Conjoined Tendon Transfer vs Modified Bristow in a Glenoid Bone Loss Model: A Biomechanical Study

OBJECTIVES: Over the last ten years there has been a resurgent interest in the use of coracoid transfer procedures for the treatment of traumatic, anterior glenohumeral instability of the shoulder. Bankart repair, whether arthroscopic or open, is the gold standard for treatment of the majority of anterior glenohumeral instability cases. Coracoid transfer procedures, on the other hand, are effective for patients with significant bony defects, those with poor quality anterior capsulolabral tissues, and in collision athletes with higher risk of instability after simple soft-tissue repair. Despite high rates of success in most studies involving the Bristow and other coracoid transfer procedures, complications have been reported. Screw breakage, nerve injury, non-union of the coracoid fragment, and arthritis related to improper placement of the bone block have all been documented. Eliminating the bone block would help to decrease the number of potential complications associated with the procedure. For this reason, we sought to evaluate whether the Bristow procedure or the conjoined tendon transfer (CTT) would be more effective in restoring kinematics of the glenohumeral joint in a glenoid bone loss model for anterior shoulder instability. This is the first study to our knowledge comparing a bone block with its attached musculotendinous sling to a sling alone in a glenoid bone loss model. METHODS: Utilizing fresh frozen cadavers, biomechanical testing was undertaken comparing anterior glenohumeral translation in two groups of ten specimens each with a 25% anterior glenoid bone defect. Group 1 consisted of a tensioned conjoined tendon transfer (CTT) into the defect. Group 2 consisted of a Modified Bristow bone block (BB) transfer. Cyclic testing was performed with the shoulder at 90 degrees of external rotation in both 60 and 90 degrees of abduction. RESULTS: At 60 degrees of abduction, the conjoined tendon transfer showed a 43% reduction in anterior humeral head translation from the unstable or deficient state, while the bone block transfer showed a 12% reduction. This difference was statistically significant. At 90 degrees of abduction, the conjoined tendon transfer showed a 59% reduction in anterior humeral head translation from the unstable or deficient state, while the bone block transfer showed a 38% reduction. This difference trended towards significance. (Please see table 1. for full details/data) CONCLUSION: A tensioned conjoined trendon transfer in the setting of 25% anterior glenoid bone loss exhibited a statistically significant reduction in anterior glenohumeral translation at 60 degrees of abduction and a nonsignificant reduction at 90 degrees of abduction versus a Modified Bristow bone block transfer. Biomechanically, we feel the musculotendinous sling of the conjoined tendon functions in place of the anterior and anterior/inferior capsulolabral structures, including the anterior band of the IGHL, which are typically deficient in the setting of recurrent anterior instability. This represents the initial step in determining if this procedure could be a viable alternative to a bone block transfer in the setting of anterior glenohumeral instability with glenoid bone loss.