Regional subdivision of the articular surfaces of the tibiofemoral joint of the knee. The femur and tibia are divided into lateral (L) and medial (M) regions, with the trochlear groove of the femur considered part of the M region. Region S represents the portion of the tibia beneath the tibial spines. The femoral and tibial surfaces are further subdivided into anterior (A), central (C) and posterior (P) regions. Region A of the femur corresponds to the patellofemoral articulation; region C the weight bearing surface and region P the posterior convexity that articulates only in extreme flexion. Region C of the tibial surface corresponds to the uncovered portion between the anterior and posterior horns of the meniscus centrally and the portion covered by the body of the meniscus peripherally.