INDICATIONS

Patients may incur massive periacetabular bone loss for a variety of reasons. The most common cause is periprosthetic osteolysis secondary to excessive PE wear requiring revision surgery. Two classification systems exist for evaluating periacetabular bone loss. These classifications are based on the severity of bone loss and ability to obtain implant fixation. Generally, previously mentioned means of acetabular reconstruction other than CTACs are appropriate for Paprosky classes I to IIIA and American Academy of Orthopaedic Surgeons (AAOS) classes I and II, whereas Paprosky type IIIB and AAOS type III and IV defects are well suited for CTAC implantation. In the two largest existing series, patients who were considered to have Paprosky type IIIB or AAOS type III or IV periacetabular bone loss were selected for CTAC reconstruction.

THE IMPLANT

A standard CT scan of the pelvis with or without implants in place with 3-mm cuts from the anterior superior iliac spine to the obturator foramen and 5-mm cuts for the remainder of the pelvis is obtained. The uncompressed data are then recorded on a CD-ROM and sent to the implant manufacturer. The manufacturer (Biomet or DePuy, Warsaw, IN) will provide detailed instructions on surgeon request. Metal subtraction software is used to create a three-dimensional, one-to-one model of the hemipelvis for the surgeon to analyze ( Fig. 51-1 ). The engineers rely on markings of the flanges made on the pelvic model by the surgeon, and a clay prototype of the component is subsequently prepared. The head center location is chosen based on patient-specific considerations, including leg length discrepancy, planned retention or revision of the femoral component, length of contralateral leg, and size of the current acetabular component and should be specified in the initial order. Generally, the vertical head center location is established by first determining the approximate anatomic position of the head center using the superior aspect of the obturator foramen as a reference point. The remaining bone of the anterior and posterior columns determines the head center in the coronal plane, whereas the flange geometry and component face diameter guide the position of the head center in the sagittal plane. The component face orientation is established by setting the abduction and anteversion angles of the cup. The abduction angle is established using the plane of the obturator foramen as a reference. The anteversion angle is established using the plane of the iliac wing and the obturator foramen as references.

A, Anteroposterior radiograph of a patient with a failed hip arthroplasty and massive periacetabular bone loss. B, Photograph of the three-dimensional reconstruction of the patient’s hemipelvis. C, Photograph of the computer-assisted design model of the patient’s hemipelvis.

Once the design of the implant has been finalized, the surfaces of the titanium alloy stock are milled. The blank of wrought titanium alloy bar stock is prepared using a hemispheric inner geometry that is compatible with standard modular acetabular component liners.

The iliac and ischial flanges contain multiple rows of screw holes for 6.5-mm screws. Five or six screw holes in the ischial flange are preferred because this area has proven to be the most common site of fixation loss. Two rows of three or four screw holes have proven sufficient for fixation of the iliac flange ( Fig. 51-2 ). The pubic flange is smaller in size and contains no screw holes.

Photograph of the computer-aided drafting (CAD) model of a patient’s hemipelvis (with the clay model of the custom triflanged acetabular component [CTAC] in place) and the final design of the CTAC to be implanted.

The implant inner geometry is machined to create a modular locking mechanism that can accept any of the modular PE or metal liners typically available for standard acetabular components. The bone interface of the CTAC, including the flanges, is coated with a porous ingrowth surface to foster osseointegration. Current CTAC designs allow for easier insertion at the time of operation and provide space behind the implant for additional bone graft. An important design characteristic is creation of a central dome that has intimate contact with the remaining ilium superiorly to reduce shear stresses on the three fixation flanges.

OPERATIVE TECHNIQUE

The CTAC is best implanted through an extensile posterolateral approach with or without a trochanteric osteotomy. Care must be taken to protect the superior gluteal artery and nerve when the gluteus minimus and medius are elevated from the iliac wing. In patients with severe acetabular component protrusion, a retroperitoneal approach initially may be required to carefully free the iliac vasculature from the protruded acetabular component, followed by revision THA through a separate extensile posterolateral or transtrochanteric surgical approach ( Fig. 51-3 ). The sciatic nerve is protected by extending the hip when the hamstring origin is cleared from the ischial tuberosity.

Preoperative anteroposterior radiograph (A) and photograph (B) of the three-dimensional reconstruction of the hip of a patient with severe intrapelvic migration of a loose acetabular component in which a retroperitoneal approach was initially done to free the iliac vessels from the protruded acetabular component.

The gas-sterilized, three-dimensional pelvic model should be referenced on the sterile field for appropriate implant placement. Bone should be removed intraoperatively to match the bone removed on the hemipelvic model as determined preoperatively. This is typically the thin rim of bone surrounding a portion of the remaining acetabulum.

Insertion of the CTAC may be initiated with insertion of the ischial or iliac flange. Initial insertion of the iliac flange is facilitated by translation of the hip proximally, with some flexion, to relax the abductor musculature followed by rotation of the ischial and pubic flanges into position while the hip is extended. Fixation should be initiated with screws in the ischial flange, where the bone is typically the poorest and osteolysis is common. The iliac flange is then fixed with screws, again protecting the superior gluteal neurovascular structures. The trial or final modular acetabular liner may be inserted at this time. In cases of pelvic discontinuity, the surgeon has two choices. The implant may be made to lie in situ in the pelvis, or a planned reduction is prepared. When the in situ method is chosen, the implant and its cup position are planned for fixation into the defect without reduction. If a reduction is planned, the CTAC is made with the cup in the reduced position. In this method the iliac screws are placed first to pull the flange down into intimate contact with the bone, which reduces the discontinuity and rotates the inferior half of the hemipelvis into correct orientation relative to the superior hemipelvis. Additional posterior column plating should be considered in patients in whom a pelvic discontinuity exists, and initial planning of the CTAC must take this into consideration.

Bone graft is used to fill remaining cavitary defects medial to the CTAC and is typically placed both before and after CTAC placement (through remaining gaps between the component and peripheral rim of the remaining acetabulum). Cement augmentation of ischial screws may be necessary in patients with severe ischial osteolysis. Postoperatively, patients are managed with protected weight bearing until initial radiographic signs of bone graft incorporation are present.