Suture Slippage in Knotless Suture Anchors as a Potential Failure Mechanism in Rotator Cuff Repair

PurposeTo quantify the strength of suture fixation of knotless suture anchors in relation to the anchors' pullout strength and to compare these results with the static friction between different sutures and anchor materials.MethodsSuture slippage within the anchor and pullout strength of 4 different knotless suture anchor models were assessed in a bovine bone model. Furthermore, the peak force before onset of slippage of different sutures trapped between increasingly loaded 4-mm rods made of commonly used anchor material (polyetheretherketone, poly-L-lactide acid, metal) was assessed.ResultsIn all but 1 of the tested anchors, there was a relevantly lower load needed for slippage of the sutures than to pull out the anchor from bone. The mean load to anchor pullout ranged between 156 and 269 N. The load to suture slippage ranged between 66 and 109 N. All sutures were better held between the metal rods (mean, 21; 95% confidence interval [CI], 19.2 to 23.3) than with polyetheretherketone rods (mean, 17; 95% CI, 15.7 to 18.1) or poly-L-lactide acid rods (mean, 18; 95% CI, 17.6 to 18.4).ConclusionsIn the case of suture anchors that hold the sutures by clamping, the hold of the suture in the anchor may be far lower than the pullout strength of the anchor from bone, because the sutures just slip out from the anchor through the clamping mechanism. This is well explained by the low static friction achieved between the tested sutures and the test rods made of anchor materials.Clinical RelevanceThe use of knotless suture anchors appears quick and easy to perform; however, most of the anchor systems could not even reach half of the anchor pullout strength from bone before suture slippage occurred.