New Upper Limb Prosthetic Doo-Dads from ISPO
As noted in last month's introductory comments, 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.
Electric lock for Body Powered Elbow
This is not a new idea, as the folks at Northwestern University designed an electric lock module some years ago that could be retrofitted to the Hosmer E-400 elbow. Unfortunately, it is not commercially available at this time. This is unfortunate, as locking and unlocking with body power is one of the most difficult aspects of upper limb prostheses for many amputees. And, we go nuts as prosthetists trying to find control sites that are simultaneously unobtrusive, easy to control independently, and that don't restrict the useful range of motion of the ipsilateral shoulder.
The folks from Otto Bock have just stepped in to fill this void with a new version of their body powered ErgoArm elbow system. It was on display in Glasgow and is slated to be available worldwide "shortly". I am told it can be retrofitted to existing units in the field as well.
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Like many of their recent developments, it uses colored coding plugs to change the method for actuation. The display model was set up for two-site lock-unlock and that worked extremely easily for me. In this mode, a biceps signal might trigger the lock while a triceps signal would release it. Interestingly, the elbow will be delivered with the standard Spectra cable release as well so the amputee will have that option too.
Other control options include co-contract and relax, slow-fast rate differential, single site lock-unlock toggle, and a range of switch options. This should be a great feature for the higher-level amputees who traditionally wind up with a harness that is very snug and somewhat restrictive, as well as for the amputee who just wants less harnessing hassles.
John Billock CPO of Warren, Ohio has decades of success fitting transhumeral patients with a body powered elbow and myo hand. I try to emulate John's success, and this has become my preferred method for creating a lightweight, responsive, and practical system for most patients. From now on, if I am using an ErgoArm elbow and an electric Terminal Device, I will routinely suggest the use of the electronic lock feature since it can be set up to use the same control sites that are required for the TD.
Finally, I noticed that the latest ErgoArm setups have a much nicer method for making fine adjustments to the unlocking cable. A simple plastic piece is included that reminds me of the tensioners that were used on my old camping tent to adjust the tension at each stake. During a recent fitting, this made determining the optimal length for the unlocking cable much simpler.
It is will worth spending some extra time fine-tuning the locking cable when you fit an ErgoArm as many amputees learn to use the "Slip-Stop" feature with a little practice. In addition to the familiar lock-unlock toggle, the amputee can also decide to pull the cable only about a millimeter. This relaxes the tension on the wrap clutch that locks the elbow, and the forearm slowly drifts down due to gravity. As soon as it reaches the desired location, the amputee simply relaxes and the wrap clutch locks the arm in position immediately. For some with high levels of loss, this has proven to be an easy way to correct for slightly over-shooting the intended locking position. The video clip below illustrates how this looks at the time of initial fitting.
AFB Slip Stop: Pulling the elbow lock cable just a millimeter allows the forearm to drift down; relaxing automatically stops further motion when the wrap clutch re-engages.
New Ring Harness
Speaking of a new twist on an old idea, Bock was also showing the first advancement in ring harnesses in decades. I am told this is not yet available but the pre-production sample on the manikin looked quite intriguing.
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The version on display was set up for a body powered elbow and hook terminal device, but closer inspection revealed that there are three small countersunk holes in the plastic ring. Each of those represents another possible origination for a Perlon control cable, so one harness should be suitable for a broad range of amputees and their needs.
If you look closely at the photo, you will notice that there is a small white plastic block that connects the harness to the prosthesis and also anchors the Perlon cabling. John Billock has used Perlon cabling on his hybrid systems for decades but it has always been a hassle to find the right connectors. This new harness should come most of the necessary hardware built in, so this will make trying a Perlon cable in lieu of the more traditional metal cable/housing more practical.
I generally try a Perlon cabling system for hybrid arms too. This new harness should make setting up and ErgoArm elbow far quicker and easier than was the case previously. I prefer the European custom of routing the elbow flexion cable along the medial aspect of the humeral section because it is less visible and therefore less objectionable to the amputee.
With Perlon, I copy the German practice of using just a small D-ring located about mid-humeral level on the medial midline as a guide. This eliminates the bulk, hassle, and robotic appearance of the traditional US metal cable housing entirely. I know this is hard to visualize, so I have attached a short video from a recent collaborative fitting showing from a posterior view how this harness/cable routing works. The video shows routing clear Perlon plastic cable from the Control Attachment Strap through a small medial D-ring and attaching it to the lateral proximal aspect of the forearm provides a biomechanically effective and unobtrusive method for elbow flexion control.
