[Bone stress. A hitherto neglected factor in hip endoprosthesis design]

Growing interest in hip endoprostheses is giving rise to new ideas on form and materials, although it is not precisely known what forces the proximal femur is subject to. To find out more about these forces, we made computerized analyses of the stresses on some different types of prostheses in an artificial standardized femur. The E-module amounts to about 14,000 N/mm. Following measurements in tension-optic trials, we inserted Student gages (EA-13-060LZ-120) at 16 points around the proximal femur; these transmitted the pressure, tensile and torsion stresses to a multipoint measuring instrument (UPM 60) and on-line to a computer. The quality of implantation was monitored by X-ray. With loads varying from 0 to 1.5 kN in 250-N steps we made five readings to reproduce the measurements, without having any material failure. To ensure an absolutely reproducible outcome we used an artificial standardized femur (KNF by Mecron), which without prostheses shows the typical and well-known (Pauwels, Perren) pressure tensions medially and tensile stresses laterally, without crossing the zero line. A distinctly reduced stress profile was seen for pressure and tensile stresses with the ribbed system prosthesis, which might be due to the anchor and collar. Nevertheless, the torsion tensions should be noted. As expected, the shaft systems without anchor and collar show a lesser degree of stiffness. The Mecroblock MR system produces preponderantly pressure stresses, while the CLS (Spotorno) prosthesis sets up more torsion tensions in different directions.(ABSTRACT TRUNCATED AT 250 WORDS)