COMPONENT DESIGN

The stem described in this chapter includes a taper over the axisymmetrical portion of the stem and two proximal wedges, one medial/lateral and one anterior/posterior (Fig. 20-1). The end result is a proximally fitting, rotationally secure implant. These are straight stems that require machining of the bone for optimal fit and fill. The preparation is done in two steps. The tapered reaming machines the proximal femur and ensures an interference tapered fit in the intertrochanteric area and proximal diaphyses of the femur. Broaching is not cutting the bone but rather compressing an existing cancellous bone and creating a slightly undersized cavity for the double wedge. The end result is a very tightly fitted stem that transfers load proximally and minimizes proximal stress shielding.

FIGURE 20-1 Cementless tapered stem.

PREOPERATIVE PLANNING

Preoperative planning helps the surgeon in preparation for the procedure. Radiographs are necessary and include a low centered anteroposterior view as well as a lateral view. I prefer a Lowenstein lateral view, which is done with the affected hip and knee flexed, in abduction, and in external rotation with the hip, knee, and ankle placed flat on the radiographic table. The x-ray beam is directed onto the affected hip over the lesser trochanter perpendicular to the proximal femur. The benefits of this view include reproducibility and good definition of the bow of the femoral diaphysis, but it may not give the desired orientation for determining the version of the femoral neck. The value of preoperative planning is manyfold: it incorporates the estimation of prosthetic size, calculation of leg length and offset restoration, and preoperative determination of femoral neck resection level. This prosthesis is suitable for any patient with hip arthritis, including osteoarthritis, avascular necrosis, and rheumatoid arthritis, independent of the bone type. Patients who are not candidates for the use of a tapered stem are those with distorted proximal femoral geometry, such as patients with excessive anteversion secondary to developmental dysplasia of the hips, patients who have had a proximal femoral osteotomy, and the occasional patient with a significant metaphyseal-diaphyseal size mismatch. A risk of overlengthening may arise in patients with large patulous canals requiring a large size implant accompanied with a long femoral neck. Fortunately these exceptions are few and may be best served with cemented implants.

TECHNIQUE

The implementation of the tapered stem can be carried out through a number of standard anterior or posterior surgical approaches, including the recently popular minimally invasive approaches. Results seem to be less favorable when two small incisions are used. I prefer a small incision and a posterior approach, as described here. It is necessary to see the entire cut surface of the femoral neck before the preparation of the femur is begun. To minimize the risk of varus placement and undersizing of the component, after the neck osteotomy a drill hole is created in the trochanteric fossa, thereby ensuring a lateral start and direct access into the femoral canal. Any residual femoral neck is then cleared with a box chisel by opening the introitus of the femoral neck. Use of a lateralizing reamer is also helpful in minimizing the potential for varus implantation. This is particularly true with hips with a significant varus femoral neck-shaft angle.

Tapered reaming is the first step; the surgeon should start with a reamer that is at least two sizes smaller than the templated size. The surgeon should feel the cortical contact with the final reamer, and he or she should inspect the final reamer to be certain that the contact is along the tapered part of the reamer proximally and not nearly distally. The presence of cancellous bone remains within the teeth of the reamer is helpful to confirm this (Figs. 20-2 and 20-3

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The Cementless Tapered Stem Femoroacetabular Osteoplasty Reconstruction of Acetabular Bone Deficiencies Using the Antiprotrusio Cage The Cemented Stem Revision Total Hip Replacement The Cemented Stem

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