Pull-out strength of Caspar cervical screws

Area of Science:

• Orthopedic Surgery
• Biomechanical Engineering
• Spinal Fusion Techniques

Background:

• Anterior cervical instrumentation is crucial for bone fusion in cervical spine surgery.
• The Caspar system is commonly used, with posterior vertebral cortex purchase often recommended.
• Limited biomechanical data exist to support this recommendation.

Purpose of the Study:

• To biomechanically evaluate the necessity of posterior vertebral cortex purchase for Caspar screw fixation.
• To compare the pull-out strength of Caspar screws with and without posterior cortical penetration.

Main Methods:

• Caspar screws were inserted into 43 human cadaveric cervical vertebral bodies.
• Screws were placed either to the posterior cortex or penetrating it by 2 mm.
• Pull-out tests were performed at a rate of 0.25 mm/s to measure force and deformation.

Main Results:

• Mean pull-out load was 375 N without posterior cortical penetration and 411 N with penetration (nonsignificant difference).
• Mean deformation to failure was 1.41 mm without penetration and 1.56 mm with penetration (nonsignificant difference).
• Failure typically occurred via clean pull-out, with cancellous bone attached to screw threads.

Conclusions:

• Posterior cortical penetration does not significantly enhance Caspar screw pull-out strength in an isolated vertebral body model.
• Further biomechanical studies are warranted before altering current Caspar screw insertion techniques.
• The findings question the necessity of strict posterior cortical purchase for Caspar screw stability.