Partially cemented Anca DualFit hip stems do not fail in simulated active patients

BACKGROUND: Partially cemented hip stems have been introduced to offer the advantages of cemented stems on the short-term, and of cementless ones on the long-term. One such device, the AncaDualFit, has been thoroughly validated pre-clinically under average loading conditions. Concerns recently arose concerning the long-term endurance of such implants in active patients. In fact, it was suspected that cyclic loads applied by demanding patients could lead to fixation failure. METHODS: The long-term performance of the AncaDualFit partially cemented stem was studied in vitro, using a validated protocol that simulated the loads of 24 years of an active patient. Inducible and permanent micromotions were measured in five specimens and compared against two well-established cemented stems, and a cementless stem (the original design from which was derived). Cement damage (fatigue cracks) due to cyclic loading was quantified and compared against the cemented stems. FINDINGS: Inducible and permanent micromotions of the AncaDualFit partially cemented stem were slightly larger that the cemented stems, but much smaller than the cementless one. The migration, however, indicated a clear trend towards stabilization. Cement damage was minimal, even if compared to the most successful cemented stems. Short cracks were observed only near the cement inlet, but did not propagate in the two cement pockets. INTERPRETATION: The partially cemented AncaDualFit stem can safely withstand very severe loading without significant damage, thanks to the design of the two anterior and posterior cement pockets. Results are reassuring also in comparison with other successful designs. Thus, it can be safely used, even in active patients

Pre-clinical assessment of the long-term endurance of cemented hip stems. Part 1: effect of daily activities (a comparison of two load histories)

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