Effect of screw type and distribution on the torsional stability of 3.5 mm string of pearls locking plate constructs

To evaluate the effects of screw type (mono- [M] versus bicortical [B]), number, and position on torsional stability of String of Pearls (SOP) locking plate constructs.

In vitro mechanical study.

SOP plates (n = 32) were applied to bone models and divided into 8 groups named according to screw type (M or B) and position in each fragment relative to the fracture gap starting at the outermost plate hole. Positive and negative controls were MMM and BBB, respectively. Specimens were non-destructively tested in torsion. Compliance and angular deformation were statistically compared (P < .002).

The MMM construct was most compliant (P < .001). Compliance decreased in groups with a single bicortical screw (P < .001). Compared to the MMM group, torsional compliance decreased in constructs where a single monocortical screw was replaced with a bicortical screw (P < .001). Compared with a centrally positioned bicortical screw, constructs with a bicortical screw in either outer- or innermost position were 15% and 23% less compliant, respectively (P < .001). Addition of a second bicortical screw/fragment further decreased compliance (P < .001). No significant difference was found between groups with 2 bicortical screws. The BBB construct was least compliant (P < .001). Group responses for angular deformation followed the same pattern of significance recorded for compliance.

A minimum of 1 bicortical screw/fragment should be used to increase torsional stability of 3.5 mm SOP constructs. Positioning this screw at the inner- or outermost positions relative to the fracture is preferred, with the innermost position providing the greatest improvement in stability. Should further torsional stability be desired, increasing the number of bicortical screws is recommended. Clinically, these results may assist with preoperative planning of various fracture patterns.