Sagittal Femur-Pelvis Balancing Mechanism and Linear Correlation Chain in Patients with Spinal Disorders: A Radiographic Retrospective Analysis.

Study design: A retrospective study.

Objective: To investigate how the femurs and pelvis collaboratively maintain sagittal balance and energy-efficient alignment by investigating sagittal parameters.

Summary of background data: Sagittal parameters are increasingly acknowledged as fundamental determinants in sustaining balance and energy-efficient postures. Among numerous parameters, pelvic morphology and retroversion compensation are critical in restoring sagittal imbalance caused by aging and pathological changes. Despite the significant role of the femurs in the sagittal plane, relevant research remains lacking.

Methods: Standing lateral full-length radiographs, including the upper femurs, were obtained from 368 consecutive patients with symptomatic spinal pathologies. Sagittal plane parameters of the thoracic spine, lumbar spine, pelvis, and femurs-such as angulation, tilt, and offset-and pelvic morphology were measured to reflect the sagittal profile. All participants completed the Oswestry Disability Index (ODI) and Visual Analog Scale (VAS) questionnaires to evaluate Health-Related Quality of Life (HRQOL). Relationships between radiographic and clinical parameters were assessed using Pearson correlation coefficients.

Results: This cohort (154 males, 214 females) had a mean age of 64.6±11.0 years, height of 163.2±8.5 cm, and BMI of 25.8±3.8 kg/m². The ODI is correlated with the tilt and offset parameters, except those of the pelvic and lumbar regions. Correlations were observed among all tilt and offset parameters regarding offset values, except for TT with PT and LT, and TO with PO and LO. These correlations pertain to the symmetry of S1. From the cephalad T1S to the caudal FT, all parameters exhibit the highest correlation with their adjacent parameters.

Conclusion: A linear correlation chain exists in the sagittal plane in patients with spinal disorders. The back tilt of a longer femur increases posterior offset more efficiently, shifting the center of gravity backward in patients with spinal disorders. Anterior pelvic tilt relative to the femur reduces the Pelvic-Femur Angle (PFA) to maintain lumbar lordosis and economic sagittal alignment. These collaborative compensatories are crucial for understanding sagittal balance and alignment of the spine, pelvis, and lower extremities.