An investigation into the effects of mechanical loading patterns from everyday activities on long term stability after cementless hip replacement surgery.

The number of Total Hip Replacement (THR) surgeries carried out per year is increasing year on year as people live longer and partake in a more active lifestyle. Other factors such as obesity and weight problems are also increasing and adding to the number of surgeries carried out. Aseptic loosening is the most frequent complication for THR for both cemented and cementless prostheses. However, the long term stability of the cementless prostheses is not well documented. In vitro studies of cementless prostheses have all shown primary stability of the prosthesis whereas the long term stability of the prosthesis and the corresponding effects of mechanical loading have not been investigated. Of the published literature that focused on numerical modelling only Jung et al. (2004) was found to evaluate long term stability and the loosening of cementless prostheses with most other studies concentrating on either primary or secondary stability. Motion analysis was carried out on 23 subjects consisting of 12 THR subjects, 6 Birmingham replacement subjects and 5 healthy subjects. Walking, sitting, standing, up steps and down steps motions were carried out by each subject during the testing. Using musculo skeletal modelling and motion data, average hip joint loading forces for THR subjects were calculated for each motion. These loads were then used as the hip joint forces for the experimental and numerical modelling throughout the study. Statistical analysis was carried out on the forces which highlighted statistical significance between a number of forces when comparing the following groups, male vs. female, large prosthesis head size vs. standard prosthesis head size, anterior surgical approach vs. posterior surgical approach, healthy hip joints vs. replaced hip joints in replacement subjects and THR subjects vs. Birmingham subjects vs. healthy subjects. Four of these significant differences were investigated further with numerical models to determine if any difference in stresses were found at the interface. There are limitations associated with loading patterns in the existing published experimental simulators as most previous research has used a dynamic ramp load which does not take into account realistic forces created at the hip joint such as walking gait often having 2 peaks during one cycle. An experimental rig was designed, fabricated and commissioned. Three pneumatic pistons were used to create the hip joint forces and also change the angle throughout a cycle. Three experimental models were conducted with varying loading conditions. The results demonstrated that both elastic and plastic displacements occurred at the interface due to the mechanical loading. The displacements recorded indicate that the mechanical loading could lead to the loosening of the prosthesis. However, it is important to note that none of the three models became unstable even after 5 million cycles. However, these results do show that the mechanical loading can change the position of the implanted prosthesis over a long period of time. Finally, the effect of the loading directions was investigated using numerical modelling and the average and maximum peak joint forces for each motion were calculated from the musculo skeletal modelling results. The loading direction was also varied in each model to investigate the effects of each loading direction. The Y direction (anterior/posterior) was identified as the main force associated with high stresses that may lead to instability and aseptic loosening while the X direction (medial/lateral) force was seen to play a role in reducing the stresses on the interface surface especially in the neck region where high stresses occurred. The Y direction was also found to be a greater indicator of possible loosening rather than the load magnitude which is used in literature to date.