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The instability of the patellofemoral joint in the transverse plane was studied in a
two dimensional mathematical model with deformable contact surfaces. Articular surfaces, muscles, retinaculum and patellar tendon, and articular cartilage were simulated as orthogonal polynomials, strings with known forces, tensile springs, and a set of compression springs,
respectively. The resulted nonlinear equations, including three equilibrium
equations and one constraint equation for each cartilage spring, were solved using Newton-Raphson method. Results indicate that total dislocation of the patella only occurs if the joint's soft and hard tissues are both abnormal. Lateral displacement of the vastus medialis attachment to the patella, was appeared to be the most effective
surgical procedure for treatment of patellar instability disorders.