Development of orthopaedic treatment models for in silico trials

The use of cementless stems in total hip arthroplasty surgeries is becoming increasingly common. However, despite advancement in the field, aseptic loosening as a consequence of inadequate osseointegration is one of the main causes of failure. Under physiological loading, insufficient primary stability leads to relative movements between bone and implant, causing the tissue at the interface to differentiate into fibrous tissue, resulting in pain and the need for revision surgery. This study aims to develop an in silico method integrated with in vivo experiment results to predict the long-term stability of cementless hip stems, considering both relative bone-implant micromotion and the osteoinductivity of coatings. Initially, a Finite Element model of a rabbit tibia implanted with titanium alloy pins was developed using bone-to-implant contact data from an in vivo study. The model incorporated a finite state machine to simulate contact changes at the interface, based on relative micromotion, stress, and gap distance, with calibration achieved through in vivo data on the maximum bridgeable gap. A simulated push-out test predicted the axial load for pin mobilisation, revealing a bridgeable gap of 50-80 μm and a push-out strength between 19 and 21 N (3.4–5.4 MPa), aligning well with previous experimental findings of 4 ± 1 MPa. Subsequently, the model was adapted to a patient-specific human femur model implanted with cementless hip stems to predict aseptic loosening risk. By incorporating a finite state machine, time-dependent osseointegration, and adjustable parameters for coating effects, the model successfully simulated enhanced osseointegration and loosening prevention for coated stems.