Enlargement of the "cone of economy" as a biomarker of decompensation in unbalanced adult spinal deformity patients.

Abstract

Purpose: Dubousset et al. proposed that the "cone of economy" (COE) in the inverted pendulum model of human standing increases in size significantly in patients with balance impairment. The clinical significance of COE size measured by motion capture as a postural sway index in adult spinal deformity (ASD) patients remains unclear.

Methods: Twenty-three patients undergoing ASD surgery in a single institution as well as five healthy male participants were enrolled. The motion-capture of the surface reflection markers above the C7 and S1 spinous processes was performed using infrared cameras, and the most appropriate biomarker of postural stability was determined based on the measurement reliability and the correlations with center of pressure (COP) parameters measured by the force plate. The measurement was repeated 6-months later to examine the postoperative change.

Results: Among the various COE parameters, the root mean square (RMS) area of C7 spinous process showed the high measurement reliability with strong correlation with RMS area of COP (ICC (3, k): 0.713, rho = 0.606, p < 0.001). Twelve patients showed their RMS area one standard deviation larger than the mean of healthy participants (23-40 years, 1.8 ± 1.3 cm²). These decompensated patients were older than the others (79.6 vs. 68.2 years old, p = 0.006), and showed the significant reduction in COE size (6.1 vs. 3.2 cm², p = 0.02) 6-months post-operatively.

Conclusion: Motion-capture based size measurement of COE was a useful assessment tool for postural stability in ASD patients, showing the significant postoperative reduction in the subgroup of ASD patients with preoperative breakdown of balance compensation mechanisms.