Numerous steps and technical issues are involved in performing an acetabular revision. Success depends on intimate contact between the implant and the remaining host bone and on creating a stable mechanical construct that minimizes micromotion, promotes bone ingrowth, and distributes the physiologic forces evenly to the surrounding acetabulum or pelvis. As in primary total hip arthroplasty (THA), the revised prosthesis must have the correct spatial orientation to avoid impingement or dislocation while restoring the hip center to its customary anatomic position to re-create normal hip kinematics. Restoration of the normal hip center requires the acetabular component to be placed as close as possible to the Kohler line and the inferomedial part of the component to be placed near the acetabular teardrop.
Preoperative planning is crucial when revising an acetabular component. Before commencing surgery, the surgeon must gain knowledge about prior surgical implants and the cause of THA failure. It is helpful to obtain serial radiographs from earlier procedures. Preoperative estimation of the volume and location of bone loss helps to determine the need for supplemental allograft bone, specialized equipment, and particular implants during revision surgery.
Fixation methods for acetabular reconstruction may be biologic or nonbiologic. Nonbiologic reconstruction relies on immediate stability of the construct without osseointegration of the component into host bone. Examples of nonbiologic techniques include cementing a polyethylene cup directly; cementing a cup with a bulk allograft, roof ring, or cage; and impaction grafting.
Biologic fixation refers to direct contact of a biologically compatible component with host bone, which promotes osseointegration and long-term fixation. Examples of this method include porous metal cups and augments (e.g., Trabecular Metal Acetabular System, Zimmer, Warsaw, Ind.). We prefer biologic fixation because of its superior durability, but osseointegration can be difficult to achieve in cases of massive bone loss. Successful osseointegration requires adequate construct stability and contact with host bone. Stable fixation with minimal micromotion (<50 μm) promotes bone formation within the pores. At least 50% surface area contact between the component and viable host bone is considered necessary for stable osseointegration. However, this value is somewhat arbitrary because the quality and location of bone and the type of metallic component used influence the minimum requirement.
Paprosky developed a treatment algorithm to guide the surgeon during preoperative planning. The choice of implant, reconstruction method, and supplemental biologic fixation depend on component migration and the extent of bone loss (see “Algorithm”). The reconstruction options described here are based on currently used implants with or without bone graft substitutes.
Liner Exchange With or Without Bone Grafting
Isolated liner exchange with or without bone grafting is indicated only in special circumstances. Most of these patients are asymptomatic and liner wear, with or without osteolysis, is detected on routine follow-up radiographs. Surgical intervention is warranted when progressive osteolysis is established radiographically or when patients become symptomatic to prevent catastrophic failure resulting in further complex reconstructive operations. Computed tomography (CT) of the pelvis can determine the extent of the acetabular defect.
After adequate surgical exposure is achieved, the polyethylene liner and any supplemental screw fixation are removed. Stability of the component is tested with manual force, and if motion or significant malposition is detected, the acetabular component is removed and revised completely. If the component is stable, an attempt is made to pack the osteolytic defect with morselized graft through the remaining screw holes. For larger lesions not accessible by this technique, a small, superolateral cortical window is made, allowing access for bone grafting. At our institution, we pack morselized bone allograft into these defects. After this step, a new, highly cross-linked polyethylene liner is secured into the locking mechanism or cemented into the cup if the locking mechanism is not competent.
Most acetabular revision cases can be managed with an isolated hemispherical or elliptical component. An acetabular component can be used alone when the hip center has not migrated more than 3 cm proximally, which is equivalent to a Paprosky type I or II defect or a Gross type II defect. Before selecting an uncemented cup, the surgeon must assess pelvic discontinuity by applying a caudal force to the ischium and visualizing any motion between the superior and inferior hemipelvis. If pelvic discontinuity exists, an acetabular component cannot be used alone.
To determine the level of the true acetabulum, a retractor is placed in the obturator foramen, which represents the inferior border of the acetabulum. Identification of the ischium and pubis can help in localizing the hip center. Larger sizes of sequential hemispherical reamers are used until the anterior and posterior columns are engaged and bleeding cancellous bone is exposed. The depth of the medial wall remaining before reaming can be determined by using a small drill bit to carefully drill through the inner wall and measure with a depth gauge to 2 to 3 mm. Trial components are used to assess stability and percentage of the component lacking coverage.
Cavitary bone defects are packed with morselized allograft bone or local autograft from previous reamings. The bone defects are packed with spherical balls or a reamer of a smaller diameter on the reverse setting. A hemispherical press-fit acetabular shell 1 mm larger than the previous reamer or line-to-line reaming for an elliptical cup provides initial fixation. Multiple acetabular screws are added to augment fixation and minimize micromotion. Bone loss of the medial wall is usually acceptable as long as a peripheral rim of bone remains to provide an adequate rim fit. This situation usually can be managed by using an extra-large “jumbo” cup, which is defined by an outer diameter of more than 66 mm. These cups promote greater initial stability and provide increased surface area for contact with host bleeding bone.
Revision of large, contained lesions with less than 50% host bone contact have traditionally been fixed with roof reinforcement or antiprotrusion cages. Advances in technology have produced implants made of or coated with porous metals, and they are more biocompatible with host osteoblasts and superior to traditional porous surfaces. With an increased material elasticity and coefficient of friction, the high-porosity metal implants enhance initial fixation of the component ( Fig. 44.1 ). Close monitoring is needed to detect early cup migration before ingrowth can occur.
Cementless Component With Metallic Augmentation
With excessive proximal and lateral migration (>3 cm) of the component, there is usually inadequate bone stock or insufficient peripheral rim to secure a hemispherical cup alone. Treatment options include a cementless cup with structural bulk allograft, a dual-geometry monoblock component, and a cementless cup with a superior metallic augment or placement of an implant with a high hip center. However, we do not think a high hip center is a good choice.
Porous metal augments are available in several sizes to closely match the dimensions of a segmental defect. These augments have the biologic benefits of ingrowth and stability as described earlier. When supplementing with a metallic augment, reamers are used to delineate the anatomic acetabulum, and a hemispherical trial component is used to create partial stability. A superior metal augment is used as a buttress augment in cases of segmental bone loss or as an inlay augment in patients with an oblong cavitary bone loss ( Fig. 44.2 ). A reamer of an appropriate size can be used to prepare the defect bed to better accommodate the augment. The augment is secured to the acetabulum with screws, and the hemispherical cup is impacted into the acetabulum in a press-fit fashion and later secured with screws. Bone graft can then be used to fill defects around the cup and augment and the polyethylene liner cemented into the correct orientation.