The design consists of three main units; an adapter that mates to the socket of residual limb, an adapter that connects to the extremity assembly, and nut that draws the two adapters together.  The nut's threads engage with the threads on the Lower adapter, and when tightens draws the Socket Adapter and Lower Adapter together, which also engages a spline in the center of the device.  At this point the spline and the threads of the device share the load of the person's movement.  Also when the nut is fully tightened a spring loaded Detent Button enages the castellations in the nut thus locking the nut into position and disallowing  the nut from being able to accidentally untighten.

The original concept for this coupler was developed by team Spline, Richmond VA during the VA Innovation Creation Series in 2015.

This model has quite a few improvements to the original concept as follows:

1. The threads of the main nut have be re modeled and made courser for easier threading that is less prone to binding.  
2. The threaded area of the coupler has been increased slightly for more cross sectional area on the threads to reduce the possibility of thread tear out.
3. A thru hole has been added so that devices that require a thru hole in the pyramid are compatible with the coupler.
4. The spline in the center of the device has be reproduced using a smoothed spline to reduce stress during use, and thus increase strength.
5. A detent button has been added to the lower adapters.  This button is spring loaded and is designed to lock the nut in place once it has been tightened down.  This makes accidentally loosening the nut much less likely.
6. A suite of lower adapters has been included in the design;  one with a male pyramid on the bottom, one with female pyramid on the bottom, and one generic one that could be threaded to accept an off the shelf pyramid or modified to add anything else as well.

Assembly of the design is as follows:

1. Print using titanium or other appropriate metal the following parts; (a) 1 each Socket Adapter, (b) 1 each Nut, (c) (Appropriate number and type) of Lower Adapter, (d) (Quantity to match number of Lower Adapters) of Detent Buttons.
2. Shape the threads of both the Nut and Lower Adapter such that a good thread lead in is achieved.
3. Print using ABS or other sturdy plastic the following parts; (e) (Quantity to match number of Lower Adapters) of Blanks.
4. Obtain the following; (f) 1 each Square Profile O-ring (McMaster P/N: 4061T254 or similar)[http://www.mcmaster.com/#4061t254/=ypzk8l], (2XNumber of Lower Adapter) of Compression Springs (McMaster P/N: 9434K25 or similar)[http://www.mcmaster.com/#9434k25/=ypzohu]
5. Apply o-ring to the top surface of the Socket Adapter where the Nut will rest (Opposite the side with the spline feature).  The O-ring provides a seal from dust and dirt and maintains constant compression which provides friction to prevent untightening of the Nut.
6. Apply the nut with threads facing downward over the top of the Socket Adapter.
7. Apply standard flat head socket head screws to affix the Socket Adapter to the socket's threaded holes.  Note that the square feature on the spline in the center of the adapter should face forward on the socket.
8. Insert both the Detent Button and Blank into the cutouts on either side of the appropriate Lower Adapter.  Note that their is only one Detent Button per lower adapter and its orientation is based on whether the person's residual limb is on the right hand or left hand side of the body.  (Some filing may be necessary to obtain satisfactory fit.
9. Using a needle nose plier insert compression springs behind the blank and button into the corresponding circular features behind the button and blank.  There is a matching feature on the lower assembly.
10. (Optional) If a female pryamid Lower Adapter is used thread the four set screw holes using a M6x1 tap.
11. Assemble the remaining prosthetic device as normal.