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

Introduction. Planning of the stem antetorsion angle (SAA) is difficult with radiograph before THA. 3D THA planning software with CT is useful for planning the cup and the stem implantation angles before THA. However, even using the 3D planning software, we sometimes experience the different SAA during surgery compare to the planned SAA. The purpose of this study was to compare the implanted SAA with the preoperative planned SAA, which was planned by using 3D THA planning software. Materials and Methods. CT evaluation was performed in 44 patients (5 males) who underwent primary THA. The mean age at surgery was 67 years (range 26–85 years). The mean BMI at surgery was 24.1kg/m. 2. (15.6–31.7kg/m. 2. ). Forty-one patients had osteoarthrosis, 2 patients had osteonecrosis, and 1 patient had femoral neck fracture. All surgeries were performed in the supine position with the direct anterior approach. The OrthoPilot imageless navigation system (BBraun/Aesculap) was used during surgery. Excia stem was used in 34 patients and Bicontact stem was used in 10 patients. Planning of the surgery was performed using 3D THA planning software (ZedHip, Lexi). After surgery, SAA was measured with CT by the same 3D THA planning software. SAA was evaluated by comparison of the planned values before surgery with the CT measured values. Also, the shape of the femur and the stem were evaluated. Results. The mean SAA of the preoperative planning was 29.6±5.6 degrees (mean±sd) [range 20.4–42.8 degrees]. The mean SAA after surgery was 29.8±5.6 degrees [10.7–49.7]. The mean difference between postoperative SAA and planned SAA (post. minus pre.) was 0.2±7.6 degrees [−16.0–24.9]. The mean SAA of the Bicontact stem was 25.9±8.8 degrees and the Excia stem was 30.9±9.1 degrees. The difference between postoperative SAA and planned SAA of the Bicontact stem was −1.8±6.2 degrees and the Excia stem were 0.8±8.0 degrees. Ten patients showed more than 5 degrees of antetorsion after surgery compared to the planned SAA. Among them, Excia stem was used in nine patients. There were 11 cases of champaigne-flute type, 29 cases of normal type, and 4 cases of steovepipe type femurs. The mean difference between postoperative SAA and planned SAA with champaigne-flute type, normal type, and stovepipe type were 3.8±7.7 degrees, −0.5±7.6 degrees, and −4.7±3.9 degrees, respectively. Discussion. In this study, the mean difference of SAA between postoperative values and preoperative planned values were very small. 3D planning software is useful for predicting SAA before surgery. The Excia stem showed greater difference of SAA and greater antetorsion compare to the Bicontact stem. Also, Champaign-flute cases tend to show greater difference of antetorsion compare to the other types of femur. The shape of the stem and the shape of the femur affected the antetorsion angle of the stem. When choosing an implant before surgery, surgeon better to consider the characteristic of the stem and the shape of the patients’ femur