ACL (front) and PCL (back): two bundles each, bright when tautCartilage: plateau and menisciLateral compartment in glassA right human knee, adult size (ruler: 1 tick = 5 mm), as a seated teaching model: the femur is strapped in two cradles and the tibia hangs from the ligaments alone. The collateral ligaments and capsule are removed, and the lateral condyle, plateau and meniscus are drawn in glass. The ACL (about 31 mm long here) runs from the front of the tibia up and back to the femur; the PCL (about 37 mm, thicker) runs from the back of the tibia up and forward. Crossed, they form a four-bar linkage with the bones: the crossing point is the joint's instant centre, so as the knee bends the femur rolls back on the tibia (here 23 mm from 0° to 135°). The condyle's curve is derived from that linkage, not drawn by hand. Bundle lengths come from the same geometry: the ACL's posterolateral bundle is tight near straight and slackens as its anteromedial bundle takes over; the PCL's posteromedial bundle is tight near straight, its anterolateral bundle in deep flexion. Click or Space cuts the ACL, and the tibia slides about 7 mm forward under a Lachman-type pull near 25°; cut the PCL and it sags back about 10 mm at 90°. A planar model: real knees also rotate, and the lateral condyle rolls back more than the medial. The tour bends the knee in 5 s, about 3× slower than standing up from a chair.
~32 mm × 10 mm前交叉韧带（ACL）长 × 宽ACL length × width~38 mm × 13 mm后交叉韧带（PCL）长 × 宽PCL length × width~2,160 N年轻成人 ACL 极限拉力（Woo 1991）ACL failure load, young adults (Woo 1991)0–135°主动屈膝范围（被动约 140°+）active flexion range (passive ~140°+)23 mm本模型股骨后滚 0→135°femoral roll-back in this model, 0→135°~2×PCL 横截面约为 ACL 的 1.5–2 倍PCL cross-section vs ACL (~1.5–2×)