Preclinical assessment of the long-term endurance of cemented hip stems. Part 2: in-vitro and ex-vivo fatigue damage of the cement mantle.

Fatigue damage in the cement mantle surrounding hip stems has been studied in the past. However, so far no quantitative method has been validated for assessing ex-vivo damage and for predicting the in-vitro risk of cement fracture. This work presents a method for measuring cement damage; the cement mantle was sliced and sections were inspected with dye penetrants and an optical microscope. Cracks were counted, measured, and classified by type in each region of the cement mantle. Statistical indicators (in total and per unit volume of cement) were proposed that allow quantitative comparison. The method was first validated on two implant types with known clinical success rate, which were tested in vitro using a physiological loading profile (described in Part 1 of this work). The most relevant indicators were able to detect statistical differences between the two designs. Retrieved cement mantles (the same design as one of the in-vitro stems) from revision surgery were also processed with the same inspection method. Excellent qualitative and quantitative agreement was found between the in-vitro generated fatigue damage and the cracking pattern found in the ex-vivo retrieved cement mantles. This demonstrated the effectiveness of the cement inspection protocol and provided a further validation to the in-vitro testing method.