The loads during daily activities contribute to fixation failure of cemented hip stems. In-vitro preclinical testing so far has consisted of simulating one or two conditions. Only a small percentage of hip implants fail, with a higher failure rate in most active patients. The goal was to define a procedure to assess the long-term effect of the lifestyle of a reasonably active patient on implant micromotions. Thus, a cyclic load of constant amplitude is unsuitable. All activities inducing high loads were included, to replicate the most critical scenario in terms of fatigue. The following motor tasks were simulated: stair climbing and descending, car entry and exit, bathtub entry and exit, and stumbling. An in-vitro simulation running for 15 days was able to replicate the load peaks occurring in 24 years of patient activity. Inducible micromotion and permanent migration were monitored. The load history was successfully applied to two different designs with known clinical performance, yielding significantly different micromotions for the two types. Results from the present load history were compared against a simpler profile including only stair climbing. Results showed that the new load profile is more sensitive to differences and can more easily discriminate between different designs. Part 2 of this work describes a further validation against retrieved implants.

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.

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