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.