Characterization of Acetabular Cup Insertion Forces in Cancellous Bone Proxy for Validation of an Invasive Sensing Model and Development of Automatic Prosthesis Installation Device: A Preliminary Study

Abstract

Total hip replacement is a widespread medical procedure, with over 300,000 surgeries performed each year in the United States alone. The vast majority of total hip replacements utilize press fit fixation. Successful seating of the implant requires a delicate balance between inserting the implant deep enough to obtain sufficient primary stability, while avoiding fracture of bone. To improve patient outcomes, surgeons need assistive technologies that can guide them as to how much force to apply and when to stop impacting. The development of such technology, however, requires a greater understanding of the forces experienced in bone and the resulting cup insertion and implant stability. Here, we present a preliminary study of acetabular cup insertion into bone proxy samples. We find that as the magnitude of force on the acetabular cup increases, cup insertion and axial extraction force increase linearly, then nonlinearly, and finally plateau with full insertion. Within the small nonlinear zone, approximately 90% of both cup insertion and extraction force are achieved with only 50% total energy required for full seating, posing the question as to whether full seating is an appropriate goal in press-fit arthroplasty. For repeated impacts of a given energy, cup displacement and force experienced in bone (measured force profile—MFP) increase correspondingly and reach a plateau over a certain number of impacts (number of impacts to seating—NOITS), which represents the rate of insertion. The relationship between MFP and NOITS can be exploited to develop a force feedback mechanism to quantitatively infer optimal primary implant stability.