I Have a Patient With Severe Segmental Bone Loss in the Superior Dome of the Acetabulum. What Should I Do to Reconstruct This Defect?
Porous, noncemented acetabular implants in revision surgery have provided far superior results than cemented components and should be the method of choice for most acetabular revisions. However, a noncemented acetabular component relies upon biological fixation (ingrowth) to provide long-term stability. Revision of an acetabulum in the setting of severe segmental bone loss continues to provide a challenging problem. In Paprosky Type IIIA and IIIB acetabular defects, the remaining acetabular rim may not provide the support needed to maintain absolute component stability to allow for bone ingrowth.1
The exact amount of biological fixation needed to provide durable implant stability is unknown. Most surgeons would agree that 50% to 60% of implant to host bone contact is needed—as viewed in the coronal plane on an anteroposterior radiograph.2
In Paprosky Type I and II defects, the acetabular rim will usually support a hemispherical cup with less than 30% of the cup uncovered. This may allow for press-fit fixation alone. Nevertheless, screw supplementation should be considered whenever absolute stability cannot be achieved by the press-fit.
Micromotion between the implant and the host bone, as little as 40 to 50 μm, can lead to meager or no bone ingrowth and subsequent failure.2 Added peripheral screw supplementation has provided durable acetabular implant fixation with excellent long-term results when less than 30% to 50% of the cup is uncovered.3 In contrast, Paprosky Type III bone defects had a 19% failure rate when a hemispherical cup with peripheral screw augmentation was used.3
Several options have been studied to allay this complicated problem. Structural “number 7” allografts, antiprotrusio rings and cages with morcellized allograft, custom triflange components, and oblong cups have provided excellent short-term results but have led to loosening and failure at mid- to long-term follow-up.4 Distal femoral or proximal femoral bulk allograft with a cage has been successfully used when the implant contacts less than 50% host bone.5,6 However, this nonbiological fixation may lead to failure as the severity of the defect worsens with time.
In contrast, modular tantalum metal augments can be used to support a hemispherical shell when less than 50% of host bone is available for biological fixation. Tantalum augments are porous and have a low modulus of elasticity, providing an optimal environment for bone ingrowth. Short-term results have been very promising and the biological fixation achieved may lead to durable fixation in the long term.5
Preoperative radiographs are studied for component migration. Superolateral or superomedial migration greater than 2 to 3 cm in reference to the superior obturator line with mild to moderate ischial lysis is consistent with Paprosky Type III acetabular defects (Figure 29-1). Computed tomography scans may be obtained to further identify bone loss and are needed if custom triflange components are to be utilized. Preoperative laboratory evaluation may include a C-reactive protein (CRP) and an erythrocyte sedimentation rate (ESR) and are correlated with intraoperative frozen sections and cell counts to evaluate for the presence of infection.
Figure 29-1. Preoperative radiograph of Paprosky Type III defect.
Positioning of the patient is critical and the orientation of the pelvis relative to the floor must be known, as the normal landmarks are often distorted. A posterolateral approach is frequently used and can be made extensile by an extended trochanteric osteotomy.
If there is spherical bone remodeling, some Type 3 defects may be treated with jumbo shells with dome screw fixation. However, in revision cases with Type 2 or Type 3 defects, oblong remodeling can occur and hemispherical cups require structural allograft or trabecular metal augments superiorly. Moving the hip center superiorly and using a hemispherical cup is another option, but this is less desirable. Custom triflange implants may also be used and do provide biological fixation; however, they are expensive and require significant time to manufacture. Tantalum metal augments can be customized by reaming or with the use of a burr and are readily available. We currently prefer tantalum metal augments over the use of structural allograft.
A posterolateral approach is frequently preferred with a posterior capsulotomy. If increased exposure is needed, a capsulectomy is performed. An extended trochanteric osteotomy will assist in the exposure of the acetabulum or removal of the femoral implant, if necessary.
The acetabulum is exposed in a subperiosteal fashion to include the posterior column and the ilium. Removal of the existing components are carried out and a systematic debridement of the acetabulum is undertaken to asses the remaining bone stock.
An attempt is made to identify the original hip center and limited reaming is carried out to achieve 2 points of contact, usually posteroinferiorly and anterosuperiorly. This allows for an intimate contact between the implant and host bone and may provide some inherent stability.
Tantalum metal augments can be added for additional fixation prior to or after the implant has been placed. A majority of the time the augment can be added after the cup has been placed and provisionally fixed with screws, usually on opposite sides of the implant.5,6 The posterosuperior quadrant of the acetabulum is the safest for screw
placement. The augment-cup interface is secured with polymethylmethacrylate and the augment is secured to the host bone with 6.5-mm cancellous screws (Figure 29-2). If the host bone or augment has a mismatch, either can be contoured using a burr to optimize the surface contact area to promote bone ingrowth.
Figure 29-2. Postoperative radiograph at 1 year after hip revision with tantalum metal augment and screw fixation.
Postoperatively, patients are instructed to partially weight bear on the operative leg with use of a walking aid for a minimum of 6 weeks. Patients must also utilize hip precautions with a hip abduction orthosis for 6 weeks due to the risk of dislocation associated with revision surgery.5
1. Paprosky WG, Perona PG, Lawrence JM. Acetabular defect classification and surgical reconstruction in revision arthroplasty: a 6-year follow-up evaluation. J Arthroplasty. 1994;9(1):33-44.
2. Sporer SM, Paprosky WG, O’Rourke M. Managing bone loss in acetabular revision. J Bone Joint Surg Am. 2005;87:1620-1630.
3. Weeden SH, Paprosky WG. Porous-ingrowth revision acetabular Implants secured with peripheral screws: a minimum twelve-year follow-up. J Bone Joint Surg Am. 2006;88:1266-1271.
4. Chen W, Engh CA, Hopper RH, Mcauley JP, Engh CA. Acetabular revision with use of a bilobed component inserted without cement in patients who have acetabular bone-stock deficiency. J Bone Joint Surg Am. 2000; 82:197-206.
5. Weeden SH, Schmidt RH. The use of trabecular metal implants for Paprosky 3a and 3b defects. Presented at: AAHKS Meeting; November 2006; Dallas, TX.
6. Weeden SH, Schmidt RH. The use of tantalum porous metal implants for Paprosky 3A and 3B defects.
J Arthroplasty. 2007;22(6):151-155.