title: Head-locking durability of fixed and variable angle locking screws under repetitive loading.
abstract: Polyaxial locking screws are increasingly applied in fracture fixation. To investigate the durability of the head-locking mechanism, the removal torque of variable angle (VA) and fixed angle (FA) stainless steel and titanium locking screws was investigated without and after a cyclic loading test. Stainless steel (St) and titanium (Ti) 2.4 mm orthogonally inserted FA screws and 2.4 mm VA screws inserted in different inclinations (0°-15°) (n = 6 per group) were locked at 0.8 Nm. Removal torque was determined without (W) and after (A) cyclic loading (sinusoidal load, 5 Hz, constant amplitude of 25 N, up to 10'000 cycles, or failure). Significant differences in-between the groups were detected by Student's t-test (p < 0.05). Except VA Ti in 0deg and FA, all groups exhibited a drop in removal torque below the insertion torque without and after cyclic testing. The removal torque was (St: FA W:0.81 Nm ± 0.04 A:0.72Nm ± 0.04; VA0deg W:0.73 Nm ± 0.04 A:0.65 Nm ± 0.05; VA15deg W:0.51 Nm ± 0.05 A:0.50 Nm ± 0.08; Ti: FA W:0.82 Nm ± 0.03 A:0.70 Nm ± 0.04; VA0deg W:0.80 Nm ± 0.02 A:0.72 Nm ± 0.05; VA15deg W:0.55 Nm ± 0.03 A:0.54 Nm ± 0.06). In all groups, the removal torque after cyclic testing did not drop below 16% of the removal torque without cyclic testing. No head loosening was observed after cyclic testing. Stainless steel and titanium 2.4 mm fixed and variable angle locking screws provide a stable and lasting head-locking mechanism. © 2015 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:949-952, 2016.
authors.affiliation: AO Research Institute Davos, Clavadelerstrasse 8, CH-7270 Davos, Switzerland.|Department of Trauma, Hand and Reconstructive Surgery, University Hospital Jena, Germany.
authors.forename: Mark
authors.initials: M
authors.lastname: Lenz
authors.affiliation: AO Research Institute Davos, Clavadelerstrasse 8, CH-7270 Davos, Switzerland.
authors.forename: Dieter
authors.initials: D
authors.lastname: Wahl
authors.affiliation: AO Research Institute Davos, Clavadelerstrasse 8, CH-7270 Davos, Switzerland.
authors.forename: Ivan
authors.initials: I
authors.lastname: Zderic
authors.affiliation: AO Research Institute Davos, Clavadelerstrasse 8, CH-7270 Davos, Switzerland.
authors.forename: Boyko
authors.initials: B
authors.lastname: Gueorguiev
authors.affiliation: Department of Orthopaedic Surgery-Hand and Upper Extremity Service, Massachusetts General Hospital, Boston 02114, Massachusetts.
authors.forename: Jesse B
authors.initials: JB
authors.lastname: Jupiter
authors.affiliation: AO Research Institute Davos, Clavadelerstrasse 8, CH-7270 Davos, Switzerland.|Institute of Health and Biomedical Innovation, Queensland University of Technology, Brisbane, Australia.
authors.forename: Stephan M
authors.initials: SM
authors.lastname: Perren
mesh_terms: D001863:Bone Screws; D005593:Fracture Fixation, Internal / Q000295:instrumentation*; D016474:Weight-Bearing
keywords: fixed angle locking; locking durability; polyaxial locking; screw removal torque; variable angle locking