Biomechanical comparison of parallel versus convergent screw fixation in Zadek osteotomy

Zadek osteotomy is an increasingly utilized treatment for insertional Achilles tendinopathy (IAT), although an optimal fixation strategy remains unclear. This cadaveric study compared biomechanical outcomes of parallel and convergent screw fixation constructs following Zadek osteotomy. We hypothesized that convergent screw fixation would demonstrate greater stability.

Five paired fresh-frozen human calcanei (10 specimens) underwent simulated Zadek osteotomy, receiving paired parallel and convergent screw fixations on contralateral limbs. Specimens were tested using a servohydraulic system and motion tracking. Axial loading was applied until construct failure. Primary biomechanical outcomes included construct stiffness, load at 2 mm displacement, ultimate load to failure, and displacement at failure. Comparisons were performed using paired t-tests. One specimen was excluded from final analysis due to premature failure.

No significant differences were observed between parallel and convergent constructs for any biomechanical outcome. Mean stiffness was 1415 ± 1337 N/mm for parallel constructs and 1360 ± 952 N/mm for convergent constructs (p = 0.96). Load at 2 mm displacement measured 707 ± 289 N and 892 ± 401 N, respectively (p = 0.54). Ultimate load to failure was 915 ± 266 N and 1065 ± 401 N, respectively (p = 0.55). Displacement at failure was similar between groups (p = 0.45). The combination shear-compression sliding at the osteotomy interface was the predominant mechanism of failure in both groups.

Parallel and convergent screw constructs demonstrated comparable initial fixation strengths following Zadek osteotomy, suggesting screw trajectory may have limited impact on construct stability. Further studies are needed to validate these findings clinically.