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.

Automated summary

This study observed the paratenons associated with the cruciate ligaments in human fetuses and found that the paratenons contained abundant blood vessels. The cruciate ligaments were mainly embedded in the paratenons, and differences in prenatal knee movements may lead to differences in the thickness of the paratenons among fetuses. Newborns may have limited knee flexion due to the burden of the thick paratenons, and slight twisting or rotation can disrupt the paratenons and accelerate cavitation.