Acetabular Revision With Metal Cup Augments or Cage Construct

Key Points

Several key steps are required to maximize the chance of a successful reconstruction and durable long-term fixation:
- Obtain excellent acetabular exposure and visualization.
- Minimize bone damage during prior implant removal.
- Cancellous bone grafting of contained bone defects.
- Maximize implant contact on host bone.
- Gain rigid initial fixation of the implant to allow subsequent bone ingrowth (with adjunctive fixation or support as needed).

Major bone loss involving the acetabulum can vary from minor cavitary deficiencies to massive segmental bone loss and associated fracture or nonunion of the acetabulum. Bone damage can occur gradually as a result of component loosening and migration, particulate-related osteolysis due to wear or corrosion, or as a sequela of periprosthetic infection. Whatever the cause and time course of the bone loss seen of the acetabular side, several key steps are required to maximize the chance of a successful reconstruction and durable long-term fixation:

obtaining excellent acetabular exposure and visualization
minimizing bone damage during prior implant removal
cancellous bone grafting of contained bone defects
maximizing implant contact on host bone
gaining rigid initial fixation of the implant to allow subsequent bone ingrowth (with adjunctive fixation or support as needed)

Excellent Exposure

The choice of surgical approach at the time of revision can be influenced by prior surgical exposure and resulting soft tissue deficiencies, the area and extent of exposure required for prior component removal (on both the acetabular and the femoral side), and surgeon preference. The vast majority of surgeons predominately use either a posterolateral approach, some variation of a direct lateral approach, or an extended greater trochanteric osteotomy for most major revision procedures. It is helpful to use the same basic exposure for both primary and revision procedures and to learn how to make those approaches extensile as needed. A lateral extended trochanteric osteotomy (performed from posterior to anterior) is an essential addition to the posterolateral approach. For those who use a direct lateral approach, a Wagner osteotomy (performed from lateral to medial) may be used. This osteotomy elevates the anterior portion of trochanter and the proximal femur for direct femoral access and femoral component removal. Regardless of the surgical approach used, excellent exposure and visualization of the peripheral rim of the acetabulum is very helpful for implant removal, defect assessment, and acetabular reconstruction.

Implant Removal

A well-fixed cup may require removal because of malposition, instability, incompatibility with modern implants, or infection. For well-fixed cementless sockets, division of the bone at the implant interface is required. The polyethylene liner can be removed with an implant-specific extraction tool if available, a screw into the liner to serve as a jack, or by reaming out the polyethylene with a small acetabular reamer. Screw removal is usually routine with the appropriate sized screw driver, but may require a carbide burr for the removal of any broken or stripped screw remnants and may require a broken screw removal set after the cup is out. Removal of the well-ingrown shell can be greatly facilitated by the use of a modular curved blade acetabular implant removal system sized to the cup diameter. Such systems allow implant removal with less bone loss than occurs when hand tools, osteotomies, or pencil tip burrs are used alone.

Cancellous Bone Grafting

After assessing the bone defects present, minimal acetabular reaming is done to remove prominent areas or high spots and enlarge the acetabular cavity just enough to maximize reamer contact on intact host bone. Residual defects or cavitated low spots are first curetted of any fibrous tissue and then grafted with cancellous bone from the acetabular reamers or with morselized allograft bone. This process facilitates the restoration of the overall bone stock for the future.

Maximize Implant Contact on Host Bone

In less severe bone defects, good contact can be achieved against host bone by careful reaming of the largely intact acetabular cavity. When larger cavitary or major segmental defects are present, contact by the hemispherical cup against the acetabulum may be limited and may lack mechanical stability, especially in smaller patients where the anterior to posterior dimensions of the acetabulum and associated anterior and posterior columns may preclude reaming for a very large or jumbo cup that might otherwise fill up any superior defects present. Large segmental defects in patients of all sizes may require either a structural allograft or a metal acetabular augment used as a prosthetic structural bone graft. Porous metal augments increase contact area by the porous acetabular construct against host bone while also increasing initial mechanical support. For irregular or elliptical acetabular defects in smaller patients, an irregularly shaped acetabular component is the best way to maximize support on host bone. The intraoperative assembly of a modular component can provide such an irregularly shaped component, customized to the patient's bone defects.

Achieve Rigid Initial Implant Fixation to Host Bone

Although uncemented highly porous hemispherical acetabular components designed to achieve bone ingrowth can be used for the reconstruction of the vast majority of revision cases, this is made possible by having a workable plan and special implants available to supplement the mechanical fixation of the cementless cup when fixation with multiple screws alone proves inadequate. A mechanically stable and motionless interface between the host bone and the porous surfaces of the implant is required over the initial weeks after surgery for bone ingrowth to occur, regardless of the type of porous surface used. As bone deficiency increases, the challenges of achieving rigid cup fixation also increase, especially if the quality of the remaining host bone is compromised (as is commonly the case because of the combined effects of prolonged disuse from impaired mobility and stress shielding around prior relatively rigid implants). By using a number of the available options for added fixation of a hemispherical cup, a stepwise approach to achieve enhanced fixation of a porous bone ingrowth revision acetabular component is possible when deemed necessary.

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