In the first stage of the current study, 14 fresh specimens from above-the-knee amputations were examined by magnetic resonance imaging arthrography. In the second part of the study, these amputation specimens and 38 embalmed knees from cadavers were dissected. The types of the joint and insertion of the biceps femoris tendon and the other structures were observed. Of 14 fresh specimens from above-the-knee amputations examined by magnetic resonance imaging arthrography, nine had a clear communication between the proximal tibiofibular joint and the knee. It was difficult to distinguish the anterior tibiofibular ligament in 30 specimens as a separate band because it was fused intimately with the biceps femoris tendon. Of these specimens, 24 had an oblique type of joint. Because of this communication, the proximal tibiofibular joint might be construed as the fourth compartment of the knee to explain subtle knee problems.