Design and optimisation of patient-specific porous femoral stems

Total hip replacement surgery is a well-established intervention that restores joint function and provides excellent outcomes for patients. In some cases, bone resorption caused by stress shielding leads to implant loosening. Stress shielding occurs because implants are much stiffer than bone and transmit a significant proportion of the load, leaving the surrounding bone to carry less load compared to an intact femur.

To address stress shielding, we aimed to design patient-specific additively manufactured porous femoral stems with reduced stiffness. Diamond lattice structure test specimens of varying porosities were manufactured and tested to measure elastic moduli and yield strengths. These properties were used in subsequent implant optimisation based on finite element analysis. Four implant templates were created based on the region of the implant that was assigned porous material. These templates were referred to as fully porous (FP), proximally porous with a solid distal end (PP), solid distal shell (DS), and fully solid shell (SS). In addition, the elastic modulus within the porous region was assigned either a linear or a radial distribution, resulting in eight possible implant designs.

Optimisation yielded six distinct solutions, which were evaluated based on the reduction in stress shielding and micromotion at the bone implant interface. While all implant designs reduced stress shielding compared to a solid implant, only the PP and SS stems were predicted to pass the standard fatigue test for femoral stems (ISO 7206-4). Implant-bone micromotion was conducive for bone implant integration for all designs, with the potential exception of the fully porous stem. Of the implants that were predicted to pass the fatigue test, the linear proximal porous stem resulted in the largest reduction in stress shielding (9.5% for walking, 8.1% for stair climbing).

Based on patient-specific computational models, the porous region of the stems influenced the reduction of stress shielding compared to fully solid stems. Considering implant fatigue failure and bone-implant micromotion, the PP and SS templates were found to be the most suitable design approaches to address femoral bone stress shielding after total hip replacement. Further investigation is required to fully comprehend the implications of porous femoral stems on long-term implant stability.