August 2009

In the Moment: O&P


istock 1676706

istock 1676706

by Jordana Bieze Foster

Feedback boosts OA bracing
Audio signal helps improve mechanics

The next frontier in knee osteoarthritis bracing could well be a device that doesn’t just position the joint but also tells the patient how to move correctly.

Researchers from Purdue University tested this concept using an offloading knee brace equipped with a goniometer, an accelerometer, and a feedback system that emits an audio signal when key gait measures fall outside a normal range. In nine healthy young women, the researchers found that use of the brace with feedback significantly lowered rates of loading at initial contact compared to the brace alone or to an unbraced condition.

The results, published in the August issue of the Journal of Biomechanical Engineering, add to the growing body of literature supporting the use of real-time feedback to correct faulty mechanics that can contribute to joint pain and degeneration.

In the Purdue study, the researchers’ objective was to decrease loading rate through a feedback mechanism linked to tibial acceleration and knee angle at 50 msec prior to initial contact. A 2005 study from the same group found that smaller knee flexion angles and greater downward vertical acceleration at the tibia contributed to loading rate, which in turn increases the risk of osteoarthritis.

In the August study, subjects wearing the feedback device heard a signal when their knee flexion angle was less than 5º or their tibial acceleration was more than 8.6 m/s2. The women were told to change their gait pattern if necessary to minimize the signaling, but were not told specifically what adjustments to make. Three gait trials per leg were performed for each condition; five minutes of acclimation was allowed prior to each brace trial.

The feedback condition differed significantly from the two other conditions for knee flexion angle at and 50 msec prior to initial contact, tibial acceleration at initial contact, and rate of loading. Walking velocity, however, did not differ significantly between conditions, suggesting that kinematics rather than walking speed were responsible for the decreased loading rate.

The Purdue study is only the most recent example of how real-time feedback can be used for biomechanical retraining.

Researchers at the University of Wollongong in New South Wales, Australia, developed an Instrumented Knee Sleeve (IKS) to improve knee flexion angles on landing, a potential risk factor for anterior cruciate ligament injury. The sleeve, described to American researchers in 2005 at the annual meeting of the American Society of Biomechanics,  produced an audible tone when the desired degree of knee flexion was achieved.

A pilot trial found that knee flexion angle increased by an average of 8º following a six-week training period in the device, but progress on the project has been tabled due to a lack of funds.

More recently, researchers from the University of Delaware have had success using visual feedback for gait retraining in patients who suffer from patellofemoral pain or who are at risk for tibial stress fractures or osteoarthritis. Patients run or walk on an instrumented treadmill with a monitor that visually tracks key biomechanical variables. They are instructed in ways to alter their at-risk mechanics, and are able to see the results on the monitor in real time.

Results presented at multiple society meetings have been encouraging. The tibial shock retraining protocol has been shown to significantly decrease vertical and torsional loading, and the patellofemoral retraining protocol has been shown to reduce excessive hip adduction and relieve pain (see “Patellofemoral pain takes turn in spotlight”). And a case study presented at the 2007 ASB meeting suggested that a similar technique can correct varus malalignment and reduce the external knee adduction moment in patients at risk for osteoarthritis.

Study suggests ankle device effect is mechanical, not neuromuscular

Ankle braces increase rotational joint stiffness but appear to do so passively rather than through neuromuscular activation, according to research from the University of North Carolina at Chapel Hill.

Investigators analyzed the effects of two ankle braces (one lace-up, one semi-rigid) on 28 physically active subjects, 14 of whom had unilateral functional ankle instability. A medial-lateral swaying cradle device allowed for measurement of ankle stiffness in response to a transient oscillation perturbation (a weighted ball dropped onto the cradle lateral to the ankle being tested).

The lace-up brace was associated with 14% more stiffness than an unbraced ankle, while the semi-rigid brace was associated with 17% more stiffness than the lace-up condition. All between-condition differences were statistically significant.

However, the researchers found that neither brace had a significant effect on preactivation levels in the peroneus longus, peroneus brevis, tibialis anterior, or soleus muscles. The findings, published in the July-August issue of the Journal of Athletic Training, suggest the effect of ankle bracing is primarily mechanical.

Hip strength, symmetry differentiate transtibial amputees who exercise

The demands of recreational exercise for transtibial amputees are particularly evident in the hip musculature, according to research from the Karolinska Institute in Stockholm, Sweden.

Swedish investigators analyzed hip strength in three active transtibial amputees, four sedentary transtibial amputees, and nine active able-bodied individuals. “Active” was defined as exercising recreationally at least three times per week. Concentric and eccentric hip flexion and extension strength was tested in both limbs on an isometric dynamometer, at 60º/s and 120º/s.

Hip flexion and extension strength was significantly more asymmetrical in the inactive amputees than the active amputees, the intact limb being 49% stronger in the former and the residual limb being 8% to 14% stronger in the latter.

Flexion/extension strength ratios were similar in the inactive amputees and the able bodied subjects, but were significantly lower in the active amputees, a sign of overdeveloped hip extensors relative to the hip flexors. In adddition, peak torques in the residual limbs of the active amputees were greater than in the able-bodied subjects.

The findings were published in the September issue of Prosthetics & Orthotics International.

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