Risk Factors Of Proximal Junctional Kyphosis And Rod Fracture After Osteotomy In Ankylosing Spondylitis-related Kyphosis

Chapter 1 Risk factors of proximal junctional kyphosis after osteotomy in ankylosing spondylitis-related kyphosisObjective To investigate the risk factors and clinical features of proximal junctional kyphosis(PJK)after osteotomy in thoracolumbar kyphosis secondary to ankylosing spondylitis(AS).Methods A retrospective review of 133 AS patients with thoracolumbar kyphosis who underwent correction surgery between January 2002 and December 2015 with a minimum of 2-year follow-up was performed,including 118 males and 15 females.The mean age was 35.1± 9.8 years(range,18-63 years).Clinical data and radiographic parameters were assessed for the risk factors of PJK.Clinical data included age,sex,body mass index,types of osteotomy and fusion levels.Radiographic parameters were measured preoperatively,postoperatively and at each follow-up including sagittal vertical axis(SVA),thoracolumbar kyphosis(TK),lumbar lordosis(LL),sacral slope(SS),pelvic tilt(PT)and proximal junctional angle(PJA).Patients were divided into PJK group and No PJK group according to the change in PJA.Comparison of clinical and radiographic data were performed between the 2 groups using Student’s t-test and Chi-square test.Results With the average follow-up duration of 3.6 ± 2.2 years(range,2 to 15 years),PJK was observed in 11 patients.The mean time course from surgery to the development of PJK was 3.7 ± 4.1 years(range,0.3 to 15 years)with PJA increased from 5.5°± 9.7° preoperatively to 21.2°± 9.5° at the time of PJK.Compared with the No PJK group,PJK group showed lower age(P<0.01)and higher proportion of patients performed with SPO(P<0.01).Preoperative PJA and SVA were greater in the No PJK group(P<0.05).There were 3 types of PJK presented in the PJK group including pseudoarthrosis(N=2),compression fracture(N=3)and progressing thoracic kyphosis due to the natural history of AS(N=6).At the final follow-up,revision surgery was performed in 1 of the patients with PJK of progressing thoracic kyphosis due to the natural history of AS.Conclusion With a minimum of 2 years follow-up,the rate of PJK in AS was 8.3%.PJK could be presented in the characteristics of pseudoarthrosis,compression fracture and progressing thoracic kyphosis due to the natural history of AS.The age at initial surgery and types of osteotomy were the risk factors of PJK.The patients with lower preoperative PJA and SVA were found to have a higher risk of PJK.Therefore,intraoperative fusion extending to the region with higher degree of kyphosis combined with the restoration of normal curve of thoracic kyphosis would be helpful in decreasing the incidence of PJK.Chapter 2 Risk factors of rod fracture after osteotomy in ankylosing spondylitis-related kyphosisObjective To investigate the risk factors for rod fracture(RF)after pedicle subtraction osteotomy(PSO)for thoracolumbar kyphosis secondary to ankylosing spondylitis(AS).Methods This is a retrospective single-center study.AS patients who underwent PSO for thoracolumbar kyphosis between January 2002 and December 2016 were included.Patients with a minimum of 2 years follow-up or the development of RF were enrolled.Recruited patients were divided into the RF group and the No RF group based on whether they developed RF.Patient demographics,operative data,radiographic parameters and adequate ossification of the anterior longitudinal ligament(ALL)were analyzed to determine the risk factors for RF.For RF patients,the fusion status at the PSO level,time course to the development of RF,site of RF and corresponding solution were also recorded.Demographic data,including age,sex,body mass index and smoking status,were summarized.The surgical data analyzed included the levels of osteotomy,fusion levels,upper instrumented vertebra,lower instrumented vertebra,osteotomy site,rod material,rod diameter and rod contour angle(RCA).Radiographic parameters included the sagittal vertical axis(SVA),thoracic kyphosis(TK),lumbar lordosis(LL),sacral slope(SS),pelvic tilt(PT)and pelvic incidence(PI).Radiographic parameters were measured at baseline,immediately after the operation and at the final follow-up.Adequate ossification of the ALL at the PSO level was defined by a total bony bridge.Adequate ossification of the ALL was also measured at baseline,immediately after the operation and at the final follow-up.Results RF occurred in 11(8.9%)of the 123 recruited patients.Solid fusion at the PSO level was found in all patients in the RF group.The average duration to the onset of RF was 31.4 months(range,12 to 68 months).All RF occurred at or immediately adjacent to the PSO level.The RCA was greater in the RF group than in the No RF group(27.8°vs 22.9°,P=0.031).A greater proportion of patients with a rod diameter of 5.50 mm was found in the RF group than in the No RF group(100.0%vs 68.8%,P=0.033).There was a larger proportion of patients with adequate ossification of the ALL at the final follow-up visit in the No RF group than in the RF group(67.0%vs 27.3%,P=0.018).Multivariate analyses demonstrated that the RCA(odds ratio,1.174;95%confidence interval,1.018-1.354;P=0.028)and adequate ossification of the ALL at the final follow-up visit(odds ratio,0.079;95%confidence interval,0.014-0.465;P=0.005)were independent factors for RF.Notably,revision surgery was performed among 6 patients,while conservative treatment was used for the remaining 5.Conclusion In AS patients following PSO for thoracolumbar kyphosis with solid fusion at the PSO level,the incidence of RF was 8.9%.Rod diameter was identified as a risk factor for RF.Furthermore,the RCA was identified as an independent risk factor for RF.In contrast,adequate ossification of the ALL around the PSO level at the final follow-up visit was identified as an independent protective factor for RF.