Accuracy of the planned vs achieved position of a cementless hip stem: a finite element study

The implant of an hip stem into the femoral medullary cavity consists of three steps: the resection of the femoral head, the rasping of the femoral canal and the placement of the stem. Priority for cemtless hip implants, in order to achieve a good level of primary stability, is the accuracy with which the stem is positioned in the host bone. An erroneous initial positioning could lead to the implant instability promoting the ultimate failure of the implant 1. Initial excessive relative micromotions at the bone-implant interface may inhibit the bony in-growth and secondary long term fixation 2,3. The final objective of the early researches along this line is to arrange a set of instruments to predict the primary stability in the pre-operative planning moving toward a less and minimally invasive surgical technique. Nevetheless, even assuming a perfect surgical planning, there is still the practical problem of correct positioning of the stem in the femur during surgery. Aim of the present study was to asses the sensitivity of the relative bone-implant micromotions, stresses and strains to the implant position as planned and achieved by the surgeon respectively before and after the operation. For this purpose, the subject-specific finite element (FE) model of a cadaveric femur, accounting for patient and surgeon, was derived from pre-operative and post-operative CT scans. The overall aim was to verify if the pre-clinical planning correctly matches the achieved implant stability conditions and hence if it can be considered as a powerful tool to train the surgeon in taking the appropriate clinical decisions.