CYCLIC LOADING OF TRANSOSSEOUS ROTATOR CUFF REPAIRS - TENSION OVERLOAD AS A POSSIBLE CAUSE OF FAILURE

Previous experimental studies of failure of rotator cuff repair have i nvolved a single pull to ultimate load. Such an experimental design do es not represent the cyclic loading conditions experienced in vivo. We created 1 cm X 2 cm rotator cuff defects in 16 cadaver shoulders, rep aired each defect with transosseous simple sutures, and cyclically loa ded the repairs by a servohydraulic materials test system actuator at physiologic rates and loads (rate of 33 mm/sec to a load of 180 N.). A progressive gap was noted in each specimen, for a 100% rate of failur e of the repairs. A 5 mm gap developed at an average of 25 cycles, and a 10 mm gap developed at an average of 188 cycles. The central suture always failed first and by the largest magnitude. This study suggests that rotator cuff tears that are repaired with a ''tension overload'' of a portion of the muscle-tendon units will undergo gradual failure with physiologic cyclic loading until the normal resting lengths of th e muscle-tendon units are restored. This ''controlled failure'' of the repairs may explain residual defects that have been demonstrated by u ltrasonography and by arthrography in patients with ''successful'' rot ator cuff repairs. Clinical implications are that: 1) rotator cuff tea rs should be repaired without tension if possible; and 2) transosseous bone tunnels should ideally extend distal to the weak metaphyseal bon e so that purchase is obtained into cortical bone for greater fixation strength.