In vitro load history to evaluate the effects of daily activities on cemented hip implants

The loads acting on the hip joint during daily activities contribute to the failure of the fixation of cemented hip stems. An in vitro analysis of newly designed prostheses is necessary prior to in vivo clinical trials. In vitro pre-clinical testing procedures up to now have consisted in simulating only one or two conditions. The goal of this work was to define a procedure to assess the long-term effect of the most stressing activities on the integrity of the cement mantle. Thus, a cyclic load of constant amplitude is not acceptable. All activities inducing high loads need to be included, so as to replicate the most critical scenario from a fatigue point of view. The following activities were included in the load history: stair climbing and descending, car entry and exit, bathtub entry and exit, and stumbling. Load values and direction were assigned to each activity, based on the literature. A typical week was defined for a patient, based on statistics from the literature. An in vitro simulation running for 2 weeks was able to replicate the load peaks occurring in 24 years of patient activity. Stem-cement elastic micromotion and permanent migration are continuously recorded at 5 locations. The cement mantle is inspected by means of dye penetrants after test completion to quantify the fatigue damage in the cement mantle. The load history was successfully applied to two different designs and is therefore ready for future applications to evaluate the long-term performance of the new prostheses and the effects of daily activities on implant outcomes.