SOM and KYM Hip Prostheses (Mizuho)

In the 1970s, the introduction of hip prostheses from Europe spread to Japan, but European hip prostheses designed by McKee-Farrar, Charnley, Weber-Huggler, Müller, and others had problems with fitting the hip joints of Japanese patients, most of which were affected by osteoarthritis secondary to hip dysplasia. Shikita was the first to clinically use the Charnley-type hip prosthesis with a metal head versus a polyethylene cup in Japan in February 1968; he switched to the Müller and Weber-Huggler types because of the poor design of the Charnley-type hip prosthesis, which resulted in wear due to the increased surface pressure of the small 22 mm diameter head and dislocation due to the limited range of motion [1]. Concurrently, because polyethylene wear is related to the survival of the artificial hip joint due to wear or breakage and foreign body reactions caused by wear debris, he conducted research on improving wear resistance by cross-linking polyethylene using an electron beam or gamma-ray irradiation and investigated the introduction of high-density polyethylene (HDP), ultra-high molecular weight polyethylene (UHMWPE), and other polymeric materials, such as carbide [2]. Shikita developed the SOM (Shikita-Onishi-Mizuho) hip prosthesis, a domestically produced prosthesis using COP stainless steel (CoNiCr alloy), which had already been reported to be an excellent material for bone fracture fixation devices by Sunami et al. [3]. COP stainless steel is an excellent material that can be polished to achieve an appropriate level of sphericity and surface roughness of the femoral head for sliding parts. The diameters of the femoral head were 28 mm and 32 mm, and a 36 mm head was also fabricated for the large acetabulum to make it like the anatomical form of the hip joint. The stem design was a straight type, and the femoral neck had three different lengths, according to Müller, to reduce the dead space and prevent dislocation by adjusting the leg length. Three types of stem lengths (large, medium, and small) and a mobile polyethylene femoral head around the neck trunnion in combination with a metal articulation cup (KYM: Kumamoto-Yao-Mizuho), like the Weber-Huggler type, were also developed by Hirota to enhance compatibility. The SOM type has been used clinically since 1971 [3]. In the early stage, HDP of HI-ZEX5000B was used as the cup material, RCH1000 (GUR412) of UHMWP was used to improve the wear resistance property [4], and a high dose of gamma-ray irradiated UHMWPE in air was used in 1972. KYM has been clinically used with a 37-mm polyethylene head and a COP cup from 1973 [5].