Fractional Curve in Adult Spinal Deformity: Is it a Driver of or a Compensation for Coronal Malalignment?

This was a retrospective review of the multicenter adult spine deformity database.

The objective of this study was to investigate the role of the fractional curve (FC) on global coronal malalignment.

Despite being very common, the role of the coronal FC as either a driver or compensation for global coronal malalignment is not well documented.

Patients with the following characteristics were extracted from a prospective multicenter database: lumbar/thoracolumbar (TL) major coronal curve >15 degrees, apex at T11-L3, lower end vertebra at L3 or L4, above 45 years old, and FC >5 degrees. In addition to the classic radiographic parameters, baseline analysis included Cobb angle, pelvic obliquity (PO), fractional ratio (fractional Cobb/main Cobb), the sum of PO and FC, as well as the coronal Qiu classification. Curves distribution (TL vs. FC) were compared across the 3 Qui types, and the role of the FC was investigated.

A total of 404 patients (63 y old, 83.3% female) were included: 43 patients were classified as type B, 120 as type C, and 241 were coronally balanced (type A). Compared with the balanced patients, type C patients had similar major TL Cobb angles but significantly larger fractional Cobb angles (17.5 vs. 22.3 degrees, P<0.001). By opposition, type B patients had significantly larger major TL Cobb angles (49 vs. 41 degrees, P=0.001) but smaller fractional Cobb angles (P<0.001). PO>5 degrees in the same direction as FC was more common in type B patients (20%) than in type C patients (7.5%), which suggests the preferential role of pelvic compensation.

Our findings challenge the idea that FC is only a compensatory curve below a main lumbar or TL curve. In type B patients, FC acts as a compensation mechanism but fails to maintain coronal alignment despite the presence of PO. In type C patients, however, the lumbosacral FC acts as a primary driver of coronal malalignment.

Level III.