Ankylosing spondylitis (AS) is a chronic inflammatory disease, generally involving the sacroiliac joints and spinal column and advancing from caudal to cranial vertebrae [1, 2]. A fixed thoracolumbar kyphosis is the most common deformity that causes difficulty standing, walking, looking horizontally, and lying flat on one’s back. In severe cases, visceral compression may also cause intra-abdominal complications or impair respiratory function . Surgical correction of the kyphosis is necessary for many AS kyphosis patients. Recently, pedicle subtraction osteotomy (PSO) or vertevertebral column decancellation (VCD) has been applied to correct thoracolumbar kyphosis secondary to AS [4–7]. Selection of the upper instrumented vertebra (UIV) and lowest instrumented vertebra (LIV) determines the proximal and distal fusion level. In other patients with spinal deformity, reducing fusion level can preserve more spinal motion [8, 9]. However, in AS patients whose spine is fixed, reduction of fusion level still contributes to the operation and could be economical. We conducted studies to investigate the optimal selection of LIV and UIV relative to the osteotomied vertebra (OV) for common kyphosis caused by AS.
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