In vitro long-term fatigue endurance of the secondary "Cement Injection Stem" hip prosthesis.

A secondary cementation hip stem (Cement Injection Stem; Aesculap, Tuttlingen, Germany) was designed to reduce the risk of fat embolism, and achieve precise implant position and high-quality cement mantle. A validated long-term in vitro simulation was carried out that replicated 24 years of activity of a very demanding patient. Inducible and permanent micromotions were monitored. The cement mantle was sectioned and inspected for signs of fatigue damage. The stem-cement interface was inspected for fretting. Results were compared against previously published results for a conventionally implanted stem with comparable design (Centrament; Aesculap) from which this project was derived. Comparable micromotions were found (slightly larger proximally, in correspondence to the precured centralizer). No sign of fretting was observed. All fatigue damage indicators were comparable or significantly better than those for the conventionally implanted stem. The few cement cracks found were mainly localized in proximity of a proximal drainage hole. It is foreseen that when this detail is optimized, long-term endurance will further improve.