• Bang-Ping Qian, Jun Jiang, Yong Qiu, Bin Wang, Yang Yu, and Ze-Zhang Zhu.
• Department of Spine Surgery, the Affiliated Drum Tower Hospital of Nanjing University Medical School, Zhongshan Road 321, Nanjing 210008, China. E-mail address for B.-p. Qian: qianbangping@163.com. E-mail address for J. Jiang: spine821107@gmail.com. E-mail address for Y. Qiu: scoliosis2002@sina.com. E-mail address for B. Wang: wbin113@126.com. E-mail address for Y. Yu: yuyangdr@126.com. E-mail address for Z.-z. Zhu: zhuzezhang@126.com.
• J Bone Joint Surg Am. 2014 Nov 19;96(22):e188.

BackgroundPelvic retroversion is one of the mechanisms for regulating sagittal balance in patients with a kyphotic deformity. This retroversion is limited by hip extension, which prevents the pelvis from becoming excessively retroverted, achieving a sacral slope of <0°. However, a negative sacral slope can be found in some patients with ankylosing spondylitis with thoracolumbar kyphosis. The purpose of this study was to analyze this finding.MethodsWe performed a retrospective review of 106 consecutive Chinese Han patients with ankylosing spondylitis with thoracolumbar kyphosis treated at our center from October 2005 to October 2012. Forty-one patients in whom the upper third of the femur was clearly visualized on lateral radiographs were analyzed. Seventeen had a sacral slope of <0° (group A) and twenty-four had a sacral slope of ≥0° (group B). Eight sagittal parameters were measured and compared between the two groups. Correlations among sacral slope, the femoral obliquity angle, and the other sagittal parameters were analyzed.ResultsMean global kyphosis, lumbar lordosis, pelvic tilt, the sagittal vertical axis, and the femoral obliquity angle were significantly larger in group A than in group B, whereas mean pelvic incidence and sacral slope were significantly smaller in group A (p < 0.05 for all). Global kyphosis, lumbar lordosis, pelvic tilt, and the sagittal vertical axis were significantly negatively associated with sacral slope but positively associated with the femoral obliquity angle, whereas pelvic incidence was significantly positively associated with sacral slope but negatively associated with the femoral obliquity angle (p < 0.05 for all). The femoral obliquity angle was significantly negatively associated with sacral slope (p < 0.05).ConclusionsNegative sacral slope does exist in Chinese Han patients with ankylosing spondylitis with thoracolumbar kyphosis. This appears to be caused by severe kyphosis, an initially small sacral slope, and pronounced tilting of the femoral shaft as a result of knee flexion, resulting in the pelvis becoming further retroverted.Copyright © 2014 by The Journal of Bone and Joint Surgery, Incorporated.

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