The effect of the insertion angle and pulling angle on the pullout strength of all-suture type anchors - A biomechanical study -

Abstract

Introduction: The pullout strength of the all-suture type anchor (ASA) at the bone-anchor interface was measured based on the angles of anchor insertion and anchor pulling. Data were evaluated with regard to the deadman theory. Materials and Methods: Synthetic sawbones of two different densities (0.16 g/cm2 and 0.32 g/cm2, representing low and high bone density) with a 3-mm-thick cortical bone model attached on one side were used. ASAs for rotator cuff repair were inserted at 45º, 60º, 70º, or 90º to the surface. Sutures were pulled at two different angles from the surface: 45º (modeling the physiologic pull of the supraspinatus) and 90º (modeling pulling out during knot tying). Pullout tests were conducted using a mechanical testing machine, and the maximum load to failure (N) and failure mode were recorded. Five consecutive tests for each insertion and pulling angle combination per sawbone were conducted (80 tests total). Results: Pullout strength for high-density bones was significantly higher than that for low-density bones (p = 0.001). For low-density bones, there was no significant difference in ASA pullout strength for insertion angle degree. However, more vertically inserted ASAs showed stronger pullout strength for high-density sawbones. Pullout strength of ii anchors inserted at 90º and 75º was significantly higher than that for anchors inserted at 45º, regardless of pulling angle (all p < 0.05), but marginally significantly higher than that for those inserted at 60º (p = 0.313, 0.06). The pullout strength of anchors pulled at 45º was higher than that for those pulled at 90º (all p < 0.05). Conclusions: This study suggests that ASAs desirable insertion is vertical rather than at the deadmans angle. However, the ASA showed stronger pullout strength when pulled in the physiologic direction of the supraspinatus tendon rather than in the knot-tying direction, corresponding to the deadman theory. Implanting ASAs vertically may be beneficial clinically when performing knot-tying during surgery and when it is pulled by the supraspinatus tendon after surgery.