UC San Diego

After injury that causes rupture of the anterior cruciate ligament (ACL), knee joint mechanics are altered, and post-traumatic osteoarthritis (PTOA) often ensues. During movement, the ACL-deficient knee exhibits increased anterior translation and internal rotation of tibia, causing specific regions of articular cartilage (AC) of the femoral condyles (FCs) to be exposed to increased loading and sliding. The resultant tissue-level contact biomechanics may cause direct mechanical or mechanobiological damage to AC and meniscus (MEN) tissues. While traditional histopathological analysis of vertical AC sections indicates that surface fibrillation and fissures can progress to tissue erosion, the limited surface area of such sections makes it difficult to assess how knee joint pathomechanics leads to AC damage in PTOA. Previously, en face imaging of the AC surfaces of normal and ACL-transected (ACLT) rabbit knees stained with India ink revealed areas of damage. In addition, radiological imaging in vivo has clarified joint and tissue mechanics of human knees. The overall aim of this work was to elucidate the biomechanics and mechanobiology of PTOA in rabbit knees due to ACLT by relating detailed patterns of AC surface damage to sites of abnormal loading and sliding. The following three studies were completed.In knees of adult rabbits subjected to ACLT in vivo and analyzed at 4 weeks, the patterns and extent of cartilage surface degeneration were mapped by 2-D en face high resolution imaging and 3-D histology. Six AC crack patterns, Haze, Dash, Transverse line, Longitudinal line, Reticular Sawtooth, and Broad Streak, indicating progressive states of deterioration and typical of human AC, were detected on ACLT FCs, and registered to 3-D histological features (roughening, fibrillation, horizontal and vertical fissures, and erosion). The site-specificity of patterns suggest progression pathways and mechanically mediated mechanisms of AC damage. In previously frozen rabbit hindlimbs, either subjected to ACLT or maintained intact ex vivo, the effects of cyclic loading to cause movement-like extension-flexion was assessed. Bone positions obtained by marker-based tracking during articulation indicated that ACLT led to increased anterior tibial translation. AC damage patterns were consistent with in vivo changes at 4-weeks. These effects of altered knee mechanics, in the absence of cell metabolism or metalloproteinase activity, suggest that knee pathomechanics can directly cause AC surface damage in PTOA. In previously frozen rabbit knees, the effects of ACLT ex vivo on AC and MEN contact was assessed using a custom micro-computed tomography jig that provided knee loading to mimic extension and allowed visualization of joint biomechanics. ACLT knees exhibited increased internal rotation and anterior translation of the tibia, posterior translation of MEN, and shifting of strain at contact. These changes are consistent with knee destabilization and areas of AC damage after ACLT. The above results have several implications. The identification of en face AC damage patterns at 4 weeks in the rabbit in vivo ACLT model suggests opportunities for early interventions to prevent such damage and subsequent deterioration. The recognition of distinct patterns of AC damage at sites subjected to more loading or more sliding indicates multiple biomechanical mechanisms by which instability contributes to PTOA. The mechanical basis of such damage, and possible interventions, may be tested in the ex vivo limb model. The experimental tie between biomechanics at the joint and tissue scale, facilitated by the custom micro-computed tomography jig, helps defines the local kinematics and tissue strains that can be studied in more detail to address both direct biomechanical damage or abnormal mechanobiology leading to AC and MEN damage in ACL-deficient knees.