Experimental and numerical approaches for investigating femoral surgical procedures

In the total hip arthroplasty, the surgeon applies impact forces using a mallet while performing broaching, which is the process of preparation of the femoral canal. Understand the order of magnitude of these forces it is important to study new methods of broaching, such as instruments with controlled impaction, and avoid possible complications during surgery. In the literature were developed test benches that replicate impaction conditions, using cadaveric bones stripped of soft tissue or polyurethane bones. The bones are usually fixed to laboratory benches of unknown stiffness and the hammer tip is instrumented with a load cell. The aim of the first part of the thesis is to study the state of the art of test benches that measure the femur impact forces during surgery, to find the specifications necessary to build one, with the focus on the load cell. There is a wide range of values of impact forces but the common order of magnitude is of tens of kN, depends on the test setup. It is made a research to find a load cell with essential specifications, in terms of maximum capacity and acquisition rate, that permit to obtain a correct measure of the forces. The second part of the thesis It was analysed an innovative fracture device, an osteosynthesis screw called Variable Fixation Locking Screw (VFLS, Biomech Innovations), characterized by a resorbable polymeric sleeve that changes its mechanical properties and mass during the fracture-healing time. In the literature, a study has tested the behaviour of these screws in two different configurations, one with six standard screws and one with six VFLSs, in order to investigate if they provide a rigid and a progressive dynamic fixation. It was used a test force applied on polymeric bone-substitutes segments, with a fracture gap, and instrumented with a standard osteosynthesis plate and either six screws. The first step was the replication of this fracture gap scenario in a FE model, performing the validation with the mesh convergence, and the results were compared with those found in the previous research. The FE analysis is done on the same model and for the two configurations, one with six standard screw and one with six innovative screws, in which it is also performed the degradation of the sleeve for five percentage steps of the elastic module decrease. The configuration with the standard screws results stiffer than the innovative one, as expected, because the sleeve degradation creates a dynamic fixation. Another FE analysis to test these two configurations is performed on a 3D model of a Sawbone femur (Sawbones, Pacific Research Laboratory Inc., Vashon, WA, USA) that is reconstructed from a CT scan. Different gaps of femur fracture are examined, varying the gap distance and considering a low elastic module in this area. The forces applied on this last model are taken from a database that provides physiological values and consent to replicate a more realistic scenario. The results of the configurations are compared to understand the behaviour of the VFLSs in different fracture cases.