Structure of the human tibialis anterior tendon

The structure and vascular pattern of the human tibialis anterior tendon wa s investigated using injection techniques, light and transmission electron microscopy and immunohistochemistry. From the well vascularised peritenon, blood vessels penetrate the tendon tissue and anastomose with a longitudina lly oriented intratendinous network. The distribution of blood vessels with in the tibialis anterior tendon was not homogenous. The posterior part of t he tendon had a complete vascular network that extends from the musculotend inous junction to the insertion at the first metatarsal and medial cuneifor m bones. In the anterior half, the tissue was avascular in a zone with a le ngth of 45-67 mm, This zone was covered by a single layer (similar to 30 mu m) of oval shaped cells. Transmission electron microscopy showed that these cells have the characteristics of chondroid cells. This region was stained by Alcian blue at pH 1 which indicates a high concentration of acid glycos aminoglycans and immunohistochemical staining for chondroitin-4-sulphate, c hondroitin-6-sulphate and aggrecan was positive, However, immunostaining fo r the typical cartilage specific type II collagen within this zone was nega tive. The location of the avascular zone corresponds to the region where th e tibialis anterior tendon wraps around the superior and inferior retinacul a which serve as fibrous pulleys. This is the region where most spontaneous ruptures of the tibialis anterior tendon occur. The presence of fibrocarti lage within gliding tendons is a functional adaptation to compressive and s hearing forces. In contrast to reports from the literature about the struct ure of gliding tendons wrapping around a bony pulley, the gliding zone of t he tibialis anterior tendon has only a narrow layer of chondroid cells and proof of type II collagen is lacking.