Structure and ultrastructure of the bone/ligament junction

Abstract

The tibial insertion of the patellar ligament of rat was investigated at light microscopy, scanning electron microscopy and transmission electron microscopy. Until the point of insertion, the patellar ligament showed the typical structure of a tendon. In proximity to the insertion, the ligament increased in diameter and was gradually infiltrated by a different, cartilage-like matrix. Its tenocytes became progressively rounded and displayed some characteristics of chondrocytes. Tendon fibres crossed this fibrocartilage and arrived well beyond the mineralization front. Finally, they appeared to interweave with the tibial diaphysis bone. These fibres were always distinct from the interposed extracellular matrix and exhibited a different pattern of mineralization from the matrix. Our observations indicate that fibrocartilage does not supersede tendon but merely infiltrates it. Thus, the tendon does not splay during articular movement and the tensile stress is redistributed across the insertion area by increasing the mechanical coupling among adjacent fibres. At the same time, this mechanism provides a structural, uninterrupted connection from tendon to bone which is consistent with the biomechanical requirements it has to withstand.