Biomechanics of the ageing human knee

The knee joint is an integral component of the musculoskeletal system, aiding the absorption and transition of weight bearing forces. It is often subjected to injury or disease, with osteoarthritis (OA) being the most prevalent disease, particularly amongst the elderly population. It is now understood that OA is a whole-joint disease affecting the entire osteochondral unit at a molecular and cellular level; however to what extent this effects material properties is mostly unexplored. This thesis firstly aimed to comprehensively review the current knowledge of whole human knee joint material properties in young versus old and healthy versus OA samples, and their subsequent macro-scale application into existing finite element (FE) models. Results indicated unambiguous gaps in the literature for material properties, particularly evident in the aged and OA samples. Consequently, existing human knee FE models apply material properties from a variety of animal and human cohorts, obtained from differing anatomical localities and diverse cadaver demographics, reducing the biological accuracy of resultant mechanical behaviour predicted from such models. Secondly, this thesis aimed to determine the effects of multiple freeze-thaw cycles on cartilage material properties in an attempt to justify a reliable storage and perseveration technique for future work. Results showed that cartilage can undergo up to three freeze-thaw cycles without statistically compromising the integrity of samples. Although data should be interpreted and subsequently applied to future research with consideration in relation to its particular application due to high biological variability across samples. Finally, this thesis aimed to collect and analyse new primary material property data of spatially distributed cartilage, subchondral bone and trabecular bone by nanoindentation techniques, and the four primary knee joint ligaments by tensile testing. Samples were obtained from cadaveric specimens with a wide age range (31 – 88 years) and OA grade (International Cartilage Repair Society grades 0 – 4) to provide varying demographics that were evidently missing from the literature. Cartilage shear storage and loss modulus and subchondral bone elastic modulus significantly decreased with increasing age and grade of OA. Furthermore, a change in cartilage shear storage and loss modulus was correlated with a change in subchondral bone elastic modulus in site-matched samples. Trabecular bone elastic modulus was not correlated with age or OA. Results also showed preferential regional development of OA in the medial knee compartment and a decrease in cartilage shear storage modulus at site-specific locations. Additionally, the anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL) material properties had correlations with age, and linear and failure mechanics showed some correlations with increasing OA grade. The medical collateral ligament (MCL) and lateral collateral ligament [LCL) failure mechanics also showed some correlated with an increase in age and OA grade. This thesis has provided, for the first time, whole-joint multiple tissue material properties from the same cadavers during ageing and disease, concluding that both age and OA affect the material properties of the entire osteochondral unit. Such valuable data can be applied to future FE modelling of the human knee to produce more accurate predication of mechanical behaviour. Current data can also be applied therapeutically, including the use of biomimetic materials, joint replacement and pharmacological interventions.