Development and initial validation of a novel thread design for nonlocking cancellous screws

Abstract

High failure rates have been associated with nonlocking cancellous screws with a typical buttress thread in patients with osteoporotic bone. This study aimed to develop a novel thread design and compare its fixation stability with that of a typical buttress thread. Nonlocking cancellous screws with a novel thread design (proximal flank angle of 120 degrees, a flat crest feature, a tip‐facing undercut feature) and nonlocking cancellous screws with a typical buttress thread were manufactured using stainless steel. Fixation stabilities were evaluated individually by the axial pullout and lateral migration tests, and they were evaluated in pairs together with a dynamic compression plate in an osteoporotic bone substitute (10 PCF polyurethane foam per ASTM F1839) under cyclic craniocaudal and torsional loadings. Pullout strength and lateral migration resistance for the individual screw test and the force, torque, and number of cycles required to achieve specific displacement and torsion for the multi‐screw test were comparatively analyzed between both screw types. A finite element analysis model was constructed to analyze the stress distributions in the bone tissue adjacent to the threads. The biomechanical test revealed the novel undercut thread had superior axial pullout strength, lateral migration resistance, and superior fixation stability when applied to a dynamic compression plate under cyclic craniocaudal loading and torsional loading than those in the typical buttress thread. The finite element analysis simulation revealed that the novel thread can distribute stress more evenly without high‐stress concentration at the adjacent bone tissue when compared to that of a typical buttress thread.

Biomechanical comparison of the undercut thread design versus conventional buttress thread for the lag screw of the dynamic hip screw system

“…The buttress thread design screw can produce microfractures at the bone-screw interface, as it is more likely to stripping ( Stahel et al, 2017 ). Previous studies have validated that the undercut thread can effectively reduce stress concentration and stress shielding and that it possesses superior fixation stability in both lateral and torsional loading ( Feng et al, 2022a ; Feng et al, 2022b ). In the present study, the undercut thread exhibited higher resistance to migration, compared to the buttress thread, in the biomechanical test simulated physiological load of the hip joint.…”

“…The material properties of the cancellous bone, cortical bone, and screw are summarized in Table 1 ( Benli et al, 2008 ; Saba, 2012 ; Lee et al, 2016 ). The material properties were all defined as homogeneous, isotropic and linear elastic ( Feng et al, 2022a ). The cancellous bone and screw were simulated as osteoporotic cancellous bone and stainless steel, respectively ( Benli et al, 2008 ; Oftadeh et al, 2015 ; Lee et al, 2016 ).…”

“…The major complication of DHS is screw cutout, usually due to lateral migration of the lag screw relative to the femoral head ( Taheriazam and Saeidinia, 2019 ). Our previous researches have demonstrated that the undercut thread design for cancellous bone screw exhibited better lateral resistance than the standard buttress cancellous bone screw ( Feng et al, 2022a ). In addition, compared with the buttress thread, the undercut thread can make the stress distribution of the bone around the thread more uniform, and effectively avoid stress shielding, thus facilitating the process of osseointegration ( Feng et al, 2022b ).…”