Improving stability of atlantoaxial fusion: a biomechanical study

Abstract

Purpose: The incidence of atlanto-axial injuries is continuously increasing and often requires surgical treatment. Recently, Harati developed a new procedure combining polyaxial transarticular screws with polyaxial atlas massae lateralis screws via a rod system with promising clinical results, yet biomechanical data is lacking. This biomechanical study consequently aims to evaluate the properties of the Harati technique.

Methods: Two groups, each consisting of 7 cervical vertebral segments (C1/2), were formed and provided with a dens axis type 2 fracture according to Alonzo. One group was treated with the Harms and the other with the Harati technique. The specimen was loaded via a lever arm to simulate extension, flexion, lateral flexion and rotation. For statistical analysis, dislocation (°) was measured and compared.

Results: For extension and flexion, the Harati technique displayed a mean dislocation of 4.12° ± 2.36° and the Harms technique of 8.48° ± 1.49° (p < 0.01). For lateral flexion, the dislocation was 0.57° ± 0.30° for the Harati and 1.19° ± 0.25° for the Harms group (p < 0.01). The mean dislocation for rotation was 1.09° ± 0.48° for the Harati and 2.10° ± 0.31° for the Harms group (p < 0.01). No implant failure occurred.

Conclusion: This study found a significant increase in biomechanical stability of the Harati technique when compared to the technique by Harms et al. Consequently, this novel technique can be regarded as a promising alternative for the treatment of atlanto-axial instabilities.

Keywords: Atlantoaxial instability; Biomechanics; Cervical fracture; Cervical spondylodesis; Odontoid fracture.

Fig. 1

Visualized screw position of the osteosynthesis technique by Harati et al. (a, b) and Harms et al. (c, d)

Fig. 2

Embedded specimen from lateral (a) and posterior (b) view

Fig. 3

3D-printed casting guide to achieve a 13° casting line

Fig. 4

3D-printed supports with a central ball bearings-joint against rotational shear forces, 2 telescoping rail as lever arm, 3 sample, 4 initial load of 50 N, 5 x–y-table, 6 load cell of the testing machine

Fig. 5

Comparison of the dislocation (range of motion (ROM) °) for flexion/extension (a), lateral flexion (b) and rotation (c)