Biomechanics of Biodegradable Subacromial Balloon Spacer for Irreparable Superior Rotator Cuff Tears

Background: A subacromial balloon spacer is an option to treat irreparable rotator cuff tears. We hypothesized that the balloon would restore glenohumeral contact pressure, the acromion-humeral interval, and deltoid load to intact values after a simulated irreparable supraspinatus tear in a cadaveric model. Methods: Fourteen cadaveric shoulders (mean age at the time of death, 67.9 years) were tested using a custom test frame. In this frame, glenohumeral contact pressure, the acromion-humeral interval, and deltoid load were measured using a digital sensor, a MicroScribe, and a spring scale, respectively. Test conditions included the intact shoulder, a small supraspinatus tear, supraspinatus repair, repair plus balloon, an irreparable supraspinatus tear (rotator cable-insufficient), and an irreparable tear plus balloon. Load was applied in a simulated neutral arm position (balanced) and active shoulder abduction (unbalanced). Results: When the balloon was inflated over the irreparable supraspinatus tear in the balanced condition, glenohumeral contact pressure increased by 122% (p = 0.006) compared with that for the irreparable tear at 0 degrees of abduction and by 94% (p = 0.046) at 60 degrees. In the unbalanced condition, pressure decreased in the irreparable tear condition after the balloon was inflated, restoring pressure to close to that in the intact state. The balloon did not restore glenohumeral contact area to that in the intact shoulder in either the balanced or the unbalanced condition. The irreparable tear displaced the humeral head superiorly in the unbalanced condition, decreasing the acromion-humeral interval. The balloon moved the head inferiorly by a mean (and standard error of the mean) of 6.2 +/- 1.3 mm (p < 0.001) at 0 degrees of abduction, 4.4 +/- 1.3 mm (p < 0.001) at 30 degrees, and 3.0 +/- 0.8 mm (p < 0.001) at 60 degrees. The balloon increased the deltoid load after an irreparable tear by 8.2% (p = 0.022) at 0 degrees, 12.6% (p = 0.002) at 30 degrees, and 11.1% (p = 0.008) at 60 degrees. Conclusions: In a cadaveric model of an irreparable supraspinatus tear, a balloon spacer restored intact-state glenohumeral contact pressures at most abduction angles, lowered the humeral head, and increased deltoid load at postoperative time 0.