For patients missing a hand, one of the biggest challenges to regaining a high level of function is the inability to rotate one's wrist (pronate and supinate). These are essential movements involved in everyday tasks such as using a door handle, a screwdriver, a knob, or simply turning over a piece of paper. For those missing their hand, these are much more awkward and uncomfortable tasks, and current prosthetic technologies offer only limited relief to this problem.
A person with forearm amputation can use a motorized wrist rotator controlled by electric signals from the remaining muscles; however, those same signals are also used to control the prosthetic hand. As a result, patients can only activate either the prosthetic wrist or the hand at one time and have to switch back and forth. Furthermore, patients get no sensory feedback, so they have no sensation of the hand's position or movement.
A new artificial joint works instead with an osseointegrated implant system. An implant is placed into each of the two bones of the forearm — the ulnar and radius bones — and then a wrist-like artificial joint acts as an interface between these two implants and the prosthetic hand. Together, this allows for much more natural movements, with intuitive natural control and sensory feedback.
Patients who have lost their hand and wrist often still preserve enough musculature to allow them to rotate the radius over the ulnar — the crucial movement in wrist rotation. A conventional socket prosthesis, which is attached to the body by compressing the stump, locks the bones in place, preventing any potential wrist rotation, and thus wastes this useful movement.
The new device offers a much more natural range of movement, minimizing the need for compensatory movements of the shoulder or torso, which could dramatically improve the day-to-day lives of many forearm amputees.
Restoring the full range of movement to all degrees of freedom in which the forearm bones can move was not necessary — the key parameter for returning a naturalistic wrist motion is the axial, or circular, motion of the ulnar and radius bones.