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Purpose: (1) Evaluate the associations between L1-pelvic angle (L1PA) and both sagittal vertical axis (SVA) and T1-pelvic angle (T1PA), and (2) assess the clinical impact of L1PA., Methods: A single-institution retrospective cohort study was undertaken for patients undergoing adult spinal deformity (ASD) surgery from 2013 to 2017. Ideal L1PA was defined as (0.5xPelvic Incidence)-21. Pearson correlation was performed to compare L1PA, SVA, and T1PA. Univariate/multivariate regression was performed to assess the effect of L1PA on mechanical complications, controlling for age, BMI, and postoperative pelvic incidence-lumbar lordosis mismatch (PI/LL). Due to the overlapping nature of patients with pseudarthrosis and rod fracture, these patients were analyzed together., Results: A total of 145 patients were included. Mean preoperative L1PA, SVA, and T1PA were 15.5 ± 8.9°, 90.7 ± 66.8 mm, and 27.1 ± 13.0°, respectively. Mean postoperative L1PA, SVA, and T1PA were 15.0 ± 8.9°, 66.7 ± 52.8 mm, and 22.3 ± 11.1°, respectively. Thirty-six (24.8%) patients achieved ideal L1PA. Though the correlation was modest, preoperative L1PA was linearly correlated with preoperative SVA (r 2  = 0.16, r = 0.40, 95%CI = 0.22-0.60, p < 0.001) and T1PA (r 2  = 0.41, r = 0.62, 95%CI = 0.46-0.76, p < 0.001). Postoperative L1PA was linearly correlated with postoperative SVA (r 2  = 0.12, r = 0.37, 95%CI = 0.18-0.56, p < 0.001) and T1PA (r 2  = 0.40, r = 0.62, 95%CI = 0.45-0.74, p < 0.001). Achieving ideal L1PA ± 5° was associated with a decreased risk of rod fracture/pseudarthrosis on univariate and multivariate regression (OR = 0.33, 95%CI = 0.12-0.86, p = 0.024). No association between achieving ideal L1PA and patient-reported outcomes was observed., Conclusion: L1PA was modestly correlated with SVA and T1PA, and achieving ideal L1PA was associated with lower rates of rod fracture/pseudarthrosis. Future studies are warranted to better define the clinical implications of achieving a normal L1PA., Level of Evidence: III., (© 2023. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)

Purpose: To assess the impact of screw density on: (1) rod fracture/pseudarthrosis, (2) proximal/distal junctional kyphosis/failure (PJK/DJK/PJF), and (3) deformity correction judged by sagittal vertical axis (SVA) and T1-pelvic angle (T1PA)., Methods: A single-center, retrospective cohort study of patients undergoing adult spinal deformity (ASD) surgery from 2013 to 2017 was undertaken. Screw density was calculated by dividing the number of screws placed by the total instrumented levels. Screw density was then dichotomized at our calculated mean density to ≥ 1.65 and < 1.65. Outcomes consisted of mechanical complications and the amount of correction obtained., Results: 145 patients underwent ASD surgery with 2-year follow-up. Mean screw density (range) was 1.6 ± 0.3 (1.00-2.00). The most common levels with missing screws were L2 (n = 59, 40.7%), L3 (n = 57, 39.3%) and L1 (n = 51, 35.2%), located mainly along the concavity in 113(80.0%) patients and apices in 98 (67.6%) patients. Rod fracture/pseudarthrosis: 23/32 (71.8%) patients with rod fracture and 35/46 (76.0%) with pseudarthrosis had missing screws within two levels of the rod fracture/pseudarthrosis. Logistic regression showed no significant association between screw density and rod fracture/pseudarthrosis., Pjk/f: 15/47 (31.9%) patients with PJK and 9/30 (30.0%) with PJF had missing screws within three levels of the upper instrumented vertebra (UIV). Logistic regression showed no significant association between screws density and PJK/F. Correction obtained: linear regression failed to show any significant association between screw density and SVA or T1PA correction., Conclusion: These findings showed that no significant association was found between screw density and mechanical complications or the amount of correction obtained, though approximately 3 out of 4 patients with rod fracture/pseudarthrosis had missing screws at or within two levels of the pathology. The prevention of mechanical complications is likely multifactorial and subject to both patient's characteristics and surgical techniques., Level of Evidence: III., (© 2023. The Author(s), under exclusive licence to Scoliosis Research Society.)

Purpose: (a) Describe the time course of each mechanical complication, and (b) compare radiographic measurements and preoperative patient-reported outcome measures (PROMs) among each mechanical complication type., Methods: A single-institution case-control study was undertaken of patients undergoing adult spinal deformity (ASD) surgery from 2009-2017. Exposure variables included patient demographics, operative variables, radiographic measurements, and preoperative PROMs, including Oswestry Disability Index (ODI), Numeric Rating Scale Back/Leg-pain scores (NRS-Back/Leg), and EuroQol-5D (EQ-5D). The primary outcomes were occurrence of a mechanical complication and time to complication. Due to overlapping occurrence, rod fracture and pseudarthrosis were grouped into one composite category., Results: 145 patients underwent ASD surgery and were followed for at least 2 years. 30/47 (63.8%) patients with proximal junctional kyphosis (PJK) required reoperation, whereas 27/31 (87.1%) patients with pseudarthrosis/rod fracture required reoperation (63.8% vs. 87.1%, Χ 2  = -0.23, 95% CI -0.41, -0.05, p = 0.023). Cox regression showed no significant difference in time to reoperation between PJK and rod fracture/pseudarthrosis (HR = 0.97, 95% CI 0.85-1.11, p = 0.686). Distal junctional kyphosis (DJK) (N = 3; 2 reoperation) and implant failures (N = 4; 0 reoperations) were rare. Patients with PJK had significantly lower Hounsfield Units preoperatively compared to those with pseudarthrosis/rod fracture (138.2 ± 43.8 vs. 160.3 ± 41.0, mean difference (MD) =  -22.1, 95% CI -41.8, -2.4, p = 0.028), more prior fusions (51.1% vs. 25.8%, Χ 2  = 0.253, 95% CI 0.41, 0.46, p = 0.026), fewer instrumented vertebrae (9.2 ± 2.6 vs. 10.7 ± 2.5, MD =  -1.5, 95% CI -2.7, -0.31, p = 0.013), and higher postoperative thoracic kyphosis (TK) (46.3 ± 12.7 vs. 34.9 ± 10.6, MD = 11.4, 95% CI 5.9, 16.9, p < 0.001). Higher postoperative C7 sagittal vertical axis (SVA) did not achieve a significant difference (80.7 ± 72.1 vs. 51.9 ± 57.3, MD = 28.8, 95% CI -1.9, 59.5, p = 0.066). No differences were seen in preoperative PROMs., Conclusion: Patients with pseudarthrosis/rod fracture had a higher reoperation rate compared to those with PJK with similar time to reoperation. Moreover, patients with PJK had higher postoperative TK, lower Hounsfield Units, more prior fusions, and fewer instrumented levels compared to those with pseudarthrosis/rod fracture. The results of this single-institution study suggest that even though mechanical complications are often analyzed as a single group, important differences may exist between them., Level of Evidence: III., (© 2022. The Author(s), under exclusive licence to Scoliosis Research Society.)