Partial correction of Cobb angle prior to posterior spinal instrumentation

Background: A substantial contribution to the overall surgical correction o f Cobb angle has been observed to occur prior to securing the instrumentati on. Knowledge specific to the amount of correction prior to instrumentation is scarce in the medical literature. If significant correction is due to t he positioning and muscle stripping during exposure of the spine, questions arise about the usefulness and need for extensive rod-rotation maneuvers t o further straighten the spine. This study quantifies the extent of correct ion achieved from standing to prone, with the spine exposed before and afte r instrumentation. Materials and Methods: Eleven patients with the diagnosis of adolescent idi opathic scoliosis (AIS) and a right thoracic major curve were included in t he study. Intraoperative changes in Cobb angle were measured before and aft er instrumentation, as well as postoperatively. The patients underwent post erior spinal instrumentation by the same surgical team using the rod-rotati on techniques. Radiographs were taken prior to surgery, intraoperatively be fore and after instrumentation and postoperatively within one week from sur gery. Cobb angle measurements were performed by the same examiner. Results: The median preoperative Cobb angle of 60 degrees +/- 14 degrees (4 8-90 degrees) corrected to a median of 26 degrees +/- 22 degrees (10-80 deg rees) on the right bend film, to a median of 55 degrees +/- 12 degrees (30- 70 degrees) intraoperatively after exposure, and to a median 30 degrees +/- 10 degrees (20-46 degrees) after rod-rotation/instrumentation and fixation . The follow-up standing radiograph median Cobb angle was 40 degrees +/- 14 degrees (9-56 degrees). A median intraoperative correction of 28 degrees w as obtained, 10 degrees of which was prior to the rod rotation and instrume ntation. High variability was observed in the percentage contribution of pr e-instrumentation release with a median of 42%+/- 25% (0-67%). Conclusion: Approximately one-third of the total correction occurred prior to instrumentation being applied, and even though it was variable and subst antial, the actual surgical rod rotation and instrumentation maneuver provi ded the majority of correction.