Mechanical properties of the anterior cruciate ligament chronically relaxed by elevation of the tibial insertion.

The effects of stress deprivation on the mechanical properties of the anterior cruciate ligament were studied in a canine model. Fifty-eight mature mongrel dogs were divided into two groups. In the relaxed group (n = 30), the tibial insertion of the anterior cruciate ligament in each right knee was surgically elevated proximally 3 mm from the tibia; the elevation was anatomically reduced in the sham group (n = 28). In order to obtain control data, the left knee in each dog was left untreated. A femur-anterior cruciate ligament-tibia complex was excised from each knee for biomechanical tests at 6 or 12 weeks after surgery. To simplify data analysis, the treat/nontreat ratio (the ratio of the data obtained from the treated knee to that from the nontreated knee) was used. The cross-sectional area of the ligament increased significantly in both groups; the area in the relaxed group (average treat/nontreat ratio = 1.37) was significantly larger than that in the sham group (1.16) at 6 weeks but not at 12 weeks. The treat/nontreat ratio of tensile strength in the relaxed group significantly decreased to 0.67 and 0.58 at 6 and 12 weeks, respectively; in the sham group, it significantly decreased to 0.79 at 6 weeks but subsequently increased to 0.87 at 12 weeks. Only at 12 weeks was a significant difference observed between the two groups. This study demonstrated that, in the anterior cruciate ligament, stress deprivation results in a rapid increase in the cross-sectional area, although this effect disappears by 12 weeks, and in a decrease in mechanical strength, although a relatively long period of more than 6 weeks is required for the deterioration.