How Many Anchors to Use on Arthroscopic Bankart Repairs? A Biomechanical Study of Postage-Stamp Glenoid Fractures.

BACKGROUND: Deciding how many anchors to use in a Bankart repair is challenging because of the desire to enhance stability while avoiding postage stamp fracture of the glenoid rim.

METHODS: This controlled laboratory study investigated load to fracture of the anterior glenoid rim after drilling holes of varying number and diameter and inserting anchors of two different types and sizes, with and without perforation of the medial cortex of the glenoid, creating postage stamp fractures using a metallic humeral head that was compressed against the anterior glenoid rim. A destructive model with a servohydraulic load frame was used to test 46 synthetic scapulae with compressive strength and elastic modulus similar to that of a human glenoid. Load to fracture of the intact glenoid was compared to groups with a varying number of anchor holes of different diameters, with anchors of different sizes and types, and with anchors perforating or not perforating the glenoid medial cortex. Percentage of force to fracture an intact specimen was used to identify relative risk of fracture: low risk >75%, moderate risk 75%-50%, and high risk <50% of intact load.

RESULTS: The load to fracture intact glenoids was 1,276 ± 42 N. Loads decreased linearly as the number of holes drilled on the glenoid rim increased. Compared with the 1.6-mm group, the 3.0-mm group had significantly lower glenoid rim strength in specimens with 4, 5, and 7 holes (p=0.013, 0.032, and 0.045, respectively). All-suture anchors in 1.6-mm holes did not alter the glenoid rim strength and up to 5 anchors were associated with low risk of fracture. Load to fracture was significantly higher with 3.0-mm rigid core bio-absorbable anchors with 4 anchors (1081±6 N) compared to the 4-hole condition (838±107 N; P=0.033). Perforating the glenoid medial cortex with 5 1.6-mm anchors significantly weakened the glenoid rim to 58% of intact (P=0.012). Perforating the medial cortex weakened the glenoid rim to 52% and 42% (P<0.001 for both) of intact in the 3.0-mm 4-anchor and 5-anchor constructions, constituting moderate and high risk of fracture, respectively.

CONCLUSION: Up to 5 1.6-mm all-suture anchors and 4 3.0-mm bio-absorbable rigid-core anchors were associated with low risk of fracture of the glenoid rim. Smaller diameter all-suture anchors best preserved structural integrity of the glenoid rim, while tunnel enlargement and perforation of the glenoid medial cortex were associated with moderate or high risk of postage stamp fracture.