New method for measuring area of spinal canal after double-door laminoplasty.

In the double-door laminoplasty procedure used for patients with cervical myelopathy, techniques to obtain a wider spinal canal have been tested. We hypothesized that making the lateral gutters as lateral as possible may be effective for obtaining a wider spinal canal. In this study we aimed to test our hypothesis by using a new measurement method to assess the postoperative cross-sectional area of the spinal canal. Fifty-six vertebrae were operated on in 11 patients, using the STSS spacer (Asahi Optical, Tokyo, Japan). On computed tomography (CT) scans, the most posterior edge of a space-occupying lesion was defined as point A and a horizontal line through point A was drawn parallel to the posterior wall of the vertebral body (line X). Two oblique lines were drawn along the medial margin of the bilateral split laminae (lines R and L). The triangular area enclosed by lines X, R, and L was defined as the substantial area of the spinal canal (SASC). A horizontal line through the median points of the bilateral facet joint in the anteroposterior direction was defined as line B. According to the relative position of line X to line B, two types of vertebra were observed: In type 1, line X was located ventral to line B; in type 2, line X was located dorsal to line B. The mean data values for type 1 vertebra were: SASC, 133.1 mm2; ATD, 22.9 mm; angle R, 61.3 degrees; and angle L, 61.8 degrees. The mean data values for type 2 vertebra were: SASC, 99.8 mm2; ATD, 20.1 mm; angle R, 58.6 degrees; and angle L, 57.5 degrees. There were significant differences between types 1 and 2 vertebrae in values both for SASC and ATD (P < 0.01). There were no significant differences between types 1 and 2 vertebrae regarding angles R and L. The difference in SASC in the two types appeared to arise from the difference in ATD. In other words, the degree of enlargement of the spinal canal was determined not by the inclination of the bilateral split laminae, but by the transverse diameter. This result supports the validity of our hypothesis. In a double-door laminoplasty, to obtain a wider spinal canal, the lateral gutters must be made as lateral as possible (i.e., at the medial border of the facet joints, especially in type 2 vertebra). On the preoperative CT scan, the relative positions of the large-volume lesion and the facet joint must be noted.