Biomechanical Evaluation of Variable-Angle Locking Systems. A Micro-CT Analysis

Area of Science:

• Orthopedic surgery
• Biomechanical engineering
• Implantology

Background:

• Polyaxial locking screws have improved fixation for challenging bone stock and periprosthetic fractures.
• Traditional polyaxial systems require precise alignment between screw heads and plate holes.
• Ensuring construct integrity with polyaxial fixation is crucial in orthopedic surgery.

Purpose of the Study:

• To evaluate the mechanical properties of various polyaxial locking screw systems.
• To correlate screw-plate engagement parameters with mechanical performance.
• To assess the impact of insertion angle on construct strength.

Main Methods:

• Testing polyaxial systems with screws inserted at angles of 0, 5, 10, and 15 degrees.
• Micro-computed tomography (micro-CT) for quantifying average thread engagement (ATE) at the screw-plate interface.
• Cantilever bending tests to determine construct strength.
• Scanning electron microscopy for detailed interface examination.

Main Results:

• Maximum bending strength was highest with standard (0-degree) insertion across all systems.
• Off-axis insertion significantly reduced bending strength for point-loading thread-in and cut-in screws.
• Locking cap screws demonstrated consistent bending strength irrespective of insertion angle.
• Micro-CT confirmed reduced ATE for off-axis screws, with locking cap mechanisms showing the highest ATE.

Conclusions:

• The mechanical performance of polyaxial locking plates can be compromised by reduced bending strength with off-axis insertions.
• Surgeons should consider the specific characteristics of each polyaxial system when determining insertion parameters.
• Locking cap screw designs appear to offer more robust performance across varying insertion angles.