Biomechanical comparison of a novel C1 posterior locking plate with the harms technique in a C1-C2 fixation model.

Study Design: A biomechanical testing protocol was used to study atlantoaxial fixation techniques in a human cadaveric model.

Objective: To compare the in vitro biomechanics of locking plate fixation of the posterior arch of C1 to C2 laminar screw fixation, with that of conventional C1 lateral mass to C2 pars screw fixation.

Summary Of Background Data: Current methods of atlantoaxial fixation pose a risk to neurologic and vascular structures. A novel posterior locking plate for C1 was designed, that when rigidly linked to C2 translaminar screws may offer alternative C1-C2 fixation with greatly decreased surgical risk. No comparative in vitro biomechanical testing has been previously done to evaluate the feasibility of this method.

Methods: Cadaveric and CT assessments of the thickness of the C1 ring were performed. Seven spines (C0-C4) were evaluated in flexion-extension, left-right bending, and left-right axial rotation in a cadaveric C1-C2 fixation model. Three conditions were evaluated: (1) intact spine, and after odontoidectomy, (2) C1 plate to C2 laminar screw fixation, (3) C1 lateral mass to C2 pars screw fixation. Flexibility and motion data were compared using a 1-way RM analysis of variance and Student-Newman-Kuels tests.

Results: Anatomic data indicated that 6 mm of screw purchase was viable for C1 plate fixation. Both the Harms and C1-plated conditions significantly reduced global flexibility in flexion-extension and left-right axial rotation. Motion at the C1-C2 level was significantly reduced for all loading modes for both instrumented conditions with the exception of the C1 plate in right bending. No significant differences occurred between the 2 fixation methods.

Conclusion: A novel C1 posterior locking plate was designed and tested in a C1-C2 fixation model. The C1 locking plate technique functioned in an equivalent manner to the existing Harms technique. The C1 plate may be a viable alternative that is technically less demanding with decreased surgical risk.