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Horton's Stance Control Orthosis: Self-Locking Joint

The most intriguing orthotic concept that I saw was also being developed by a group of ABC Certifees: Horton's Stance Control Orthosis. Hortons is a small private facility in Little Rock, Arkansas, and they have licensed the rights to a self-locking joint originally developed by engineers from the National Aeronautical and Space Administration in nearby Huntsville, Alabama. There is a small posting about this project on the NASA web site at: http://techtran.msfc.nasa.gov/new/kneelicense.html.

The guys from Hortons were showing examples of a pediatric and adult KAFO using the current version of the lock. The joint heads are rectangular, and about twice the thickness of typical single axis joints. They attach to sidebars in the conventional manner, so fabrication should be pretty straightforward.

The release mechanism on display used a tough plastic push-rod to actuate the lock. In my hands, it seemed to engage and release positively and securely. The actuating mechanism is a second pull of thermoplastic in the heel region of the shank segment of the KAFO.

Under normal conditions, the knee is unlocked and the leg can move freely in the sagittal plane. Immediately after heel contact, the thermoplastic "stirrup" is displaced upward by body weight. This pushes on the plastic rod that engages the knee lock.

The knee remains locked throughout most of stance phase, releasing automatically just after heel off - when the stirrup is free to return to its normal position. According to the Horton's folks, this simple mechanical actuation has looked very promising in preliminary clinical trials.

To my eye, the present actuating assembly is a bit bulky. But, it should be fairly straightforward to downsize it with a little engineering expertise, if this release algorithm proves applicable clinically. And, it seems much less "finicky" and more durable than other actuator concepts I have seen through the years.

For at least the past century, the dream of every orthotist and rehabilitation engineer has been to design a practical, safe, reliable, and low cost method to automatically lock and unlock the knee joint during ambulation. Almost every major "think tank" in recent decades has confirmed that this remains one of the most significant orthotic challenges remaining today, as the report from the NCMRR on P&O Research for the 21st Century confirms [www.oandp.org/jpo/64/64100.asp].

What are the chances that a private clinical practice in the southland will be the first to successfully meet this challenge? No one knows at this point, but it is exciting to see such seminal work being done in a "real world" setting. Practitioner-generated developments are often unsophisticated but almost always possible to implement in a clinical practice. More elegant solutions sometimes suffer from implementation limitations.

At the most recent ISPO Congress, in Amsterdam in 1998, Ambrose of Holland was showing a different style of lockable KAFO triggered by ankle motion, called the "UTX Swing". I think this was theoretically available in the USA through their distributor in Minnesota, but I'm not aware of any actual clinical applications in the States.

The "UTX Swing" is a unilateral locking system, and therefore has the same limitations for adult applications as any other unilateral orthosis. The requirement for at least 15 degrees of plantarflexion motion at the ankle to disengage the lock further limited the applicable patient population. And, their plan was to make it available only on a central fabrication basis. But, it is a very lightweight and intriguing concept in its own right. For more information on all the Ambrose designs, go to www.ambroise.nl; click on "Products" to find the UTX device.

The concept from Horton's avoids these limitations: it is basically an individual component that can be fabricated successfully in any P&O lab. The heel stirrup trigger allows it to be used with solid ankle or free motion ankles. And, the double locking configuration makes the orthosis structurally stronger and significantly reduces the risk to the patient in the event of a lock failure.

Horton's also showed a pediatric version of the locks, which were appropriately downsized. Their pediatric sample was also dual locking system, but I presume a typical pediatric single sidebar version could be made if the loading was low enough to make this safe for the child.

While pondering the applicability of an autolocking othosis, it should be remembered that controlling stance stability and then releasing it is only half of the challenge of restoring ambulation. Cadence responsive swing damping is the other half, and very little work has been done in this area to date. But, a reliable and low cost self-locking orthosis is certainly a "first step". And, we know that many folks with paralysis walk very slowly due to the range of their disability, so they will have minimal swing control requirements.

To me, one of the most promising applications for Horton's approach would be for adults who could probably walk with floor reaction AFOs under controlled situations but today must use locked KAFOs for safety reasons. Many unilateral involvements, including paresis or paralysis secondary to polio and similar diseases, present with this clinical picture. Let's hope that Horton's and NASA are on to something here, and that we will soon have a new element in our armamentarium of orthotic joints.

To contact the folks at Hortons for more information, you can visit their web site at www.hortonsoandp.com.