• Shin Oe, Yu Yamato, Tomohiko Hasegawa, Go Yoshida, Tomohiro Banno, Hideyuki Arima, Koichirou Ide, Tomohiro Yamada, Keiichi Nakai, Kenta Kurosu, and Yukihiro Matsuyama.
• Division of Geriatric Musculoskeletal Health and Department of Orthopedic Surgery Hamamatsu University School of Medicine, Hamamatsu, Shizuoka, Japan.
• Spine. 2023 Jun 15; 48 (12): 843852843-852.

Study DesignRetrospective study.ObjectiveTo determine whether an occupancy rate of a pedicle screw (ORPS) <80% in an upper instrumented vertebra (UIV) is a risk factor for UIV fracture (UIVF).Summary Of Background DataThe ratio of the length of the pedicle screw to the anteroposterior diameter of the vertebral body at the UIV is defined as ORPS. Previous studies showed that the stress on the UIV is reduced to the greatest degree when ORPS is >80%. However, it remains unclear whether these results are clinically valid.Patients And MethodsA total of 297 patients who had undergone adult spinal deformity surgery were included in the study. The group with an ORPS ≥80% and <80% was defined as the H (n = 198) and L (n = 99) group, respectively. Propensity score matching and logistic regression analysis were used to evaluate the association between ORPS and the development of UIVF adjusting for confounders.ResultsThe mean age of both groups was 69 years. The average ORPS in the L and H groups was 70% and 85%, respectively. The incidence of UIVF was 30% in group L and 15% in group H ( P < 0.01). In addition, the 99 patients in group H were subdivided into 2 groups according to whether the screws penetrated the anterior wall of the vertebral body: 68 patients had no penetration (group U ), whereas 31 patients showed evidence of penetration (group B ). A total of 10% and 26% of the patients in the U and B groups, respectively, experienced UIVF ( P < 0.05). Logistic regression analysis indicated that ORPS <80% was significantly associated with UIVF ( P = 0.007, odds ratio: 3.9, 95% CI 1.4-10.5).ConclusionTo reduce UIVF, screw length should be set with a target ORPS of 80% or higher. If the screw penetrates the anterior wall of the vertebral body, the risk of UIVF is greater.Copyright © 2023 Wolters Kluwer Health, Inc. All rights reserved.

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