The effect of bone mineral density on anchor function and its implication on the pull-out strength of trans-osseous sutures and Ultrafix rotator cuff anchors

Background An experimental study was carried out to assess the influence of Bone Mineral Density (BMD) of the humeral head, neck and greater tuberosity in seven pairs of fresh frozen humeri on the pull out strength of the Ultra-Fix Anchor (Stryker,Inc,USA) and the standard intra-osseous sutures used to repair rotator cuff tears. Method An anchor and a transosseous suture were placed in a trough, created with a 6 mm burr in the cuff footprint of each humeral head by the same specialist shoulder surgeon(L.N.). The technique for intra-osseous suture and anchor localisation were standardised using a 5 Metric (No 2) braided polyester suture (Ethibond, Ethicon, UK). Both fixation methods were subjected to tensile testing with the humeri mounted at 30° of abduction corresponding to the direction of anatomical pull of the supraspinatus muscle. Results and discussion There was a significant linear correlation (p = 0.007) between Ultra-fix anchor pull-out strength and humeral head BMD as well as between bone suture failure strength and humeral neck BMD (p = 0.015) There was no correlation between the BMD of the greater tuberosity and failure strengths of both fixation methods. (p = 0.23 for anchor and p = 0.29 for bone suture) The Ultra-fix anchor was significantly stronger in pull-out when compared to the bone suture (p = 0.05). Conclusion The mean pull-out strength of the Ultrafix rotator cuff anchor is significantly higher when compared to the traditional bone suture technique for anchorage into a proximal humerus trough (141.7 N and 111.6 N, respectively).There is a significant correlation between the total BMD of the proximal humerus (head, neck and greater tuberosity regions) and the pull-out strength of both the anchor (p = 0.039) and bone sutures (p = 0.024).