Acetabular bone loss with chronic pelvic discontinuity: Acetabular cup-cage vs cage-augment reconstruction

Background

Pelvic discontinuity is defined as a dissociation of the proximal and distal halves of the acetabulum associated with variable amounts of bone loss. This dissociation results in multiple challenges for revision total hip arthroplasty. Pelvic discontinuity can be acute or chronic. Often it can occur intraoperatively, and in these circumstances, it is usually treated with open reduction internal fixation using plate fixation followed by conventional acetabular components for total hip arthroplasty (see Chapter 10 ). More often, however, it is a result of chronic bone loss because of osteolysis and loosening of an acetabular component which requires reconstruction of bone stock and ensuring adequate acetabular coverage prior to insertion of an acetabular implant.

Common techniques used to treat this clinical entity include morselized or structural allograft plus internal fixation using a plate or an ilioischial anti-protrusio cage. According to Petrie et al. four principles need to be addressed when chronic pelvic discontinuities are treated: restoration of the continuity of the acetabulum by reconnecting the ilium and ischium, bone-grafting of the osseous deficiencies, optimizing the contact of remaining bleeding host bone to component surfaces with ingrowth potential and obtaining a mechanically stable reconstruction. Complication and failure rates have historically been reported as up to 50% and are primarily attributed to non-biological fixation of the cages and relying on screw fixation alone.

As a result of the high complication rates associated with the use of morselized or structural allograft plus the use of an ilioischial anti-protrusio cage, the use of trabecular metal cup-cage reconstruction for chronic pelvic discontinuity has been adopted and shows improved results. The rationale for this technique is the placement of a highly porous shell in contact with a mixture of bleeding host bone and morselized allograft. A titanium cage that is placed in conjunction with an acetabular cup provides load relief for the acetabular component while ingrowth and stabilization occur. The trabecular metal shell is used like a structural graft, and the cage allows the cementation of a polyethylene liner in the correct anatomical position.

The use of structural allografts has decreased in recent years because of technical difficulties, complications related to allografts, and guarded long-term results. ^, Mid-term results after reconstruction with highly porous metal augments have been uniformly promising; thus, there has been more enthusiasm for their use. In situations with severe uncontained acetabular defects with minimal bleeding host bone to support osseointegration of a hemispherical component alone, an acetabular cup in combination with highly porous metal augments can be used. Often cup-augment constructs are utilized in instances where there is not enough bleeding host bone to stabilize a highly porous acetabular shell, but stabilization of an augment is possible. The augment provides stabilization of the cup and provides an implant for bone ingrowth.

The benefits of metal augments compared with allografts are the improved properties for bone ingrowth, elimination of the risk of resorption, avoidance of disease transmission, and the convenience of off-the-shelf use. The augment is cemented to the cage, and theoretically, when bone ingrowth into the augments occurs, some stress is taken off the cage. The disadvantages of metal augments are that they do not restore bone stock, and there is a potential for third-body wear because of debris produced by fretting between the metal augment and a loose cage or cup. Premise: In this chapter, we compare the use of a cup-cage reconstruction versus a modular porous metal augment in conjunction with an ilioischial cage for the treatment of chronic pelvic discontinuity.

Surgical technique

Required equipment

Acetabular reconstruction, either with cup-cage or cage-augment constructs, in cases with chronic pelvic discontinuity is performed with an off-the-shelf highly porous hemispherical acetabular shell, modular porous metal augments, and a titanium reconstruction cage ( Fig. 16.1 ).