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Independence Through Humeral Rotation In the Conventional Transhumeral Prosthetic Design

Joseph J. Ivko, Sr.

ABSTRACT

This article discusses a new design in achieving humeral rotation in the body-powered prosthesis for the transhumeral bilateral and unilateral amputees. The humeral rotator (H.R.) unit has the ability to rotate 220°: 165° to the medial range and 55° to the lateral range. It affords positive locking in all positions to overcome side pressures. Side pressure is any force that causes the prosthetic arm to involuntarily move to the medial or lateral side.

Working models have been constructed and tested to evaluate the concept. The result was a 75% improvement in function, efficiency, and appearance. To increase the independence of a transhumeral amputee, it is recommended that the H.R. unit be used whenever possible.

Key Words: Prosthetic design, humeral rotation.

This article describes the development of the only body-powered humeral rotator that increases the user's independence and confidence in function and efficiency¹ (Figures 1a , 1b , 1c , and 1d ).

One of the problems a bilateral transhumeral amputee faces is correctly positioning the forearm and terminal device to perform basic activities of daily living. This rotation is accomplished by pulling or pushing the friction turntable against a fixed object within the environment.

Tests involving 51 activities performed by persons with intact arms showed humeral rotation is needed for these tasks. Most tasks required rotation in the range of 20° to the lateral range and 40° to the medial range² (Figures 4 , 5 , 6 , 7 , 8 , 9 , 10 , and 11 ). Significant use of humeral rotation is needed to function in our society.³

An electric-powered rotator has been attempted in the development of the Utah Artificial Arm, however, no further development has been reported.⁴

All commercial mechanical elbows use friction mechanisms to provide a torque against humeral rotation. Hosmer Corporation and USMC use a cork disk between the proximal surface of the elbow and the distal surface of the lamination collar, compressed by a nut and washers on a threaded stud.

None of the commercially available rotators have positive locking to overcome side pressure, except for Boston Elbow II and MK. 14 Mechanical Elbow. They both apply manual locking, applicable only to the unilateral transhumeral population.

Methods

The H.R. unit can be used by all transhumeral amputees. The system consists of a lower turntable made of 304 Stainless Steel; a Stainless Steel needle bearing; a Stainless Steel thrust bearing and washers; and a single spring-action lock housing and lock pin. The upper turntable is a lamination collar modified to accommodate a lock housing, a lock pin, and pin stops to provide rotation to 220°⁴ (Figures 1a , 1b , 1c , and 1d ).

At this point, it is important to note that the friction-free cable system is an important part in activating the H.R. unit.⁵ The humeral section and forearm need to have cabling revised from the lateral side to the medial side of the arm.

How to Operate the H.R. Unit

Shoulder elevation and arm extension is used to lock and unlock the elbow is. The very same motion is used to unlock the H.R. unit, with further arm extension involving the anterior deltoid muscle to unlock the H.R. lock. Once the rotator is unlocked, the arm automatically rotates to the medial range of motion. This rotation is caused by pressure being applied to the harness strap across the scapula going to the cable. Slight pressure is applied to the harness while arm extension is applied to unlock the H.R. unit (Figure 3 ). The same motion of further arm extension is used to return the arm to the neutral position, unlocking the H.R. unit and pushing the forearm against the body until it is in neutral position.

Analysis

The H.R. unit performs quietly and is easy to use. The alignment of the lower and upper turntables ensures accurate positioning at all times. The positive lock ensures stability and confidence to overcome side pressures (Figures 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , and11 ).

The E-400 Hosmer Elbow and similar elbows require a stud replacement to accommodate 1/4-in. to 3/8-in. length to install the H.R. unit. The upper turntable needs to be modified to accommodate the lock housing and lock pin, and pin stops are needed to regulate rotation. This system can be used on a new prosthesis or retrofitted on an existing prosthesis.

Testing

An applied force to create a shear of the locking pin on the H.R. unit was applied in increments of 50 foot-lb. When a total of 350 foot-lb. of pressure was applied to the prosthetic arm in which the H.R. unit was installed, the prosthesis was destroyed. The amount of force needed to create a failure in the system (to shear a lock pin), is 650 foot-lb. The H.R. unit has been tested by bilateral transhumeral amputees for 20 years without problems or failures.

Summary

Conventional body-powered prostheses, wherever possible, are far more functional and efficient. The reaction-time to response-time is immediate, and noise level in body-powered prostheses is held to a minimum. Body-powered prostheses, when fitted with the H.R. unit and friction-free cable system are dependable, stable, functional, and efficient (Figures 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , and11 ).

Acknowledgements

Our very special thanks to Craig W. Heckathorne, M.S.S.E., Research Engineer at the Prosthetic Research Laboratory at Northwestern University, Chicago, Illinois, for his invaluable assistance in researching material to complete this article.

References:

1. Ivko JJ, Ivko RM. Using body power to achieve humeral rotation. Prosthetic Arm Having Humeral Rotation, U.S. Patent #3,526,007.
2. Taylor C. The biomechanics of the normal and of the amputated upper extremity. Klopsteg PE, Wilson PD [eds.] In: Human Limbs and Their Substitutes, New York: McGraw-Hill, 1954;169-221.
3. Engen TJ, Spencer WA. Development of Externally Powered Upper Extremity Orthotics. Final Report. Texas Institute for Rehabilitation and Research, Houston, Texas. January, 1969.
4. Jacobsen SC, Knutti DF, Johnson RT, Sears HH. Development of the Utah Arm. IEEE Transactions on Biomedical Engineering. BME 1984;29(4),249-269
5. Esparza W, Ivko J. Friction free cable system: alternative cable system for transhumeral-level conventional prostheses. Journal of Prosthetics and Orthotics. 1997;9:135-136.