Change in Cobb angle of each segment of the major curve after posterior vertebral column resection (PVCR): a preliminary discussion of correction mechanisms of PVCR

Posterior vertebral column resection (PVCR) is an effective technique for treating severe rigid spinal deformities, and no other osteotomy is capable for such an excellent corrective effects. The purpose of this study was to discuss the correction mechanisms of PVCR.

Forty-six patients with severe rigid spinal deformities undergoing PVCR were retrospectively analyzed. According to a routine posteroanterior supine entire spine radiograph performed before and after surgery, the major curve at coronal plane was divided into three segments factitiously: upper segment (from the superior endplate of the upper vertebra of the major curve to the inferior endplate of the upper vertebra adjacent to the resected vertebra), middle segment (from the inferior endplate of the upper vertebra adjacent to the resected vertebra to the superior endplate of the lower vertebra of the resected vertebra), and lower segment (from the superior endplate of the lower vertebra of the resected vertebra to the inferior endplate of the lower end vertebra of the major curve). Cobb method was used to measure the curvature of the major curve and each segment. We analyzed the changes of the Cobb angle in the major curve and each segment. We also analyzed the correlation between the placement of pedicle screws and deformity correction.

The Cobb angle of the major curve decreased from 110.1 ± 18.1° to 51.0 ± 17.3° (p < 0.05) after surgery (decreased by 59.1 ± 16.4°), the mean correction rate was 54.1 ± 12.2% (p < 0.05). The Cobb angle of the middle segment decreased by 28.1 ± 14.7° (p < 0.05), the contribution rate was 49.1 ± 27.3%. The upper and lower segments decreased by 15.7 ± 13.1° and 15.3 ± 12.4°, respectively (p < 0.05). There were no significant differences in the contribution rate between upper and lower segments (25.2 ± 16.6% vs. 26.3 ± 22.6%) (p > 0.05). 22 patients were instrumented with at least one pedicle screw in the adjacent upper and lower vertebras of the resected vertebra and gained a better corrective effect in comparison with the others (p < 0.05). The data also indicated that deformity correction was closely related to the numbers of the pedicle screws (r = 0.82, p < 0.05).

In conclusion, the middle segment offered the highest contribution rate to the deformity correction of the major curve, but at the same time the spinal cord was angulated in this segment. So, it is dangerous to gain too much deformity correction in the middle segment. Because spine would shorten and the tension in spinal cord would decrease after vertebral column resection, a better correction effect could be gained in upper and lower segments at a low risk of spinal cord injury. But it was actually too hard for such rigid spinal deformity. It could gain a better corrective effect and stability by placing more pedicle screws at major curve, especially at the upper and lower vertebras adjacent to the resected vertebra, but sometimes it was difficult to place enough pedicle screws in severe rigid spinal deformities.