title: Concept of variable angle locking--evolution and mechanical evaluation of a recent technology.
abstract: Applications for fracture-adapted screw positioning offered by variable angle locking screws are increasing. The locking strength of the variable angle locking mechanism at different insertion angles was compared to conventional fixed angle locking screws. Stainless steel (S) and titanium (Ti) variable and fixed angle 2.4 mm locking screws, inserted at different inclinations (0°-15°), and locked at 0.8 Nm were subjected to a load-to-failure test. Ultimate failure moment at the screw-head interface and failure mode of the screws were determined. Significant differences were detected by one-way ANOVA (p < 0.05). Stainless steel and titanium variable angle locking screws inserted at 2°-10° inclination exhibited a failure moment comparable to the 0° position (S 1.67± 0.04 Nm; Ti 1.67 ± 0.14 Nm) and failed predominantly at the screw neck, with the head remaining fixed to the plate. Inserted at 15° inclination, screws revealed a lower failure moment (S 1.33 ± 0.06 Nm, Ti 1.58 ± 0.05Nm), and failed predominantly by breakout of the head thread. Fixed angle locking screws inserted at an inclination >2° did not lock properly in the plate hole, providing insufficient locking strength. Variable angle locking screws offer a stable head-locking mechanism at different inclinations, comparable to the locking strength of orthogonal inserted fixed angle locking screws. Marginal inclinations >15° should be used with care.
authors: AO Research Institute Davos, Clavadelerstrasse 8, 7270, Davos, Switzerland.|Department of Trauma, Hand and Reconstructive Surgery, University Hospital Jena, Germany.
authors: Mark
authors: M
authors: Lenz
authors: AO Research Institute Davos, Clavadelerstrasse 8, 7270, Davos, Switzerland.
authors: Dieter
authors: D
authors: Wahl
authors: AO Research Institute Davos, Clavadelerstrasse 8, 7270, Davos, Switzerland.
authors: Boyko
authors: B
authors: Gueorguiev
authors: Department of Orthopaedic Surgery-Hand and Upper Extremity Service, Massachusetts General Hospital, Boston, Massachusetts, 02114.
authors: Jesse B
authors: JB
authors: Jupiter
authors: AO Research Institute Davos, Clavadelerstrasse 8, 7270, Davos, Switzerland.|Institute of Health and Biomedical Innovation, Queensland University of Technology, Brisbane, Australia.
authors: Stephan Marcel
authors: SM
authors: Perren
mesh_terms: D001863:Bone Screws; D019544:Equipment Failure Analysis
keywords: fixed angle locking; locking stability; locking strength; polyaxial locking; variable angle locking