Surgical Options for Acetabular Reconstruction

CHAPTER 51 Surgical Options for Acetabular Reconstruction


Custom Components




Current treatment options for massive acetabular defects encountered in revision total hip arthroplasty (THA) are numerous but often associated with inconsistent clinical results and substantial complication rates. These reconstructive options include creation of a high hip center with a standard acetabular component, jumbo hemispherical acetabular components, bipolar hemiarthroplasty, acetabular impaction bone grafting combined with a hemispherical cup, massive structural allografts, oblong acetabular components, modular trabecular metal cups with augments1,2 and noncustom acetabular reconstruction rings.3 Difficulties associated with these methods in patients with massive periacetabular bone loss have included loss of fixation, component fracture, hip instability, and gait alterations caused by failure to restore functional hip biomechanics.


Another implant option designed to limit these failure mechanisms is a custom triflanged acetabular component (CTAC). This custom component is created using a thin-cut CT scan of the pelvis and is designed with a central dome that fits into the central acetabular defect, contacting the ilium superiorly. Three flanges project from the central dome to provide additional fixation on the remaining ilium, ischium, and pubis. A polyethylene (PE) or metal liner is placed into the central dome using a modular locking mechanism. This chapter describes the indications, design methods, surgical technique, and results of the CTAC used to reconstruct massive acetabular defects in revision THA.




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.



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.



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.



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.

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Mar 10, 2016 | Posted by in Reconstructive surgery | Comments Off on Surgical Options for Acetabular Reconstruction

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