Introduction

Presently there is no consensus on the tear pattern and which anchor location to use for a partial SB repair. The purpose 1) Evaluate two partial tear patterns of the superior half of the SB tendon and their strain behavior, 2) Evaluate anchor position in repair of a partial SB tears. The hypothesis was 1) Full thickness, partial SB tears are more likely to propagate vs. partial-thickness, partial SB tears 2) Medial (articular) anchor will result in higher biomechanical parameters than lateral (bursal) location in SB partial tear repair

Methods

5-matched pairs of human cadaveric shoulders were tested using an Instron machine and video-digitizing-system. The proximal humerus was potted and mounted creating a perpendicular loading angle between the Instron and SB tendon footprint. 10 markers were placed on the articular side (2 rows x 5 columns) for strain analysis. 10N-preload followed by cyclic loading from 10-100N for 30 cycles at 1mm/second, followed by load-to-failure.
Two partial tears: partial and full-thickness facet-1 tears were tested. Tears were extended into facet-2 creating identical full-thickness partial-SB tears. Tears were repaired with a double-loaded suture anchor and mattress suture. Anchor position was randomized to one of two locations, medial or lateral, in the 1st-facet of SB tendon.

Results

After normalizing strain of each specimen post-tear creation to intact specimens, we found a negative strain gradient from the tear edge to the remaining inferior tendon in full-thickness facet-1 tears (-0.7, R2=0.78, p=0.04) and a positive strain gradient for partial-thickness facet-1 tears (0.7, R2= 0.54, p=0.13). There were no statistically significant differences between the two anchor locations for linear stiffness, yield-load, or energy absorbed to failure. Medial repair anchor location had a significantly higher ultimate load vs. lateral anchor location.

Discussion

Full-thickness tears involving the first facet have a higher propensity for propagation than partial thickness tears, indicating a clinical need for repair. The anchor position for subscapularis repair did not have an effect on biomechanical parameters. For patients with osteoporotic bone or concerns of slow bone healing, a medial anchor position may provide improved resistance to suture anchor pull-out or tendon repair failures.