Variation of cervical-diaphiseal angle: biomechanical effects

In the last few years, the realization of customized orthopaedic devices seems one of the most important goals to be accomplished by biomedical industries. Requirements on products which fit the best as possible with the anatomical and physiological characteristics of patients made bio-engineers go towards design and realization of implants comprised of modular components with different sizes in order to allow changes in geometrical parameters. This strategy yield simplifications of implant procedures and made it possible to optimize implant geometry resulting from surgery. Last generation hip prosthesis is an excellent example of the aforementioned design approach since the implant it is characterized by the modularity of insertion of the necks and the stems. This allows us to change the geometric configuration of the neck as far as it concerns its length and/or the value of the cervical-diaphiseal (CD) angle. It appears that such a large variety of products released on market, makes it necessary to study biomechanical effects related to the variation of any geometric parameters. In view of this, the present work aims to investigate on the biomechanical effects produced by variation of the CD angle in the range between 125° and 135°. It was found that as the value of CD angle decreases, the force applied by the pelvic-trochanteric muscles experiences a 10% reduction while the load level in correspondence of the coxo-femoral joint is 7% lower. Stress distribution in the implant changed as well: normal stresses in the neck and in the stem respectively decrease by 16% and 7% while bending moment in the stem increases by 19%.