Mechanical Partial-Hand Prostheses

Point Designs Point Digit

We don’t normally cover non-bionic devices, but with a lack of affordable options for partial-hand amputees, we’re making an exception.

Body-Powered Partial-Hand Prostheses

A body-powered partial-hand prosthesis is basically a mechanical extension of the residual natural hand.

Naked Prosthetics Body-Powerered Partial Hand Prosthesis

Source: https://www.thelondonprosthetics.com/our-clinic/insights-and-case-studies/naked-prosthetics-body-driven-devices-innovative-t

Its design objective is the same as that of its bionic counterpart, i.e. to allow users to grasp and hold objects. But it achieves this using strictly mechanical means powered by flexing the residual hand’s remaining muscles.

The degree to which this is possible, including the amount of force generated, depends in large part on the remaining musculature. However, this can be amplified through an ingenious use of leverage.

Naked Prosthetics is a leader in this field and has mastered these techniques, as the following video demonstrates:

To measure its product’s effectiveness, Naked Prosthetics followed up with 102 patients one year after they were fitted with its devices. 95 of those patients still wore their devices daily, which is an extremely high retention rate for any prosthesis.

These devices are superb examples of engineering and innovation, but they are not ideal if there is insufficient residual hand structure to support them, or if the user lacks the necessary muscular control.

Ratcheting Mechanical Partial Hand Prostheses

A ratcheting mechanical hand prosthesis does not require direct muscular control because it does not actively participate in gripping objects. Instead, the user bends the mechanical digits around an object using his or her other hand. A finely tuned ratcheting mechanism locks the fingers in place, resulting in a firm grip.

Here is a short video from one of the leaders in this field, Point Designs, that provides a close-up look at their ratcheting mechanism:

Here is a short video showing how quickly an experienced user can position the fingers:

And here is an example of how reliable and effective the device can be once the fingers are properly positioned:

The physical requirements for this type of device are minimal. First, there must be enough residual hand to support the prosthesis. Second, the user must have sufficient use of his or her other hand to bend the mechanical fingers into place around a target object and then release the grip afterward (typically by pressing a release button).

The advantages of this type of partial hand prosthesis are:

  • high durability because of the purely mechanical nature of the digits;
  • high load capacity once the fingers are locked into place.

We should note that Naked Prosthetics also has a ratcheting finger called the “GripLock Finger”. We just wanted to show you the products from both leading companies in this space.

How to Choose Between the Different Partial-Hand Prosthesis Options

The most important step to choosing the right partial-hand prosthesis is to find a prosthetist who is intimately familiar with all the available technologies. Partial-hand solutions are complex. Working out the best solution for your situation and objectives involves a lot of nuances, and there is nothing like experience when dealing with nuance.

That aside, here are some general points to keep in mind:

  • Bionic solutions are appealing because they are more intuitive and augment your hand strength with electrical power, but they’re also fragile. Bionic fingers don’t stand up well to repeated heavy loads or challenging conditions involving water, a lot of dust, etc. If you face these challenges, you’ll at the very least need two solutions: a bionic prosthesis for light duties and something else for heavier duties.
  • If you do choose a bionic solution, focus a lot of attention on the myoelectric control component, both in terms of selecting your solution and training to use it. To understand why, please read our article on Myoelectric Control Systems.
  • If you opt for a non-bionic solution, the shape and condition of your residual hand will likely dictate whether you need a body-powered versus a ratcheting device.

Related Information

For a list of all partial-hand options, including bionic options, see All Partial Hand Options.

For a comprehensive description of all current upper-limb technologies, devices, and research, see our complete guide.

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