BioMechanics
August 2006

Postural stability measurement balances technology and treatment
A dynamic assessment of sway may help practitioners design treatment for a range of patients.

By: Michelle Butera

http://www.biomech.com/showArticle.j...leID=192700650

Measuring and evaluating postural stability is an effective way for practitioners to diagnose balance problems and to decide on treatment for their patients at risk for falling. New tools for measuring available data give practitioners a more complete picture of how their patients move and can help researchers develop new methods for treating stability issues. This in turn can advance evaluation, rehabilitation, and performance for people with postural stability problems.

Continually changing technology is challenging some of the processes used to provide data for assessing movement. New ways to analyze the comprehensive data available from testing devices such as the Biodex Stability System and the NeuroCom International Balance Manager System are being developed.

The NeuroCom EquiTest system comprises a movable visual surround and a force plate that can both move backward and forward and rotate. The system protocol includes the Sensory Organization Test (SOT) and the Equilibrium Score (ES), a composite of the results of the SOT. The SOT looks at how proprioception affects postural sway. The ES averages the weighted scores of a patient's performance under the six sensory conditions of the SOT, which all take place in the sagittal plane. It identifies changes of coordination and interaction of the visual, somatosensory, and vestibular systems that are the focus of the SOT.

But there are some important balance and stability factors the ES doesn't consider. The ES assumes a maximum anteroposterior sway angle of 12.5 degrees before a normal person will fall. But that assumption doesn't take into account individual differences due to age, sex, height, or weight. By averaging the single maximum points of anterior and posterior sway, the ES also can't distinguish between small amounts of sway in each direction and a large sway angle in one direction and none in the other. A subject who, say, can control posterior movement (no sway) but not anterior (large sway angle) is more likely to fall than someone who moves a little in both directions-but they may both have the same ES.

A newer measure proposed for use in conjunction with the SOT is the Postural Stability Index (PSI).1,2 A composite of movements in several directions, the PSI uses data obtained through the SOT to augment what can be learned from the ES. The PSI also calculates an average sway angle. But by incorporating sway across the time of the trial in the final number, the PSI can give a fuller picture of a subject's functional balance.

Outcomes and findings

A study to test the efficacy of the PSI was conducted at the facilities of the VA Medical Center in East Orange, NJ.1 The balance of 20 subjects, 10 with chronic fatigue syndrome (CFS) and 10 armed services veterans with no known illness, plus 10 healthy controls was assessed on the NeuroCom EquiTest system. Prior to testing, none of the subjects had been diagnosed with any balance problems, though the CFS patients and vets were chosen because of links between CFS and balance problems and the fact that the veterans had some of these symptoms. None took any medications that are known to cause problems pertaining to balance. Each subject participated in three identical 20-second trials for each of the six conditions. Researchers calculated ES and PSI outcomes at five-, 10-, and 20-second intervals to assess how time in the trial would affect final scores. For both measures, a score of 100 represents perfect stability: the lower a subject's score (on either measure), the more impaired his or her balance.

ES and PSI both have a place in the measurement of postural stability and AP sway but the PSI was created because of a need for deeper analysis of postural stability. It can offer more practical, reliable, and valid information for a few reasons.

Clinicians generally accept that ankle stiffness is directly related to postural stability. The more rigid a patient's ankles, the less stable the patient. The ES considers the foot and ankle as one unit with the body swaying above the ankle. The PSI recognizes movement of the foot independent of the ankle for a more accurate representation of how ankle stiffness (less independent foot movement) affects balance. The researchers found that PSI decreased as ankle stiffness increased, suggesting a negative correlation between ankle stiffness and postural stability. The ES does not have this capability because it does not take ankle stiffness (defined as "the rate of change of torque at the ankle with respect to the displacement (in radians) of the COM [center of mass]") into consideration.2

The study found that a dependable estimate of stability could be obtained with the PSI even if a subject fell during testing. Because only the extremes of movement are included, the ES for a subject who falls is zero for that trial. This is not true with the PSI because movement through the whole of a trial is considered. In addition, of 2000 data points collected from the SOT, only two values of data for each trial (maximum anterior sway and maximum posterior sway) are used to form the ES. The PSI, on the other hand, uses all the data points plotted by the SOT to measure stability patterns.

The Postural Stability Index is one of many tools that analyze information gathered from force plate technology and can provide unique information to the practitioner.

Practical use

A number of patients can benefit from the results of posturography testing as practitioners can analyze the paths of neurological and vestibular problems and evaluate existing balance disorders.

According to the American Geriatric Society,3 one-third of the population in the U.S. over the age of 65 experience unintentional falls.

"When problems with sluggish reflexes, the sensory system, the vestibular system, visual changes, and minor weakness occur, there is an increased chance of a fall," said Ray Hedenberg, a physical therapist with South Valley Physical Therapy in Denver. "The EquiTest gives information about the patient's ability to use sensory systems and how the information is integrated through the midbrain, cerebellum, and brain stem and then coordinated with the motor systems so the patient can respond in an appropriate fashion," he said.

Hedenberg works with elderly patients to more effectively use the vestibular system and try to reactivate and use the function they have. Posturography systems can look at postural strategies that patients use to maintain their center of gravity on unstable surfaces.

"That information is particularly helpful with geriatric patients to help a clinician customize treatment based on assessment findings," he said.

The information from the SOT is useful, he said, in determining whether patients display an appropriate response pattern to maintain balance under particular conditions.

"If they are using an inappropriate strategy, that puts the patient at a very high risk for falling," he said.

Strength and range of motion play a role in effective postural strategies, as does a patient's cognitive level. Sometimes the automatic responses of patients with Alzheimer's disease and dementia can be retrained to safely recruit their muscle groups and sensory systems and be effective even without full cognitive ability.

More than one million people in the U.S. are living with Parkinson's disease. Difficulty with balance is one of the symptoms of the condition.4 With no cure available yet, medication is the best form of treatment for the neurological disorder.

Researchers at Stanford University Medical Center studied the role of medication in postural stability for Parkinson's patients in order to determine which medications and which dosages were most helpful.5 Posturography equipment can help track progress and note changes medication dosage may have on a patient's tendency to sway. The researchers used the SOT to gather data on postural sway before and after patients were medicated. They found a correlation between medication and tremors.

Sports and sway

Posturography may offer opportunities for evaluating athletic performance and rehabilitation after injury. Postural stability and neuropsychological testing are becoming integral parts of postconcussion assessment in athletes.6 Given that one in five high school football players sustains a concussion each year and 50% of college athletes have a history of concussion,7 postconcussion evaluation is important in determining whether an injured athlete can return to play. General concussion assessment recommendations of the Atlantic Coast Conference (ACC) for collegiate sports include general cognitive and postural stability testing to determine the severity of concussion. Once an athlete has been assessed postrecovery and determined to be symptom-free, return to play can be considered.8

Kevin Guskiewicz led a recent study on postural stability and neuropsychological deficits after concussion in college athletes. He used the SOT to look at postural stability assessment for return to play. He concluded that the acute balance deficits demonstrated by athletes with cerebral concussion were likely the result of ineffective use of information from the vestibular and visual systems.9 These balance deficits can affect performance and place the athlete at risk.

Limits of posturography

High equipment costs have so far kept force-plate posturography technology from making the leap from research facilities to rehabilitation practices.

"The expense associated with high-tech equipment can be something of a deterrent to a clinic or a clinician," Hedenberg said. "That includes not just NeuroCom equipment, but also isokinetic or motion analysis equipment."

Reimbursement is allowable for dynamic posturography and there is an evaluation code for Medicare, but unfortunately not all insurance companies follow Medicare's lead.

"With today's reimbursement environment, it is a risk to buy the systems with high cost because you may not get reimbursed and there is the worry that you won't be able to pay for it," Hedenberg said. "We deal with what we have and utilize our clinical skills. That's unfortunate but that's the way healthcare is today."

Advancements in biomechanical devices are changing postural stability research and evaluation. With new additions like the PSI, data collection capabilities are evolving. Research will gain from newly available information and practitioners fortunate enough to have access to a dynamic posturography equipment system have a richer assessment tool.

Michelle Butera is a freelance writer based in San Francisco with a BS in kinesiology.

References

1. Chaudhry H, Findley T, Quigley KS, et al. Measures of postural stability. J Rehabil Res Dev 2004;41(5):713-720.

2. Chaudhry H, Findley T, Quigley KS, et al. Postural stability index is a more valid measure of stability than equilibrium score. J Rehabil Res Dev 2005;42(4):547-556.

3. Rubenstein LZ. Falls and balance problems. Patient Education Forum, American Geriatrics Society. http://www.americangeriatrics.org/ed.../falling.shtml, accessed March 2006.

4. Parkinson's Action Network. About Parkinson's: What is Parksinson's disease. http://www.parkinsonsaction.org/abou...parkinsons.htm, Accessed June 2006.

5. Bronte-Stewart HM, Minn AY, Rodrigues K, et al. Postural instability in idiopathic Parkinsons disease: the role of medication and unilateral pallidotomy. Brain 2002;125(Pt 9):2100-2114.

6. Moser RS. Knock Knock: concussions from sports injuries. N J Med 1998;95(11):27-29.

7. Hausdorff JM, Rios DA, Edelberg HK. Gait variability and fall risk in community-living older adults: a 1-year prospective study. Arch Phys Med Rehabil 2001;82(8):1050-1056.

8. Guskiewicz KM. Sport-related concussions: Are we headed in the right direction. The Atlantic Coast Conference (ACC). Sports Sciences, October Features: Soccer. www.theacc.com/genrel/100104aac.html, accessed June 2006.

9. Guskiewicz KM, Ross SE, Marshall SW. Postural stability and neuropsychological deficits after concussion in collegiate athletes. J Athl Train 2001;36(3):263-273.