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Related Experiment Video

Updated: Mar 19, 2026

Minimally Invasive Treatment for Thoracolumbar Burst Fracture Using Sagittal Alignment Screws and A Trauma Reduction Device

Minimally Invasive Treatment for Thoracolumbar Burst Fracture Using Sagittal Alignment Screws and A Trauma Reduction Device

Published on: November 8, 2024

The biomechanical cost of variable angle locking screws.

John E Tidwell¹, Evan P Roush², Cassandra L Ondeck²

¹UCSF Fresno, Department of Orthopaedics, 2823 Fresno Street, Fresno, CA 93721, USA.

|June 22, 2016

Summary

Variable angle (VA) locking plates offer flexibility but have lower rotational strength than standard systems. Using VA screws perpendicular to the plate maximizes strength, while increasing the angle reduces it.

Keywords:

Biomechanical testing Distal femur fracture Locking plates Polyaxial Variable angle

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Published on: September 19, 2018

Area of Science:

Orthopedic surgery
Biomechanical engineering
Implant design

Background:

Variable angle (VA) locking plates allow screw insertion within a "locus of vectors" to avoid anatomical obstacles.
Clinical failures of VA constructs due to screw rotation at the plate/screw interface have raised biomechanical concerns.
This study investigates the mechanical properties of a common VA locking mechanism compared to standard systems.

Purpose of the Study:

To evaluate the biomechanical performance of a variable angle (VA) locking mechanism.
To compare the rotational strength of VA locking screws at various angles against standard locking screws.
To determine the effect of screw angulation on the screw-plate interface strength.

Main Methods:

VA locking screws were inserted into distal femur fracture plates at angles from 0° to 15°.
A control group used standard locking screws and plates.
Maximum moment and moment at reference displacements at the screw/plate interface were measured.

Main Results:

VA screws locked perpendicular (0°) exhibited the highest maximum moment and reference displacement moments within the VA system.
VA screws demonstrated significantly lower moments compared to standard locking screws (p<0.001).
Screw-plate interface strength decreased linearly with increasing off-axis angle (approx. 0.4 Nm/°), with significant decreases observed at 5°, 10°, and 15° compared to 0°.

Conclusions:

Standard locking systems offer superior resistance to rotational failure compared to VA systems.
VA systems achieve maximum rotational resistance when screws are inserted perpendicular to the plate.
Increasing screw off-axis angle in VA systems significantly reduces rotational resistance, potentially impacting clinical outcomes in distal femur fractures.