Concept of variable angle locking--evolution and mechanical evaluation of a recent technology.

Area of Science:

• Orthopedic biomechanics
• Surgical implant technology
• Materials science in medical devices

Background:

• Variable angle locking screws offer increased flexibility for fracture fixation.
• The locking strength of these screws at various insertion angles needs evaluation compared to traditional fixed angle designs.

Purpose of the Study:

• To compare the locking strength and failure modes of variable angle locking screws against fixed angle locking screws at different insertion inclinations.
• To assess the biomechanical performance of stainless steel and titanium locking screws.

Main Methods:

• Variable angle and fixed angle 2.4 mm locking screws (stainless steel and titanium) were inserted at 0°-15° inclinations.
• Screws were locked at 0.8 Nm and subjected to load-to-failure testing to determine ultimate failure moment and failure mode.

Main Results:

• Variable angle locking screws at 2°-10° inclinations showed failure moments comparable to 0° insertion, failing at the screw neck.
• At 15° inclination, variable angle screws had reduced failure moments and failed by head thread breakout.
• Fixed angle screws inserted at >2° inclination failed to lock properly, exhibiting insufficient strength.

Conclusions:

• Variable angle locking screws provide a robust locking mechanism across various inclinations, matching the strength of orthogonally inserted fixed angle screws.
• Care should be exercised when using variable angle locking screws at inclinations greater than 15° due to potential compromises in locking strength.