Tribology of hemiarthroplasty

Hip hemiarthroplasty (HA) is a conservative treatment for hip diseases (e.g. osteoporosis, femoral head necrosis) and injuries (e.g. femoral neck fractures) compared to total hip replacement (THR). HA is commonly used in elder patients who have low activity requirements and can be delivered with reduced blood loss, shorter operation time, and hospital stay. HA has been developed to unipolar and bipolar prostheses, and the range of motion has been improved with new designs of the bipolar prostheses. However, satisfaction of HA is not always positive due to the erosion, degradation of the acetabular cartilage, and the dislocation of femoral head which have reported in clinical studies. Hence, the mechanical factors which affect the tribological properties of articular cartilage in HA have been investigated experimentally in an in- vitro simulation of HA. In a simple geometry HA model, the articular cartilage coefficient of friction, deformation, and wear have been examined in a cartilage pin on metal plate model. The loading time, contact stress levels, contact areas, stroke lengths and sliding velocities were set as the input variables, and it was found that: 1) coefficient of friction was time- and load- dependent in short-term (1 hour) testing; 2) wear was load-dependent in long-term (24 hours) testing; 3) contact area only affected the coefficient of friction when both fluid and solid phase of cartilage supported the load due to the different aspect ratio; 4) the stroke length and sliding velocity affected the coefficient of friction only under low level of contact stress (≤2 MPa), but affected the cartilage wear under all levels of contact stresses. In an entire simulation of HA, the acetabular cartilage coefficient of friction, surface roughness, wear, and deformation have been studied with metal or ceramic heads articulating against acetabular cartilage in a pendulum friction simulator. The clearance between the head and acetabulum, loading types, and prosthesis design were set as the input variables, and it was demonstrated that: 1) a trend of decrease of the coefficient of friction was shown with increasing clearances under both constant and dynamic loading; 2) severe cartilage severe damage was observed when radial clearance was >1.8 mm under constant loading; 3) ceramic heads decreased the cartilage wear, and change of surface roughness compared to metal heads; 4) a novel bipolar design with a thin metal cup and metal head failed to reduce the coefficient of friction due to the cup locking in the acetabulum. The methodologies and findings in this research can currently adopted for related future studies, additionally the anatomic pendulum friction natural joint simulation can be used for the pre-clinical studies in less invasive procedures in the future.