Characterisation and In Vitro Simulation of the Natural Hip

Abnormal hip joint morphology, associated with diseases such as femoroacetabular impingement (FAI) or developmental dysplasia of the hip (DDH), is thought to be a precursor of osteoarthritis (OA) in the hip. Changes in joint morphology alter the loading pattern through the hip, which results in damage to the tribological interface, including labral tears and/or labral- cartilage separation. Evidence shows that early intervention to repair the labrum is more beneficial than labral excision; however scientific understanding of the tissue and the effect of surgical treatments are limited. It is hypothesised that an in vitro natural hip simulation system could be used in biomechanical testing of hip joint tissues as well as generating labral damage which could be used to assess current and new surgical treatment methods for the labrum.
Initial quantitative assays revealed human and porcine labral tissue to have higher collagen content but lower water and GAG content than articular cartilage. Histological staining identified the structure of collagen within the labrum and cartilage, as well as the dispersion of GAGs, in human and porcine tissue. Slight differences were seen between the two species with the human labrum containing more connective tissue compared to the porcine labrum, which was primarily composed of fibrocartilage, and less GAGs. Mechanical tests identified little variation between the compressive properties of the human and porcine labrum however, larger differences were identified in the tissues tensile properties, where by the human labrum was stronger than the porcine labrum. Labral tissue was also found to be weaker in compression in comparison to cartilage tissue.
An in vitro natural hip model was successfully developed using clinically relevant conditions. The cup inclination angle was set at 45 °, a full ISO14242 gait cycle was applied to the joint with a peak load of 750 N, to account for porcine tissue. No labral or cartilage damage was observed after 10800 cycles. In vitro labral damage was also successfully developed, by increasing the acetabular cup angle to 60 ° and increasing the load by 50 %. The model was run through the full gait cycle for a minimum of 10800 cycles. Damage was classified using the Outerbridge and Lage systems. All types of labral damage outlined in the Lage classification system were identified within the model. Labral damage was found to progress from labral flattening, to radial fibrillation followed by longitudinal peripheral tears.
The methodology and findings within this study can be used in future studies and can be advanced to mechanically test the soft tissues of the hip in situ, as well as the effect of labral damage on the functions of the hip joint and potential labral treatments.