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˚À15˚), 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˚À10˚inclination exhibited a failure moment comparable to the 0˚position (S 1.67AE 0.04 Nm; Ti 1.67 AE 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 AE 0.06 Nm, Ti 1.58 AE 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.

Biomechanical comparison of anatomical plating systems for comminuted distal humeral fractures

“…The design of recent locked plating systems provides the opportunity for poly-axial screw placement. While this enhances the ability to capture individual fracture fragments by placing the screws accordingly, it involves the risk of earlier screw loosening [27]. The findings from this study suggest that the poly-axial mechanism has no biomechanical disadvantages in terms of construct stiffness and fatigue failure.…”

“…The findings from this study suggest that the poly-axial mechanism has no biomechanical disadvantages in terms of construct stiffness and fatigue failure. Previous studies even suggested that poly-axial locking mechanisms are more forgiving in terms of screw stability for deviation from correct screw inclination than fixed angle locking mechanisms [27].…”

Head‐locking durability of fixed and variable angle locking screws under repetitive loading

“…Despite a significant drop of the removal torque in some groups after cyclic loading with significance related to small standard deviations, over 84% of the initial removal torque of fixed, and variable angle locking screws persisted. A loosening of the locked head would implicit a nearly complete drop of the removal torque, since loosened head threads provide no torque resistance during unscrewing …”

“…For this study, a modification of a previously published test setup was used . Stainless steel and Titanium‐Aluminum‐Niobium (TiAl6Nb7) 2.4‐mm, variable angle locking screws of 30 mm length (ref.…”

“…In contrast, variable angle locking screws allow head locking in an inclined position. A previous study with destructive quasi‐static mechanical tests revealed construct failure due to overloading of fixed and variable angle locking screws in terms of bending, breakage, or breakout . Variable angle locking screws provide a good screw head purchase with an ultimate strength comparable to orthogonally inserted fixed angle locking screws .…”