System

OHMG Overview

An osseointegrated prosthetic is an artificial limb that is directly connected to the bone of the target limb. This type of connection is typically far better than a common socket connection. Socket connections can cause a lot of pain and irritation to the user, as they rely on the socket having a tight grip on the target limb. As a result, I made the decision to use an osseointegrated connection system.

The Osseointegrated Human-Machine Gateway (OHMG) is a system devised through a collaborative effort by Max Ortiz-Catalan and many of his esteemed colleagues in Sweden. It utilizes electrodes implanted within muscles groups surrounding the osseointegrated fixture. These electrodes read the signals sent from the brain to the amputated forearm. They pick up on these signals and treat them as data, using them as indicators of which "muscles" in the forearm are meant to be moved. This data is then sent to the prosthetic limb and that movement is simulated in the moving of a finger or the rotating of the arm, for example. This technology has been provided to actual amputees who are currently using the system at extremely high levels of dexterity. However, it has only been used for upper limb prosthetics, and has yet to be put into practice for lower limb amputees. When I interviewed Alexander Thesleff, a PhD student working with Max Ortiz-Catalan on transfemoral implementation of the OHMG, he told me that lower limb prosthetics are lagging far behind upper limbs in terms of both research and application. This was the inspiration for my change in focus.

Joints

The joint system that I designed consists of three main parts. First is the osseointegrated implant rod which connects directly to the prosthetic knee joint. The data travels through wires in the center of the rod into the joint, where it is processed and turned into transferable signals. These signals then tell the joint how to move. This joint design is capable of simulating the exact movements of a human leg. It can rotate along the y-axis to an angle slightly larger than 90º, just as a knee would. It can also rotate along the x-axis. This is not normal movement for a knee, as horizontal rotation mainly comes from the hip. However, movement in the lower leg is instrumental in making this rotation–it generates momentum, stability, and enough strength to rotate our legs nearly 90º. So, in order to simulate this movement and help the prosthetic user to rotate their leg in this direction, I decided to have the joint do so. The 3" diameter spherical figure in the renderings to the left is the knee joint itself. It is the part that connects to the lower leg and rotates in the previously described ways. The cylindrical shape is where the sphere is housed. It also contains the circuitboards that work with the OHMG to process the signals from the nerves and the motors that rotate the sphere.

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