Biomechanical Comparison of Conventional Technique Versus Oblique Screw Placement Under Axial and Torsional Loading—An In Vitro Study of Humerus Bone Surrogate

Abstract

The study specifically aims to investigate a novel method to increase strength of bone plate and screw system fixation in orthopedic surgery via placing screws at an angle rather than in a transverse direction as in the conventional technique. Biomechanical tests were performed in axial compression and torsion for humeral bone fixation construct using a bone plate and screws system. Fifteen humeral surrogate bone models were utilized to have 3 groups (N = 5). Humerus bone with a simulated 6 mm fracture gap at midshaft was fixated with a 4.5 mm dynamic compression plate and six bi-cortical screws. Three fixation configurations, which were different by the number of oblique screws, were employed. In the control group (C), there was none oblique screw. The single oblique (SO) group had one oblique screw at each end of the plate. The double oblique (DO) group has two oblique screws. All oblique screws were inserted at 30° angle with respect to transverse plane. All groups were tested under axial and torsional loads in a non-destructive cyclic test prior to destructive test. One-way ANOVA test was used to detect significant differences between the group means with 95% confidence interval (p ≤ 0.05). None of the sample failed the 10,000-cycle fatigue test. The axial stiffness of group SO and DO were superior to that of group C (556.4 ± 52.4, 655.3 ± 24.6, and 632.1 ± 70.2 N/mm (p = 0.05) for group C, SO, and DO respectively). The values for yield load and ultimate load were also reported, but they were not significantly different. The post-cyclic torsional stiffness of group C, SO and DO were 0.59 ± 0.01, 0.62 ± 0.02, and 0.63 ± 0.03 Nm/° (p < 0.05). In summary, the oblique screw placement increased the axial stability of the bone construct while it did not alter significantly the torsional stability. When compared between SO and DO groups, test values were relatively equivalent. Using two consecutive oblique screws was not proven to be more effective than using one oblique screw at plate end.