was used in 25, autogenous iliac crest graft in 3, and allograft in 5 patients. Cemented acetabular components were used, and no supplemental graft fixation was employed. Recently, these same patients were reassessed clinically and radiographically, now at an average of 12.8 years following initial reconstruction. 29 Six hips showed evidence of definite cup loosening, and four of these demonstrated progression of the protrusio. Ninety percent of the hips were stable clinically. Graft resorption was not evident in any hip. Eaton and Capello reported satisfactory results using corticocancellous autogenous iliac crest grafts in seven surface replacement arthroplasties. 19 Fixation of the graft to the ilium was obtained by cortical screws. There was no graft failure at the 14-32 months follow-up evaluation. Ritter and Trancik reported one acetabular component loosening in 20 dysplastic hips using lateral acetabular autogenous femoral head graft without any supplemental screw fixation and using cemented acetabular components. 69 The average follow-up evaluation was 5 years, and there was no graft failure. Sanzen and associates reported no evidence of lateral resorption in 20 of the 32 autogenous grafts used to supplement deficient acetabular roof in primary THA. TM The median follow-up evaluation was 52 months. Sufficient resorption jeopardizing the bone support of the socket was present in three hips. Two of these progressed to component loosening and re-

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quired revision surgery. Three additional sockets were revised for loosening.

Osteonecrosis Autogenous bone graft without prosthetic reconstruction of the hip joint has been applied to the treatment of early stage osteonecrosis. Core decompression with or without bone grafting has been reported with mixed success. 12, 75, 76. a7 The insertion of a strut graft following core decompression may add a biomechanical support to prevent further collapse. It may also contribute cellular and/or matrix factors that would enhance revascularization and bone repair. Long-term success of bone grafting in osteonecrosis of the hip has not been uniform. Bonfiglio and Roke reported success in 78.4% of 116 drilling and bone grafting operations. 3 Only 20 hips were idiopathic aseptic necrosis while the others were secondary to previous fractures. The mean follow-up evaluation was six years. Meyers reported encouraging results using muscle pedicle graft in stage I and II patients in a short-term follow-up study. 59 Current data suggest that the best clinical results using core decompression and grafting are in early stage osteonecrosis prior to articular surface collapse. However, the question of whether collapse would have occurred if left untreated always remains. Application of vascularized grafts in hip reconstruction has not been widely utilized. Recently, Urbaniak reported promising clinical results after a minimum follow-up evaluation of four years in 85 patients with femoral head osteonecrosis treated with decompression and vascularized fibular graft, a° Clinical evaluation demonstrated a successful outcome in 75% of the patients, and the complication rate was 11%. Ten percent of the patients have been converted to THA. The rationale for this technique includes decompression and grafting of the necrotic femoral head with a vascular pedicle fibular bone graft that should more effectively support the subchondral plate and revascularize the infarcted area than using a standard nonvascularized autograft strut. The best prognosis can be expected in patients with osteonecrosis in the early stages. In summary, the most predictable application for autogenous bone graft in THA is in correcting acetabular deficiency during primary THA. It is critical to use corticocancellous graft with stable fixation to the host ilium when reconstructing superolateral defects. Supplemental fixation is not necessary in reconstructing central defects if the peripheral contact with

host bone is maintained. Protected weight-bearing during the postoperative period is important to minimize stresses on the graft as it undergoes a period of transient physiologic weakness during incorporation. The role of bone grafts in conjunction with cementless acetabular components remains to be fully determined. Initial implant stability with maximal possible contact with host bone are crucial. Bony ingrowth into the component, even with autograft, is unlikely.

AIIograft Autogenous bone is undoubtedly the best graft material for management of skeletal defects. The supply, size, and shape of autograft however, is limited. Donor site morbidity may further detract from the practicality of these grafts. Segmental osseous and osteochrondral allografts have become an important adjunct to orthopaedic reconstructive surgery following tumor resection over the past 15 years. Mankin and associates reported 70% good or excellent clinical results in 9I patients who had undergone allograft reconstruction primarily for tumors. 55 The complication rate was significant including a 13.2% infection rate and a 16.5% graft fracture rate. Two of these cases were performed for failed THAs, although their results were notspecified. Despite considerable complications, the efficacy of allograft reconstruction in restoring significant bony defects was established by these investigators.

Acetabular Reconstruction Bone allografts have had their widest application in revision total hip arthroplasty, as compared to antografts in primary arthroplasties. Clinical studies have shown acetabular component loosening with progressive bone loss to be a significant long-term problem.3a, 4a Adequate bone coverage of the prosthetic component is essential for successful long-term fixation. Failure to properly restore the anatomic hip rotation center is also associated with predictably high loosening rate. 45" 86 Autograft from the iliac crest is limited in quantity. Defects in the ilium donor site may compromise the fixation of the bone graft to the host pelvis. Bilateral hip arthroplasties would further limit the use of the contralateral pelvis as a donor site. Acetabular defects can be classified into two broad

Bone Graft and THA

categories, segmental or cavitary. ~ Type I segmental defects represent loss of the bony acetabular rim. A type II cavitary defect represents loss of bone with the acetabular rim intact. A central segmental defect represents loss of the medial wall, whereas medial cavitary defect implies excavation of the medial acetabulum, without violation of the medial cortex. Type III deficiencies represent a combination of cavitary and segmental defects. Type IV defects are pelvic discontinuity, and type V is arthodesis. Approaches to restoring bony continuity and support for the prosthetic component should be individualized according to the defect type and available graft material. For type I segmental defects, reconstruction with solid bulk graft is essential for prosthetic support. The intact wall in cases with cavitary defects (type II) provides the necessary mechanical support for the acetabular component. These deficiencies can be managed with solid or particulate grafts, cement, or a protrusio socket. In type III (combined) defects, reconstruction of the peripheral rim should be addressed first, and similar principles as in type II reconstructions can be followed to address the remaining cavitary defect. In type IV defects, the pelvic continuity must be restored first by stabilizing the anterior and posterior columns of the host peivis with solid grafts fixed with acetabular reconstruction plates and screws. Any remaining segmental and/or cavitary defects can then be addressed. Femoral condyle or whole-pelvis allografts are good choices for these cases with severe bone loss. Harris reported good or excellent results in 8 of I2 patients using frozen femoral head allograft to supplement acetabular reconstruction in revision total hip arthroplasty. 39 Graft fixation to the host ilium was secured by bolts. Recently, Jasty and Harris reported additional follow-up on their experience with allografts in acetabular reconstruction. 43 Thirtytwo percent of the acetabular components were loose radiographically at a mean follow-up of 5.9 years. Graft resorption was evident in nearly 80% of the surviving hips. Long-term success of this technique remains to be fully determined. Oakeshott and colleagues reported the results of 69 acetabular revisions using allografts. 63 There was a variety of allografts used, including both structural and morselized grafts. All grafts were sterilized by radiation, and stored by deep-freezing. Morselized and structural grafts incorporated equally well at a mean follow-up of 20 months (range, 6-72 months). There was no apparent difference in clinical outcome between cemented or cementless acetabular components. However, erosion of the graft and migration of the prosthesis were consistently associated with bipolar prostheses, regardless of the type or location

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of the graft. No cemented or cementless cup showed migration when used with a lateral shelf or major segmental allografts. The incidence of prosthetic migration in morselized graft was not specified. Trancik and associates reported good clinical outcome among 21 hips that required allografts during cemented acetabular revision surgery. 79 Allografts were removed under sterile conditions, stored deepfrozen, and fixed with cancellous screws to the host ilium. The mean interval to incorporation was i 1.8 months (range, 5-18 months). Single photon-emission computed tomography was performed in I2 hips. Eleven of the 12 scans showed symmetrical uptake indicating successful incorporation. One aiMgraft collapsed and required revision. Two additional sockets showed progressive radiolucencies, but patients remained asymptomatic. There was no infection. Fuchs and colleagues reported the results of acetabular revision using metallic shell or Muller ring. 26 In 81% of the cases the acetabular deficiency was type III. Frozen allograft was used in 44 of the 68 hips. There was no significant difference in graft incorporation between autograft and allograft (55% autograft, 53% allograft, 66% combined autograft and allograft) at 2-5 years follow-up. Ninety percent of the patients had good or excellent clinical results. Recently, Ganz and Zehntner reported 79.6% survival probability in 56 acetabular revisions using frozen femoral head allograft and reinforcement ring. 2s There were however, eight late infections (14%), one attributed to graft contamination, and the others to late hematogenous sepsis. Allograft reconstruction of deficient acetabulum using a bipolar prosthesis has been advocated by some investigators, provided that the peripheral support is adequate. 72'73 Scott reported 10 of 19 hips with radiographic evidence of bipolar migration at a mean 2-year follow-up. 73 Recently, Wilson and associates reviewed 32 hips reconstructed using allograft and bipolar prosthesis, s4 The average graft incorporation time was 10.5 months (range, 3-15 months). Two noncontained peripheral grafts were considered failures. Migration of the bipolar component was evident in all cases. The best results were obtained in central, contained defects. Noncontained grafts, solid or morselized, uniformly had greater superomedial migration of the component. Twenty to thirty percent of the initial thickness of the graft was lost during the early phase of weight-bearing. Our experience at The Hospital for Special Surgery showed that reconstruction with bipolar prosthesis and morselized bone allograft was associated with a high rate of failure.7 Unsatisfactory results were seen in 11 of 18 hips at a mean follow-up of 35 months

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(range, 2 4 - 6 0 months). Fifty-five percent of these 11 failures showed complete resorption of the bone graft at an average interval of 24 m o n t h s (range, 8 - 3 6 months). Questions concerning the use of cementless, porous in-growth sockets in the presence of graft supplementation are multiple. Obtaining initial compon e n t stability and b o n y ingrowth for long-term fixation are the major goals. Brick and associates reported an 18% failure among 28 consecutive reconstructions using cementless socket and bone allograft, at a m e a n follow-up of 2.3 years. 6 The average time to graft failure was 8 m o n t h s (range, 2 - 1 5 months). Eighty percent of all the sockets had evidence of migration. Morselized grafts used to reconstruct large peripheral defects fared the worst. Galante reported on 139 consecutive acetabular revisions using a cementless fiber-coated titaniumalloy c o m p o n e n t with a m i n i m u m follow-up of 3 years. 27 Seventy percent of the hips required morselized bone graft for acetabular reconstruction. Bulk structural allograft was used only in four cases. Seven sockets have been revised, four for recurrent dislocation, two for infection, and one for aseptic loosening. Only 1 4 % of the sockets showed no radiolucency. However, no hip demonstrated progression of radiolucency after one year postrevision. Preoperative staging of b o n y deficiency did not correlate with overall clinical or radiographic outcome. This technique was most applicable to cavitary and contained defects. Chandler and associates recently reported the long-term result of solid acetabular grafts in 38 revision THAs. ~4 The average follow-up was 9.5 years (range, 6 . 2 - 1 8 . 7 years). Twenty autografts and 18 allografts were used. The allografts were deep-frozen, and without irradiation sterilization. Fixation of the grafts was accomplished by cancellous screw or interference fit. Cemented components were used in 34 cases. Six hips required rerevision to date (16%). A1lografts were used in five of the six failed hips, all of which were solidly incorporated radiographically and as determined during revision surgery. Two additional hips showed major allograft resorption (one infected and one asymptomatic). Survival of solid acetabular autografts was 97.4% at a mean 9.5 years follow-up. Schutzer and Harris recently reported no fixation failure in 54 hips reconstructed with placement of cementless sockets at a high hip center without using bone graft. TM The m e a n cup height (vertical distance from the hip center to inferior border of the tear drop) was 46 m m as compared to a m e a n of 14 m m for the anatomic center. These reconstructions were not excessively lateral. Despite high placement, 50% (27

hips) were lengthened compared to preoperative status. Overall good clinical scores were obtained at a mean follow-up of 36 months ( 2 4 - 6 4 months). Forty-six percent of the sockets had no radiolucency, while the other 54% had nonprogressive radiolucency in one or two zones. No socket showed complete radiolucency. The best clinical results have been achieved using allograft in cavitary and contained acetabular defects. Cementless acetabular components are equally effective as cemented prostheses during short-term follow-up. Migration is a significant complication using the bipolar prosthesis. This technique is best in conjunction with solid central grafts, and its use may be limited to the elderly inactive patient with adequate peripheral support. Regardless of the bone graft type or component design, the basic principles of obtaining intimate host bone contact with the graft, stable component fixation, and restoration of anatomical hip rotation center, must be achieved to maximize successful long-term clinical result. The efficacy and durability of reconstruction at a high hip center without bone grafting remain to be fully determined.

Femoral Reconstruction Significant proximal femoral bony defects pose a considerable challenge to surgical reconstruction. Several methods have been used, including custom prostheses, segmental allograft, and osteoarticular whole-joint allografts. 4" 50 Substantial proximal femoral bone loss is associated with failure of primary total hip arthroplasty, if the indication of revision surgery is protracted or neglected. It is more c o m m o n after multiple revisions. McGann and Harris reported favorable clinical result in 5 cases using massive frozen femoral allograft reconstruction for deficient proximal femoral bone stock, at a mean follow-up of 30 months. 58 Head and associates reported improvement in 73% of the 22 hips that required proximal femoral reconstruction using segmental freeze-dried allograft during revision surgery. 4° The allograft length averaged 15 cm (range, 6 - 2 4 centimeters). Nine patients (36%) required reoperations for complications (5 graft-host nonunions, 3 acetabular loosenings, and 1 femoral prosthetic fracture). The m e a n f o l l o w - u p interval was relatively short at 28 months. Jofe and colleagues reported 70% excellent or good results in 28 patients using segmental frozen allografts for tumor resection and reconstruction, at 2 - 1 5 years of follow-up. 44 Thirteen patients had an allograft-prosthesis c o m b i n a t i o n , and 15 patients

B o n e Graft and THA

had osteoarticular allografts. Fourteen complications occurred in these 28 allografts (4 fractures, 4 infections, 1 had both infection and fracture, 2 nonunions at the host-graft junction, and 1 unstable hip joint). Harris and associates recently reported 77% good clinical results in 30 allograft-prosthesis composite reconstructions at a m e a n follow-up of 42 months.36 Complications included one each of allograft fracture, graft-host junction n o n u n i o n , and delayed union. There was no infection. Patients with a healed trochanteric fragment to the allograft had good abductor function (overall 4.1/5.0 manual muscle testing). Direct soft tissue repair of the abductor mechanism fared worse (overall 3.2/5.0 m a n u a l muscle testing). Union of bone and soft tissues directly to an allograft represents a maj or advantage over the use of mefallic prosthesis, which do not permit such repair. Emerson and colleagues reported encouraging resuits in revision surgery using multiple strut allografts and circlage wires for proximal femoral defects in 28 patients with a m i n i m u m follow-up of 2 years. 2° Radiographically, 80% of the strut grafts had united to the host femur. Sixteen percent were stable but not united, and 4% of the grafts showed resorption. Instability of the hip joint remained a significant problem (25%). An additional application of bone allograft is for femoral and acetabular endostea] osteolysis. Gie and colleagues recently reported a technique of morselized allograft tight packing of the femoral canal, reaming, and insertion of a cemented femoral prosthesis. In 57 hips with cystic defects of the proximal femoral shaft, excellent component fixation was obtained and there was no prosthetic fixation failure at a mean follow-up time of 3 years. 33 Currently, an efficacious approach to reconstructing the proximal femur with deficient bone stock includes (1) strut allograft with retention of as much host bone and greater trochanter as possible, and (2) a longstem femoral component to achieve good distal fixation. Whenever possible, the distal stem should be press-fit into the host femur to encourage union at the graft-host junction. Stability of the proximal segmental allograft around the prosthetic stem is best achieved with cement. Autogeneous grafting of the junction site is recommended. An oblique or stepcut osteotomy, or a lateral neutralization plate will further enhance junction stability. The abductor mechanism resting length must be restored to maximize proper soft tissue tension, good muscle strength, and stability to minimize dislocation. If in doubt a postoperative brace may be helpful in assuring stability. Postoperative protected weight-bearing is necessary to enhance trochanteric and host-graft

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junction union, especially if cementless implants are used. Increased follow-up t.ime is required to fully determine the long-term success of these difficult reconstructions.

Synthetic Osseous Substitutes Synthetic bone substitutes such as hydroxyapatite and tricalcium phosphate, alone or in combination with autogeneous bone or bone marrow, represent another alternative for promoting osseous repair. These synthetics lack substantial mechanical integrity to withstand physiologic loads, possess no osteoinductive potential, but are capable of osteoconduction. 8 Clinical application has been in filling posttraumatic cystic defects, 9 and in spine surgery. 15 These materials may have application in reconstructing acetabular and femoral cavitary defects. Early clinical results have been promising in hydroxyapatite-coated femoral components for cementless total hip arthroplasty. 3°

Conclusion Bone grafts have facilitated difficult reconstructions of the hip joint. Their increasing application is being evaluated in revision THA. The biologic and biomechanical properties of autografts and allografts are predictable in most situations. The process of incorporation involves a sequence of events that can be influenced by graft selection, surgical technique, and postoperative care. Many of the clinical failures associated with bone grafts are avoidable and others can be minimized by better understanding the biology of graft incorporation. The biologic potential, biomechanics, and risks or complications associated with different osteogenic materials are varied and largely predictable. The choice of graft material and surgical technique best suited to a specific set of reconstructive requirements must be individualized. The clinical success will improve with better understanding of the limitations of bone grafts, optimization of their full biologic and biomechanical potential, and adhering to surgical principles and postoperative rehabilitation that appropriately reflect the physiology of the incorporation process.

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