Failure mode of suture anchors as a function of insertion depth in human cadaver humeri (SS-48)

      Rotator cuff integrity after repair is a major determinant of a good functional result and residual deficiencies have been found in greater than 50% of complex tears. Different modes of suture anchor failure have been identified and may occur due to anchor displacement or pullout, suture breakage, knot slippage, or suture pulling though the tendon. Recently, another mode of anchor failure has been identified in the bovine model by suture cutting though the bone when an anchor is placed deeper than recommended. However, this result has not been reproduced using human cadaveric shoulders. The purpose of this study was to evaluate the biomechanical stability of a suture anchor placed deep within the rotator cuff footprint of human cadaver humeri. Methods: Metallic screw-in anchors loaded with a single, number 2 braided nonabsorbable polyester suture were placed in the infraspinatus footprint of eight human cadaver humeri (87 ± 3 years) at two depths. Standard placement had the threads countersunk 3 mm below the bone surface. The deep anchors doubled this depth to 6 mm. The sutures were placed under 10 N of preload and the knots were placed equidistant from the bone hole exit and a smooth metallic rod used for mechanical testing. Specimens were cyclically loaded between 10 to 45 N to a maximum of 500 cycles and then, if still intact, loaded at 0.5 mm/sec to failure. Total displacement after cyclic testing (mm) and ultimate failure load (N) were analyzed with a one-way ANOVA (P < .05). Failure mode and location were recorded after testing. Results: The total displacement after cyclic loading was significantly different between repair depths. The deep anchors attained a total displacement of 8.4 ± 2.4 mm while the standard anchors had a final displacement of 5.7 ± 1.4 mm (P < .03). There was no difference in ultimate failure load between depths, with the deep anchors failing at 144 ± 14 N and standard anchors failing at 143 ± 13 N. The deep anchors were noted to translate to the cortical margin within the first few cycles. Both anchor positions seemed to rotate at the cortical surface due to the orientation of loading as testing progressed. Failure occurred at the eyelet in two tests, while the remaining tests exhibited anchor translation/rotation and some suture cut through. Discussion: Burying suture anchors beyond the specified insertion depth is inadvisable based on these data. Excessive anchor depth may lead to early clinical failure by the suture cutting through bone. Both anchor depths displayed rotation and translation at the cortical surface and some migrated above the joint surface during physiologic cyclic loading. The rotation and translation may induce early deficiencies in the cuff repair while the migration may limit joint range of motion and induce joint arthrosis. Due to poor bone quality, these rotations/translations are of specific concern in the osteoporotic population. Improved anchor eyelet designs may lessen the possibility of abrasive degradation of the suture, and abrasion-resistant materials may decrease the risk of suture fretting.