Development of patient-specific numerical musculoskeletal model to assess patellar resurfacing

Osteoarthritis is the most common joint disease, with the knee being the most commonly affected joint. It is usually found in elder population, affecting more than a third of the people aged over 65 years old. Total knee arthroplasty (TKA) is the most satisfying surgical technique for severe cases of osteoarthritis. However, although most of its postoperative complications appear after 10-15 years, they can cause severe problems, sometimes even to the point of not allowing a revision surgery. Different problems come from either resurfacing or non-resurfacing the patella, without clear advantages one over the other. Non-resurfaced patella problems are less severe than the problems for the resurfaced case, but occur more frequently. In non-resurfaced patella there is a high rate of postoperative anterior knee pain (AKP) while in the resurfaced there is a moderate rate of patellar component loosening and a low rate of extensor mechanism rupture or patellar fracture, among others. A numerical model which simulated a squat movement was created. Strain was evaluated in the patellar bone. Different strain levels were found in each of the cases, with much higher values in the resurfaced case. The reason for this increase in strain values was mainly due to the change of section and constitution of the patella. Additionally, the lever arms of the quadriceps muscles were slightly reduced, during nearly the whole squat movement, in the resurfaced case, which resulted in further increasing the mechanical load suffered by the patella. Contact patterns and forces in both cases were found to be congruent with data found in literature. However, it had to be taken into account that patellar component geometry also plays a special role in determining the contact surface, and each design has its own unique contact pattern. Results showed that the non-resurfaced case had a better outcome from a mechanical point of view, since no dangerous strain limits were reached in the patellar bone. However, anterior knee pain origin is currently unknown and although it might have to do with the mechanical properties and parameters of the system, the way it is linked to them is currently unclear. Thus, nowadays it is not possible to assess this issue with only a solid mechanics simulation.