Today there is an increased interest in powered prostheses with improved functionality. Every time new features are made available, the profession is forced to address anew the challenge of coupling the hard structures of a prosthesis to the soft tissues of the remaining limb. Usually added function equals added weight. To accommodate this weight we have been forced to revisit socket designs. Often, when a user complains of too much weight, it simply means that the socket does not fit. In this issue, the most experienced people in the field address how to improve the way sockets attach to the user. Most of the older techniques of collecting information with a plaster cast overlook the anatomy underneath. In this issue, you will learn how prosthetists can improve the comfort of the prosthetic fit while at the same time increasing the ability of the socket to transmit forces and torques to the terminal device (TD) and to other aspects of the prosthesis used to interact with the environment. Even the best sockets and liners fail to address some of the other major problems of amputees. I have outlined below some of the areas that still need to be studied. Some of these issues are being addressed at this time by individual machine designers, prosthetists, and surgeons; however, until more people are addressing them and the results are published, users will continue to struggle.
CAPTURING USER-GENERATED ROTATION ABOUT THE LONG AXIS OF THE REMAINING LIMB
At present an elbow disarticulation leaves the epicondyles intact which permits the prosthetist to preserve internal and external rotation. In shorter amputations the humerus can still rotate under full natural muscle control; however, there is no easy way to couple this rotation to the prosthesis. Solving this problem needs cooperation between the surgeon, the prosthetist, and the patient. In a few cases the new method of osseointegration1 will allow direct coupling of the prosthesis to the remaining limb. There are some contraindications to this procedure. It should be limited to persons who will be scrupulous in their hygiene where the implant passes through the skin. Further the implant requires considerable time. An alternative is to install artificial epicondyles at the end of the remaining humerus. This procedure has been done in Norway2 and the optimal shape for the implant is still under study. A third option where there is sufficient bone or where bone can be harvested from other areas of the body is the Marquardt angulation osteotomy.3 This procedure, long used in Germany, is now being adapted by surgeons in the United States. The field eagerly awaits the appearance of a article outlining the best protocols for each level of limb loss.
When direct coupling is not practical, rotation of the humerus in the socket may be sensed to control motor-driven internal rotation. Members of Todd Kuiken’s group at the Neural Engineering Center for Artificial Limbs in the Rehabilitation Institute of Chicago are currently studying how an implanted magnet might be used to generate the required control information.
Finally, there is a need to improve upon the Rimjet™ internal rotation lock. A universal lock is needed that can work with the elbows of several manufacturers.
Many traumatic amputations result in a remaining limb with some degree of prosupination. Although the prosthetist can often retain this motion in wrist disarticulation cases, little study has been applied to the question of how the surgeon might assist in recapturing this motion in shorter amputations. New socket designs coupled with roll-on liners will surely add to the number of amputees who retain useful prosupination. This is much more than simple mechanical or electrical rotation. The motion becomes truly useful when the user’s proprioception reports position in space and when the socket accurately reports the forces and torques applied to the TD.
COMPENSATING FOR GRAVITATIONAL TORQUES
Torques Across the Elbow Joint
The prosthetics field paid little attention to the loads caused by the weight of the TD and the distal parts of a prosthesis until electric hands came on the scene. The introduction of the Otto Bock Automatic Forearm Balance Elbow has made people think more about this problem. However, the two most important places where weight prevents function still have not been addressed. The very short transradial amputee can now be fit with a roll-on liner that will allow improved suspension. In addition new socket designs permit fitting these cases. The problem that will not go away is the short moment arm for lifting a heavy TD. Here is where compensation around the joint is needed. The compensating elbow has the following requirements.
- Two thin hinges with an arrangement for aligning the axes with the anatomical axis of elbow rotation.
- A compensation mechanism to fit into the distal socket space.
- A compensating mechanism that applies no torque at full extension and maximum torque at 90°.
- The amount of compensation needs two forms of adjustment. At the time of fitting, gross compensation must be set for the length of the prosthetic limb and the likely TDs that will be used. In addition the user needs to be able to further tune the compensation for special projects like working overhead.
Torques Across the Shoulder Joint
There is always a dilemma when fitting the very short transhumeral amputee. Often the subject has a full range of motion—the problem is lack of sufficient moment arm to permit working out front especially with an electric TD. The issues for this level of compensation are as follows:
- Both flexion and abduction need to be available to the user even if only flexion is compensated.
- The loads to be compensated must be transferred from the upper arm to a frame type socket.
- The axes of the flexion and abduction hinges must pass through the center of rotation of the glenohumeral joint, a typical orthotic requirement.
- The hinges must be thin so that the prosthesis does not get too bulky.
- The mechanism for compensating flexion loads must fit in the upper arm distal to the remaining limb.
- Depending on how the user forward flexes at the shoulder doing common tasks, the zero angle of compensation needs to be adjustable.
- No wings are required on the upper arm socket which can be optimized to couple the flexion and abduction moments from the short residuum to the socket.
BETTER WRISTS FOR ORIENTING THE TERMINAL DEVICE
No discussion of the challenges still remaining in upper extremity prosthetics would be complete without mentioning wrist orientation. With the recent arrival of improved electric hands, lack of wrist control becomes ever more obvious. The technology to use the “original” muscles to control all three wrist motions exists now. The control engineering community needs to complete the development of pattern recognition schemes and multisite control systems; and mechanical engineers need to give amputees a compact, responsive, and robust powered wrist.
1. Branemark R, Branemark PI, Rydevik B, Myers RR. Osseointegration in skeletal reconstruction and rehabilitation: a review. J Rehabil Res Dev
2. Witso E, Kristensen T, Benum P, et al. Improved comfort and function of arm prosthesis after implantation of a Humerus-T-Prosthesis in transhumeral amputees. Prosthet Orthot Int
3. Marquardt EG, Neff G. The angulation osteotomy of above-elbow stumps. Clin Orthop