In vitro fatigue crack analysis of the Lubinus SPII cemented hip stem

摘要

Fatigue damage of cement mantle of cemented hip replacements has been extensively studied. Different conclusions concerning the micro-cracking mechanism has been reported. An in vitro study on commercial cemented synthetic femurs was conducted to determine if micro-cracking formation of the cement mantle would preferentially be at the cement-stem interface or at the bone-cement interface. A stair climbing fatigue loading cycle was applied during one million cycles at 2 Hz on commercial stems. Three hip replacements with the Lubinus SPII cemented stem were studied. After the experiments each femur was sectioned at eleven sections, with an average of 5 and 10 mm of offset between each one of them and analysed with dye liquid penetrates. The results (micro-cracking and debonding) were influence by the geometries of the reamer and the stem and by technical replacement procedure. The micro-cracking formation initiated at the stem-cement interface towards the cortical bone of the femur. For a cement mantle thickness of more than 5 mm, cracking initiated at the cement-bone interface and depended on the geometry of the reamer. At the medial side of the femur more significant cracks and accumulated damage was observed at the stem-cement interface. Correlation between the formation of cracks and minimum cement thickness was determined. A critical cement mantle thickness was observed for 0.5 mm, where a relevant sign of damage was assessed. The analysis of crack-density and accumulated damage showed that fatigue induced cement damage and depended on the axial position of the stem and increase proximally.