Structural Allografts With or Without Cages
Acetabular defects can be relatively small and contained like those observed in patients with subchondral cysts, erosive changes caused by inflammatory arthritis, pigmented villonodular synovitis (giant cell tumor of tendon sheath), aseptic loosening and rarely, benign epiphyseal tumors such as chondroblastoma or giant cell tumor of bone. Observation and surveillance over time generally are recommended, depending on the location of the defect, the underlying cause of bone loss and its location. Larger lesions, especially those associated with progressive bone loss or tumors and defects in the weight-bearing dome of the acetabulum require further investigation and treatment. Usually these defects, surrounded by normal bone, are treated with open or percutaneous grafting procedures, either bone autografts or allografts. Based on previous studies,[1,2] allograft bone is preferred. In both animal (nonhuman primate) and human studies, the authors demonstrated that cortical bone of certain size dimensions (100–300 μ) incorporates as well as autograft bone. Moreover, this bone, when packed tightly in osseous defects, has radiographic features that are reminiscent of polymethylmethracrylate (PMMA) but unlike PMMA, incorporates over time. As incorporation progresses, the graft material assumes the same radiodensity as the surrounding bone, advancing from the periphery to the center of the defect.
For larger defects, especially those in the weight-bearing dome of the acetabulum, structural grafts are necessary to achieve adequate stability. Medial superior dome defects in the acetabulum are observed in patients with inflammatory arthritis, particle disease caused by polyethylene wear or fretting metal wear debris, and rarely, osteolysis and superior and medial implant migration caused by bipolar hemiarthroplasty. In these situations, the defect is either thoroughly curetted, or in those patients with especially large defects, reaming is employed to remove the damaged bone and to create a geometrically stable cup and cone fit for a structural allograft on which a metal-backed acetabular component can be placed and stabilized with screws into the ilium (Figure 1).
Figure 1.
A, Anteroposterior radiograph of a right proximal femoral replacement with a bipolar head in a 73-year-old woman with a dedifferentiated chondrosarcoma. B, Anteroposterior pelvic radiograph 2 yr later showed proximal migration of the implant across the ilioischial line with an obvious protrusion defect. The patient had significant pain at this time. C, Anteroposterior radiograph of the pelvis demonstrating conversion of the bipolar hemiarthroplasty to a total hip replacement using a femoral head structural allograft to fill the superior and medial acetabular defect. D, Anteroposterior and lateral radiographs of the left hip showing the femoral head allograft outline (arrows) filling the acetabular defect and placement of a metal cup over the graft and fixing the implant into the ilium with screws.
For large uncontained defects, (pelvic discontinuity) the structural graft can be augmented with an acetabular cage. In one study by Regis et al.[3] over an 18-year period, surgery was performed on 97 hips with extensive periacetabular bone loss using bulk allografts and Burch-Schneider antiprotrusion cages. Sixty-five hips were available at a mean follow-up of 14.6 yr. Twenty-nine patients (32 hips) died of unrelated causes. Nine cages required rerevision because of infection (three), aseptic loosening (five), and flange breakage (one). The average Harris hip score improved from 33.1 points preoperatively to 75.6 points at follow-up (P<0.001). Radiographically, graft incorporation and cage stability were detected in most hips, 48 and 52 hips, respectively.[3]
Abolghasemian et al.[4] reviewed results in 44 consecutive patients (50 hips) who underwent acetabular re-revision after a failed previous revision that was done using structural or morcellized allograft bone with a cage or ring for uncontained defects. In summary, they observed a fairly high rate of aseptic loosening for hips with pelvic discontinuity and less when the allograft had been in place for longer intervals of time. They concluded that the use of a cage or ring over a structural bone graft for large uncontained acetabular defects could restore bone stock and allow future revision surgery in certain patients.[4] Although structural bone grafts, cement and cages have been used for patients with large metastatic deposits in the acetabulum, for themost part, these patients are better served with cage or augment-cemented constructs that create immediate stability and allow early mobilization and are unaffected by radiotherapy.
Curr Orthop Pract. 2019;30(6):501-509. © 2019 Lippincott Williams & Wilkins
