Preliminary Biomechanical Proof of Concept for a Hybrid Locking Plate/Variable Pitch Screw Construct for Anterior Fixation of Type II Odontoid Fractures

STUDY DESIGN.: A human cadaveric biomechanical proof-of-concept study. OBJECTIVE.: To test whether adding a locking plate to the anterior surface of C2 attaching directly to the interfragmentary screw may reduce potential for anterior screw cutout and improve construct strength. SUMMARY OF BACKGROUND DATA.: The most common mode of failure for screw fixation of dens fractures is via cutout at the anterior body of C2. METHODS.: A human, cadaveric model of type II dens fractures was created and fixed using either a headless, fully threaded variable pitch screw (FTVPS) or a screw with an attachable locking plate construct (LPC). Following quasistatic loading to failure, stiffness and load to failure were compared using t tests. Mode of failure was determined from radiographical and gross inspection. RESULTS.: Load to failure was greater for the LPC than for the FTVPS alone (498 N vs. 362 N, P = 0.04). The LPC consistently failed via compression of cancellous bone posterior to the lag screw, whereas the FTVPS constructs failed via cutout of the screw from the anterior C2 body. CONCLUSION.: Locking plate supplementation of anterior screw fixation of type II odontoid fractures improves construct strength and changes the failure mechanism from anterior screw cutout to posterior displacement of the screw. An attachable locking plate/interfragmentary screw construct may improve clinical outcomes for these fractures.