Biomechanical studies of the medial collateral ligament (MCL) in the knee have suggested that regional variations exist both in the structure and in the mechanical behavior of the MCL, particularly between the central midsubstance and bony insertion site regions. Although this type of heterogeneity has been commonly reported for soft connective tissues, reasons for this heterogeneity have not been clarified.

A new combined experimental and theoretical approach was used to investigate the degree of heterogeneity of the normal rabbit MCL and to evaluate the relative contribution of geometry, microstructure, material properties, and tethering of ligament to bone. Protocols for in vitro tensile tests and scanning electron microscopy were developed to identify variations in surface strain and internal structure along the length of this ligament. Two- and three-dimensional finite element models of a normal rabbit femur-MCL-tibia complex were also used to study regions of the ligament inaccessible in the in vitro tensile tests such as the ligament-bone junctions. The two-dimensional model was used to study how the degree of heterogeneity in surface strain is affected by changes in material properties and tethering, such as those that might occur after injury and/or during healing.

The experimental studies revealed that surface strains varied along the length of the MCL, but were not necessarily higher at the ligament-bone junctions than in the midsubstance. The two-dimensional model demonstrated that the magnitude and distribution of surface strains was affected by tethering, more so than by material properties or geometry. Regional variations in microstructure were also observed;​ fibers in the epiligament and at the insertion sites were aligned transverse to those in the midsubstance region. Thus, measurements of ligament properties appear to depend on measurement location and the integrity of ligament-bone junctions.

The results of this study suggest that for future studies on the MCL, or other ligaments, one should identify the degree of heterogeneity of its structure and tensile behavior. Further, one needs to identify how the heterogeneity, in addition to the mechanical properties, changes after injury. In particular, changes in ligament tethering should be noted as these changes could strongly affect measured ligament properties.