How stiffness and distal interlocking of revision hip stems influence the femoral cortical strain pattern.

BACKGROUND

Stress shielding and nonphysiological load transfer after primary or revision total hip replacement (THR) prepare the ground for resorptive bone remodeling. The quality of the bone stock influences the risk of periprosthetic fractures and the severity of future revision surgeries. The question of whether or not bending stiffness and distal screw interlocking influence load transfer of a modular revision hip stem with a solid, hollow, and hollow-slotted stem extension led to the conception of this experimental study. The results were compared with a standard hip stem for primary THR.

METHODS

Revision stems were implanted in photoelastically coated composite femora. Cortical strain mapping was conducted before and after insertion of the implants under standardized loading conditions, considering the relevant muscle forces. Statistical analysis was based on a 95% confidence interval and a variance analysis for repeated measurements.

RESULTS

Significant stress shielding was observed after insertion of all types of hip stems compared with the intact femora. There was also a marked difference between strain alterations induced by standard and revision hip stems. With revision stems, the most distinct stress shielding effects were registered with the solid stem extension, particularly in the femoral diaphysis. Distal interlocking screws only had a local action on strain pattern and tended to enhance stress shielding at the midstem area when using the more flexible components.

CONCLUSION

More flexible revision stems provide a cortical strain pattern of the femur closer to the preoperative status. This may reduce resorptive bone remodeling in the long term. However, any type of revision stem tested in this study caused higher stress shielding than the hip stem for primary THR, especially in the diaphyseal region medially and laterally. With sufficient proximal anchorage, the influence of distal interlocking screws on the femoral strain pattern was localized.