Cyclic loading of anchor-based rotator cuff repairs: Confirmation of the tension overload phenomenon and comparison of suture anchor fixation with transosseous fixation

Previous experimental studies of failure of rotator cuff repair have involved single pull to ultimate load. Such an experimental design does not represent the cyclic loading conditions experienced in vivo. We created 1 x 2 cm rotator cuff defects in 16 cadaver shoulders, repaired each defect with three Mitek-RC suture anchors (Mitek Surgical Products, Inc, Westwood, MA) using simple sutures of No. 2 Ethibond, and cyclically loaded the repairs by a servohydraulic materials test system actuator at physiological rates and loads (rate of 33 mm/s, lead 180 N). A progressive gap was noted in each specimen, for a 100% rate of failure of the repairs. The central suture always failed first and by the largest magnitude, confirming tension overload centrally. One specimen exhibited combined bone and tendon failure, but the other 15 specimens failed through the tendon. Overall, the repairs failed to 5 mm and 10 mm at an average of 61 cycles and 285 cycles, respectively. Half the specimens were less than 45 years of age and had a 5-mm and 10-mm failure at an average of 107 and 478 cycles, respectively. The other half were over 45 years of age and failed to 5 mm and 10 mm at an average of 17 and 91 cycles, respectively, indicating more rapid failure of the rotator cuff tendons in the older group, and this was statistically significant (P ≤.02). Comparison of suture anchor fixation in this study with transosseous bone tunnel fixation in a previous cyclic loading study at this institution indicates that bone fixation by suture anchors is significantly less prone to failure than bone fixation through bone tunnels (P = .0008). Changing the bone fixation from bone tunnels to suture anchors effectively transferred the weak link from bone to tendon.