An in-vitro medium term simulation of hip hemiarthroplasty

Hemiarthroplasty of the hip is a common procedure to treat fracture of the neck of femur, which commonly affects the elderly. The treatment involves replacing the femoral head with a metallic component, leaving it to articulate against the cartilage of the acetabulum. This can commonly lead to rapid cartilage erosion. There have been several studies that examined the tribology of cartilage under a variety of contact mechanics conditions, the majority of these studies use simple geometrical models and none of the current in-vitro studies have examined cartilage tribology for longer than a two hour period. This is a relatively short period which is a limitation with regards to drawing conclusions that might affect clinical practice. Therefore, the overall aim of this thesis was to investigate the in-vitro tribological effect of varying clearance of hip hemiarthroplasty over a four day period. This involved developing a medium term in-vitro simulation system, which needed to be validated.

There were five areas that required development and validation for the proposed in-vitro tribological simulation system. Firstly, the animal acetabulum that was used to represent human tissue needed to be chosen. This was performed by comparing the mechanical and geometrical properties of the hip joint and femoral head cartilage between human and a variety of animals. Secondly, the lubricant which was based on previous literature was validated in terms of maintaining sterility, preventing tissue degradation and its ability to mimic synovial fluid. Through performing this validation, it was found that contamination significantly decreases friction coefficient and hence, needed to be eliminated. Therefore, the third area of validation was the aseptic dissection technique. The fourth area that needed to be addressed was to design, develop and validate the “vessel” that will house the in-vitro simulation system. The final area of validation was the contact mechanics of the hemiarthroplasty simulation system, which needed to be similar to that seen in-vivo.

During hemiarthroplasty surgery, the size of the metallic femoral head is chosen with the aim to best match the geometry of the natural femoral head. The diameter of the natural femoral head is commonly measured using either callipers or circular templates. However, 2 mm diameter sizing increments are commonly used in clinical practice; which could potentially lead to increasing the radial clearance between the femoral head and acetabulum by 1 mm. Therefore, the effect of mismatching the femoral head geometry with the acetabulum by 1 mm radial clearance was investigated tribologically in the medium term using the validated system.

It was found that by mismatching the hip hemiarthroplasty geometry by an extra 1 mm radial clearance; friction coefficient, frictional shear stress and bone exposure significantly increased. There was no significant difference in cartilage thickness. However, through histological assessment, the cartilage exhibited large undulations with fractures to the cartilage surface. Biochemical assessment also showed that there was a significant decrease in glycosaminoglycan concentration compared to fresh cartilage when the geometries were mismatched.

In conclusion, increasing the radial clearance between the femoral head and acetabulum by an extra 1 mm could have a significant effect on the longevity of the hemiarthroplasty implant. This could have clinical implications due to 2 mm diameter sizing increments that are commonly used in clinical practice. Therefore, tighter control over sizing the correct hemiarthroplasty implant must be performed to improve clinical outcomes. It was also found by allowing the cartilage to recover, the friction coefficient and frictional shear stress significantly decreased, which could have a major factor in the wear. Therefore, for future longer term in-vitro tribological studies, the effect of cartilage recovery must be taken into consideration.