The application of a dual-lead locking screw could enhance the reduction and fixation stability of the proximal humerus fractures: a biomechanical evaluation

Abstract

Introduction: Bicortical screw fixation, which penetrates and fixes the near and far cortex of bone, has been conventionally used to achieve compressive fixation for fracture using screws. Open reduction and internal fixation using the locking plate are widely used for treating proximal humerus fractures. However, minimal contact between the bone and the locking plate can lead to an insufficient reduction. Theoretically, a dual-lead locking screw with different leads for the screw head and body could enhance the reduction and fixation stability of fragments in proximal humeral fractures without bicortical fixation, and achieve additional compression at the bone-plate-screw interface. This study assessed the insertion mechanics of the lead ratio of the dual-lead locking screw and its effect on the fixation stability of the proximal humerus fracture.

Methods: A Multi-Fix® locking plating system composed of $\varnothing$ 3.5 mm locking screws and a locking plate was used to make a locked plating for Sawbone bone blocks and fourth-generation composite humeri. Two different types of Sawbone bone blocks were used to simulate the osteoporotic (10 PCF) and normal cancellous (20 PCF) bones. The lead of the screw head thread ($L_{\mathrm{head}}$) was 0.8 mm, and that of the screw body ($L_{\mathrm{body}}$) was 0.8, 1.25, 1.6, 2.0, and 2.4 mm, whose lead ratios ($R_{\mathrm{lead}}=L_{\mathrm{body}}/L_{\mathrm{head}}$) were 1.0, 1.56, 2.0, 2.5, and 3.0, respectively.

Results: The dual-lead locking screw elevated the compression between the locking plate and the bone. The elevation in the compression due to the dual-lead thread became weaker for the cancellous bone when the lead of the screw body was more than twice that of the screw head. The plate/humerus compression with strong bone quality withstood higher dual-lead-driven compression.

Discussion: A dual-lead locking screw of $L_{\mathrm{body}}=1.25\mathrm{mm}$ ($R_{\mathrm{lead}}=1.56$) is recommended for maximum rotational stability for the locked humerus plating. The screws with over $L_{\mathrm{body}}=1.6\mathrm{mm}$ ($R_{\mathrm{lead}}=2$) have no advantage in terms of the failure torque and maximum torsional deformation. Any locking dual-lead screw with a body thread lead of <1.6 mm ($R_{\mathrm{lead}}=2$) can be used without the risk of bone crush when surgeons require additional compression to the locked cancellous bone plating.

Keywords: bone crush; compression; dual-lead; insertion torque; lead ratio; locked plating; proximal humerus; stability.