Advanced method of quantifying the cone of economy may allow practitioners to determine appropriate treatment options

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A new method to quantify the boundaries of Dubousset’s cone of economy, the centre of mass displacements, and the amount of sway within the cone of economy along with the energy expenditure for a specific patient, has been developed and evaluated by Ram Haddas and Isador Lieberman of the Texas Back Institute (Plano, USA).

Lieberman recently presented the results of this study at the 19th Annual Conference of the International Society for the Advancement of Spine Surgery (ISASS; 3–5 April 2019, Anaheim, USA).

Haddas and Lieberman set out to evaluate this method to quantify the dimensions of the cone of economy and the energy expenditure associated with maintaining a balanced posture within the cone of economy in adult degenerative scoliosis (ADS) patients compared to matched non-scoliotic controls. They note that ADS patients exhibit a variety of postural changes within their cone of economy, involving the spine, pelvis and lower extremities, in their effort to compensate for the altered posture.

They defined balance as the ability of the human body to maintain its centre of mass within the base of support with minimal postural sway, and note that the cone of economy refers to the stable region of upright standing posture. In general, though, the brain, through the righting reflex, will sacrifice focal alignment to optimise global balance within the cone of economy. They also mentioned that the underlying assumption is that deviating outside of a patient’s individual cone challenges the balance mechanisms and consumes more energy.

In this non-randomised, prospective, control cohort study, Haddas and Lieberman included 15 ADS patients and 15 non-scoliotic volunteers, all of whom performed a series of functional balance tests. The investigators measured the dimensions of the cone of economy and the energy expenditure related to maintaining balance within the cone of economy using a human motion video capture system and dynamic surface electromyography.

The investigators describe how the ADS patients included in the study presented more centre of mass range of sway in the sagittal (ADS: 1.59cm vs. H: 0.61cm; p=0.049) and coronal (ADS: 2.84cm vs. H: 1.72cm; p=0.046) directions in comparison to the non-scoliotic controls. Additionally, they note that ADS patients presented with more centre of mass (ADS: 33.3cm vs. H: 19.13cm; p=0.039) and head (ADS: 31.06cm vs. H: 19.13cm; p=0.013) overall sway in comparison to the non-scoliotic controls. Finally, scoliosis patients expended more muscle activity to maintain static standing, as manifest by increased muscle activity in their erector spinae (ADS: 37.16mV vs. H: 20.31mV; p=0.050), and gluteus maximus (ADS: 33.13mV vs. H: 12.09mV; p=0.001) muscles.

“The strength of this study relates to the objective nature of this form of functional balance testing,” the investigators tell Spinal News International. “In addition this is an important first step in understanding the relationship between spinal balance and function,” they add.

Haddas and Lieberman conclude that this method of cone of economy measurement “will enable spine care practitioners to objectively evaluate their patients in an effort to determine the most appropriate treatment options, and in objectively documenting the effectiveness of their intervention.” Currently, Haddas and Lieberman are using this method to explore the effect of surgical intervention in adult degenerative scoliosis, cervical spondylotic myelopathy, spondylolisthesis, sacroiliac dysfunction and chronic low back pain patients.

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