Differences in femoral anatomy have been partially ascribed to gender differences. Traditionally, femoral stems for THA have been designed across an entire population including both males and females. The purpose of this study was to compare the applicability of two femoral stem systems in male and female populations via preoperative templating.

Methods: All patients seen during a single month who presented complaining of knee pain had screening pelvis x-rays. These x-rays formed a consecutive cohort of hips for the templating study. During templating, the acetabular component was placed in a fully medialised position at 45o of abduction. The center of rotation was marked. The femoral neck osteotomy was set at 15 mm proximal to the lesser trochanter. Templates of equal magnification were utilized for both systems. System 1 had a double tapered wedge body design, a fixed 135o neck-shaft angle with two different offsets (6 mm difference) and two different neck lengths (4 mm difference). There were 7 head options with different lengths. System 2 had the same body design with a modular neck offering 20 different offsets/lengths and 7 different neck-shaft angles, with only one head option. Neck length and offset were independent of body size for both systems. Based upon templating, the categories were: No obvious advantage of either system, System 1 preferred, System 2 preferred, Neither system appropriate. Preference was determined based upon providing at least one additional length or offset option, and avoiding the extra extended offset option in System 2 based upon the theoretical risk of disassociation due to extremely high moments.

Results: There were 20 female patients contributing 40 hips and 27 males contributing 54 hips. Among the males, there was no obvious advantage in 20/54 hips (37%), System 1 was preferred in 11/54 hips (20.4%), System 2 was preferred in 15/54 hips (27.8%), and neither system was appropriate in 8/54 hips (14.8%). In addition, System 1 could have been used in 33/54 hips (61.1%), while System 2 could have been used in 42/54 hips (77.8%). Overall, 46/54 male hips (85.2 %) could be implanted with this stem. Among the females, there was no obvious advantage in 17/40 hips (42.5%), System 1 was preferred in 1/40 hip (2.5%), System 2 was preferred in 13/40 hips (32.5%), and neither system was appropriate in 9/40 hips (22.5%). In addition, System 1 could have been used in 22/40 hips (55%), while System 2 could have been used in 31/40 hips (77.5%). Overall, 31/40 female hips (77.5 %) could be implanted with this stem.

Discussion: Significantly, there are gender differences in applicability of femoral stems. Specifically, more neck length and offset options seem to be required for females. One criticism of this study would be that the neck osteotomy length was fixed. In practical application, surgeons frequently adjust the level of the neck osteotomy to successfully reconstruct the hip. Further study is necessary to determine the role of neck-shaft angle, bone quality and adjustment of neck osteotomy height.

Unicompartmental arthroplasty of the knee (UKA) is technically challenging because the prosthetic devices must function in concert with a mostly normal joint. Malalignment is common, leading to patient dissatisfaction and early failures. However, UKA remains attractive as a temporizing treatment in early disease. Until now, resurfacing UKAs were performed with free-hand techniques. This study is only the second report investigating the use of a tactile guidance system (TGS—essentially, a robotically assisted surgery) for the performance of UKA.

Methods. The first 20 patients who underwent resurfacing using a Mako Surgical Inc. TGS system by a single surgeon were studied. Surgical goals were to place the components to replicate closely the patient’s native bony architecture. The surgical plan was completed on a workstation, and then executed with the TGS system through a mini-arthrotomy. Stelkast, Inc resurfacing components were implanted with methymethacrylate. Intraoperative measurements of component position were obtained. Pre- and postoperative radiographs were also measured for alignment correction, change in angulation of the joint line relative to the femoral and tibial anatomic axes, femoral component alignment relative to the femoral anatomic axis, and change in tibial slope.

Results. All cases could be completed as planned. None were converted to a full arthrotomy. None required conversion to a different implant. There were no failures of the TGS, associated navigation, or the CAT-scan based preoperative plan.

Intraoperative measurements showed an average femoral component position of 0.89+3.36 degrees of varus relative to the mechanical axis, with 62.5% being varus and 37.5% being valgus. The average femoral component flexion was 11.1+2.11 degrees, with no outliers (less than 5 degrees; greater than 15 degrees). The tibial component position was 4.60+1.76 degrees of varus, with all components in varus as desired. There was an average of 5.00+2.37 degrees of slope, with 25% outliers (less than 3 or greater than 7 degrees).

Postoperative measurements showed an overall limb alignment correction of 4.29+2.60 degrees, femoral joint line change of only 0.43+0.49 degrees, and an overall component alignment relative to the anatomic axis of 4.54+3.77 degrees of valgus. On the tibial side, the joint line varus was corrected by 3.00+2.04 degrees and the slope was changed by 4.29+3.24 degrees, including 19% outliers (less than 3 degrees, more than 7 degrees). However, 33% of the outliers were outliers preoperatively as well. Interestingly, the bone level after resection on the tibial side averaged 5.36+3.00 degrees of varus, suggesting that component placement must be carefully watched.

Discussion. TGS seems to be extremely accurate and precise in recreating individual patient anatomy. This also applies to cases in which the patient anatomy dictates placement of components in so-called “outlier” positions. It is unknown whether these “outlier” positions really translate into poorer outcomes. Impressively, there were no failures to execute the intended surgical plan and no failures of the TGS system. Future research will attempt to correlate component placement in native anatomical positions with functional outcomes and failures, as well as cost-effectiveness of the system.