CHAPTER OUTLINE
Acetabular bone deficiency is routinely encountered during revision total hip arthroplasty (THA). Factors influencing treatment include severity of bone loss, location of any defects present, and the quality, location, and vascularity of the host bone that remains. A final critical element affecting treatment choice and success rate is the amount of host bone (versus graft) available for cup support and fixation after acetabular preparation. Successful management of major bone deficiency requires careful assessment of the defect present, selection of the optimal reconstruction method, bone preparation efforts that maximize support for the revision component on host bone, stable secure initial cup fixation, and preservation or restoration of normal anatomy whenever possible.
INDICATIONS AND CONTRAINDICATIONS
Several systems for classification of acetabular bone defects have been proposed, but the system proposed by Paprosky has proven helpful in the development of treatment plans by helping predict the support that may be expected for the revision component after removal of the failed device.
Paprosky Classification
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Type I: No superior migration of failed implant, minimal lysis, Kohler’s line intact
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Implies near primary situation with >90% host bone support of cup
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Type II: Under 3 cm superior migration, intact columns; implies at least 70% host apposition
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A: Contained cavitary loss with intact rim and floor with mild elevation of hip center
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B: Slightly higher hip center but still has superior support
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C: Superior and medial migration of the hip center but still with superior support; Kohler’s line broken
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Type III: massive bone loss with more than 3 cm upward migration
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A: “Up and out”—medial support possible, superior support lost
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B: “Up and in”—no medial or superior support, Kohler’s line broken
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Treatment Options Using a Modular Acetabular Reconstruction System
After removal of the failed implant, defect assessment, and acetabular preparation, bone grafting is routinely performed to reconstitute bone stock. Morselized cancellous graft is preferred, and structural allograft is used only when required because of massive bone deficiency. Hemispherical porous ingrowth cups fixed with multiple screws are currently the workhorse method for acetabular revision and can be applied successfully in more than 90% of revision cases encountered. Use of a hemispherical ingrowth socket with multiple screws is always our preferred and first choice, and alternatives are sought only when this method cannot be made to work or fixation status is tenuous. Use of a tantalum cup allows placement of extra screw holes and added screws via holes created with a carbide burr. Cementation of the polyethylene liner provides added stability by creating a “locking screw” effect. Once the acetabular screws are covered by cement, this prevents backing out or angulation of screws under loading.
In selected cases of massive bone loss, irregular bone defects, or impaired host bone quality (e.g., from prior radiation), alternative methods may be required. A modular system using a hemispherical cup with multiple screws in combination with matching acetabular augments can be used to fill large segmental or cavitary defects in critical support locations, allowing cup placement at or near the anatomic level. Situations involving pelvic dissociation, poor bone quality, or more massive defects that preclude achievement of stable cup support (even with augments) on host bone can be managed with a combination antiprotrusio cage and ingrowth cup (the so-called cup-cage combination).
PREOPERATIVE PLANNING AND TECHNIQUE
Descriptions of the advantages and disadvantages and the surgical technique recommendations for each of the components of this modular revision acetabular system follows.
Acetabular Augments
Advantages
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Provide stable support for hemispherical cup when critical segmental defects exist
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Avoid structural allograft and the potential for graft resorption with resulting loss of mechanical support
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Help increase contact area of porous ingrowth material and cup construct against host bone
Disadvantages
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New method with only relatively short-term data to support efficacy (Nehme and colleagues, 2004)
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Potential exists for disassembly if loss of fixation occurs, with possible generation of particulate debris
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May make removal difficult if needed and the implant is secure (e.g., in the case of infection)
Surgical Technique Recommendations
Surgical technique recommendations are as follows:
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Defect extent, location and mechanical support for the cup on host bone after initial preparation determines if and where augments are used.
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Try to reconstruct the hemispherical acetabular cavity first using augment (type II or III configuration as detailed later), then place the socket in optimal orientation within the reconstructed acetabulum.
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It may be easier to place the cup first and then add the augment second, especially to superolateral rim defects (type I construct as detailed later).
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Maximize contact area of implant construct against intact host bone.
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Use cancellous graft for residual bone defects and augment fenestrations.
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Try for rigid (screw) fixation of augments to host bone when possible.
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Use rigid augment fixation to the cup to avoid motion and debris (via cement). No cement is used or desired between the augment or cup and host bone.
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Use cup fixation with screws into the ischial area and inferior portion of the posterior column (zone 3) as well as the usual multiple dome or iliac screws.
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Cementation of the acetabular liner over the top of screw heads will give a “locking screw” type of fixation by making screws into fixed-angle devices less likely to back out or angulate (especially in osteoporotic bone).
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A tantalum cup allows creation of “extra “ screw holes using a carbide bur, if the standard hole position and/or the screws there fail to give sufficient fixation or purchase.
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In an initial 16 cases of acetabular augments used during revision THA, none were found to be loose clinically or radiographically after a minimum of 2 years (Nehme and colleagues, 2004).
Patterns of Augment Placement
For type I configurations, see Figs. 48-1 to 48-3 .
