The relationship between laminoplasty opening angle and increased sagittal canal diameter and the prediction of spinal canal expansion following double-door cervical laminoplasty.

Purpose: To clarify the relationship between laminoplasty opening angle (LOA) and the increase in sagittal canal diameter (SCD) in double-door cervical laminoplasty (DDCL) and to predict the increase in SCD using the resulting formula.

Methods: We analyzed 20 patients with multilevel cervical spondylotic myelopathy who underwent DDCL between September 2010 and January 2013. The pre- and post-operative parameters of the cervical spinal canal were measured by computed tomography. We deduced a formula describing the relationship between LOA and the increase in SCD and used it to predict the increase in SCD of these patients as LOA increased.

Results: When the C3-C7 LOA was 25°-45°, the magnitude of the increase in SCD was notable (increases of 3.08-5.6 mm compared with the pre-operative SCD). When the C3-C7 LOA was more than 45°, the magnitude of the increase in SCD was relatively smaller; the increase in C3-C7 SCD with a 55° LOA was merely 0.4 mm more than with a 45° LOA. When LOA was 30° at C3-C6 or 40° at C7, the increase in SCD was more than 4 mm. When the C3-C6 LOA was 40°, SCD increased by more than 5 mm.

Conclusions: The formula accurately showed the relationship between LOA and the increase in SCD in DDCL. Based on the LOA, increases in SCD following C3-C7 laminoplasty can be accurately predicted using this formula. This enables DDCL based on accurate individual LOAs, which prevents inadequate or excessive opening.