Clinical and Radiographic Evaluation of Sagittal Imbalance: A New Radiographic Assessment

In this article, we describe a case series study involv­ing a new radiologic evaluation of sagittal imbalance. We review the current radiologic assessment of sagittal imbalance and introduce a new radiologic evaluation that helps in ruling out hip flexion contracture as the pri­mary cause of sagittal imbalance and the type and level of spinal osteotomy required to regain sagittal balance.

Sagittal imbalance is important in spinal deformity assessment. Studies have confirmed that overall clini­cal outcomes and patient satisfaction with surgery were best in cases that resulted in an increase in lumbar lordosis.

For this study, radiologic assessment of sagittal imbalance was conducted on a long, 14×51-inch upright lateral plain radiograph that included the proximal femur and the entire spine. The radiograph was taken with the arms at 45° forward flexion and the hips and knees fully extended. The femoral axis line was drawn and extended cephalad. The C7 offset, the perpendicular distance between the femoral axis line and the center of C7, represented the degree of sagittal imbalance. The angle between the femoral axis line and a line extending from the center of C7 to the vertebra at the level of the proposed osteotomy—the Seattle angle—predicted how much correction was required to bring the C7 plumb in line with the femoral axis and to decrease the C7 offset, thus regaining sagittal balance. The proposed method was used to evaluate 10 consecutive patients who required spinal osteotomies to regain sagittal balance. Preoperative and postoperative plain radiographs were assessed twice, at a 6-week interval, by an independent spine surgeon and a musculoskeletal radiologist. Cohen κ correlation coefficients were used to calculate intraob­server and interobserver reliability.

The 2 reviewers’ intraobserver reliability was excellent (κs = 0.98, 0.93). Interobserver reliability was lower but good (κ = 0.76).

Inclusion of the proximal femur on the long upright lateral plain radiograph of the entire spine and iden­tification of the relation between the femoral axis line and the center of C7 are important in evaluating sagittal imbalance. Excellent intraobserver reliability, coupled with good interobserver reliability, suggest that this new radiologic assessment method can be helpful in preop­erative assessment of sagittal imbalance.