Introduction: Rod fracture (RF) after posterior corrective fusion surgery in patients with adult spinal deformity (ASD) is one of the most frequent and serious complications. Many reports have been published to investigate the pathomechanism of RF and to resolve this devastating problem. However, the association between RF and preoperative hip joint function after corrective fusion surgery in ASD patients is unknown. The purpose of this study was to investigate the association between the occurrence of RF and hip joint function in ASD patients who underwent corrective fusion surgery.

Methods: A retrospective cohort study was conducted at a single hospital for ASD patients who underwent corrective fusion surgery between September 2014 and May 2019. The surgery was performed on a pedicle-screw basis posteriorly. Lateral lumbar intervertebral fusion (LLIF) was accompanied in some patients. Only autogenous bone graft was used in all the patients. Any bone morphologic proteins were not grafted concurrently. All the surgeries were performed by the same single attending orthopaedic spine surgeon (OS). Using the SRS-Schwab Classification, the ASD was defined as the following: 1) sagittal vertical axis (SVA) was 40 mm or more, 2) pelvic incidence minus lumbar lordosis (PI-LL) was 10 degrees or more. Preoperative demographic and radiological characteristics, as well as surgical characteristics, were assessed. In addition, hip function (hip extension range of motion [ROM] and hip extensor strength) was measured. The outcome in this study was the occurrence of RF within one year postoperatively. The association between hip function and occurrence of RF within one year postoperatively was assessed by multivariable analysis. Multivariable analysis was adjusted for several confounding factors (gender, age, body mass index, SVA, PI-LL, number of fusion levels, rod diameter, and material). The level of significance was set at P<0.05.

Results: A total of 37 subjects with ASD (34 women, three men; age range 56-79 years) had a full set of all data and were included in this study. Preoperative radiological characteristics were SVA, 139.2±78.4mm; PI-LL, 37.3±9.1 degrees. Postoperative RF occurred in 16 patients (43%). Multivariable analysis showed that hip extension ROM was an independent risk factor for the occurrence of RF after corrective fusion surgery (OR, 0.82; 95%CI, 0.70-0.95).

Discussion: The current study found that hip extension ROM is an independent risk factor for the development of postoperative RF in ASD patients after corrective fusion surgery. The hip joint is most adjacent to the spine and is considered important as a compensatory mechanism for sagittal imbalance. This plays an important role to complement the dysfunction of the fixed spine, especially after corrective fusion surgery. The result in this study is useful for many health care providers who are responsible for ASD patients scheduled for corrective fusion surgery. Maintaining adequate hip extension ROM in ASD patients scheduled for corrective fusion surgery may be a strategy to prevent postoperative RF.