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 chal lenges those using them to apply objective criteria for device selecti on. 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 compari son of the suture anchors currently available. Using a fresh never-fro zen porcine femur model, 10 samples of each of the 14 different anchor s tested were inserted into each of the three different test areas: di aphyseal cortex (usually 3- to 4-mm thick), metaphyseal cortex (usuall y 1- to 2-mm thick), and a cancellous bone ''trough''. The suture anch ors were threaded with 0.018-inch stainless steel wire or, for anchors requiring a more flexible suture, 0.018-inch stainless steel 1 x 7 wi re 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, S tate) 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 scr ew (Linvatec, Largo, FL), Acufex TAG Wedge, Acufex TAG Rod 2 (Acufex M icrosurgical, Mansfield, MA), Statak models 1.5, 2.5, 3.5, 5.0, and 5. 2 (Zimmer, Warsaw, IN), Arthrex ESP (Arthrex inc, Naples, FL), Arthrot ek Harpoon, and Arthrotek LactoSorb (Arthrotek, Warsaw, IN). The avera ge failure strength of each of these anchors in the diaphyseal cortex, metaphyseal cortex, and cancellous bone is reported. Using Duncan's m ultiple range test, the Statak 5.0, 5.2, Revo screw, and Mitek G4 show ed 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 a nchors in all locations showing very little variation in failure stren gth despite variations in location or bone density. Mode of failure (a nchor pullout, suture cutout of the anchor, or wire breakage) was gene rally consistent for each anchor type, although some variations occurr ed in different test areas. The size of the insertion hole is clinical ly 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 n on-screw anchors resulted in lower mean failure strengths in cancellou s bone. Those anchors suitable for a bioabsorbable construct (ESP, Mit ek G3, LactoSorb, Acufex TAG Rod 2, and Acufex TAG Wedge) were evaluat ed together. In this group, for diaphyseal cortex, larger drill holes resulted in higher mean failure strengths. Considering the metallic an chors in cancellous bone, larger drill holes lead to lower mean pullou t strengths.