Analysis of the elastic bending characteristics of cementless short hip stems considering the valgus alignment of the prosthetic stem

Abstract Background The resultant hip force causes a varus torque which must be compensated by a shear force couple depending on the stem alignment of the prosthesis. Since the prosthesis is substantially less flexible than the bone, the interior of the femur is stiffened over the entire prosthesis length. The present study thus aims at analyzing short-stem prostheses for its elastic bending characteristics, considering inappropriate valgus alignment of the prosthetic stem. Methods Five short stem prostheses were implanted each in synthetic femora in a standardized manner – in neutral and valgus stem alignments. Bending movements were recorded applying a tilting torque M X of ±3.5 Nm in medio-lateral direction. Variance analyses and Friedman tests were used. A P-value Findings Bending movements b 1 -b 6 showed significant differences (P  Interpretation Regarding the elastic bending behavior we see a relevant influence of the stems´ design. We conclude that the short-stem principle does not necessarily require the shortest possible prosthesis but rather a long and thin extending stem tip to optimize the lever ratios, ensuring a more physiological bending behavior of the femur. In addition, without sufficient anchoring of the prosthesis, the valgus stem alignment could favor tilting of the implant and should therefore be avoided.