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 afte r surgical correction were analyzed and string test measurements made by th ree observers in 55 surgical cases (13 surgical types). Objectives. The purpose of the study was to assess the effect of various co rrective maneuvers for spinal deformity on the spinal canal length. Summary of Background Data. When perioperative neurologic deficit occurs, t he surgeon removes implants because they are displaced into the spinal cana l or the canal has been lengthened. It is important to know the effect cert ain 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 a nd posterior vertebral body lines were measured. The adjusted coronal line was the assumed path of the spinal cord starting at the midportion of the v ertebral body at the top and the bottom of the deformity and then in betwee n, hugging the pedicles as closely as possible while staying inside the ped icles. Adjustments for magnification were made. Results. Anterior compression instrumentation without cages (n = 5) consist ently shortened the spinal canal (mean delta -6.67 +/- 2.30 mm, P = 0.003), whereas instrumentation with cages (n = 13) lengthened the canal (mean del ta 10.54 +/- 7.58 mm, P = 0.0003). Thoracic curves treated by posterior cor rective forces (n = 14) demonstrated lengthening of the canal (mean delta 1 0.14 +/- 5.23 mm, P = 0.0001), large (n = 5) curves (81-140 degrees, mean d elta 13.47 +/- 7.05 mm), and medium (n = 7) curves (50-80 degrees, mean del ta 8.43 +/- 3.24 mm). Conclusions. Many deformity correction maneuvers, although they do not dire ctly include application of posterior or anterior distraction forces, do in directly lengthen the spinal canal.