JBJS | Instructional Course Lectures, The American Academy of Orthopaedic Surgeons

The Journal of Bone & Joint Surgery, Volume 81, Issue 2

Instructional Course Lecture | February 01, 1999

Instructional Course Lectures, The American Academy of Orthopaedic Surgeons - Total Acetabular Allografts*†

WAYNE G. PAPROSKY, M.D.‡, WINFIELD, ILLINOIS; TODD D. SEKUNDIAK, M.D., F.R.C.S.(C)§, WINNIPEG, MANITOBA, CANADA

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An Instructional Course Lecture, American Academy of Orthopaedic Surgeons

The Journal of Bone & Joint Surgery. 1999; 81:280-291

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Introduction

Although the number of total hip arthroplasty procedures has remained relatively constant in the last few years, the prevalence of these implants in the general population is increasing because of the longer life spans of the patients. Thus, these implants are subjected to the stresses resulting from increased levels of activity for longer periods of time. Mechanisms of failure include infection, instability, wear, loosening, and fracture^9,29,31,37.

Historically, the use of a press-fit, porous ingrowth implant has provided the best results after revision acetabular arthroplasties, irrespective of the mode of failure^{5,9,21,23,26,27,29,31,34,37,38,42,44}. Wear and loosening of the implant are the most common causes of failure. These modes of failure are of concern as wear and its associated osteolytic process tend to precede loosening and consequent symptoms. Patients therefore may delay medical attention until bone loss becomes massive. It is essential to quantitate the amount of bone loss as this determines the type of acetabular reconstruction that is needed.

Attempts at reconstruction of acetabula with severe defects have included the use of a bilobed component, a threaded cup, a bipolar stem, a cup inserted with cement, and a porous ingrowth cup^{5,9,21,23,26,27,29,31,34,37,38,42,44}. Universally, such procedures have led to poor results as there is not enough host bone to support the implant. If the appropriate type of allograft is selected for the specific acetabular defect, acetabular reconstructions can be successful in these revision procedures³⁶. Acetabular bone grafts serve multiple functions. They help to provide initial stability for the acetabular component and allow placement of the component closer to the normal hip center. It is hoped that, in the long term, such grafts will prolong the survival of the component and restore bone stock to the host pelvis.

Autogenous bone graft has the advantage of generating osteogenic, osteoinductive, and osteoconductive potentials while minimizing the transmission of disease⁷. However, the amount of autogenous bone graft that is available for an acetabular revision is small, and thus such bone-grafting is not a feasible option in hips with severe acetabular defects. Allogenic bone and bone substitutes are alternatives. These grafts provide only an osteoconductive potential. Morseled grafts fill voids in contained cavitary defects. The graft helps to support the component initially and eventually restores bone stock once the host has incorporated the graft¹⁴. Bulk grafts are used initially to reconstruct deficient rims or columns, which give support to the component. Ultimately, these grafts may restore bone stock by uniting to the host bone and may act as a scaffold for possible future revisions¹². It is unlikely that a bulk graft will become completely viable and remodel^39,40.

During preoperative planning, surgeons must be able to recognize what type of acetabular defect is present so that they can choose the best mode of reconstruction. The use of a classification system for acetabular defects before an operation allows surgeons to select from these different modes. There are a number of classification systems that have not been validated and therefore lead to wide intraobserver and interobserver error in the identification and grading of acetabular defects. Most classification systems differentiate between contained cavitary defects and noncontained structural defects.

The classification system of the American Academy of Orthopaedic Surgeons, which has been previously described⁸, is organized with regard to the descriptive nature of the defect, but the type of defect is determined on the basis of an intraoperative assessment. Defects are classified as segmental, cavitary, combined, pelvic discontinuity, or arthrodesis. The system of Gross et al.¹² classifies defects as contained or noncontained, and noncontained defects are categorized as minor or major. We support the use of a classification system that is based on the assessment of an anteroposterior radiograph of the pelvis according to four criteria (Table I)³. There are four additional classification systems that describe similar types of defects^6,10,13,17.

Without unanimous agreement with respect to the description of the types of acetabular defects, it is difficult to compare reconstructive procedures critically. Historically and pragmatically, contained acetabular defects are treated, without considerable difficulty, with use of a routine acetabular component. However, large noncontained defects present a reconstructive dilemma. Because systems for the classification of acetabular defects have not been validated, the definition of a large acetabular defect is variable. It is therefore difficult to assign specific reconstructive procedures to specific types of defects. As a result, the classification system of the American Academy of Orthopaedic Surgeons⁸ has raised concern because type-I, II, III, and IV acetabular defects can all be treated with a structural allograft and yet can be very different kinds of defects^{15,18,32,44,45,48}. One of us (W. G. P.) and colleagues²⁹ and Gross et al.¹² attempted to develop classification systems that quantitate the amount of host bone remaining and correlate it with the remaining structural rim that is able to support an acetabular component. In both classification systems, a severe acetabular defect (type IIB according to the system of Gross et al.¹² and type IIIB according to that of one of us and colleagues²⁹) is one in which at least 50 percent of the host bone is missing, and attempts to fit an acetabular component into such a defect will lead to a high rate of failure^12,26. The authors of these classification systems recommended the use of a structural allograft to reconstruct the defect and to support the fixation of an acetabular component^4,11.

Because specialized radiographs or computed tomography scans are cumbersome to make and difficult to evaluate, a preoperative anteroposterior radiograph of the pelvis is used routinely to assess acetabular defects. Judet radiographs may allow some additional assessment of the anterior and posterior columns, but they have not proved to be of benefit in our practice. The anteroposterior radiograph is analyzed according to four criteria: migration of the hip center, ischial osteolysis, destruction of the teardrop, and disruption of Kohler's line. These criteria are quantitated to determine the structural integrity of the acetabular walls and columns. By knowing the extent and location of bone loss, the examiner can determine the appropriate type of acetabular reconstruction.

With less severe acetabular defects, revision with a porous-coated acetabular component can be very successful^28,29,37,43. Morseled or structural grafts can be used to augment host bone stock but are unnecessary for fixation of the component. These revisions can be as successful, with regard to the functional results, as a primary hip replacement and can almost be treated as such. However, when a hip has a more severe acetabular defect—that is, when at least 30 percent of the host bone is missing—an attempt to press-fit a porous-coated implant is possible but more difficult and less reliable. The use of a structural allograft is recommended to reconstruct a deficient rim or column³⁶ as it helps to maintain the position of the component and to promote the fixation of the component to the host bone. Such a deficiency is classified as type IIIA according to the system of one of us and colleagues²⁹, type IIA according to that of Gross et al.¹², or type I or III according to that of the American Academy of Orthopaedic Surgeons⁸.

When at least 50 percent of the host acetabulum is missing, attempts to obtain fixation of a porous-coated implant have a high rate of failure^11,32. There is not enough host bone to allow fixation of the implant. Allografting is one means with which to restore acetabular bone and to provide a fixation point for the acetabular component. These acetabular defects (type IIIB according to the classification system of one of us and colleagues²⁹, type IIB according to that of Gross et al.¹², or type III according to that of the American Academy of Orthopaedic Surgeons⁸) are the most severe and the most difficult to treat^4,11,20. Fortunately, only a small number of revision procedures involve such defects.

Reconstruction cages or antiprotrusio rings now allow the surgeon more options. The role of the acetabular reconstruction cage still needs to be defined as only short-term and intermediate-term follow-up studies have been done^5,33,47. These devices have been used in a number of different settings. Rosson and Schatzker³³, in a study of forty-six hips in which a ring had been inserted, reported a success rate of 89 percent after five years of follow-up. Zehntner and Ganz⁴⁷ reported that the rate of survival of fifty-five hips in which a ring had been used was 80 percent at ten years. These hips, however, were reconstructed with use of Müller rings, which are unable to span the more severe defects that we described. Berry and Müller² reported on the use of the Burch-Schnieder cage in forty-two hips that had a type-III defect according to the classification system of the American Academy of Orthopaedic Surgeons⁸. After an average duration of follow-up of five years, aseptic loosening and infection had developed in five hips each.

A reconstruction ring that is used to treat a severe defect must span the defect and allow for adequate fixation proximal and distal to the defect. With a cage, screw fixation can be attempted into the remaining dome of the acetabulum, but the cage must be secured to the ilium with extensions that accept screws. An extension into the ischium is secured with screws, impaled, or hooked into the remaining bone (Fig. 1). It is essential to understand that these fixation points augment the stability of the cage. For the cage to be successful, it must be supported by the remaining host acetabular columns or have adjunctive columnar support created with the use of a structural allograft^2,25,30,47. We do not know of a reported series in which a reconstruction cage was used with a massive structural allograft. Use of the reconstruction cage, in conjunction with a structural allograft, dissipates the forces through the remaining pelvis, which may improve the union of the graft to the host bone by preventing excessive transmission of force through the avascular graft. In addition, bone may still reconstitute, as the remaining host bone is being loaded. The structural allograft, after it has united to the host bone, also acts to support the reconstruction cage and to prevent possible late metal fatigue.

*Printed with permission of the American Academy of Orthopaedic Surgeons. This article will appear in Instructional Course Lectures, Volume 49, American Academy of Orthopaedic Surgeons, Rosemont, Illinois, March 2000.

†No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article. No funds were received in support of this study.

‡Department of Orthopaedic Surgery, Central DuPage Hospital, 25 North Winfield Road, Winfield, Illinois 60190.

§Section of Orthopaedic Surgery, University of Manitoba, St. Boniface Hospital, Winnipeg, Manitoba R2H 3C3, Canada.

‡Department of Orthopaedic Surgery, Central DuPage Hospital, 25 North Winfield Road, Winfield, Illinois 60190.

§Section of Orthopaedic Surgery, University of Manitoba, St. Boniface Hospital, Winnipeg, Manitoba R2H 3C3, Canada.

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TABLE I CLASSIFICATION OF ACETABULAR DEFECTS³

*Minimum = at least three centimeters proximal to the superior transverse obturator line, and marked = more than three centimeters proximal to the superior transverse obturator line. †Mild = zero to seven millimeters distal to the superior transverse obturator line, moderate = eight to fourteen millimeters distal to the obturator line, and severe = fifteen millimeters or more distal to obturator line. ‡Grade I = lateral to Kohler's line, grade II = migration to Kohler's line, grade II+ = medial expansion of Kohler's line into pelvis, grade III = migration into pelvis with violation of Kohler's line, and grade III+ = marked migration into pelvis. §Mild = minimum loss of the lateral border, moderate = complete loss of the lateral border, and severe = loss of the lateral and medial borders.

Type of Defect	Superior Migration of Hip Center*	Osteolysis of Ischium†	Medial Migration of Hip Center‡	Osteolysis of Teardrop§
I	Minimum	None	None	None
IIA	Minimum	Mild	Grade I	Mild
IIB	Minimum to marked	Mild	Grade II	Mild
IIC	Minimum	Mild	Grade III	Moderate or severe
IIIA	Marked	Moderate	Grade II+ or III	Moderate
IIIB	Marked	Severe	Grade III+	Severe

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TABLE I CLASSIFICATION OF ACETABULAR DEFECTS³

Type of Defect	Superior Migration of Hip Center*	Osteolysis of Ischium†	Medial Migration of Hip Center‡	Osteolysis of Teardrop§
I	Minimum	None	None	None
IIA	Minimum	Mild	Grade I	Mild
IIB	Minimum to marked	Mild	Grade II	Mild
IIC	Minimum	Mild	Grade III	Moderate or severe
IIIA	Marked	Moderate	Grade II+ or III	Moderate
IIIB	Marked	Severe	Grade III+	Severe

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FIG1:: Fig. 1 Photograph of an antiprotrusio cage (DePuy, Warsaw, Indiana), showing the proximal and distal extensions, which allow fixation into the remaining host bone in the ilium and the ischium. If fixation is not possible in the ischium, impaling the extension into the ischial body provides adequate stability.

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FIG2:: Fig. 2 Drawing of a type-IIIB acetabular defect²⁹, showing complete loss of medial bone, loss of bone at the superior confluence of the anterior and posterior columns, and loss of the anterior and posterior aspects of the inferior rim.

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FIG3:: Figs. 3 through 7: Drawings showing the steps of the operative procedure. Fig. 3: The type-IIIB acetabular defect²⁹ is reamed spherically to create a tongue of bone superiorly to accept the allograft and to size the defect.

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FIG4:: Fig. 4 The acetabular transplant is cut (broken lines) to allow for acceptance into the host. The actual positioning of the cuts is determined by the amount of remaining host bone.

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FIG5:: Fig. 5 The medial aspect of the graft is sculpted with a reamer or a burr to create a groove that is in contact with the tongue of bone (as created in Fig. 3).

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FIG6:: Fig. 6 The graft is impacted into position, after which 6.5-millimeter cancellous-bone screws are placed in a proximal-to-medial direction to maintain the position.

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FIG7:: Fig. 7 The coronal view of the transplant with screw fixation. The cup is cemented into the allograft.

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FIG8:: Fig. 8 Radiograph made two years after an acetabular reconstruction with an allograft transplant. Because of the lack of abductor muscles, a constraining polyethylene cup was inserted to prevent dislocation.

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FIG9:: Figs. 9-A and 9-B: Anteroposterior radiographs of a patient who had failure of a cemented acetabular component in the left hip. Fig. 9-A: Radiograph made before the reconstruction procedure, showing the disruption of Kohler's line, loss of the teardrop, and osteolysis of the ischium extending distally more than fifteen millimeters and superiorly more than three centimeters proximal to the transverse obturator line. A type-IIIB defect²⁹ is present.

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FIG9-B:: Fig. 9-B Radiograph made four years after transplantation of an acetabular allograft, showing maintenance of the position of the cup and the allograft.

Preoperative Planning

A severe acetabular defect (type IIIB according to the classification system of one of us and colleagues²⁹, type IIB according to that of Gross et al.¹², or type III according to that of the American Academy of Orthopaedic Surgeons⁸) is identified preoperatively so that the proper planning can be undertaken to ensure a successful procedure. An anteroposterior radiograph of the pelvis is assessed according to the four previously described criteria. Each of these radiographic criteria represents a separate section of the acetabulum, and the degrees of the involvement of these sections are combined to classify the acetabular defect. If 50 percent of the host bone is determined to be missing, the surgeon must consider the use of grafting techniques to reconstruct the acetabulum. Recognition of the defect is critical so that the surgeon does not begin the operative procedure without the necessary preparation. It also allows the surgeon to refer the patient to a facility where the personnel have a great deal of experience with these reconstructions.

Migration of the hip center is measured from a horizontal line drawn through the most superior extent of the obturator foramen on the anteroposterior radiograph. If this distance is more than three centimeters, then the normal hip center has migrated at least two centimeters from its normal position. This finding indicates that the confluence of the anterior and posterior columns is in jeopardy, with an absence of superior bone to support the component. Ischial osteolysis is assessed on the basis of the distance that the osteolysis has extended into the body of the ischium, as measured from the same transverse line drawn through the superior part of the obturator foramen. If the osteolysis is severe (extending fifteen millimeters or more distal to this line), then the posteroinferior column is also jeopardized and is not able to support the fixation of the component. The anteronferior column is assessed by determining the extent of disruption of the radiographic teardrop. Although the teardrop is not a true anatomical structure, its lateral and medial margins are created by cortical bone from the true medial wall of the acetabulum and the inner wall of the true pelvis, respectively. With both its lateral and its medial margin disrupted, this part of the acetabulum is also unable to support a component. Finally, if Kohler's line is violated, indicating that the component has migrated into the pelvis (grade-III or III+ migration [Table I]), with no intervening osseous shell, then the defect is type IIIB according to the system of one of us and colleagues²⁹, type IIB according to that of Gross et al.¹², or type III according to that of the American Academy of Orthopaedic Surgeons⁸, and acetabular grafting is indicated (Fig. 2).

The size of an acetabular defect is commonly underestimated when a component is well fixed because the bone destroyed by the instruments and by the removal of the component is not taken into account. It is essential that the surgeon forecast the removal technique and the likelihood of bone loss so that the appropriate type of reconstruction components and equipment can be available. A minor columnar or contained defect associated with a large porous-coated acetabular cup can easily progress to a severe noncontained columnar defect if haste to remove the component precedes proper technique.

Severe acetabular defects can be associated with substantial medial migration of the component or adherent bone cement, and additional investigations to assess the intrapelvic structures must be considered. Currently, there are no concrete recommendations as to when or how to evaluate the position of the component relative to the iliac vessels, pelvic nerves, or ureter. We have used preoperative computed tomography scanning with arterial infusion to assess the pelvic structures. Conventional angiography or intravenous pyelography can also be considered when there is concern. If suspicion is raised, an adjunctive retroperitoneal approach needs to be used at the time of revision to aid in the removal of the components.

If a severe acetabular defect is suspected and transplantation of allograft bone is considered, the graft should be obtained with use of the guidelines recommended by the Musculoskeletal Council of the American Association of Tissue Banks²⁴. An allograft obtained from the same side of the pelvis of a cadaver of a young man is chosen for fit and structural integrity. Caution should be exercised so as not to grossly mismatch the size of the graft to the host because the bulk of the graft may preclude insertion or the graft may fail to span the defect. Morseled allograft is also recommended for impaction at the host-graft junction as well as into any remaining cavitary defects. Large-fragment screw sets and a reconstruction plate are needed for fixation of the graft and for possible repair of an associated pelvic discontinuity. An antiprotrusio cage with iliac and ischial extensions can also be used for fixation of the graft and for repair of a discontinuity. As with any revision procedure, the appropriate instruments for the revision and any removal devices specific to the prosthesis to be extracted are required.

A severe acetabular defect, by definition, is massive with marked volumetric and structural bone loss. The use of a reconstruction cage in isolation is not recommended for this type of defect because the defect is so large and noncontained that the cage has no columnar support. The repetitive loading causes the cage to flex and leads to metal fatigue or pull-out of the screws. The structural graft acts to support the cage and to dissipate stresses on the hip joint.

Operative Technique

Transplantation of a total acetabular allograft is performed with the patient in the lateral position and with a standard posterolateral approach. The proximal extent of the incision is directed to the posterior superior iliac spine and distally along the lateral aspect of the femur. Another investigator⁴¹ recommended a triradiate approach to gain access to the pubic ramus, but we have found this to be unnecessary. Visualization of the entire acetabular rim, iliac wing, pubis, and ischium is possible with a posterolateral approach. Extensile exposure is essential, and an extended trochanteric osteotomy or a similar osteotomy is considered if access to the acetabulum is difficult⁴⁶. After débridement and removal of the components, synovial white-blood-cell counts, gram-staining of specimens of synovial fluid, and histological examination of frozen sections of inflammatory tissues should be performed immediately to exclude the possibility of infection. Even with this extensive preoperative and intraoperative assessment, an occult periprosthetic infection can be difficult to identify. If there is any suspicion of infection on the basis of the patient's history, physical examination, or operative findings, it is necessary to stage the reconstruction in order to prevent the propagation of the infection. Performance of the total acetabular allograft procedure in the presence of a concomitant periprosthetic infection will lead to catastrophic early failure and substantial morbidity in an already sick patient.

Débridement of the acetabular pseudocapsule proceeds until the acetabular rim is completely exposed. All acetabular membranes are removed. Posterior dissection must be subperiosteal to protect the sciatic nerve. Any remaining posterior part of the capsule is maintained to act as a leverage point for a self-retaining retractor and as a protective tissue plane for the sciatic nerve. The retractor can then be placed with one blade between the capsule and the acetabulum posteriorly and against the femur anteriorly to displace the femur and allow visualization of the acetabulum. With large defects, anterior dissection must be undertaken judiciously as the femoral neurovascular bundles or branches of the bundles can be encountered. The trochanteric osteotomy allows for easier exposure in this direction if difficulty is encountered. The removal of the component, cement, debris, and tissue is performed routinely. It is essential to comprehend the inherent difficulties in the removal of the component and to have available the appropriate removal devices specific to the component.

After removal of the acetabular component, débridement of the acetabulum must be completed. It is essential to understand that these bone defects occur as a result of a degenerative, not a traumatic, process. Bone defects can be masked by scarring in the form of reactive membranes and fibrous tissue. A pelvic discontinuity can be missed as it is difficult to visualize on preoperative radiographs, and it is also insidious intraoperatively because of scarring. Irrespective of the type of acetabular defect, a pelvic discontinuity must first be stabilized before additional reconstruction can be considered. A pelvic reconstruction plate is used to bridge the defect between the ilium and the ischium. It is fashioned along the posterior column and fixed with screws to provide stability. Once the plate has been applied, the preparation of the transplant can proceed. An antiprotrusio cage can be used instead of the plate if screw fixation can be achieved both proximal and distal to the discontinuity. (This technique will be described.)

Slight reaming is done to remove the reactive debris, size the defect, and create a spherical shape for acceptance of the trial component. With the trial component inserted, the amount of bone coverage can be estimated and correlated with the preoperative templating. When the most optimum trial component has been inserted and at least 50 percent of the cup is uncovered, then an acetabular allograft is needed to obtain a press-fit. Use of a trial component at this stage allows the fit of the acetabular component to be judged manually, and the surgeon can determine the quality of the bone that remains and its ability to support a press-fit acetabular component rather than being concerned with the percentage of the acetabular cup that is covered. With the most severe acetabular defects, attempts at press-fitting an acetabular trial component, irrespective of its diameter, will be unsuccessful, and macromotion of the trial component will occur when it is manipulated. By initially reaming the acetabular defect, the surgeon also creates a hemispherical shape in the remaining host bone, which provides a fixation point for the graft and allows sizing of the diameter of the rim (Fig. 3).

Additional dissection superiorly, posteriorly, and inferiorly is required for placement of the allograft. Dissection is always subperiosteal to prevent neurovascular damage. Posterior and anterior dissection was previously described.

Exposure of the acetabular rim must be complete. To allow for fixation of the graft along the remaining intact iliac wing, the abductor muscles are removed with use of blunt dissection with a periosteal elevator and a sponge. Maintaining subperiosteal dissection allows the superior gluteal vessels and nerve to be preserved. Dissection should not extend into the greater sciatic foramen as disruption of the vessels can cause uncontrolled bleeding. Inferior exposure is required along the lateral wall of the ischium, which later will act as a contact point for the reconstruction.

The acetabular transplant is shaped to buttress the host bone. The superior pubic and ischial rami of the graft are cut at a point distal to the acetabular confluence with a length remaining to fill the defects in the host pelvis. These graft extensions buttress the remaining host pubis and ischium and provide additional support. The iliac crest of the transplant is cut in a curvilinear manner from the greater sciatic notch to the anterior inferior iliac spine (Fig. 4). This gives the graft ample room to accept fixation without the bulk of excess bone and without the concern that fixation devices will enter the acetabular cavity. The flange of the iliac crest is fashioned to allow the best positioning of the screws for purchase into the remaining host bone. Repeated fitting and trimming of the graft is performed to ensure the appropriate amount and placement of bone resection. Resection should be slight initially in order to maintain as much structural bone as possible and to avoid the possibility of undersizing the graft, which can lead to poor fixation, poor stability, early resorption, and ultimate failure.

When the host and the graft are prepared, the aim is to create a tongue-in-groove mortise that ensures intimate contact and stability of the construct. The tongue of the mortise is created by the remaining rim of the host acetabulum. The groove is created in the medial aspect of the graft where contact is made with the rim of the host acetabulum (Fig. 5). Transfer of load from the graft to the host bone then occurs along this mortise and may aid in the biological union of the graft to the host. The mortise prevents excessive stresses at the screw-fixation points, allowing for improved stability.

A high-speed cylindrical burr, with or without a female reamer that is sized two millimeters larger than the last male reamer used in the defect in the host acetabulum, is utilized to score the medial side of the transplant. This creates a hemispherical defect, 1.0 to 1.5 centimeters deep on the inner table of the graft, which will accept the remaining rim of the host acetabulum. The position of the groove in the graft is compared with the host bone and typically extends from the anterior inferior iliac spine to the ischium. The cylindrical burr is used to sculpt the groove to accept the irregularities in the host rim and to ensure that there is a tight tongue-in-groove fit. The medial portion of the graft should not be thinned enough to weaken the articular surface or the outer table of the graft as this leads to weakening, and the possibility of fracture, of the graft. The transplant is seated in place and manipulated to ensure that it is in its most stable position.

Once the graft has been trimmed, it is impacted in place with use of an acetabular reamer or impactor. The transplant should be manipulated by hand to ensure that there is no toggling. The priority in the positioning of the transplant should be to obtain stability of the construct. The position of the articular portion of the transplanted acetabulum is secondary as the component can be repositioned to ensure joint stability. Commonly, the transplant is positioned more vertically than the normal acetabulum.

The construct is temporarily stabilized with one or two Steinmann pins. Placement of the pins and screws is started in the superior rim of the iliac wing of the graft. The aim is to direct the screws in a proximal-to-medial direction along the force vectors of the joint (Fig. 6). This allows compression at the host-transplant junction, optimizes contact, improves stability, and promotes union. The screws must be placed in the best structural bone without interfering with the insertion of the component. The confluence of the host acetabular columns has the best structural integrity, even in a failed acetabulum, as it continues to accept load resulting from the vertical migration of the component. Screws placed superior to this point provide weaker fixation as the iliac wing thins. It is optimum to use three or four screws with bicortical purchase in the host ilium. Large-fragment, partially threaded cancellous-bone screws are used to lag the transplant to the host while allowing the bicortical purchase in the thin iliac wing of the host. Once fixation is complete, manipulation should not produce motion of the construct. Washers are used to prevent the graft from being crushed and to ensure the compressive force. The tongue-in-groove joint created between the host and the graft markedly augments the fixation so that stability is maintained with the screw fixation.

The acetabular transplant is prepared by removal of cartilage and a minimum amount of subchondral bone. Overreaming is avoided as it weakens the bone and causes a fracture. As the graft is avascular, the acetabular component must be inserted and fixed with cement (Fig. 7). Large anchor holes for the cement should be avoided as they also cause fractures by creating stress-risers. Anchor holes also might promote the intrusion of the cement at the host-transplant interface if the medial wall of the transplant is broached. This can prevent osseous union of the graft and possibly lead to early failure.

Use of a reconstruction cage can be considered to augment the fixation of the allograft when there is complete absence of the posterior column of the host acetabulum. The host acetabular shell is prepared as previously described. The transplant then is fashioned so that the cage is supported along the posterior column. The acetabular transplant is cut in the coronal plane and is trimmed to allow intact host acetabular bone to be in contact with the cage. Shaping of the graft reduces the bulk of the reconstruction. After multiple fittings and trimmings, the graft is initially fixed to the host ilium with screws or pins. Acetabular reamers are judiciously used to remove any redundant articular bone and cartilage from the transplant. Reaming also determines the outer diameter of the proposed cage. A cage with a diameter that allows intimate contact with the host, and more importantly with the graft, but does not necessitate overreaming and possible weakening of the bone is chosen. With the disparity between the quality of the host bone and that of the transplant, the reamer can easily ream eccentrically, producing an additional columnar defect in the host.

Once the spherical defect has been created, morseled allograft is impacted into the remaining defects and into the junction between the host and the transplant. A finishing reamer or a basket reamer on a T-handle can be used to manually impact and direct the grafts into the defects. An impactor equal in size to the outer diameter of the cage is chosen to ensure that the morseled graft acts only as a filler to improve the stability of the fixation of the cage to the host or graft bone. Excessive allograft should not be used as it positions the acetabulum laterally and prevents solid fixation of the cage to the remaining structural bone.

The cage is then manipulated into position, which can be an arduous task as the proximal and distal extensions of the cage impinge on bone and soft tissues. We suggest the use of a malleable cage so that it can be contoured to the transplant and the host bone. A sharp retractor placed in the iliac wing retracts the abductor muscles and protects the superior gluteal structures when the proximal extent of the cage is placed. Inferiorly, the extension from the cage can be placed either on the outer aspect of the ischial tuberosity and secured with screws or impaled into the substance of the ischium from the acetabular aspect. As with whole acetabular transplants, the reconstruction cage should be placed for optimum fit and stability. The actual position of the polyethylene component can be altered to improve the stability of the joint. Positioning the cage and molding its shape usually takes a number of attempts.

Reconstruction with use of a cage achieves stability by press-fitting the cage against the acetabular transplant and the remaining acetabular rim, iliac wing, ischium, and pubis of the host. The cage initially is fixed through the holes in its dome to ensure close approximation to the host bone and the transplant. The initial screws should be placed superiorly, and subsequent screws should be placed posteriorly and anteriorly to avoid early migration of the construct with weight-bearing. Additional screws are then secured in the ischium and the iliac wing. Once the construct has been stabilized with screws, additional stability is achieved by pressurizing cement through the remaining screw-holes in the cage.

Before the acetabular liner is cemented into the transplant or the cage, trial reductions are performed to determine the stability of the hip. Because of the massive nature of the acetabular defect, it is possible to misjudge the orientation of the components that allows a safe range of motion of the hip. Trial reductions also show if there is impingement between the acetabular construct and the femoral component. Impingement can lead to dislocation or subluxation of the joint through a levering effect and can be avoided by trimming the acetabular graft or cage. When there is marked loss of soft tissue, a constraining polyethylene liner can improve the stability of the hip and prevent possible dislocations (Fig. 8). The limb lengths are estimated, and the depth of the insertion of the femoral component is also assessed at this time. As previously discussed, the position of the transplant is usually more vertical and retroverted than that of the normal acetabulum. Therefore, the acetabular liner must be placed in a more horizontal and anteverted position than the transplant or cage, leaving the posterior-superior quadrant of the liner uncovered. Some cages have an augmented wall in this position to support a buttress of cement, which is used to cover this quadrant of the liner.

Postoperative Care

Rehabilitation after an acetabular transplantation must be more guarded than that after a routine acetabular revision arthroplasty. This approach not only protects the osseous reconstruction but also allows time for the soft tissues to heal and strengthen. If the soft tissues were not already damaged from the multiple previous operative procedures and lack of use, the extensive exposure itself causes some damage. To prevent the possibility of postoperative dislocation, an abduction orthosis is worn for a period of three months. When there is an obvious absence of muscle control, as determined intraoperatively or postoperatively, a knee-ankle-foot extension is worn. All patients are routinely managed with a brace because of these concerns. Toe-touch weight-bearing is continued for the first three months, and progressive weight-bearing is continued for an additional three months. The patients are evaluated radiographically and clinically once each month for the first three months and then at six months, biannually for two years, and then annually.

Antibiotic prophylaxis with vancomycin (one gram every twelve hours) is continued for five days after the operation. This coverage is extended if wound drainage continues past this point. Rehabilitation proceeds routinely as for any other hip arthroplasty procedure.

Clinical Results

MacDonald et al.²⁰ and Kwong et al.¹⁹ reported on transplantations of whole acetabular allografts that had been performed for the treatment of acetabular defects that were type IIIB according to the classification system of one of us and colleagues²⁹. The initial diagnosis was osteoarthritis for eleven patients; traumatic injury for six patients; and rheumatoid arthritis, congenital dysplasia, and radiation necrosis for one patient each. All patients had reconstruction of the hip performed, as described, with use of a whole acetabular allograft and an acetabular component inserted with cement (Figs. 9-A and 9-B). No reconstruction cages were used.

At the most recent follow-up examination, which was an average of thirty-one months (minimum, twenty-four months) postoperatively, seventeen patients were available for assessment. All successful grafts had radiographic evidence of union, and one graft had union confirmed by biopsy. Early migration of the graft was evident in most hips. Migration averaged 1.5 millimeters. One graft had migrated four millimeters during a period of thirty-six months and then ultimately failed. Two additional reconstructions failed, secondary to infection. Most grafts showed some continual minor lateral resorption. According to the modified system of Merle D'Aubigné and Postel²², the average score for pain and walking improved from 3.7 points preoperatively to 8.9 points at the time of the most recent follow-up.

The most common complication was instability; six hips had a dislocation. Five of the six hips were treated successfully with a brace, and one had a revision to a constraining acetabular liner. Two dislocations occurred anteriorly because of excessive anteversion of the polyethylene cup; one occurred secondary to noncompliance on the part of the patient; and one each was due to poor muscle control, several previous operative procedures, and a history of recurrent dislocation. If a revision is necessary, the surgeon should consider using a constraining acetabular liner (as previously described) if the abductor muscles are absent or if stability is not within a safe range.

Infection developed in four patients. Two of those patients were managed with local débridement and antibiotics (one gram of vancomycin every twelve hours for ten days), and no recurrence was evident at the time of the most recent follow-up. The third patient had a recurrence of a gram-negative infection. He was managed with suppressive antibiotics (one gram of vancomycin every twelve hours for ten days) and was asymptomatic despite radiographic evidence of failure of the acetabular component at the most recent follow-up examination. The fourth patient needed a resection arthroplasty to eradicate the infection. The concern about the recurrence of infection after allograft has been used in a two-stage reconstruction following failure due to infection has been raised, but disputed, by some authors¹.

Stiehl⁴¹ reported on twelve hips that had been reconstructed with a pelvic allograft through a triradiate approach. His reconstruction included application of a plate and stabilization of the anterior column. At the time of follow-up, fourteen to eighty-four months postoperatively, two of the reconstructions had failed because of loosening of the acetabular component, which had been press-fit into place, and two additional hips had failed because of infection, which necessitated removal of the entire graft-and-component construct. Six hips had a good or excellent result, with an average improvement in the hip score of 40 points. After eight months of follow-up, all grafts showed radiographic evidence of remodeling, which was defined as probable incorporation. The complications were similar to those in the previously described series^19,20, with an episode of dislocation in six of the twelve hips and an infection in two of the twelve.

Garbuz et al.¹¹ reported on twenty-eight reconstructions of the hip that had been performed to repair a major columnar defect (type IIB according to the system of Gross et al.¹²). Fourteen hips that had been treated with a true acetabular allograft were followed for an average of thirty-six months. Six hips needed a reoperation because of fragmentation or fracture of the graft. Five of the six acetabular grafts had a successful outcome at two years after the reoperation¹¹.

Discussion

Revision total hip arthroplasties performed by different surgeons with use of a porous ingrowth acetabular cup have produced exceptional short and intermediate-term results and now can be accepted as the standard of care^9,29,31,37. These exceptional results have been, however, in hips with small acetabular defects. Different strategies of treatment must be instituted for hips that have large acetabular defects; one alternate treatment strategy is not enough for all situations^15,16,19,32. We have reported both success and failure after the same structural allografts were used unknowingly for different acetabular defects. Reconstruction with a porous-coated acetabular cup and a structural allograft was successful, at an average of ten years, for twenty-nine (97 percent) of thirty hips that had a type-IIIA acetabular defect according to the classification system of one of us and colleagues^29,36. However, when the same reconstruction was used for ten hips that had a type-IIIB defect according to the same classification system, seven failed at an average of five years²⁹. For this reason, use of a structural allograft with a porous-coated acetabular component has been abandoned for the most severe defects. A total acetabular transplant is used for these defects.

The use of an acetabular transplant for the reconstruction of an acetabulum that has severe bone loss has been successful, but the results cannot be compared with the outstanding results obtained with porous ingrowth cups in hips with smaller acetabular defects. These most severe acetabular defects were previously considered to be unreconstructable, and a salvage operation was considered to be the best procedure. Unquestionably, reconstruction with an acetabular transplant is a massive undertaking. However, it should no longer be considered a salvage procedure. Surgeons must be alert and must keep a close watch on patients postoperatively to be sure that they maintain compliance with stringent postoperative instructions. In so doing, they can best ensure the achievement of optimum results.

Other reconstructive procedures, such as the use of an acetabular reconstruction ring or impaction grafting, have been proposed^2,30,38,47. It is important to realize that, when a severe acetabular defect is present, these reconstructive measures should be considered not as an alternative but rather as an adjunctive procedure. When an acetabular defect is severe (that is, type IIIB according to the system of one of us and colleagues²⁹), there is not enough bone to contain the cage or morseled graft. As we have discussed with respect to the operative technique, an option is to create a contained defect with the use of a partial acetabular transplant and then to augment the remaining bone loss with a reconstruction cage and morseled graft. These cages cannot be used in isolation to span a structural defect with the hope that metal will be able to provide support in the place of a nonexistent acetabular column. A structural acetabular allograft is required in conjunction with this cage to provide biological support to the cage and to prevent fatigue fractures of the cage over the long term.

As a caregiver, the surgeon should be able to recognize not only when to consider reconstruction but also when to avoid it. The patient needs the support of the surgeon and an appropriate institution that can accommodate the difficult postoperative rehabilitation and any possible complications that may ensue. Therefore, we recommend that this type of reconstruction be done in institutions that perform a large number of arthroplasties and thus have the resources to support the patient. We cannot overemphasize the diligence necessitated by these procedures to achieve a successful outcome but also the increased patient satisfaction that results from the relief of pain and the improved ability to walk.

References

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FIG4:: Fig. 4 The acetabular transplant is cut (broken lines) to allow for acceptance into the host. The actual positioning of the cuts is determined by the amount of remaining host bone.

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FIG5:: Fig. 5 The medial aspect of the graft is sculpted with a reamer or a burr to create a groove that is in contact with the tongue of bone (as created in Fig. 3).

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FIG6:: Fig. 6 The graft is impacted into position, after which 6.5-millimeter cancellous-bone screws are placed in a proximal-to-medial direction to maintain the position.

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FIG7:: Fig. 7 The coronal view of the transplant with screw fixation. The cup is cemented into the allograft.

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FIG9-B:: Fig. 9-B Radiograph made four years after transplantation of an acetabular allograft, showing maintenance of the position of the cup and the allograft.

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TABLE I CLASSIFICATION OF ACETABULAR DEFECTS³

Type of Defect	Superior Migration of Hip Center*	Osteolysis of Ischium†	Medial Migration of Hip Center‡	Osteolysis of Teardrop§
I	Minimum	None	None	None
IIA	Minimum	Mild	Grade I	Mild
IIB	Minimum to marked	Mild	Grade II	Mild
IIC	Minimum	Mild	Grade III	Moderate or severe
IIIA	Marked	Moderate	Grade II+ or III	Moderate
IIIB	Marked	Severe	Grade III+	Severe

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TABLE I CLASSIFICATION OF ACETABULAR DEFECTS³

Type of Defect	Superior Migration of Hip Center*	Osteolysis of Ischium†	Medial Migration of Hip Center‡	Osteolysis of Teardrop§
I	Minimum	None	None	None
IIA	Minimum	Mild	Grade I	Mild
IIB	Minimum to marked	Mild	Grade II	Mild
IIC	Minimum	Mild	Grade III	Moderate or severe
IIIA	Marked	Moderate	Grade II+ or III	Moderate
IIIB	Marked	Severe	Grade III+	Severe