Most early revisions in total hip arthroplasty (THA) are caused by dislocations. It has been established that surgeon experience is a major component in the need for revision surgeries. Currently, no standardised protocol for intraoperative soft tissue tension assessment exists. Furthermore, orthopaedic surgeons do not have a method of preoperative validation of trial implant configurations. To address this problem, the design of a physical simulator of the hip joint with muscle force and strain sensors is proposed, mostly made of widely available materials, manufactured using fusion deposition modeling (FDM) technology. The goal is to create a training tool for orthopaedic surgeons which will reduce the number of revision operations caused by poor hip balance. In addition, the simulator could be used to review the effects of varying implant geometries, which may reduce operation times. The simulator is hypothesised to have a similar joint balance to an in vivo hip, where an increased femoral offset will result in increased tension around the joint. In addition, the increased femoral offset is hypothesised to cause an increased external rotation moment and angle required for subluxation.

A scaling model based on anatomical data is constructed, and an additively manufactured shape is designed which mimics the tensile characteristic of muscle tissue. Sensors are added to almost each phantom muscle, to actively monitor the subjected tension and strain. The simulator is then tested by orthopaedic surgeons, performing two of their preferred movements for hip stability assessment, namely a traction and external rotation test, using three different implant configurations with varying femoral offsets. The muscle strains at the point of subluxation are reported. This experiment is then repeated, but not executed by a specialist, while using an additional implant configuration. During this second traction test, the total traction force at the foot was also measured. During the second external rotation test, the angle and moment required for subluxation were also measured at the foot.

The experiments yielded promising results. Muscle strains were recorded during the traction test for the gluteus maximus, minimus and piriformis which ranged from maxima of 10% to 26% at subluxation. For these muscles, the recorded force and strain were significantly higher (P ≤ 0.05) for the highest femoral offset compared to the lowest offset. During the external rotation test, the gluteus medius and minimus were maximally strained from 12% to 23%, while showing significantly higher force and strain for the highest offset configuration compared to the lowest offset. An increased femoral offset did cause increased tension in the joint, suggesting that the first hypothesis may be valid. In addition, the surgeons were able to correctly answer if there had been an increase or decrease in femoral offset without knowing the current implant configuration. They were enthusiastic about the potential of the simulator as a training tool, both for surgeons becoming more acquainted with the stability assessment movements and as a general method of understanding the mechanics of the hip joint, specifically how an implant configuration can influence these mechanics. This experiment was followed by an experiment where the same movements were performed, but not by a specialist...