Biomechanical comparison of four anterior atlantoaxial plate systems

The optimum fixation method to achieve atlantoaxial fusion after resection of the odontoid process remains a matter of discussion. Anterior atlantoaxial plate fixation has been described by Harms as a fixation procedure to be performed after transoral odontoid resection. In recent biomechanical and clinical studies investigators have shown that this procedure is a good alternative to established posterior atlantoaxial fixation techniques, but they have also indicated the biomechanical disadvantages of the Harms plate design. Therefore, three new anterior atlantoaxial plate designs were developed. The purpose of this study was to compare these three newly designed plate systems biomechanically with that used in Harms anterior atlantoaxial plate fixation.

Twenty-four human craniocervical cadaveric specimens were tested in flexion, extension, axial rotation, and lateral bending in a nonconstrained testing apparatus by using a nondestructive stiffness method. Three-dimensional displacement of C 1-2 was measured with an optical measurement system. Six different groups were examined: 1) control (24 specimens); 2) unstable (after odontoidectomy and dissection of the atlantoaxial ligaments; 24 specimens); 3) Harms (anterior atlantoaxial plate fixation according to Harms; six specimens); 4) subarticular atlantoaxial plate (SAAP; six specimens); 5) transpedicular atlantoaxial plate (TAAP; six specimens); and 6) subarticular atlantoaxial locking plate (SAALP; six specimens). Stiffness, range of motion, and neutral and elastic zones were determined. Compared with the Harms plate, stiffness was significantly higher when methods for placing the SAAP, TAAP, and SAALP devices were used (p < 0.05). Angular displacement of SAALPs was less than that demonstrated in any other group (p < 0.05). Stiffness values in any direction were significantly greater for the SAALP-fixed specimens than for the TAAP, SAAP, Harms, control, or unstable specimens (p < 0.05).

Experimentally, the SAAP, TAAP, and Harms plate achieved less stable fixation than the SAALP. Therefore, if transoral odontoid resection is performed, SAALP-fixed spines will provide significantly improved stability compared with previous fixation devices and methods. This may be a necessary prerequisite for a fast and uneventful osseous fusion even without additional posterior stabilization.