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Socket Design Workshop

For me, the highlight of the meeting from a scientific standpoint was the OSSUR Workshop on "Socket Design Principles". Greg Michalov CP did a very good job of explaining the Scandinavian concepts about socket configurations that have been driving the OSSUR products in recent years. The general theme was the progressive evolution from localized pressures [specific weight bearing areas and reliefs in the PTB or Quad sockets] to more global pressures in Total Surface Bearing and similar sockets. The potential payoff from using quasi-hydrostatic weight bearing principles is that the peak pressures on the skin are lower since the load is spread more uniformly across the residuum.

One major question, of course, is whether these modern theories have any basis in fact. The second half of the Workshop addressed this question by presenting a summary of the doctoral thesis work done at the University of Strathclyde by Arjon Buis of the Netherlands. Dr. Buis presented a cogent and thought-provoking summary of his research, which involved comparing the dynamic distribution of pressures in transtibial prosthetic sockets.

Of particular note was the clever method he used to visually summarize the millions of points of data measured throughout the gait cycle. Using a variation on the F-Scan sensors, Dr. Buis created a color-coded pressure map of the entire inside surface of the sockets while the amputees walked in them. In PowerPoint, he was able to "unroll" these three-dimensional maps onto the screen and present them as animated digital videos.

This was similar to watching digital equalizer output while listening to music: at first it was just pretty images. But, as you watched the video loop again and again, it was easy to see where the patellar tendon, fibular head, distal tip of the tibia, and other high-pressure areas were located. Once that was clear, it became simple to compare the effects of a PTB versus a "hydrocast" socket for the same amputee. There was no doubt that the PTB [even though the patients rated it as comfortable and well-fitting] had more localized forces and significantly higher peak pressures on the skin.

To his credit, Dr. Buis was careful to state the conclusions of his work conservatively. He suggests that pressure-casting methods are likely to be inherently more effective at reducing peak skin loads than hand casting, as well as more replicable by the novice. But, he doubts that using water pressure to pre-load the tissues during casting as he did for this thesis is the final answer. He also noted that one risk with such sockets is that the residual limb tends to "float" within the socket, resulting in rotational instability.

In many ways, this is the transtibial extension of the concepts first proposed by Dr. Robin Redhead from England back in the 1970s. Dr. Redhead used a kind of elastic stockinette as his pressure casting aid, attempting to reduce the localized ischial pressures inherent in the quadrilateral socket of that era. Unfortunately, his ideas were almost impossible to implement clinically thirty years ago, so they were hardly noticed by most practitioners. It is exciting to see that intriguing ideas from decades ago are now able to come back to life thanks to the efforts of forward-thinkers like Dr. Buis, and the application of today's technology to prosthetic and orthotic conundrums. Interested readers are welcome to contact Dr. Buis via email at: biebel.buis@wxs.nl.

Dr. Redhead's article is well worth reading for a perspective on this issue. The citation from the RECAL database, a fantastic resource maintained by the University of Strathclyde, is reprinted below. Copies of the article can be purchased for a modest cost by requesting publication #00080 from www.recal.org.uk if you don't have this back issue of the ISPO journal handy.