Deterioration of Mechanical Properties of the Autograft in Controlled Stress-shielded Augmentation Procedures

Effects of partial and complete stress shielding on me chanical properties and histology of in situ frozen pa tellar tendons were studied in 120 mature female Japa nese White rabbits that were divided into three groups: completely stress-shielded, partially stress-shielded, and sham-operated groups. In the former two groups, tendon tension was reduced to 0% and about 30% of normal force, respectively, with a polyester artificial liga ment. Tensile tests were conducted on patella-patellar tendon-tibia complexes harvested 1, 2, 3, 6, or 12 weeks after surgery. Tensile strength significantly de creased compared with the sham group to 17% and 28% at 3 and 6 weeks, respectively, in the completely stress-shielded group, and to 54% and 63% at 3 and 12 weeks, respectively, in the partially stress-shielded group. Patellar tendon cross-sectional area significantly increased to 156% and 157% at 2 and 3 weeks, re spectively, in the completely stress-shielded group and to 133% at 2 weeks in the partially stress-shielded group, compared with the sham group. Stress shielding significantly changed tensile strength, tangent modulus, and cross-sectional area of in situ frozen patellar ten don ; these changes depended on degree of stress shielding. Histologic observations indicated that remod eling occurred in the patellar tendon while there were no cells in the fascicle.