VISCOELASTIC BEHAVIOUR OF THE CANINE CRANIAL CRUCIATE LIGAMENT COMPLEX

The canine stifle joint is one of the most vulnerable joints within the musculoskeletal system and the cranial cruciate ligament (CCL) is the most susceptible ligament to rupture within the joint. When this ligament is damaged, the stifle joint becomes mechanically unstable leading to abnormal load distribution within the joint. This physiological change is associated with osteophyte formation at the joint margins, thickening of the medial aspect of the joint capsule and the medial collateral ligament, softening of the articular cartilage resulting in osteoarthritis (OA). Ligament injury can be either purely traumatic or a degenerative non-contact form. The aetiopathogenesis of non-contact cranial cruciate ligament rupture (CCLR) is unclear, however alterations in the composition of the extracellular matrix (ECM) has been implicated as one of its causes. This thesis aimed to advance the current understanding of the biomechanical behaviour of the canine CCL and investigated the contribution of proteoglycans (PGs) to the viscoelastic behaviour of the CCL. The objectives comprise of experimental and numerical studies, including the development and utilisation of a novel full-field three-dimensional digital image correlation method (3D DIC) and a representative FEM of the whole canine stifle joint. Experimental Study I on the canine CCLs was the first to focus on characterising slow strain rate sensitivity and hysteresis behaviour of the ligament at the toe-region of stress-strain behaviour. This study showed that arranging mechanical tests in different orders of strain rates resulted in different tissue response, such that tensile responses of the CCL during the ascending (increasing order of strain rates from 0.1 to 1%/min, and 1 to 10%/min) tests were significantly different from the descending tests (decreasing order of strain rates from 10 to 1%/min, and 1 to 0.1%/min). Only during ascending tests were the CCLs strain rate sensitive and hysteresis was strain rate dependent. The different tensile responses of the CCLs during the ascending and descending order of strain rate may be associated with strain history of the tissue. In Experimental Study II, two groups of the CCLs (control and treatment (PG depletion)) were tested under tensile load at slow strain rates (0.1, 1 and 10%/min). PG content in the treatment group was depleted by 21.11 ± 14.51% (p=0.45). Water content in the treatment group reduced by approximately 5.2% (p=0.048). Although there were no statistically significant values; stress-strain, tangent modulus, hysteresis and creep behaviour in the treatment was different from the control groups. Stress relaxation rate was significantly higher in the control than the treatment group (p=0.039). The lower relaxation rate in the treatment group could be associated with sGAGs which provides cross-links between collagen molecules. Hence, it is possible that an efficient depletion of PGs in canine CCLs could result in significant mechanical changes in the tissue. A full-field 3D DIC method was developed to generate five CCL-specific FEM and provide load-deformation behaviour across the middle region of the CCLs. This information was utilised to predict stress-strain behaviour of the CCLs through inverse analysis. In addition, an anatomically representative FEM of the canine stifle joint was developed and employed to investigate the joint when PGs in the CCL were depleted. Results showed reduction in joint stability in joints with depleted CCLs (p=0.56). Hence, PG content in the CCL could be one of the ECM components contributing to the mechanical behaviour of the ligament, and affecting the stability in canine stifle joints. This research leads to a better understanding of the biomechanical behaviour of canine CCL, and it is useful for researchers in the field of biomechanics and biomedical science who are seeking advanced experimental and numerical works in tissue mechanics.