Cyclic testing of arthroscopic knot security

Abstract

Purpose: Arthroscopic soft tissue repairs undergo many cycles of tensioning and relaxation before significant tissue healing occurs, and knot security under cyclic loads is essential for good results after these repairs. The current study was designed to assess the security of arthroscopic knots under stepwise cyclic loading.

Type of study: In vitro materials testing.

Methods: Three types of arthroscopic knots--sliding knots with loop reversal but without post switching (S=SxSxSxS), modified Revo knots with loop reversal and post switching twice (S=S//xSxS//xS), and the SMC knot backed by 2 throws with loop reversal and post switching (S//xS//xS)--were compared with five-throw square knots tied with an open technique. All knots were tied with No. 2 braided nonabsorbable suture around 2 aluminum rods. They were pretensioned to 10 N, and initial displacement was recorded. The knots were then cycled at 1 Hz for 9 different load levels, with 250 cycles at each level, followed by a 30-second rest period to allow recording of permanent laxity of the suture loop. The first step of cyclic loading was from 10 to 40 N, and the upper limit was increased by 10 N for each successive load level. Any knot surviving cyclical loading was pulled to failure.

Results: Ten knots of each configuration were tested. All 3 arthroscopic slip knots showed significantly less initial displacement compared with the square knots (0.26 mm compared with 0.45 mm, respectively, P <.005). All square and modified Revo knots reached the ninth (120 N) loading level, whereas only 6 of the SMC knots and 3 of the arthroscopic knots without post switching reached the ninth loading level. Furthermore, all square and modified Revo knots failed by suture breakage at the knot, whereas 8 arthroscopic knots without post switching and 5 SMC knots failed by slipping. In fact, 3 of the arthroscopic knots without post switching and 1 of the SMC knots failed during the initial cyclic loading from 10 to 40 N. The modified Revo knots reached 3 mm of permanent laxity at significantly higher load levels than the square knots tied with an open technique (96 N v 78 N, P <.005). The SMC knots reached 3 mm of permanent laxity at statistically similar load levels compared with the square knots, but showed considerably more variability. None of the arthroscopic knots without post switching survived the entire cyclic-loading protocol. Four square, 5 modified Revo, and 5 SMC knots that survived cyclic loading reached ultimate failure at statistically similar load levels (157, 156, and 152 N, respectively).

Conclusions: Post switching and loop reversal are key to arthroscopic knot security. Arthroscopic slip knots may be tighter at the time of knot construction than openly hand-tied square knots. The modified Revo knots appear to be as durable as openly hand-tied square knots regarding resistance to loosening under cyclic loading conditions. Security of arthroscopic knots without post switching is quite variable, making these knots less reliable. The SMC knot also proved not to be as reliable as the square and modified Revo knots when cyclically tested.