Surgical increase in thoracic kyphosis increases unfused lumbar lordosis in selective fusion for thoracic adolescent idiopathic scoliosis.

PURPOSE: In adolescent idiopathic scoliosis (AIS), there is a close relationship between thoracic kyphosis (TK) and proximal lumbar lordosis (PLL). The hypothesis states correction of hypokyphosis increases lumbar lordosis (LL) through increase in PLL after surgical correction of TK.

METHODS: 111 consecutive thoracic AIS, Lenke 1 or 2 who underwent posterior selective thoracic fusion with reduction by simultaneous translation on 2 rods and 2 years follow-up have been prospectively selected and analyzed. Instrumentations below L1 and anterior releases were excluded. Global TK and LL were measured by a dedicated software. Mean values were compared through T test, correlations assessed through Pearson's coefficient.

RESULTS: Global TK increased from 27° to 46° at the last follow-up (p < 0.0001) and LL from 58° to 65° (p < 0.0001). PLL increased by 8° (15°-23°), and distal lumbar lordosis remained stable (42°). The gains were higher for the Hypo-Kyphosis group than for the Normo-Kyphosis group (p < 0.001). There was a strong correlation (coef = 0.65) between TK and PLL as well as between the gain of TK and the gain of PLL (coef = 0.70). LL increased after the first postoperative month. At 1 month, there was a significant increase in pelvic tilt and decrease in sacral slope, offsetting the LL increase, and indicating a temporary pelvic retroversion.

CONCLUSIONS: Increase in TK led to increase in uninstrumented LL through increase in PLL with a continuous correlation between TK and PLL. These results allow surgeons to calculate the TK required during surgical correction of thoracic AIS to adapt LL to pelvic incidence. These slides can be retrieved under Electronic Supplementary Material.