Biomechanical evaluation of Caspar cervical screws: comparative stability under cyclical loading

Area of Science:

• Spine Biomechanics
• Orthopedic Surgery
• Materials Science

Background:

• Anterior cervical instrumentation aids bone fusion but lacks definitive biomechanical data for some techniques.
• Posterior cortical penetration is advised for the Caspar system without strong experimental evidence.
• Previous studies compared pull-out strength; this study evaluates stability under cyclical loading.

Purpose of the Study:

• To compare the biomechanical stability of unicortical versus bicortical screw penetration in anterior cervical Caspar instrumentation.
• To assess screw stability under cyclical loading simulating physiological flexion-extension.

Main Methods:

• Caspar screws were inserted into human cadaveric vertebrae using either unicortical or bicortical penetration.
• Screws were subjected to cyclical loading simulating flexion-extension for 200 cycles.
• Deformation (screw wobble) was measured over time to compare stability between groups.

Main Results:

• Statistically significant differences in deformation were observed between unicortical and bicortical groups, increasing over time.
• Bicortical purchase demonstrated enhanced stability compared to unicortical fixation, particularly with prolonged cyclical loading.
• Both groups showed significant increases in deformation over time, suggesting screw loosening at the bone interface.

Conclusions:

• Bicortical screw penetration provides superior short-term stability for Caspar system anterior cervical instrumentation.
• Potential screw loosening and interface deterioration occur with repeated flexion-extension loading, even with bicortical purchase.
• This finding raises concerns for Caspar plate use in cases of multi-column spinal instability requiring long-term fixation.