Selection of Lowest Instrumented Vertebra for Thoracolumbar Kyphosis in Ankylosing Spondylitis.

Retrospective study.To determine the optimal osteotomized vertebra (OV) and lowest instrumented vertebra (LIV) in ankylosing spondylitis (AS) kyphosis.Although most of AS kyphosis cases are treated by pedicle subtraction osteotomy (PSO), few studies have focused on the selection of the LIV relative to distal OV.We reviewed all AS kyphosis cases surgically treated at our institution between 2010 and 2013. Patients were divided into groups based on the relative position of LIV and distal OV: group OV+2, the LIV was the second vertebra below OV; group OV+3, the LIV was the third vertebra below OV; group OV+4, the LIV was the fourth vertebra below OV. The preoperative and 2-year postoperative radiographic parameters and clinical data of the former two groups were compared. In addition, if the LIV was S1, patients were included in group S1, and those remaining were included in group non-S1 (the LIV was L5 or above).None of the patients presented fixation failure. Groups OV+2 and OV+3 had similar magnitudes of kyphosis (P > 0.05) and sagittal vertical axis corrections (P > 0.05) at the last follow-up. There was no difference in the incidence of proximal junctional kyphosis (PJK) between groups (P > 0.05). Between groups S1 and non-S1, the incidence of PJK and the magnitudes of kyphosis and sagittal vertical axis corrections were not significantly different (P > 0.05). The lumbosacral visual analogue scale and the incidence of pressure sores in group S1 were higher than in group non-S1 (P < 0.05).When PSO is performed at the level of L2 or L3, the instrumentation can be limited to the two caudal vertebra that follow. Extending the fixation to more vertebra or to the sacrum does not appear to improve the stability of the instrumentation and the fusion rate, and it is not suitable to carry out PSO at L4.4.