CHAPTER OUTLINE
Component Design 159
Preoperative Planning 159
Technique 160
Postoperative Management 161
Summary 162
Cementless tapered stems can be divided into two major types: stems requiring a two-step preparation and stems requiring a single rasping step. In this chapter the focus is on the cementless tapered stems that require a two-step preparation. There are several advantages ascribed to the tapered stem. Proximal and even load transfer and conversion of shear forces into compressive forces result in minimization of stress shielding. Furthermore, if a crack fracture does occur during the insertion of the tapered stem it is almost always located in the residual femoral neck where it is easily noted by the operating surgeon. This is in contrast to the situation with more cylindrical stems in which the interference fit and fixation are more distal and most of the crack fractures occur distally where they are not visible to the operating surgeon; these distal crack fractures go unnoticed until a postoperative radiograph is performed.
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.
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 ). Distal contact can be mistaken for adequate sizing; the end result of this mistake is a slightly undersized component, which may be seen particularly in patients who have thick cortices and a narrow femoral canal. On rare occasions, a third step is added: cylindrical reaming may be needed to clear some of the bone distally to make tapered reaming easier and more predictable. Emphasis must be made to create a taper in the proximal part of the diaphysis and distal part of the metaphysis. Once cortical contact is achieved with the tapered reamer, broaching can begin. Reaming is started at least two sizes smaller than the last tapered reamer used, and broaching is done up to the final size. If the broach corresponding to the last used tapered reamer fills the proximal femur, the preparation of the canal is finished. If, however, there remains a cuff of cancellous bone, 1 to 2 mm in thickness around the broach, then the surgeon can consider moving up a size by aggressively reaming with the next size tapered reamer and then broaching with the corresponding size broach. When the preparation is completed, on inspection of the prepared canal, one frequently will see compressed cancellous bone medially, posteriorly, and laterally, but anteriorly in the proximal part of the femur there may be exposed cortical bone reflecting the posterior bow in that part of the femur ( Fig. 20-4 ). A calcar planer is helpful in creating a flush finish of the calcar to the last seated broach ( Fig. 20-5 ).
