Mechanical Partial Hand Prostheses

Point Designs Point Digit

Because the focus of our website is on human bionics, we do not typically cover strictly mechanical solutions. However, we’re going to make an exception. Partial hand amputations account for nearly 2/3 of all upper limb amputations. This equates to millions of people around the world. Bionic partial hand prostheses are ideal for these situations, but they’re also very expensive. Hi-tech body-powered and ratcheting mechanical devices, by comparison, are more affordable and well-suited for many tasks.

The Design Intent of Bionic Partial Hand Prostheses

The ultimate intent of most bionic hands is to replicate the natural hand’s functionality in every way. Indeed, we are beginning to see features that exceed human capabilities, such as exceptional grip force, extreme damage resistance, and the ability to rotate hands 360 degrees.

Bionic partial hand prostheses are not quite so ambitious. Because the human hand is so complex, each partial hand is unique. It may be missing one or more digits, parts of digits, or part of the palm — all in near countless configurations.

To address these varied needs, bionic partial-hands are modular. Each digit must be capable of interacting with other bionic digits but not dependent on them.

Vincent Partial 3 Hand

This imposes certain design restrictions. For example, a natural thumb not only opens and closes; it can also rotate laterally. Full bionic hands can duplicate this capability because they have the physical space to house dual actuators and larger motors. An independent bionic thumb unit does not have this luxury.

There are also limits on force and stress tolerance because a partial hand still has flesh and bone components.

The main objective of a bionic partial hand prosthesis is therefore to support more basic functions:

  1. The ability to grasp an object.
  2. The ability to hold onto that object while doing something with it.

It is against these more modest objectives that we will compare body-powered and ratcheting mechanical alternatives.

If you wish to further familiarize yourself with the capabilities of bionic partial hand prostheses, please see Ossur’s i-Digits products and the Vincent Partial 3 hand.

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 marketing video from one of the leaders in this field, Point Designs:

This much shorter video gives you a closer look at the actual ratcheting mechanism:

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.

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 nuance, 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 in 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 perform a lot of light-to-moderate tasks requiring two hands, and you can’t afford a bionic prosthesis, you should probably lean toward a body-powered vs a ratcheting mechanical solution. In the very least, do a few walkthroughs where you’re frequently using one hand to shape and release the other. This will at least help you understand the extra motions involved.
  • Whatever solution you choose, temper your expectations that one device will meet all your needs. We can’t yet say this with authority about partial hand prostheses because we haven’t encountered enough independent, third-party feedback. But we can tell you that the happiest users of full bionic hands are those who use different devices for different activities.

Related Information

For more information on upper limb bionics, see Understanding Bionic Arms & Hands.

For a list of competitor devices, see All Partial Hand Options.