Novel motion preservation device for atlantoaxial instability

An in vitro cadaveric biomechanical study.

To evaluate the anteroposterior (A-P) stability and the flexibility of our novel motion preservation device (MPD) using cadaveric cervical spines.

The MPD intended to restrict the A-P instability of the C1-C2 complex and to preserve the axial rotation, flexion, extension, and lateral bending was designed and produced. The stability and the flexibility of the MPD was evaluated.

Ten embalmed cadaveric specimens were loaded with pure A-P translation force and the A-P translational distances were measured. Each specimen was tested for the following 4 models, respectively: Intact (control), the Dens-removed, the MPD instrumented, and a Rod fixation system instrumented. Fifteen specimens were loaded with pure moments (up to 1.5 Nm), and the C1-C2 range of motion (ROM) was measured for flexion, extension, lateral bending, and axial rotation using a stereophotogrammetry motion analysis system.

Mean A-P translational distances were 4.26 mm in Intact, 13.1 in the Dens-removed, 5.42 in the MPD, and 2.58 in the Rod fixation. The distance values with the MPD had no significant difference compared with Intact. Mean C1-C2 ROM of Intact, the MPD, and the Rod fixation at 1.5 Nm were: 14.7, 6.96, and 2.11 degrees in flexion, 6.46, 4.72, and 2.84 degrees in extension, 3.29, 4.02, and 1.01 degrees in right lateral bending, 4.92, 4.58, and 1.84 degrees in left lateral bending, 26.4, 15.4, and 1.16 degrees in right axial rotation, and 25.6, 14.3, and 1.21 degrees in left axial rotation, respectively.

The MPD restricted the A-P instability of the C1-C2 complex. In flexion, extension, and axial rotation, the C1-C2 ROM of the MPD was about 50% of the ROM in Intact, whereas equivalent to Intact in lateral bending.