Case Report #6

Case Report #6: Acetabular Revision using a Jumbo Cup and Augmentation

Scott I. Berkenblit, M.D., Ph.D. and
Marc W. Hungerford, M.D.

The patient is a 50-year-old male who had undergone a left total hip replacement in 1980 for osteoarthritis. Two years later, he underwent revision of the acetabular component because of loosening. He then did well until several months ago, when he began to develop increasing pain in the left hip. His primary orthopaedic surgeon diagnosed massive acetabular osteolysis with loosening of the acetabular component of his prosthesis and he was referred for further management. Past medical history is notable for IV drug use approximately 20 years ago. He also had a history of ethanol abuse but had not consumed any alcohol for the past 12 years. The patient has no other active medical issues.

Physical Examination

On examination, the patient had a marked Trendelenburg limp on the left. The left lower extremity was approximately 2.5cm shorter than the right. On range of motion testing, the left hip had 95 degrees of flexion, no internal rotation, 30 degrees external rotation, and 20 degrees of abduction. Neurovascular examination of both lower extremities was normal.

Radiology Studies

AP (Figure 1) and lateral (Figure 2) views of the left hip reveal massive osteolysis about the acetabular component, which consists of a cage with cemented polyethylene liner. The component has migrated superiorly and is in significant protrusio. The femoral stem appears to be well fixed.

Revision of a loose acetabular component can present a challenging surgical problem, particularly if there is significant bone loss. Several classification systems have been developed as guides to treatment. The AAOS classification system [4] categorizes defects as either segmental, cavitary, or combined, with pelvic discontinuity and bony ankylosis added as additional categories. The classification system of Paprosky et al [7] is more complex but is more useful for determining the approach to treatment. This system depends on four radiographic parameters: superior migration of the hip center, ischial lysis (indicative of posterior column insufficiency), destruction of the teardrop (medial wall), and disruption of Kohler’s line. The present case illustrates all of these features and would be classified as Type IIIB, the most severe, using this system.

When performing an acetabular revision, the femoral component must be examined for stability and alignment. If the component is well fixed and not malpositioned, it may be left in place and only the acetabular component revised. Good clinical results have been reported with retention of a well-fixed femoral component during isolated acetabular revision [5].

One of the challenges in a case such as this is constructing an adequate superior buttress to support the new acetabular component. One option is to use a structural allograft, which is fixed to the pelvis and then reamed to accommodate a jumbo uncemented cup [6]. Alternatively, the defect can be spanned by a cage or ring [1]; morsellized bone graft is packed beneath the device and a polyethylene liner is cemented into it. Finally, an uncemented cup can be buttressed superiorly by additional metal. While oblong cups have been developed for this purpose, a more flexible approach is to use a cup with modular augments (as was done in the present case).

Porous tantalum is a relatively new biomaterial which has been used to fabricate acetabular components for both primary and revision arthroplasty applications. Currently manufactured by the Zimmer Corporation under the name “Trabecular Metal,” the material is formed by precipitation of elemental tantalum metal onto a carbon skeleton which approximates the morphology of cancellous bone [8]. The material is highly porous (75-85% by volume) but has sufficient mechanical strength to provide support for an acetabular component [8]. In a canine model, porous tantalum has been shown to reproducibly develop 40-50% bone ingrowth within 4 weeks of implantation [2,3]. For revision applications, acetabular cups as well as modular augments in a variety of sizes are available. The augments are cemented to the cup prior to implantation; both the cups and augments contain multiple holes to allow supplemental screw fixation as well.

The main theoretical advantage of porous tantalum implants is that they stimulate true bone ingrowth into the material (due to its “trabecular” morphology) rather than ongrowth onto the surface as with conventional cementless cups. The main disadvantage of using porous tantalum augments instead of bulk allograft is that the material is not replaced by bone. If removal of the component is required during a subsequent revision, this will create a new bony defect.

It was recommended to the patient that he undergo revision of his acetabular component. Because of the large osteolytic defect, the preoperative plan was to use either a bulk allograft, a Burch-Schneider ring with cemented liner, or a jumbo cup with possible augmentation in order to reconstruct the acetabulum. The risks of the procedure, including sciatic nerve injury, infection, dislocation, and graft resorption, were discussed with the patient and he elected to proceed.

The hip was approached via a direct lateral approach, using the previous skin incision. Once the hip capsule was entered, the acetabular component was found to be grossly loose and was easily removed. The femoral stem was solidly fixed and in acceptable alignment and thus was not revised. Intraoperative soft-tissue specimens were sent for frozen-section analysis and revealed no sign of infection. Thus, the decision was made to proceed with the revision arthroplasty.

With the acetabular component removed, the lytic defect was debrided and carefully examined. Overall, the defect measured approximately 10cm in largest diameter. A large superior dome defect was present, as well as a small medial wall defect. Because a portion of both the anterior and posterior columns were still intact, it was felt that a jumbo cup might gain sufficient purchase, supplemented by an augment superiorly as well as multiple screws.

A 70mm acetabular component composed of porous tantalum (“trabecular metal”) was found to fit well, with a 58x20mm augment (of the same material) positioned superiorly. The augment was cemented to the cup and the composite structure was implanted into the acetabulum and secured with multiple screws. The remaining portion of the superior defect was packed with morsellized bone allograft obtained from the bone bank. A polyethylene liner was cemented into the shell, a new femoral head was impacted onto the trunnion, the hip was relocated, and the wound was closed in the usual fashion. Postoperatively, the patient has done well, with good relief of his hip pain. Postoperative radiographs (Figures 3,4) show the acetabular cup, superior augment, and screws to be intact, with evidence of incorporation of the bone graft.

In this case, a grossly loose acetabular component with a large bony defect presented a challenging reconstructive problem. A jumbo acetabular cup with modular augment provided an alternative solution to using a structural allograft or a cage, with a successful outcome as of the patient’s most recent follow-up visit.