APS 2008 (Zimmer)

The APS Natural-Hip™ (APS) system, in which APS stands for anatomic plasma spray, is a stem that can be inserted naturally according to bone morphology and was marketed by Zimmer (Warsaw, IN, USA). The main development concepts were as follows:In the 1990s, anatomic stems were marketed to ensure secure fixation proximal to the medullary cavity and prevent excessive stress shielding. However, the anatomic stem (Zimmer, Inc.) at that time was made based on the anatomical characteristics of Caucasian individuals, and it was difficult to adapt the stem to all Japanese patients. Therefore, the need for a stem that could be easily adapted to Japanese patients was the impetus for development. First, the proximal bone morphology of patients with secondary osteoarthritis (OA), which proceeds from developmental dysplasia of the hip (DDH) and is common in Japan, was analyzed to see if it differed from normal femoral bone morphology. A computer-aided design system was used to examine the angles of inclination of the proximal medullary cortex in coronal and sagittal planes, medial and lateral, and anterior and posterior from CT data of 36 control hips and 113 hips of patients with OA secondary to DDH. It was found that 96% of secondary OA could be classified into three femoral canal morphologies [1]: Type A, which accounted for 89% of the normal morphology, was present in only 42% of the secondary OA group; type B, which had a large anterior tilt at the proximal-posterior region, was present in 26%; and type C, which had a steep proximal-medial tilt, was present in 30%. These results suggest that three types of stems with different shapes are necessary to fit each type of bone morphology in Japanese patients. Additionally, the femoral canal diameter and femoral neck length of Japanese people were used as references for stem size determination. The APS standard anteverted stem, which is suitable for type A, was marketed overseas as an anatomical stem with a 12° stem anteversion. The standard non-anteverted stem, which is more suitable for type B, has a corrected anteversion, and the body is the same as that of the standard stem; however, the anteversion is 0°. The reduced metaphysis type for type C also had 0° anteversion, a steep proximal medial tilt, and a proximal transverse section that was rounder in shape on the lateral side. This was because type C had a significantly greater number of cases with a circular shape cross-section. The latter two types were limited to the domestic market (Fig. 13.1a–c) and were sold from October 2008 to June 2018.