Five acetabular components were re-revised because of aseptic loosening at 2.6, 3.0, 4.8, 7.6, and 8.7 years postoperatively. Therefore, the ten-year survival rate with re-revision because of aseptic acetabular loosening as the end point was 78% (95% confidence interval, 74% to 82%). All of the cups that were re-revised because of aseptic loosening demonstrated incorporation of the femoral allograft, and all of the acetabular defects that were present at the time of the re-revision were consistent with Type-II defects that were amenable to treatment with a porous hemispherical shell. The shells that were used at the time of re-revision ranged in size from 68 to 72 mm. Radiographic evidence of osseous ingrowth was observed in association with all of the remaining cups but one; the latter cup was in a hip that demonstrated radiographic signs of failure at the cup-graft interface at 2.6 years postoperatively. Therefore, the ten-year survival rate with radiographic signs of acetabular loosening as the end point was 74% (95% confidence interval, 70% to 78%).
Allograft resorption around intact stable shells was classified as “none” in eleven hips, “mild” in one, “moderate” in four, and “severe” in one. Allograft resorption around shells that were re-revised was classified as “none” in three hips, “mild” in one, and “severe” in one. Although extremely difficult to ascertain on the basis of anteroposterior pelvic radiographs alone, the estimated percentage of host-bone coverage as viewed on the anteroposterior pelvic radiographs averaged 42% for the stable shells and 43% for the shells that failed clinically.
Clinical evaluation of the patients who had an intact acetabular component demonstrated improvement of the average Merle D'Aubigne and Postel hip score from 5 points preoperatively to 10 points at the time of the latest follow-up. Ten patients required no assistive devices for walking, five patients used a cane for long walks, and two patients used a cane full time. Thirteen of the patients with a stable component had no pain, while the remaining four had mild exertional pain. The one patient with radiographic signs of loosening had a postoperative dislocation that required closed reduction. This patient had not had any additional episodes of instability at the time of the most recent follow-up.
Acetabular revision in the presence of extensive bone loss is challenging and technically demanding. Successful long-term results depend heavily on the achievement of intraoperative stability of the acetabular shell. Most acetabular revisions are associated with some degree of bone loss. The extent of bone loss and osteolysis often is underestimated on plain radiographs. Therefore, the surgeon must be comfortable with alternative surgical options when unexpected amounts of bone loss are encountered.
The acetabular classification system described by Bradford and Paprosky can be used to categorize the acetabulum into one of three different types in order to assist preoperative planning, to guide treatment options, and to standardize patient reporting3. Defects associated with minimal loss of bone and structural support (Paprosky Type-I and II defects) can be reconstructed with use of a hemispherical acetabular component with or without the use of nonstructural cancellous bone graft. Excellent long-term results have been reported in association with such treatment2,9.
Type-IIIa defects are characterized by severe bone loss with superior migration of the femoral component by >3 cm cephalad to the superior obturator line. Typically, osteolysis involving the teardrop and the ischium is moderate and the Kohler line remains intact. Poor results have been observed when a cementless hemispherical shell alone has been used to reconstruct this type of defect2,10. Options to provide initial stability for bone ingrowth include the use of a hemispherical shell augmented with a structural graft, the use of an implant that accommodates the deficient superior dome (a bilobed cup or a trabecular metal cup with augmentation), or the placement of the prosthesis in a high hip center9,11. We believe that restoration of bone stock is preferable, especially in a younger patient, if complications such as graft loosening, resorption, and infection can be minimized.
The senior author (W.G.P.) previously reported excellent intermediate-term results following acetabular reconstruction of Type-IIIa defects with the use of a distal femoral allograft and a porous hemispherical shell12. In that study, forty-eight patients were reviewed at an average of 6.1 years postoperatively and the survival rate was 94% with aseptic acetabular loosening as the end point. All of the radiographically stable implants demonstrated osseous ingrowth, with no evidence of resorption of the allograft. The current study documents the continued successful results for patients in this cohort at an average of ten years postoperatively. The five components that required revision because of aseptic loosening were converted to a hemispherical shell without the need for additional structural allograft support. Therefore, we believe that our grafting technique restores acetabular bone stock and can allow conversion of a Type-IIIa defect to a Type-II defect if subsequent revision is necessary.
Other authors have reported that the use of structural allograft provides reliable results when >50% host-bone support is present. Morsi et al. reported an 86% survival rate at seven years postoperatively in a study of twenty-nine patients who had been managed with a “shelf allograft” for the treatment of superolateral deficiency13. Woodgate et al. reported an 80.4% survival rate at ten years postoperatively in a study of fifty-one hips and concluded that good results can be achieved in association with the use of structural allograft, especially if there is restoration of nearly normal hip biomechanics14.
Poorer long-term results have been reported when bulk allografts have been used in patients with <50% host-bone support. Chandler et al., in a study of twenty-four hips that had been followed for an average of more than twelve years, reported a 26% rate of revision for aseptic loosening and a 41% rate of radiographic loosening and concluded that structural allografts are rarely indicated15. Similarly, Garbuz et al., in a study of thirty-three hips in which a structural allograft supported >50% of a cementless acetabular component, reported a clinical and radiographic success rate of 55% after an average duration of follow-up of seven years16. The authors concluded that the only clinically important factor was the method of fixation and recommended the use of a roof-reinforcement ring.
Alternatives for acetabular revision in patients with a deficient dome and superior migration (a Type-IIIa defect) include either the use of a bilobed implant or a trabecular metal acetabular component with a superiorly placed trabecular metal augment. The long-term clinical results of acetabular reconstruction performed with use of trabecular metal are currently unavailable, to our knowledge. However, this material appears to allow extensive bone ingrowth with high initial frictional resistance. These material properties ultimately may change the indications for the use of a monoblock acetabular component. In contrast, the intermediate-term results associated with bilobed acetabular components have been disappointing. These implants are used in an attempt to lower the hip center of rotation and to obtain fixation in both the true acetabulum and the ilium. Chen et al., in a study of thirty-seven hips, reported a 24% failure rate at an average of forty-one months postoperatively17. Because of these poor results, they have abandoned the use of this implant. On the basis of the successful results in the present series, the senior author (W.G.P.) continues to use a structural distal femoral allograft and a porous-coated acetabular component for the treatment of Type-IIIa acetabular defects in young patients who have associated bone loss involving the superior dome. ▪
A video supplement to this article is available from the Video Journal of Orthopaedics. A video clip is available at the JBJS web site, . The Video Journal of Orthopaedics can be contacted at (805) 962-3410, web site: .
The authors did not receive grants or outside funding in support of their research or preparation of this manuscript. One or more of the authors received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity (Zimmer). In addition, a commercial entity (Zimmer) paid or directed, or agreed to pay or direct, benefits to a research fund, foundation, educational institution, or other charitable or nonprofit organization with which the authors are affiliated or associated.
Investigation performed at Central DuPage Hospital, Winfield, Illinois
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