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.