OBJECTIVE The aim of this paper was to analyze the incidence and risk factors of proximal junctional kyphosis (PJK) in patients with ankylosing spondylitis (AS) who underwent pedicle subtraction osteotomy. METHODS The records of 83 patients with AS and thoracolumbar kyphosis who underwent surgery at the authors' institution between 2007 and 2013 were reviewed. The patients were divided into 2 groups based on the presence or absence of PJK. The radiographic measurements, including proximal junctional angle (PJA), sagittal parameters, and pelvic parameters of these 2 groups, were compared at different time points: before surgery and 2 weeks, 12 months, and 2 years after surgery. Oswestry Disability Index scores were also evaluated. RESULTS Overall, 14.5% of patients developed PJK. Before surgery, the mean PJAs in the 2 groups were 13.6° and 8.5°, respectively (p = 0.008). There were no significant differences in age, sex, and body mass index between groups. Patients with PJK had a larger thoracolumbar kyphotic angle (50.8° ± 12.6°) and a greater sagittal vertical axis (21.7 ± 4.3 cm) preoperatively than those without PJK. The proportion of patients with PJK whose fusion extended to the sacrum was 41.2% (7/17), which is significantly greater than the proportion of patients with PJK whose lowest instrumented vertebra was above the sacrum. Oswestry Disability Index scores did not significantly increase in the PJK group compared with the non-PJK group. CONCLUSIONS The authors found that PJK occurs postoperatively in patients with AS with an incidence of 14.5%. Risk factors of PJK include larger preoperative sagittal vertical axis, PJA, and osteotomy angle. Reducing the osteotomy angle in some severe cases and extending fusion to a higher, flatter level would be also beneficial in decreasing the risk of PJK.
Keywords: AIS = adolescent idiopathic scoliosis; AS = ankylosing spondylitis; ASD = adult spinal deformity; BMI = body mass index; LIV = lowest instrumented vertebra; LL = lumbar lordosis; OA = osteotomy angle; ODI = Oswestry Disability Index; PI = pelvic incidence; PJA = proximal junctional angle; PJK = proximal junctional kyphosis; PSO = pedicle subtraction osteotomy; SVA = sagittal vertical axis; TK = thoracic kyphosis; TLK = thoracolumbar kyphosis; UIV = upper instrumented vertebra; ankylosing spondylitis; complication; deformity; proximal junctional kyphosis; risk factors.