Preventing Cerclage Failure: How Many Twists Are Needed to Avoid Untwisting and Maximize Fixation Strength

Background Cerclage wires are widely used for fixation of long-bone spiral and peri-implant fractures. However, the optimal number of twists to maximize biomechanical stability is not defined and is largely based on surgical experience. This study investigated the influence of twist number and wire diameter on cerclage stability. Aims To determine how twist number and wire diameter affect failure mode, stiffness, and load-bearing capacity of cerclage wires, and to identify the minimum twist number required to prevent untwisting. Methods A total of 120 stainless-steel cerclage constructs were produced using 1.0-mm, 1.25-mm, and 1.5-mm wires with 4, 6, 8, or 10 twists. Twist formation was standardized with controlled axial load of torque, followed by uniform trimming. Static tensile testing assessed stiffness, load-to-yield, elongation, and load-to-failure. Hand-twisted 1.5-mm constructs underwent cyclic loading at 700 N until elongation of 2, 3, or 5 mm or catastrophic failure. Results Across all diameters, low twist counts failed predominantly by untwisting, while higher twist counts shifted failure toward wire breakage. In 1.0-mm wires, breakage occurred only at 10twists (50%), whereas all lower twist counts failed by untwisting (p=0.040). In 1.25-mm wires, breakage appeared only at 8 and 10twists (p>0.9). Stiffness increased with twist number: 1.0-mm wires from 34.1±2.4 N/mm (4twists) to 41.5±5.0 N/mm (10 twists); 1.25-mm from 44.0±5.5 to 50.5±5.4 N/mm; and 1.5-mm peaked at 8 twists (92.8±17.0 N/mm) before decreasing at 10 twists (82.3±11.8 N/mm). Load-to-failure increased with diameter and twist number, reaching 660.7±139.6 N (1.0-mm, 10 twists), 858.0±220.0 N (1.25-mm, 8twists), and 1873.7±80.6 N (1.5mm, 10twists). In cyclic testing, six-twist 1.5-mm constructs showed the greatest durability, requiring the most cycles to reach 2,3,and 5mm elongation. Conclusion Increasing twist number consistently shifts failure from untwisting to wire breakage. Six to eight twists provide optimal biomechanical performance, with six twists representing the minimum required to prevent untwisting across all clinically used wire diameters.