Biomechanical Evaluation of a Novel Integrated C1 Laminar Hook Combined with C1–C2 Transarticular Screws for Atlantoaxial Fusion: An In Vitro Human Cadaveric Study

Objective

To evaluate the acute stability of a novel integrated C1 laminar hook (H) combined with a C1–C2 transarticular screw (TAS) with established techniques.

Methods

A novel integrated C1 laminar hook was tested. Seven human cadaveric cervical spines (C0–C3) were used. The range of motion (ROM) of C1–C2 during flexion-extension, lateral bending, and axial rotation were recorded. The specimens were tested under the following conditions: intact, destabilized (using a type II odontoid fracture model), and destabilized but with internal fixation. The following screw systems were used: bilateral C1–C2 TAS combined with the Gallie (G) technique (TAS+G), C1–C2 TAS combined with a novel integrated C1 laminar hook (TAS+H), C1 lateral mass screw and C2 pedicle screws (C2PS+C1LMS), and novel integrated C1 laminar hook combined with C2 pedicle screws (C2PS+H). The TASs were always inserted after the C2PSs. The C2PS+C1LMS and C2PS+H tests were performed alternatively, as were the TAS+G and TAS+H tests.

Results

All fixation constructs greatly improved acute atlantoaxial stability, with no significant difference among TAS+H, TAS+G, and C2PS+C1LMS (all P > 0.05). C2PS+H showed the greatest C1–C2 ROM in axial rotation, significantly different from TAS+G, C2PS+C1LMS, and TAS+H fixation models (all P < 0.05). However, there were no significant differences between C2PS+H and the other 3 models in flexion-extension and lateral bending (all P > 0.05).

Conclusions

The TAS+H technique can achieve acute stability comparable to that with the TAS+G technique for treating C1–C2 instability. The C2PS+H is a promising alternative, although it provides less stability in axial rotation than TAS+G, TAS+H, or C2PS+C1LMS.

Introduction

Atlantoaxial instability often requires solid C1–C2 segmental fixation to promote atlantoaxial fusion. Since the first description of the C1–C2 transarticular screw (TAS) by Magerl and Seeman,¹ it has been used extensively to treat atlantoaxial instability. Biomechanical tests have demonstrated its superiority over wiring techniques in terms of controlling lateral bending and axial rotation. Bilateral C1–C2 TASs combined with posterior wiring/cable bone fusion can efficiently control C1–C2 motion in all directions and is considered the gold standard for posterior C1–C2 fusion.2, 3, 4, 5

Although the use of bilateral C1–C2 TASs combined with a posterior wiring/cable technique greatly promotes bone fusion of the atlantoaxial complex, it carries a risk of spinal cord injury when passing the sublaminar wire/cable. Therefore, bilateral C1 laminar hooks have been used to replace the wire/cable in some studies.6, 7, 8, 9 TAS combined with bilateral C1 laminar hooks retained a 3-point fixation, the biomechanical stability of which is comparable to that achieved with C1–C2 TASs combined with posterior wiring/cable.¹⁰

Clinically, however, we found that the C1 laminar hook currently used was poorly matched with the C1 posterior arch, making it difficult to connect directly with the C1–C2 TAS. Therefore, we designed a novel integrated C1 laminar hook based on measurements of the C1 posterior arch on human cadaveric cervical spine specimens and computed tomography images. This hook closely matches the atlantoaxial anatomy and facilitates connection to the C1–C2 TAS. In addition, the design makes the hook's contact point on C1 more medial than that of the current C1 laminar hook, possibly improving leverage and the ability to sandwich a piece of bone effectively.

We hypothesized that TAS combined with the novel integrated C1 laminar hook could achieve the same atlantoaxial stability as is achieved with bilateral C1–C2 TASs combined with the posterior wiring/cable technique. To test our hypothesis, we assessed its biomechanical stability in human cadaveric spine and compared the results with those from in vitro internal fixation techniques in current use.