Role of hip and lumbar flexion angles in stiffness-related disabilities with activities of daily living after lumbar spine surgery

OBJECTIVE: The aims of this study were to clarify the relationship between the combined forward flexion capacity of the lumbar spine and hip joint, referred to as TrunkAflex, and Lumbar Stiffness Disability Index (LSDI) scores in patients following lumbar spine surgery, and to assess whether TrunkAflex better reflects disabilities with activities of daily living (ADL) than the number of fused segments or lumbar spine flexion alone. METHODS: This prospective, cross-sectional, multicenter study included 147 patients who underwent lumbar spine surgery and completed LSDI questionnaires. Lateral radiographs were obtained in the maximum seated flexion position. Radiographic parameters included pelvic incidence (PI)-corrected lumbar lordosis in flexion (PI-LLflex), representing lumbar spine flexion ability, pelvic femoral angle in flexion (PFAflex), representing hip joint flexion ability, and TrunkAflex, defined as the angle between the axis of the L1 vertebral body and the proximal femoral shaft, mathematically expressed as the sum of PI-LLflex and PFAflex. Correlation analyses were used to determine the relationships between LSDI and these parameters. Subgroup analyses were performed to compare the lumbar fusion (upper instrumented vertebra [UIV] at L1 or below) and thoracolumbar fusion (UIV at T12 or above) groups. RESULTS: The LSDI score was significantly correlated with the number of fused segments (r = 0.328, p < 0.01), and PI-LLflex showed a significant correlation with LSDI (r = -0.354, p < 0.01). However, TrunkAflex demonstrated the strongest correlation with LSDI (r = -0.491, p < 0.01). Subgroup analysis revealed that PI-LLflex was more influential in the lumbar fusion group, while PFAflex was more impactful in the thoracolumbar fusion group. Nevertheless, TrunkAflex consistently showed the strongest correlation with LSDI across all groups. Notably, TrunkAflex was minimally influenced by PI, making it a practical and consistent parameter for trunk forward flexion assessment. CONCLUSIONS: This study demonstrated that the combined forward flexion ability of the lumbar spine and hip joint, represented by TrunkAflex, is a stronger predictor of LSDI scores than the number of fused segments or lumbar spine flexion alone, irrespective of the fusion range. Preoperative assessment of hip joint function is particularly important when planning long-segment fusion, and adjusting surgical strategies to preserve appropriate TrunkAflex may contribute to better postoperative ADL outcomes.