An experimental study was performed in 32 adult beagle dogs to clarify the effect of nonphysiologically high initial tension on the mechanical and histologic properties of in situ frozen anterior cruciate ligaments. Both anterior cruciate ligaments in each dog underwent the in situ freeze-thaw treatment. The tibial insertion of the ligament was then made free from the tibia along with a cylindrical bone block. In the right knee, an initial tension of 20 N was applied on the anterior cruciate ligament by translocating the bone block in the distal direction. In the left knee, this bone block was anatomically reduced. Each bone block was firmly fixed with an interference screw. Ten animals were sacrificed at 6 weeks and 10 at 12 weeks. The tensile strength and the tangent modulus in the highly tensioned knee were significantly less than those in the physiologically tensioned knee at 12 weeks. Histologically, cell nuclei appeared to be spindle-shaped in the physiologically tensioned knee, while oval nuclei and focal degenerative changes with a number of vacuoles were occasionally found in the matrix in the highly tensioned knee. This study demonstrated that a nonphysiologically high tension significantly deteriorates the mechanical properties of the in situ frozen anterior cruciate ligament compared with physiologic tension.

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.

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