The ultimate strength of suture anchors

Suture anchors of various designs are gaining acceptance for open and arthroscopic procedures. The rapid proliferation of these devices challenges those using them to apply objective criteria for device selection. Comparative data on implant security in different settings, modes of failure, and ultimate failure strengths is lacking. This study was undertaken to independently develop such data for an objective comparison of the suture anchors currently available. Using a fresh never-frozen porcine femur model, 10 samples of each of the 14 different anchors tested were inserted int each of the three different test areas: diaphyseal cortex (usually 3- to 4-mm thick), metaphyseal cortex (usually 1- to 2-mm thick), and a cancellous bone “trough”. The suture anchors were threaded with 0.018-inch stainless steel wire or, for anchors requiring a more flexible suture, 0.018-inch stainless steel 1 × 7 wire braid. Tensile stress parallel to the axis of insertion was applied at a rate of 12.5 mm/second by an Instron 1321 (Instron Corp.,City, State) until failure. Average failure strength was calculated for each anchor at each test area. The anchors tested were the Mitek G2, Mitek G3, Mitek G4 (Mitek Surgical Products, Norwood, MA), Linvatec Revo screw (Linvatec, Largo, FL), Acufex TAG Wedge, Acufex TAG Rod 2 (Acufex Microsurgical Mansfield, MA), Statak models 1.5, 2.5, 3.5, 5.0, and 5.2 (Zimmer, Warsaw, IN), Arthrex ESP (Arthrex Inc, Naples, FL), Arthrotek Harpoon, and Arthrotek LactoSorb (Arthrotek, Warsaw, IN). The average failure strength of each of these anchors in the diaphyscal cortex, metaphyseal cortex, and cancellous bone is reported. Using Duncan's multiple range test, the Statak 5.0, 5.2, Revo screw, and Mitek G4 showed the highest mean failure strengths in cortical bone. The Statak 5.0, 5.2, Revo screw, and Mitek G2 had the highest mean failure strengths in cancellous bone. The Revo screw was the most consistent of these anchors in all locations showing very little variation in failure strength despite variations in location or bone density. Mode of failure (anchor pullout, suture cutout of the anchor, or wire breakage) was generally consistent for each anchor type, although some variations occurred in different test areas. The size of the insertion hole is clinically important and each anchor's performance was evaluated as a function of its minor diameter or drill hole. All screw anchors were compared and the larger the screw anchor, the higher the mean failure strengths in all three test environments. In contrast, larger drill holes for non-screw anchors resulted in lower mean failure strengths in cancellous bone. Those anchors suitable for a bioabsorbable construct (ESP, Mitek G3, LactoSorb, Acutex TAG Rod 2, and Acufex TAG Wedge) were evaluated together. In this group, for diaphyseal cortex, larger drill holes resulted in higher mean failure strengths. Considering the metallic anchors in cancellous bone, larger drill holes lead to lower mean pullout strengths.