Additive manufacturing structural redesign of hip prostheses for stress-shielding reduction and improved functionality and safety

Nowadays, the total hip arthroplasty (THA) is a widespread surgical procedure, representing the best option to restore hip joint mobility in patients suffering from trauma or joint diseases. One of the well-known possible drawbacks of THA is the stress-shielding phenomenon. Some years after the surgery, the femur starts to degrade because of its persistent unloaded condition induced by the high prosthesis stiffness, which carries the great part of the load normally taken by the bone. This condition is particularly invalidating in younger patients, with longer life expectation after the operation, requiring one or multiple additional operations to restore the proper prosthesis-bone firm connection. The present study tries to address this issue proposing an innovative prosthesis design, taking advantage of the shape freedom ensured by Additive Manufacturing techniques. Additionally, the structural integrity of the novel prosthesis is assessed using a ductile damage numerical approach. Different prosthesis geometries were investigated: one conventional and commercially available already, and two more innovative geometries. For each one, a bulk solution was compared to a lighter version characterized by an inner reticular structure with a body-centred cubic unit cell and by an equivalent density of about 5%, only feasible through the additive manufacturing fabrication. Extensive Finite Element numerical simulations were carried out to compare the percentage of the induced stress shielding for the different prosthesis geometries. Pros and cons of each geometry were pointed out and eventually the most promising solution in limiting the stress shielding phenomenon was chosen. At the same time, the structural integrity of the selected design was ensured, embedding a ductile damage model in the Finite Element analysis, calibrated on a SLM Ti6Al4V, the biocompatible alloy for the prosthesis fabrication. Structural safety was evaluated under four different loading conditions: walking, stumbling, the exceptional overload due to hammering insertion during surgery and the force which induced the collapse of the implant. Additionally, the safety margin was quantified through the definition of an overall safety factor under the maximum expected load.