Cone of economy classification: evolution, concept of stability, severity level, and correlation to patient-reported outcome scores.

STUDY DESIGN

A prospective cohort study OBJECTIVE: To determine a classification system for cone of economy (CoE) measurements that defines clinically significant changes in altered balance and to assess if the CoE measurements directly impacts patients reported outcome measures (PROMs). Preoperative functional data is a crucial component of determining patient disability and prognosis. The CoE has been theorized to be the foundation of biomechanical changes that leads to increased energy expenditure and disability in spine patients. PROMs have been developed to quantify the level of debilitation in spine patients but have various limitations.

METHODS

A total of 423 symptomatic adult patients with spine pathology completed a series of PROMs preoperatively including VAS, ODI, Tampa Scale for Kinesiophobia (TSK), Fear and Avoidance Beliefs Questionnaire (FABQ), and Demoralization (DS). Functional balance was tested in this group using a full-body reflective marker set to measure head and center of mass (CoM) sway.

RESULTS

PROMs scores were correlated with the magnitude of the CoE measurements. Patients were separated by the following proposed classification: CoM coronal sway > 1.5 cm, CoM sagittal sway > 3.0 cm, CoM total sway > 30.0 cm, head coronal sway > 3.0 cm, head sagittal sway > 6.0 cm, and head total sway > 60.0 cm. Significant differences were noted in the ODI (< 0.001), FABQ physical activity (< 0.001-0.009), DS (< 0.001-0.023), and TSK (< 0.001-0.032) across almost all planes of motion for both CoM and head sway. The ODI was most sensitive to the difference between groups across CoM and head sway planes with a mean ODI of 47.5-49.5 (p < 0.001) in the severe group versus 36.6-39.3 (p < 0.001) in the moderate group.

CONCLUSIONS

By classifying CoE measurements by the cutoffs proposed, clinically significant alterations in balance can be quantified. Furthermore, this study demonstrates that across spinal pathology, higher magnitude CoE and range of sway measurements correlate with worsening PROMs. The Haddas' CoE classification system in this study helps to identify patients that may benefit from surgery and guide their postoperative prognosis.