Computer Registration for Navigation
The instrumentation for computer navigation was calibrated while the patient was prepared for anesthesia. After the patient was anesthetized, a metal base plate for the pelvic tracker was secured with three 1/8-inch threaded pins to the thickest portion of the pelvic brim. With the patient supine, the anterior pelvic plane registration was performed by bony contact to both anterosuperior iliac spines and the pubic bone near the pubic tubercles. The skin and fat over the pubis (mons pubis) was always punctured to contact the pubic bone. In obese patients with approximately 5 mm of skin and fat over the anterosuperior iliac spines, a stab wound to the bone was made to insure bone contact. The patient was then “flipped” to the lateral position and secured with two pelvic supports and two chest supports (Sunmed, Redding, CA). The registration pointer was used to contact the two posterior supports with three points in triangular geometry to register the longitudinal axis of the body. The software can then compute the tilt of the pelvis relative to this longitudinal reference plane. The pelvic tilt permits adjustment of inclination and anteversion into the radiographic coronal plane.
The femoral pins and tracker were placed with the patient in the lateral position. The femoral baseplate was attached to the anterior lateral femur 8 cm cephalad from the superior pole of the patella and anterior to the palpable anterior edge of the iliotibial band. The pins were drilled through the anterior lateral cortex into, but not through, the medial cortex. For determination of femoral anteversion the longitudinal plane of the leg is registered. This is done after the incision and posterior approach have been undertaken with exposure of the greater trochanter but before femur dislocation. Five points are registered: the greater trochanter, the two femoral epicondyles, and two ankle malleoli. The knee must be at 90° of flexion. These points are used to determine the plane of the leg from which the software defines the femoral plane to determine femoral anteversion. In obese patients the epicondyles are difficult to identify percutaneously and this may contribute to inaccurate values of the plane of the leg and consequently femoral anteversion. The navigation system software was programmed only for the APR anatomic stem (Zimmer, Warsaw, IN) during the time of this study.
The incision is made over the posterior border of the greater trochanter, and extends proximally from the level of the vastus tubercle for 10 cm cephalad. The first incision into hip tissue is done in the gluteus maximus muscle, which is incised for 6 to 8 cm along the posterior border of the greater trochanter. The second is through the small external rotators and the posterior capsule with the leg in internal rotation. It is made as a single flap from just proximal to the quadratus femoris muscle either to or through the piriformis tendon. If the piriformis tendon is transected, the incision extends through 1 to 2 cm of the gluteus minimus muscle. In flexible hips, the piriformis tendon is preserved and an L-shaped incision is made with one arm parallel to the piriformis tendon. The hip is dislocated and the corresponding templated neck cut for restoration of leg length made. The third incision is of the inferior medial capsule, which is incised from the anterior femur to the acetabulum through the transverse acetabular ligament.
The preparation of the femur was performed first so that the anteversion of the femur was known prior to the preparation and implantation of the acetabulum. The femur is presented into the wound by positioning of long-handled retractors (Zimmer, Warsaw, IN). Femoral preparation was done by reaming and broaching. Five points of the intramedullary canal of the femur were registered with a metal registration tool. The software could then determine the position of the implants in the femoral bone by calculating the intramedullary canal relative to the plane of the leg. The anteversion of the broach (and subsequently the stem) was computed as it was implanted into the bone and displayed on the computer screen.
Three long-handled retractors are placed to obtain correct exposure of the acetabulum.The labrum is removed as is any floor osteophyte overlying the cotyloid fossa. The cortical bone of the cotyloid notch is registered as the medial wall. Two registrations of the native acetabulum are done prior to acetabular preparation. (1) The acetabulum is digitized 16 times to obtain the center of rotation and diameter of the bony acetabulum. (2) Four points on the cortical bone of the cotyloid notch digitized the medial wall. The change in center of rotation by depth of reaming in both the medial and superior directions can be visualized on the computer screen. Reaming medially is done to the cotyloid notch which medializes the center of rotation 3 to 6 mm; removal of the lunate bone and formation of a hemisphere moves the center of rotation cephalad to 5 mm. The cup is targeted to 40° ± 5° inclination and the anteversion is customized according to the femoral anteversion to provide a combined anteversion of 30° to 45° (with 5° margin of error for safe zone of 25° to 50°), lower for men and hips with low femoral anteversion, and higher for women and hips with high femoral anteversion. The coverage of the cup is to have the inferior-medial edge flush with the cortical bone of the cotyloid notch (just inside the transverse acetabular ligament) and 3 mm below the pubis cortex; the anterior-superior edge is level with the anterior iliac bone; posteriorly, it is below the ischial cortex and may be prominent by up to 3 mm posterior-superiorly.