Comparison of morphology between patients with and without developmental spinal canal stenosis to inform the feasibility of C1 lateral mass screw insertion in the atlas.

BACKGROUND

C1 lateral mass screws are increasingly being used as an effective anchoring system. Insufficient posterior arch height increases difficulty of inserting the screws using the Tan method. The general aim of our retrospective cross-sectional study was to clarify the atlas morphology of patients with developmental spinal canal stenosis to evaluate the usefulness of the Tan method.

METHODS

A computed tomography myelogram was performed in 85 Japanese patients presenting with spinal disorders. The following morphological measures at the level of the atlas were obtained: anterior-posterior length (APL), medial-lateral width (MLW), lateral mass transverse diameter (LMTD), dural sac area (DSA), spinal cord area (SCA), lateral mass height (LMH), lateral mass width (LMW), lateral mass length (LML), posterior arch length (PAL), posterior lamina height (PLH), and posterior vertebral groove height (PVGH). Patients were classified into two groups: patients with developmental spinal canal stenosis (DCS), identified by a longitudinal diameter of the spinal canal <12 mm and patients without developmental spinal canal stenosis (NDCS), identified by a longitudinal diameter of the spinal canal ≥12 mm. Measures obtained on the computed tomography images were compared between the two groups.

RESULTS

Among the 85 patients in our study group, developmental spinal stenosis was identified in 47 patients. The APL, DSA, PAL and PVGH measurements were significantly smaller among the 47 patients in the DCS group, compared to the 38 patients in the NDCS group (p < 0.01). The mean height of the PVGH was <4.0 mm in 63.5% of the total study group.

CONCLUSIONS

The normally inserted C1 lateral mass screws have a diameter of 3.5-4.0 mm, and hence, the Tan method require a posterior arch height that exceeds this value. Our study provides evidence of a narrower posterior arch in Japanese individuals with developmental spinal canal stenosis. Therefore, the safe use of the Tan method to insert C1 lateral mass screws may be limited in Japanese individuals with developmental spinal canal stenosis.