Abstract
INTRODUCTION:
Recreating the natural head center of the hip joint during hip arthroplasty is important for restoring biomechanics in order to minimize leg length discrepancies, improve soft-tissue tension, and mitigate impingement [1,2]. New tools have been developed that allow anatomical measurements and analysis of three-dimensional digital femura geometry based on CT scans [3]. The purpose of this study is to analyze the head center location of various fit-and-fill hip stem designs in relation to the natural bone head center location using a novel technique.
METHODS:
556 computer tomography (CT) images (SOMA™) of left femora were used in this study. The acetate templates of five fit-and-fill stem designs (Design 1: Secur-Fit Advanced, Stryker; Design 2: Secur-Fit Max, Stryker; Design 3: Summit, Depuy; Design 4: Synergy, Smith & Nephew; Design 5: Zimmer, VerSys Epoch FullCoat) were compared to each other to correlate stem sizes between different systems. The appropriate stem body size for each of the CT bones was established based on the medial offset of the bone 20 mm above the lesser trochanter (MO+20) and the stem medial offset at the medial resection point. Utilizing the commercially available offset heads for each design, the bone head centers and the stem head centers were plotted, aligning the central axis of the bone/stem as well as the MO+20 of the bone with the stem medial resection point. The percent of bone head centers within 1,2,3,4,&5 mm of a stem/head offset data point was calculated for all designs. Additionally, the distance from the bone head center to the closest stem/head offset data point and the average head offset used were calculated.
RESULTS:
Each of the five designs except for the VerSys Epoch FullCoat exhibits similar results for the percent of stem head centers within 3,4,&5 mm of the natural bone head centers (see Figure 2). However, Designs 1 & 2 are able to reconstruct the natural head center within 1 mm and 2 mm with a higher percentage than Designs 3,4,&5. Additionally, the average distances from the bone head centers to the closest stem/head offset data points are similar between all five designs, but the average head offset used for each design does vary (see Figure 3).
DISCUSSION:
This novel technique of using CT scans to analyze stem head center locations shows that Designs 1 & 2 are able to reconstruct the natural head center within 1 mm and 2 mm with a higher percentage compared to Designs 3,4, & 5. Further, less ‘plus’ offset heads may be used for Designs 1,3,4, & 5 to reconstruct the natural head center compared to Design 2, and subsequently, less skirted heads may be used for Designs 1,3,4,&5. In this study, the neck resection level was held constant. Preoperative templating is important to allow appropriate adjustment of the neck resection in order to properly reconstruct the natural bone head centers. This novel technique may allow for optimizing the head center position for hip designs in order to more closely reproduce the native femur head centers.
