Relationship between the angle of the opened lamina and the increase of sagittal canal diameter following double-door cervical laminoplasty

English Keywords:Cervical spine  Laminoplasty  Laminoplasty opening angle  Spinal canal sagittal diameter

English Abstract:

  【Abstract】 Objectives: To clarify the relationship between the angle of the opened lamina and increase of sagittal canal diameter in double-door laminoplasty. Methods: The formula, d=h×(sinβ/sinα-1), describing the relationship between the angle of the opened lamina and increase of sagittal diameter was deduced by using trigonometry, α indicated preoperative lamina angle, β indicated the angle of the opened lamina, h indicated vertical distance which was from the split points before surgery to the wired of bilateral hinge gutters, d indicated the increase in sagittal diameter after double-door cervical laminoplasty, which was defined as the difference between post-surgical diameter and pre-surgical diameter. 32 patients with multilevel cervical spondylotic myelopathy underwent double-door cervical laminoplasty(C3-C7 in 20 patients and C3-C6 in 12 patients) in our institution between September 2010 and April 2014. At pre-operative and 1 week post-operative, axial CT was made at each pedicle level from C3 to C7 to measure the values of α, β, h by using software(picture archiving and communication system, PACS). The actual value of d was obtained by measuring the pre-and post-operative C3-C7 sagittal canal diameter by using PACS software. The computed value of d of C3-C7 in each patient was processing the formula d=h×(sinβ/sinα-1). The differences between the data obtained by actual measurement and the data computed by the formula were compared by the paired t test. The correlation between the data obtained by actual measurement and computed by the formula was assessed by Pearson′s correlation coefficient. The accuracy of the formula was assessed. Results: The computed value of d at C3-C7 was 5.15±0.47mm, 5.39±0.47mm, 5.22±0.37mm, 5.25±0.25mm, 4.35±0.35mm respectively; the actual measured value of d at C3-C7 using PACS measurement software was 5.17±0.40mm, 5.43±0.52mm, 5.27±0.44mm, 5.29±0.28mm, 4.38±0.33mm respectively. The data obtained by actual measurement and the data evaluated by the paired t test showed the value of t computed by the formula at C3-C7 was 0.42, 0.68, 1.58, 1.38, 1.11 respectively, which showed no significant difference(P>0.05). The value of r by Pearson correlation analysis computed by the formula at C3-C7 was 0.8953, 0.8155, 0.9159, 0.7821, 0.9238 respectively, which showed a very high degree of correlation(P<0.001). Conclusions: The formula accurately reveals the correlation between the angle of the opened lamina and the increase in sagittal canal diameter in double-door laminoplasty. Based on the angle of the opened lamina, increases in sagittal canal diameter following laminoplasty can be computed by using this formula.