Altered knee joint mechanics after anterior cniciate ligament (ACL) injury are commonly speculated to lead to osteoarthritis. However, the techniques for quantifying changes in internal knee joint mechanics are limited. This study presents a mathematical model for quantifying cat patellofemoral (PF) joint contact mechanics in a normal and ACL deficient joint in-situ, and determining the distribution of joint cartilage thickness. Three-dimensional kinematics were measured precisely with varying joint loads, joint angles, and ACL integrity. Cartilage and bone surfaces were precisely measured with a newly introduced multi-station digital photogrammetry technique. Joint surfaces were aligned and modelled with a numerical technique new to biomechanics called the thin-plate spline. The model was used to determine cartilage thickness distribution, and contact patterns in the PF joint that varied with altered kinematics. It successfully estimates internal PF joint contact mechanics.