What is actually happening inside the "cone of economy": compensatory mechanisms during a dynamic balance test.

STUDY DESIGN

A nonrandomized, prospective, concurrent control cohort study.

OBJECTIVE

To further develop cone of economy (CoE) measurements by identifying compensatory mechanisms at the extremes of the CoE and comparing balance control strategies in a group of adult degenerative scoliosis (ADS) patients with non-scoliotic controls. The CoE concept was first proposed by Dubousset and is frequently referred to when assessing balance in spinal deformity patients. Recently, a method that quantifies the CoE of individual patients through 3D video kinematic and electromyography data was developed. However, this method lacks measurements that describe the motor control strategies utilized by spinal disorder patients to maintain balance.

PATIENT SAMPLE

Twenty ADS patients and 15 non-scoliotic controls.

METHODS

All test subjects were fitted with a full body marker set. Each subject performed a series of functional balance tests (Romberg's with eyes opened) while being recorded in a human motion capture system. Three-dimensional CoE dimensions, range of sway (RoS), overall sway and lower extremity and trunk range of motion (RoM) were measured and analyzed.

RESULTS

Patients with ADS demonstrated greater overall sway and RoS in the sagittal and coronal planes compared to controls. Moreover, ADS patients presented with more hip flexion and trunk flexion at maximal points of sway and more ankle, knee, hip and trunk RoM when swaying in comparison with controls.

CONCLUSIONS

ADS patients have larger CoE dimensions and increased sway when compared to non-scoliotic controls. ADS patients rely on a hip balance control "strategy" and lower extremity RoM to maintain balance, which differed from control subjects. Unlike prior attempts to define compensatory mechanisms in ADS patients, the described technique utilizes dynamic, three-dimensional measurements to define what is occurring within the CoE. By expanding on prior CoE measurements, we were able to define a unique dynamic balance control strategy for each patient.