Comparative Study of Caucasian and Chinese Femur Shapes for Evidence-Based Implant Design

Abstract

With the increasing use of 3D medical imaging, it is possible to analyze 3D patient anatomy to extract features, trends and population specific shape information. This is applied to the development of ‘standard implants’ targeted to specific population groups. INTRODUCTION Human beings are diverse in their physical makeup while implants are often designed based on some key measurements taken from the literature or a limited sampling of patient data. The different implant sizes are often scaled versions of the ‘average’ implant, although in reality, the shape of anatomy changes as a function of the size of patient. The implant designs are often developed based on a certain demographic and ethnicity and then, simply applied to others, which can result in poor design fitment [1]. Today, with the increasing use of 3D medical imaging (e.g. CT or MRI), it is possible to analyze 3D patient anatomy to extract features, trends and population specific shape information. This can be applied to the development of new ‘standard implants’ targeted to a specific population group [2]. PATIENTS & METHODS Our population analysis was performed by creating a Statistical Shape Model (SSM) [3] of the dataset. In this study, 40 full Chinese cadaver femurs and 100 full Caucasian cadaver femurs were segmented from CT scans using Mimics®. Two different SSMs, specific to each population, were built using in-house software tools. These SSMs were validated using leave-one-out experiments, and then analyzed and compared in order to enhance the two population shape differences. RESULTS An SSM is typically represented by an average model and a few independent modes of variation that capture most of the inherent variations in the data. Based on these main modes of variations, the shape features, e.g. length, thickness, curvature neck angle and femoral version, presenting largest variations were determined, and correlations between these features were calculated. Figure 1 represents the Caucasian and Chinese average models, and shows that while the length of these two models was significantly different, the AP and ML dimensions were similar, indicating a difference of morphology (other than a scaling) between the two populations. Figure 2 represents the first mode of variation that illustrates the variation of Chinese femur shape with size. As an example, the neck angle increases of 26° with an increase of 139 mm in femur length, indicative of the effect of changes in loading conditions on geometry as a function of size. CONCLUSION The advantage of using more advanced statistical analyses is that the 3D data are probed in an unbiased fashion, allowing the most important parameters of variation to be determined. These analyses are thus particularly effective to compare different populations, to evaluate how well existing implant designs fit specific populations, and to highlight the design parameters that need to be adapted for good fitment of specific populations.