The Effect of Insertion Angle on the Pullout Strength of Threaded Suture Anchors: A Validation of the Deadman Theory

Abstract

To determine the effect of insertion angle, from 45° to 135° in 15° increments, on the number of cycles withstood, the ultimate pullout strength, and the stiffness of threaded suture anchors subjected to load. Threaded anchors were inserted into polyurethane foam at angles from 45° to 135°, in 15° increments, relative to the direction of pull. Five anchors were tested at each angle. The anchors were first cycled for 30 cycles (10 each at 100 N, 150 N, and 200 N). The surviving specimens were then tensioned to failure. The McNemar test was used to compare cyclic failure rates. Paired-samples t tests were used to compare load-to-failure (LTF) and stiffness data. All P values are multiplicity adjusted by the Hommel procedure. Four of 5 anchors inserted at 45° failed during cyclic testing at a mean of 27 cycles (P= .13). One of 5 anchors placed at 60° failed after 29 cycles (P= .99). All other anchors survived cyclic testing. Mean LTF was 234 N, 243 N, 297 N, 373 N, 409 N, 439 N, and 417 N at insertion angles of 45°, 60°, 75°, 90°, 105°, 120°, and 135°, respectively. LTF was significantly less for the 60° group when compared with the 90°, 105°, 120°, and 135° groups (P < .05). LTF was significantly less for the 75° group when compared with the 105°, 120°, and 135° groups (P < .05). For the 90° group, LTF was only significantly less when compared with the 135° group (P= .022). Thedifferences in LTF between the 105°, 120°, and 135° groups were not significant. Stiffness increased from 28.13 N/mm at 90° to 43.4 N/mm at 105° (P= .03), 61.48 N/mm at 120° (P= .003), and 86.83 N/mm at 135° (P= .008). Anchors placed at more acute angles, that is, anchors placed closer to the so-called deadman's angle, failed at lower loads and provided less construct stiffness than anchors placed at angles greater than 90°. Stiffness also increased sequentially from an angle of insertion of 90° up to our maximum angle tested of 135°. For threaded metallic suture anchors, an obtuse insertion angle of 90° to 135° in relation to the line of pull of the suture and rotator cuff withstands a greater LTF and provides a stiffer construct than the more acute insertion angle advocated by the "deadman theory." This study offers a biomechanical validation for optimal placement of threaded suture anchors at an angle of 90° or more, as anatomic restraints allow, from the vector of pull of the attached suture and rotator cuff, rather than the 45° angle recommended by the deadman theory.