Does the killer turn angle in biceps tenodesis lead to a guillotine effect? A biomechanical study

The purpose of this study was to investigate whether a killer turn angle in biceps tenodesis creates a guillotine-like effect under physiological loading conditions and to evaluate its influence on tendon elongation and tunnel widening.

30 fresh ovine humeri underwent biceps tenodesis using a bicortical endobutton technique. Specimens were divided into three groups according to the bone-tendon angle at the tunnel entrance (45°, 60°, and 90°). Each construct was subjected to 500 cyclic loading cycles between 0 and 150 N. Tendon elongation and tunnel widening were measured and compared among groups.

No tendon rupture or construct failure was observed in any group. Tendon elongation and residual creep were significantly greater in the 90° group compared with the 45° and 60° groups (p = 0.001), whereas no significant difference was found between the 45° and 60° angles (p = 0.247). Tunnel widening did not differ significantly among groups, although a trend toward increased enlargement with higher angles was noted.

Sharp bone-tendon angulation increased tendon elongation during cyclic loading, suggesting the presence of a guillotine-like mechanical effect in biceps tenodesis. Reducing the killer turn angle to 60° or less appeared sufficient to mitigate this effect. Although no acute failure was observed, these findings highlight the potential relevance of the guillotine effect in shoulder surgery and underscore the importance of considering the killer turn angle, particularly in tendon transfer procedures with higher reported failure rates.

Level V, biomechanical study.