String test measurement to assess the effect of spinal deformity correction on spinal canal length

Study Design. Long cassette coronal and lateral radiographs before and after surgical correction were analyzed and string test measurements made by three observers in 55 surgical cases (13 surgical types). Objectives. The purpose of the study was to assess the effect of various corrective maneuvers for spinal deformity on the spinal canal length. Summary of Background Data. When perioperative neurologic deficit occurs, the surgeon removes implants because they are displaced into the spinal canal or the canal has been lengthened. It is important to know the effect certain constructs have on canal length because the ability of the spinal cord to adapt to canal lengthening is variable. Methods. On the coronal radiographs the concave, convex, midvertebral, and adjusted midvertebral line, and on the sagittal radiographs, the anterior and posterior vertebral body lines were measured. The adjusted coronal line was the assumed path of the spinal cord starting at the midportion of the vertebral body at the top and the bottom of the deformity and then in between, hugging the pedicles as closely as possible while staying inside the pedicles. Adjustments for magnification were made. Results. Anterior compression instrumentation without cages (n = 5) consistently shortened the spinal canal (mean delta -6.67 ± 2.30 mm, P = 0.003), whereas instrumentation with cages (n = 13) lengthened the canal (mean delta 10.54 ± 7.58 mm, P = 0.0003). Thoracic curves treated by posterior corrective forces (n = 14) demonstrated lengthening of the canal (mean delta 10.14 ± 5.23 mm, P = 0.0001), large (n = 5) curves (81-140°, mean delta 13.47 ± 7.05 mm), and medium (n = 7) curves (50-80°, mean delta 8.43 ± 3.24 mm). Conclusions. Many deformity correction maneuvers, although they do not directly include application of posterior or anterior distraction forces, do indirectly lengthen the spinal canal.