Turntable Lock for Elbow Units
Fred Sammons, B.A. *
In the conventional elbow unit (Hosmer E 400) for above elbow and shoulder dis articulation amputees, manual control of humeral rotation is permitted by virtue of cork and teflon gaskets providing mechanical friction between the top of the main portion of the elbow unit and the turntable to which the upper arm shell or socket is fastened. The amount of friction is determined by the tension maintained by the stud and attaching nut. Since humeral rotation great as to rule out easy adjustment. At the same time there must be enough resistance to rotation to accomplish most activities.
There are times when a rigid arm is desirable; for example, when climbing ladders, using a shovel for long periods, carrying an object balanced on the forearm, or carrying an object held away from the body. To provide rigidity for such demanding tasks, the North Western University Prosthetics Research Center has developed a manually controlled lock which can be mounted on the area provided for a forearm lift assist on the Hosmer E 400 elbow unit. A spring forcefully engages the locking pin in one of three holes drilled through the turntable for this purpose. Since the turntable possesses enough friction for most activities, the locking pin need only be used to overcome the tendency of the forearm to rotate gradually when shoveling, to provide the extra margin of safety when climbing vertical ladders, or to supply the rigidity needed in certain other tasks. The amputee returns the locking pin to the disengaged position when the task is completed.
Installation of the lock requires: first, drilling the indexing holes in the turntable; second, revising the plastic cap on the elbow unit and mounting the locking device; third, cutting a notch in the cork and teflon gasket to make room for the locking pin and regluing the gasket to the elbow unit.
Fig. 1 and Fig. 2 are views of the locking device, and Fig. 3, Fig. 4, and Fig. 5 show details of its installation.
The first prototype (not shown) of the lock was fitted to DM, a 38 year old farmer who is a left above elbow amputee. The lock was mounted on the posterior rim of the elbow frame. A 3/16 in. locking pin was used and has proved to be very durable. The lever which latches and unlatches the lock has been replaced because of breakage. A disadvantage was the requirement for modification of the elbow frame and extensive modification of the cork and teflon gasket. Another disadvantage was the location of the lock lever at the back of the elbow rather than at the side. The device has been worn continuously for 20 months with no malfunction in the locking pin. One unit of the second prototype (as shown in the illustrations for this article) of the lock was fitted to EA, a 38 year old farmer and bulldozer operator who is a right above elbow amputee. The device has functioned well for a period of more than 16 months, and the amputee reports that he uses il several times daily. He is able to lock and unlock the device without removing winter clothing.
Another unit of the second prototype of the lock was fitted to IS, a 40 year old farmer who is a right above elbow amputee. The device malfunctioned after six months when the elbow became free moving without the usual amount of friction. This caused excessive strain on the locking pin, which bent under the load. The pin was replaced, friction was restored, and the device has worked for 10 additional months. The amputee reports using the lock when holding materials to be butt welded, when climbing ladders, and in other situations where a static arm is required.
- Aitken, George T., T. J. Bushey, Edward R. Ford, and Fred Leonard, Upper-extremily components, Chap. 2 in Orthopaedic Appliances Atlas,Vol. 2, Edwards, Ann Arbor, Mich., 1960.
- Sammons, Fred, Elbow rotation lock, Northwestern University Prosthetics Research Center, July 1964.