Paratenon of the cruciate ligaments of the knee: a macroscopic and histological study of human foetuses

Background: The paratenon is a sheath-like connective tissue that allows the tendon to move with minimal friction. The careful removal of the paratenon along the cruciate ligaments is a critical step of knee surgery. Thus, orthopaedic surgeons and interventional radiologists consider the paratenon as a basic anatomical tissue along a ligament, not along a tendon. Materials and methods: We performed macroscopic and histological observations of cruciate ligament-associated paratenons in 43 human foetuses. Results: This tissue usually had a thick armour-like appearance that was distant from the infrapatellar fat pad. The anterior cruciate ligament, rather than the pos-terior ligament, was deeply embedded in the paratenon. The paratenon contained abundant arteries and veins and, at and near the crossing between the cruciate ligaments, had a well-developed venous plexus. Notably, there were abundant fused veins in the paratenon venous plexus, and prenatal knee movements (es-pecially rotation) seemed to restrict its blood supply, leading to the development of a large cavity by way of advancing fusion of veins in the degenerating plexus. This unique manner of cavitation likely expanded the joint cavity. Conclusions: Differences in knee movements in utero seemed to cause differ-ences in the thickness of the paratenon among foetuses. New-borns might have limited knee flexion due to a mass-effect of the thick paratenon around the cru- ciate ligaments. A slight twisting or rotation at the knee may help to release the knee, because it can break the foetal paratenon and accelerate cavitation.