Microscopical investigation of canine anterior cruciate ligament and patellar tendon: collagen fascicle morphology and architecture.

The collagen structure of the canine anterior cruciate ligament (ACL) and patellar tendon (PT) was examined by using light and scanning electron microscopy. The collagen waviness known as a crimping was found to occur in ACL and PT fascicles. This waviness, seen at the periphery of fascicles, is very smooth, and its amplitude seems to decrease from the periphery toward the fascicular center. It appears as a periodic collapse of the fascicle in two dimensions. Two models of the architectural patterns of the ACL and PT wavy fascicles are presented. The constituent collagen fibrils are either parallel or twisted relative to the fascicle axis, giving rise to planar and helical wave patterns, respectively. There is a distinct difference between the ACL and PT collagen structure. The helical wave pattern occurs in both PT and ACL while the planar waveform is found only in the centrally located ACL fascicles. In addition, there is less variability in fascicular size and density over the PT cross-section than in ACL.