Effect of stem position and length on bone-stem constructs after cementless hip arthroplasty

Aims There are concerns regarding initial stability and early periprosthetic fractures in cementless hip arthroplasty using short stems. This study aimed to investigate stress on the cortical bone around the stem and micromotions between the stem and cortical bone according to femoral stem length and positioning. Methods In total, 12 femoral finite element models (FEMs) were constructed and tested in walking and stair-climbing. Femoral stems of three different lengths and two different positions were simulated, assuming press-fit fixation within each FEM. Stress on the cortical bone and micromotions between the stem and bone were measured in each condition. Results Stress concentration was observed on the medial and lateral interfaces between the cortical bone and stem. With neutral stem insertion, mean stress over a region of interest was greater at the medial than lateral interface regardless of stem length, which increased as the stem shortened. Mean stress increased in the varus-inserted stems compared to the stems inserted neutrally, especially at the lateral interface in contact with the stem tip. The maximum stress was observed at the lateral interface in a varus-inserted short stem. All mean stresses were greater in stair-climbing condition than walking. Each micromotion was also greater in shorter stems and varus-inserted stems, and in stair-climbing condition. Conclusion The stem should be inserted neutrally and stair-climbing movement should be avoided in the early postoperative period, in order to preserve early stability and reduce the possibility of thigh pain, especially when using a shorter stem. Cite this article: Bone Joint Res 2021;10(4):250–258.

Effects of Stem Malalignment in Cementless Hip Arthroplasty: a Computational Study

Implant loosening and deformation issues contribute to the instability of the hip arthroplasty. Prosthesis stem malalignment can occur in varus, anteversion and retroversion in different degrees due to several reasons. In this study, computational analysis of cementless hip arthroplasty with different stem malalignment cases was conducted to investigate the biomechanical effects in hip arthroplasty. Five hip arthroplasty models were developed using finite element analysis which are straight/aligned model, malalignment models at varus +3°, varus -3°, sagittal flexed +3°, and sagittal extended -3°. Results show that different pattern of stress distribution was observed in each malalignment case. The varus -3° malalignment model had demonstrated the greatest risk of failure based on the resulting stress distribution and total deformation.