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Thursday, March 1, 2001

Technique to add rim support in THA reviewed

Acetabular bone loss is a challenge in revision total hip arthroplasty. Superior rim loss or an uncontained cavitary defect often is present with either an intact medial wall or an intact rim of bone on the medial wall of the acetabulum. A study in scientific exhibit 61 reviews 12 cases over an eight-year period which used a technique to medialize the acetabular cup for added rim support on host bone.

Ten women and two men presented between 1991 and 1998 with bone loss of the superior rim and migration of the cup where the medial wall either had been intact or a rim of bone existed along the medial wall below the cavitary uncontained superior defect. These patients were revised by medializing a larger acetabular component to transfer load through the medial wall more in line with the center of the articulating femoral head.

The technique included reaming to the medial wall below the superior bone defect. Then the wall was osteotomized to create a disk of bone still attached to intrapelvic musculature. Once the medial wall was penetrated, standard reamers on reverse were used to protect intrapelvic contents and prevent destruction of the central disc of bone.

The component then was inserted and if not enough support existed for press fit fixation, at least two screws were inserted to secure the shell. If more than one-third of the cup was uncovered laterally, an allograft bone block was shaped into place and fixed with screws (one patient). Even if lateral rim allograft was used, most of the superior support was provided by the intact superior rim on the medial wall. If a third or less of the cup was uncovered then morselized cancellous bone graft was used to fill the defect.

Average follow-up was 34 months and all components in this series remained in place, ranging from 12 to 60 months. Only one of the 12 patients required a block allograft for lateral cup support. The average preoperative Harris hip score was 36.5 and postoperatively increased to 89.7. No patient had mechanical failure of the cup. Average cup size for this group of patients was 62 mm (range 54 to 70).

Rather than opting to accept a higher hip center in these patients or to support the cup fully on block allograft, the investigators say that the acetabular defects in this study were handled by medializing the cup and or osteotomizing the central wall such that the acetabular component was resting on native bone along the osteotomized medial wall of the acetabulum. They observe that traditionally, structural allografts have not faired well with failure rates greater than 40 percent in five years postoperatively.

The investigators are William M. Mihalko, MD, PhD, and Leo A. Whiteside, MD, both of the Missouri Bone and Joint Center, St. Louis, Mo.

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Last modified 14/February/2001 by IS