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

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, orthopedic 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 fetuses. Results: This tissue usually had a thick armor-like appearance that was distant from the infrapatellar fat pad. The anterior cruciate ligament, rather than the posterior 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 (especially 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 differences in the thickness of the paratenon among fetuses. New-borns might have limited knee flexion due to a mass-effect of the thick paratenon around the cruciate ligaments. A slight twisting or rotation at the knee may help to release the knee, because it can break the fetal paratenon and accelerate cavitation.