Biomechanical Evaluation of the Stabilising Function of Three Atlantoaxial Implants Under Shear Loading: A Canine Cadaveric Study

Introduction
Atlantoaxial instability leads to cranial cervical myelopathy that can induce clinical signs ranging from pain to severe neurological deficits. Considerable controversy exists in the surgical management of this condition. Final success rates for ventral procedures (85%) are comparable to dorsal procedures (89%). However, it remains difficult to assess which therapy is more adequate since, to our knowledge, no biomechanical studies analysing the effect of implant fixation on the atlantoaxial joint in dogs have been performed as yet.

The aim of the present study is to compare the biomechanical properties of a ventral trans-articular lag screw fixation technique, a new dorsal (AAI Clamp, Z Medical, Tuttlingen, Germany) and a new ventral fixation device (AAI Hook plate, ASC, Tuttlingen, Germany) under sagittal shear loading, after transection of the ligaments of the atlantoaxial joint.

Material and methods
The occipito-atlanto-axial region of 10 Beagle cadavers euthanatized for reasons unrelated to the study were used. To exclude bone abnormalities, a CT scan of each specimen was performed before testing. The atlanto-occipital joints were blocked with 2 trans-articular diverging 1.8mm positive threaded K-wires. The occipital bone and the caudal end of C2 were secured by embedding one third of each bone in polymethyl methacrylate (PMMA) and mounted in a pure shear loading testing machine. Three cycles in dorsoventral direction with a constant speed of 0.2 mm/s up to a limiting force of 50 N were performed for each implant tested. The tests were performed and the range of motion (ROM) and the neutral zone (NZ) were determined with all atlantoaxial ligaments intact, after transection of all ligaments and after fixation with each implant. The testing order of implants was randomly assigned. After cyclic loading, the last tested implants were subjected to implant failure test in flexion. The mode and the required load for implant failure were then recorded.  [...]