Stress distribution inside bone after suture anchor insertion: simulation using a three-dimensional finite element method.

PURPOSE: To define stress distribution patterns inside a bone around suture anchors inserted at different angles using a three-dimensional finite element (FE) method.
METHODS: An isotropic cube model (Young's modulus, 1,380 MPa; Poisson's ratio, 0.3) was designed on a computer to standardize analysis conditions. A virtual Twinfix anchor was inserted into the cube at two different angles (45° and 90°) against the top surface. A traction force (100 N) was applied to the anchor at six different angles (15°, 30°, 45°, 60°, 75° and 90°) against the top surface. Elastic analysis was performed, and the distribution of the von Mises equivalent stress inside the cube was calculated. The highest value of the equivalent stress at each traction angle was compared between the 45° and 90° anchor insertion settings.
RESULTS: Stress concentration was most evident around proximal anchor threads, particularly on the traction side. Interestingly, stress gradually declined with an increase in traction angle only for the 90° insertion setting. At 15° and 90° traction angles, the equivalent stress was lower for the 45° insertion setting than for the 90° insertion setting. In contrast, the 90° insertion setting exhibited lower equivalent stress than the 45° insertion setting at 30°, 45° and 60° traction angles.
CONCLUSIONS: Insertion of an anchor at 90° might reduce the stress concentration around the proximal anchor threads on the traction side and provide lower equivalent stress in the middle range of traction angles (30°-60°) than insertion at 45°. This could avoid early postoperative anchor failure.