Biomechanics of Subscapularis V-shaped Tenotomy compared to Standard Tenotomy.

INTRODUCTION: Subscapularis function is critical after anatomic total shoulder arthroplasty (aTSA). Recently, however, a technique has been described that features a chevron or V-shaped subscapularis tendon cut (VT). This biomechanical study compared repair of the standard tenotomy (ST), made perpendicular to the subscapularis fibers, to repair of the novel VT using cyclic displacement, creep, construct stiffness, and load to failure.

METHODS: This biomechanical study used six pairs of fresh frozen paired cadaveric shoulder specimens. One specimen per each pair underwent VT, the other ST. Subscapularis tenotomy was performed 1 cm from the insertion onto the lesser tuberosity. For VT the apex of the V was 3cm from the lesser tuberosity. After tenotomy, each humerus underwent humeral head arthroplasty. Eight figure-of-eight sutures were used to repair the tenotomy (Ethibond Excel; Ethicon, US LLC, Somerville, NJ, USA). Specimens were cyclically loaded from 2N to 100N at 45 degrees abduction at a rate of 1 Hz for 3000 cycles. Cyclic displacement, creep, and stiffness, and load to failure were measured.

RESULTS: Cyclic displacement did not differ significantly between the ST and VT from 1 to 3,000 cycles. The difference in displacement between the V-shaped and standard tenotomy at 3,000 cycles was 1.57mm (3.66 ± 1.55 mm vs. 5.1 ± 2.8 mm p= .31, respectively). At no point was the V-shape tenotomy (VT) above 3mm of average displacement, whereas the standard tenotomy (ST) averaged 3mm of displacement after 3 cycles. Creep was significantly lower for VT in cycles 1 through 3 . For all cycles stiffness was not significantly different in the VT group compared to the ST group. Load to failure was not statistically significant in the VT compared to the standard tenotomy throughout all cycles (253.2 ± 27.7 N vs. 213.3 ± 76.04 N; p= .25, respectively). The range of load to failure varied from 100-301 N for standard tenotomy compared to 216-308 N for VT.

CONCLUSION: This study showed that VT and ST demonstrated equivalent stiffness, displacement, and load to failure. VT had the benefit of less creep throughout the first three cycles, although there was no difference from cycle 4 to 3,000. The VT had equivalent biomechanical properties to the ST at time zero, an important first step in our understanding of the technique. The VT technique warrants further clinical study to determine if the technique has clinical benefits over ST following aTSA.