MSM wrist 0.1-revision [open prosthetics]

After an initial prototype was built and after I used it for a while, two major inconveniences (1: inbus screw issue; 2: cable pull direction has to match sprocket design issue) became apparent.

Since our roadmap currently does not provide for a new major MSM wrist release in the near future and since Otto Bock has not announced a comparably sturdy design, we decided to move towards fixing the MSM wrist 0.1 prototype's initial issues.

Building on our initial prototype MSM wrist 0.1, there are a number of MSM wrist 0.1-revision features:

* sturdy design, minimal amount of parts, quick lock/release mechanism with one-directional lock / one-directional release without dedicated locking mechanics (that'll be part of the next version)

* full compatibility with ratchet-free Otto Bock standard wrist and Otto Bock standard bolts (Nutzapfen).

* cable-pull-locked wrist in either direction (counter clock wise for Otto Bock Movohook 2Grip, clock wise for Otto Bock VO system hand).

Documentation of modifications

The hand now features a sprocket with a mirrored shape:

The hand sprocket (left) and the sprocket for the hook (right) feature a reversed or mirrored design. Also you can see that we used six M4 inbus screws rather than three M3 inbus screws to keep the terminal device fixed in the sprocket.

Explanation of principle behind the modification

Why do I need a mirrored sprocket for the hand? How does that lock really work? What's behind the wrist unit's high stability under practical use despite an apparently loose design?

The hook is opened with a cable pull direction (see cable and adjacent white line) that is not parallel to the plane (curve around wrist: red line) and main rotation axis (dashed red line) of the wrist. Thus, pulling the cable will both pull the terminal device close (long green arrow) and lock the hook's wrist connector into the wrist (short green curved arrow).

The hand features a different cable pull direction (black line). Due to the angle towards wrist (red line) and main rotational axis of the wrist (dashed red line) the cable pull will also lock the sprocket connector (short blue curved arrow) and not just pull the terminal device towards the wrist (long blue arrow).

Also, the sprockets feature minimally narrowing channels for the six wrist bolts so a mild amount of wedging is enacted; furthermore, the channels of the sprockets are not totally horizontal. The trick lies in clever choice of material in combination with a wicked design that's sturdy and relatively easy to use.

That in combination allows for a high practical usage with wiggle free operation despite a simple construction. I am fully relying on this wrist unit now.

Some features:

Previous / further development see MSM wrist roadmap and MSM legal aspects / file download area. Operational and design based analysis by myself and Stephan Müller, technical analysis and work done by MDP Meili, Kaltenbach, Switzerland - all issues discussed together. If you have similar questions, get in touch with Roman Meili.

Cite this article:
Wolf Schweitzer: - MSM wrist 0.1-revision [open prosthetics]; published 08/04/2009, 14:51; URL:

BibTeX: @MISC{schweitzer_wolf_1656511401, author = {Wolf Schweitzer}, title = {{ - MSM wrist 0.1-revision [open prosthetics]}}, month = {April}, year = {2009}, url = {} }