Figure 1. The geometry of the extensor mechanism of the knee (as described in this model).

κ  =  knee flexion angle; π  =  patellar tendon angle (relative to the longitudinal axis of the tibia); ρ  =  patella tilt angle (relative to the longitudinal axis of the femur); µ  =  angle of the quadriceps relative to the femur; α  =  angle of incidence of the quadriceps tendon on the patella; β  =  angle of incidence of the patellar tendon on the patella.