Uncemented Acetabular Revision with Metal Augments or Cup-Cage Combinations

Kevin I. Perry

David G. Lewallen

INDICATIONS/CONTRAINDICATIONS

The last several decades have seen the number of complex revisions with severe acetabular bone loss increase dramatically. Fortunately, better uncemented acetabular technology has allowed many of these revisions to be treated with uncemented acetabular components alone. As such, the role for traditional antiprotrusio acetabular cages has diminished over the last decade. With the advent of highly porous metals; however, the role for uncemented acetabular components with acetabular augments and/or cup-cage constructs has expanded and now offers surgeons new options for dealing with massive acetabular bone loss in the revision setting.

The use of acetabular augments and/or cup-cage constructs is indicated when the use of an uncemented acetabular component alone can be predicted to have a high likelihood of failure. The role of antiprotrusio cages in particular has evolved considerably over the last several decades. Traditional use of the antiprotrusio cage involved placement of the cage construct against host bone with multiple screw fixation into the ileum and pelvis and cementation of a polyethylene cup into the cage. The limited potential for osseointegration of the cage into host bone, however, led to substantial failure rates of these constructs. As uncemented acetabular technology has improved, the use of isolated cage constructs has diminished. Nevertheless, innovative ways to implement the use of antiprotrusio cages in conjunction with an uncemented acetabular cup (the cup-cage construct) led to continued interest in their use. These constructs have been of particular interest in the setting of pelvic discontinuity and to enhance initial cup stability in other complex acetabular reconstructions.

Structural augments made from porous metals were initially made to replace the need for structural allografts, and their development has provided new alternatives for acetabular reconstruction. These augments not only provide structural support and stability of the uncemented acetabular component but also provide a surface that is highly conducive to bone ingrowth, which hopefully will enhance the durability of the reconstruction.

Uncemented acetabular components are undeniably technically easier to implant alone than in combination with augments or antiprotrusio cages and thus are the preferred method of acetabular reconstruction when possible. The exact amount of host bone necessary to achieve implant stability and, ultimately, bone ingrowth into an uncemented acetabular component alone is unknown, but the traditional teaching that 50% of host bone contact is needed is no longer applicable in all scenarios since the advent of porous metals. The quality and location of the host bone are integral factors to consider when contemplating uncemented acetabular reconstruction alone because successful reconstruction with modest host bone available is possible. This is especially true if the host bone available is at the acetabular rim and a good peripheral fit of the acetabular component can be achieved. Nevertheless, there are circumstances in which there is so little host bone available or the geometry of the acetabular defect dictates that a stable, uncemented hemispherical cup alone is unobtainable and ingrowth into the acetabular component unlikely without augmented fixation. In these circumstances, the use of porous metal augments alone or in combination with a cup-cage construct (1) has provided a valuable tool to gain initial implant stability and allows ingrowth of bone into the reconstruction. Additionally, in the setting of a pelvic discontinuity (2), the cup-cage construct (with or without posterior plating) is one option for acetabular reconstruction.

The main contraindication to uncemented acetabular reconstruction with the use of augments or a cup-cage construct is active infection in the hip joint.

PREOPERATIVE PREPARATION

History and physical examination are crucial elements to understand the reason for implant failure. Attention to details of the history helps the surgeon avoid repeating techniques that may have led to implant failure.

It is essential in the setting of revision total hip arthroplasty to rule out infection before proceeding with surgery. We recommend routine evaluation with an erythrocyte sedimentation rate and C-reactive protein preoperatively. If the history or physical examination are concerning for infection, preoperative hip aspiration with a cell count and culture is indicated. Careful examination and documentation of the neurovascular status of the limb and the function of the abductor musculature are essential. It is also prudent to obtain previous operative reports with implant records prior to surgery so that appropriate implants are available, especially if isolated acetabular revision is being considered.

RADIOGRAPHY

Plain radiographs should be obtained to assess the entire prosthetic hip joint. Specifically, the quality and quantity of the surrounding femoral, acetabular, and pelvic bone should be evaluated. Routine use of an anteroposterior pelvis, anteroposterior femur, and lateral radiograph of the hip is recommended. The surgeon should look for areas of acetabular bone loss, osteolysis, and any evidence of pelvic discontinuity. Radiographic signs of pelvic discontinuity on an AP pelvis view include a visible fracture line that traverses both the anterior and posterior columns, medial translation of the inferior hemipelvis, or rotation of the inferior hemipelvis (as evidenced by asymmetry of the obturator foramina). Judet views of the pelvis can be helpful to identify a true pelvic discontinuity. Computed tomography (CT) scan of the pelvis is not routinely utilized by the authors but can be helpful to further delineate and define any bony defects. Multiple classifications to define the extent of bone loss exist, but Paprosky’s classification of acetabular defects (3) is the most commonly used in North America and provides a practical classification scheme that can help guide treatment.

TECHNIQUE

The authors prefer to operate with the patient in the lateral decubitus position, though revision hip surgery can be performed with the patient supine as well. Wide preparation and draping of the extremity is essential to allow access to the entire pelvis and ileum, especially if a cup-cage construct is being considered. Intravenous antibiotics should be administered prior to incision unless
there is a high suspicion for infection, in which case, antibiotics may be held until good intraoperative cultures can be obtained.

Adequate exposure of the acetabulum and pelvis can be accomplished through a multitude of operative approaches. Surgeon preference, philosophy, and experience are the major determinants of the approach chosen. Whatever approach is chosen, however, it is important to understand that if a cup-cage construct is to be utilized, exposure of the ileum is necessary and places the superior gluteal nerve at risk. This risk is increased with the use of traditional posterior or anterior exposures. The risk of injury to the superior gluteal nerve can be minimized by the use of a transtrochanteric approach, though the authors commonly use an anterolateral approach incorporating a small osteotomy of the anterior greater trochanter.

To perform a traditional anterolateral approach, a straight lateral incision is made, incorporating any previous incisions, if possible. Dissection is carried down to the level of the iliotibial band fascia, and the fascia is incised in line with its fibers. Often, the anterior aspect of the gluteus maximus must be split in line with its fibers as well. Next, a straight osteotome is used to osteotomize a small portion of the anterior aspect of the greater trochanter. The anterior portion of the gluteus medius and the vastus lateralis are left in continuity with this segment of the greater trochanter. In most cases, the osteotomy is only a small wafer of bone, usually no more than 0.5 cm thick. This segment is reflected anteriorly, and the anterior and superior portions of the hip pseudocapsule are excised. When adequate exposure of the femoral component, femoral head, and acetabular rim are achieved, the hip is dislocated anteriorly. If revision of the femoral component is to be undertaken, removal at this point will facilitate acetabular exposure. If the femoral component is to be left in place, mobilization of the femur with further removal of the pseudocapsule may be necessary to achieve adequate acetabular exposure.

It is important to identify and protect the sciatic nerve, when possible, to minimize the risk of injury, especially when contemplating a cup-cage construct or posterior acetabular plating in the setting of a pelvic discontinuity.

Adequate exposure of the entire acetabular rim is critical to assess any and all acetabular defects. This is achieved by removing scar and pseudocapsule from the entire periphery of the acetabulum until the entire rim is visible.

Once the acetabular component has been removed, careful assessment of the residual acetabulum and any bony defects is essential. Typically, the extent and pattern of bone loss encountered at the time of surgery dictates the type of reconstruction. Meticulous inspection of both the anterior and posterior columns is critical to determine if there is a pelvic discontinuity as this will have reconstructive implications. The authors’ preferred technique to check for a pelvic discontinuity is to use a Cobb elevator and push on the inferior portion of the hemipelvis and assess whether any motion occurs between the superior and inferior portions of the acetabulum. Motion between these two segments confirms the presence of a pelvic discontinuity.

The modern considerations for acetabular reconstruction in the setting of massive bone loss (with or without a pelvic discontinuity) are an uncemented acetabular component alone, an uncemented acetabular component in conjunction with structural augments, utilization of a cup-cage construct (with or without posterior plating or structural augmentation), or a custom triflange component. The use of an uncemented acetabular component with augments and the cup-cage construct is discussed in this chapter.

STRUCTURAL AUGMENTATION

Structural augmentation of acetabular bone deficiency with the use of porous metal augments is an effective alternative to the use of allograft bone. Acetabular augments have several design features that make them reliable for reconstruction. First, they are made of porous material that has shown excellent bone ingrowth capability (4). They also allow for screw fixation and come in multiple sizes to accommodate a multitude of different defects. There are fenestrations in the augments that allows for adjunctive morselized bone graft. Lastly, they can be unitized to the acetabular shell by both screw and cement fixation providing excellent initial stability of the construct.

In our experience, there are three types of acetabular defects and corresponding augment configurations that account for the majority of defects encountered at the time of acetabular reconstruction. Type I defects are peripheral, segmental defects for which a “flying buttress” configuration is often optimal (see Fig. 24-1A-E). Type II defects include large combined cavitary and peripheral segmental

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