E. G. Theodorou, C. G. Provatidis , P. D. Megas, MD
Over the past years total hip arthroplasty has become the golden standard as far as the reconstruction of the hip joint is concerned. The younger and more active patients are increasing constantly and the use of modular implant systems is considered as one of the best available options since they provide extensive flexibility and revision possibilities. Many parameters affect the longevity and the stability of the implants but in the current study we focus on the effects of neck modularity in the overall biomechanical behavior of the bone and its femoral components. The acquisition of a Computed Tomography scan set from a cadaveric femur and the implementation of a Profemur-E system with six modular necks: straight, 8ο retroversion and 15ο anterversion (short & long) along with a typical femoral head, resulted in the development of a full scale three dimensional finite element model. Under a one-legged stance phase loading scenario, the numerical results revealed significant alterations in stresses and strains in the bone implant assembly. The neck pair positioned with retroversion produced significantly higher values, especially for neck stress and bone strains. The Ti Alloy stem had also higher stress concentrations for the long retroverted neck. The equivalent von Mises strains of the femur were examined with respect to Frost’s Law for bone growth activity. It was shown that all six models, although variations both numerically and visually existed in their distributions, had similar behavior. For the region of the lesser trochanter the use of short neck invoked lower strains with possible bone loss. Finally, two profile lines were isolated, enclosing the greater and lesser trochanter, and their corresponding node values were plotted respectively, with the short and long retroverted necks being in the upper regions of the chart. Peak (lateral side) – and some local minimum values (medial side) – of the outer cortical bone were located at the stem tip zone, in accordance with the applied load for a properly positioned stem.