New Lower Limb Prosthetic Doo-Dads from ISPO
As noted previously, I saw no gigantic technological leap forward in P&O technology at this World Congress. However, I did see several clever and promising new ideas.
New Thermoplastic Liner Material
As most readers know, the clinical application of elastomeric liners has gradually increased since the 1980s, when Ossur Kristinsson introduced his Icelandic Roll-On Suction Sockets. Although the original ICEROSS were custom fabricated over a specially modified mold of the patient's residuum, the hand fabrication required made it expensive and time-consuming and made subsequent replacements problematic.
During the 1990s, various manufacturers introduced a large array of prefabricated elastomeric liners in an effort to make them more consistent and simpler to provide. The inherent difficulty with prefab liners is that they don't fit as precisely as custom made ones, particularly when the leg shape is irregular or markedly tapered. Ohio Willow Wood's Alpha liner was the first composed of a thermoplastic elastomer that could be remolded over a positive model.
|
In Glasgow, the Swedish company Centri introduced a new type of thermoplastic material that I found quite interesting. It has a "feel" somewhat similar to Poron although it seems a bit denser. The radial stretch seems quite good as does that in the longitudinal direction. The outer surface has a glossy finish that is reported to slide easily into the socket, while the inner surface has a "sand blasted" finish that seems to cling to the skin much as silicone does. According to the Centri folks, there is no need to use a textile over the liner for donning or doffing.
What really impressed me, however, was how well it could be thermoformed without thinning out the wall thickness. The liner can be worn just as it comes out of the box, if that is comfortable for the patient. But, it can also be thermally molded over a positive plaster model at any time, and seems to conform as readily as polyethylene foams like PE-lite. Click here for the manufacturer's thermoforming instructions.
Since this is a brand new product, I couldn't find anyone outside the company who had any clinical experience with these liners to report. But, the material properties are intriguing enough that you may want to take a look for yourself. They are available with and without a distal pin attachment. Contact Centri at www.centri.se for more details.
OSSUR TALUX
|
The Scandinavians had a number of innovations at this ISPO meeting. OSSUR announced their latest Flex-Foot product, called the Talux. There was a great deal of interest among the practitioners present, and the amputees who had tested it were very enthusiastic about the smooth rollover and multiaxial motion it provided. They also felt that the forefoot spring support was just as good for normal walking as more rigid Flex-Foot products they had tried.
It looks similar to the Vari-Flex, but a block of pretty resilient polyurethane foam that seems to really enhance the range of motion separates the dual carbon fiber springs. A posterior textile tether limits dorsiflexion for midstance stability, but the Talux seems to move very easily into plantarflexion, inversion, and eversion, as well as some transverse rotation.
As was the case with the Centri liners, I couldn't find anyone outside the company who had used this foot long term and could report from clinical experience how it functions. But, the concept looks sufficiently interesting that readers may want to check it out for themselves. More details can be found at www.flexfoot.com on the Web.
TEC Harmony
|
Probably the most innovative lower limb item I saw at this meeting was the Harmony socket suspension system from the TEC folks. Because I lived in the Minneapolis area until this summer, I have been following this development for a couple of years as Carl Caspers CPO refined it and tried to figure out why it works. Carl kept us appraised of his progress at the local Academy meetings, so we knew this was coming out as a product eventually.
Many readers will recall that Carl started the current upsurge of interest in thicker elastomeric liners with his original Total Environmental Control custom liners. Carl's vision, as I understand it, has always been to use the liner-socket interfaces to solidly "link" the residual limb with the prosthesis and minimize the psuedarthrosis effect. The better we can connect the artificial leg to the amputee, the better their proprioception, control, and presumably energy efficiency in ambulation.
One of the inherent limitations in elastomeric liners that are anchored to the socket with a shuttle lock or lanyard is that they concentrate forces about the distal aspect of the residual limb, particularly when the socket displaces slightly form the force of gravity during swing phase. Marty Carlson, MSE, CPO explained this concept at the Academy Symposium on "Liners" last spring, and noted that one way to prevent this problem would be to somehow "glue" the liner into the socket after the amputee donned the leg. Unfortunately, no one has developed a reversible adhesive that would release so the patient could remove the leg!
|
The TEC Harmony uses negative pressure from a vacuum pump to anchor the outside of the liner to the socket wall. When the vacuum is released, the liner can then be easily removed from the socket for doffing. The vacuum is generated automatically when the amputee loads and unloads a special spring-loaded pylon during normal walking; one version is integrated with the OWW Pathfinder foot. Similar to a four-cycle gas engine, the Pathfinder version uses the down-stroke of the piston for socket suspension and the up-stroke for shock absorption and plantarflexion resistance.
Carl has worked with engineers at a local college to investigate the effect this has on the residual limb. Their pilot data suggests that not only does this high-vacuum between the socket and liner reduce pistoning and tissue stresses, but it also reduces the volume fluctuation of the residual limb significantly. The postulated mechanism is that the elimination of liner traction and compression eliminates the "milking" of fluids out of the residuum. If this is borne out by subsequent investigators with a large enough number of subjects to achieve statistical significance, this may be one of the most significant discoveries in lower limb prosthetic practice. Changes in residual limb volume are responsible for the overwhelming majority of skin problems amputees experience, so if this suspension also maintains a constant limb volume this will be a great breakthrough.
Additional information is available at www.tecinterface.com/products.html if you are interested.
Most Controversial "Outcomes" Paper at ISPO
Of the hundreds of papers presented at this latest ISPO meeting, the one that generated the most debate and consternation among the practitioners was the study conducted by the Prosthetic Research Study in Seattle on one of the thermoplastic gel liner systems. This was a pretty straightforward comparison of 13 transtibial amputees who alternated wearing a prosthesis with a locking liner with one using a PE foam liner. So far as I could tell, this was a well-constructed protocol with the order of the liners randomized and a three month period to collect data.
PRS study used the StepWatch Activity Monitor to record amputee walking and provide an objective measurement of their ambulatory activity.
In addition to the usual subjective opinions from the amputees, the PRS folks also collected more objective data including validated measures of socket comfort, residual limb pain, and the number of steps taken each day. Not surprisingly, the results didn't demonstrate any significant differences in socket comfort: a well-fitted prosthesis is pretty comfortable regardless of the liner material selected.
However, there was a significant difference between the two types of prosthesis in terms of the average number of hours worn per day, the number of steps taken - in other words, in the overall activity level of the amputee. The amputees were clearly more active with the PE foam liners...and the majority chose to keep the PE foam liner prostheses at the end of the study period!
Obviously, this was not an intuitive result since most people would assume that the comfort liners would be preferred and probably encourage a higher level of activity too. But, it is precisely such counter-intuitive results that teach us the most by pointing out that "common sense" is not always accurate.
I have no explanation for these results at this time, but I'm pretty confident they are valid. PRS has a very long and well respected history of conducting valid and clinically relevant studies in P&O, and their team of investigators is well versed on the field.
The Principle Investigator was Kim Coleman, and she tells me that an abstract has been submitted to present this work at the next Academy meeting. I would encourage everyone to attend the next time this study is presented at a meeting, to try to understand the details of how it was conducted and to ponder what the results tell us.
I have a hunch this may be as significant as the studies a decade ago showing that "energy storing" feet do not reduce the energy cost of ambulation at typical amputee walking speeds. We may not like such outcomes, but we need to understand them and learn why our assumptions weren't supported by objective data.
In the case of dynamic response feet, subsequent studies showed clearly that there is a significant reduction in energy cost at faster cadences. So, clinicians were right to feel that such feet "made it easier for the amputee to walk" but we were wrong to assume this was independent of their cadence. As there are more studies of elastomeric liners published, we should be able to refine our understanding in this area too.