Rupture of the cranial cruciate ligament (CCL) is the most common cause of hind limb lameness and stifle osteoarthritis in dogs, reliably leading to chronic pain and disability if left untreated. The tibial plateau leveling osteotomy (TPLO) is the most commonly performed surgical technique for treatment of CCL ruptures and functions by rotation of the proximal tibial plateau to convert shear forces into compressive forces, thus supposedly ‘eliminating’ the need for the CCL. The ideal amount of tibial plateau rotation to achieve this is unknown – the current recommendations are based on guidelines from the developer of the technique with little peer review. The technique is not perfect, as evidenced by the fact there is significant osteoarthritis (OA) progression in 100% of patients despite this surgical intervention. Interestingly, it has been shown that dogs with early partial CCL ruptures (just beginning to tear but still stable) at the time of TPLO surgery have no to minimal OA progression. Additionally, the remaining CCL in these dogs is preserved long-term, resulting in a stable stifle without late meniscal tears. These facts support the theory that the CCL is unloaded by the TPLO. Solely replacing a ruptured CCL with an intra-articular (IA) graft, as done in humans, has been evaluated thoroughly by multiple investigators: the IA graft consistently fails in the early post-operative phase due to the loads they are subjected to. Experimentally, some grafts can withstand only ~30% of the load of the normal intact cruciate. However, it is possible that by performing a TPLO at the same time as an IA graft, the load on the graft itself can be decreased, thus resulting in a stable stifle with outcomes similar to the early partial CCL tears mentioned above. This grant is the first step in exploring whether or not a TPLO with the addition of an IA CCL replacement graft would increase long-term stifle stability and decrease OA formation. The OBJECTIVE of this grant is to determine the relationship between the tibial plateau angle (TPA) and the strain on the intact CCL ex vivo. Our SPECIFIC AIMS are to 1) to measure CCL strain at decreasing TPAs during stifle loading on a materials testing system, and 2) to determine the minimum amount of CCL strain achievable by tibial plateau rotation within the confines of a TPLO. The results of this project will not only let us proceed to the next step of the project (replacement of the intact CCL with an IA graft ex vivo), but will also result in immediately useable clinical recommendations for the ideal amount of tibial plateau rotation for a TPLO.