Unicortical Button Fixation Provide Higher Strength Compared with Transosseous Repair for Subscapularis Tendon in Total Shoulder Arthroplasty.

Background: Subscapularis tendon (SSc) dysfunction following total shoulder arthroplasty (TSA) results in poor functional outcomes. There have been numerous SSc repair constructs tested biomechanically and clinically, however, none has been demonstrated as superior. Newer techniques and implants have emerged, but have not been fully tested.

Hypothesis: We hypothesized that the unicortical button (UB) fixation will provide significantly improved restoration of the anatomic footprint and biomechanical properties when compared to transosseous (TO) repair of the SSc.

Methods: A digital footprint of SSc humeral insertion was obtained in 6 pairs of fresh-frozen cadaveric shoulders using a three-dimensional (3-D) digitizer. A complete SSc tear was created, and each pair of shoulders was randomized to either SSc repair with UB or TO repair. Each specimen underwent a cyclic loading protocol followed by pull-to-failure. The failure load, elongation at failure, gapping failure, number of cycles until failure, the load at key gapping points (1 mm, 3 mm, 5 mm, and 10 mm) and the failure mode were recorded using high-resolution video recording. 3-D surfaces of the insertion footprint and repair site were obtained, and surface areas were calculated using a custom MATLAB script and laser scanner. Paired t-tests were conducted to compare differences between two repair groups.

Results: Failure load was significantly higher in the UB group (382.4 N ± 56.5 N) than in the TO group (253.6 N ± 103.4 N, p=0.005). TO repair provided higher gapping at failure (28.8 mm ± 8.2 mm) than UB repair (10.4 mm ± 6.8 mm, p=0.0017). UB repair had significantly higher load at the 1-mm, 5-mm, and 10-mm gapping compared with TO repair with p=0.042, p=0.033, and p=0.0076, respectively. There were no significant differences between elongation failure, the difference in footprint area from native to repair states, or the percentage of restored footprint area between groups. (p=0.26, p=0.18 and p=0.21 respectively) CONCLUSION: The UB fixation showed a significantly lower gap at failure, higher failure load and number of cycles until failure, and higher gap loads compared with the traditional TO repair for SSc. Although more clinical research is necessary, the UB fixation that utilizes cortical bone presents promising results.