Introduction to Upper Limb Prosthetics
Prosthetic arms or upper limb prosthetics can be inert, body-powered or bionic. The inert type of prosthetic arm is not activated and therefore serves a cosmetic function only. A body-powered prosthesis uses existing body motion to activate a movement device. The most common form of body-powered upper limb prosthetics is the shoulder harness which is used to activate the arm or hand.
Myobock® Below Elbow Prosthesis
Roger Wolfson and Associates is certified to provide the Myobock® Below Elbow upper limb prosthetic.
The Myobock® Below Elbow is a myo-electrically controlled upper limb prosthesis that includes accessories and fittings to ensure the most dependable rehabilitation results. The components of the Myobock® Below Elbow prosthesis combine optimal outer appearance with high grip force and grip speed, as well as numerous other combination and adjustment options.
DynamicArm Transhumeral Prosthesis
Amputations above the elbow constitute a special challenge since the function of the elbow has to be replaced in addition to that of the hand. The DynamicArm elbow joint makes virtually natural movements possible with its special technology. Controlled by your muscle signals, the DynamicArm is driven by an electric motor. The control unit of the DynamicArm also transmits the muscle signals to the prosthetic hand, so that you are able to rotate the wrist unit in addition to opening and closing the hand with short response times. You can extend and flex the DynamicArm as well.
We recommend combining the DynamicArm with the Ottobock VariPlus Speed. It is equipped with a high-performance drive unit, and opens and closes almost three times as fast as other electric hands. A fitting with the DynamicArm and VariPlus Speed restores numerous abilities to hold objects, grasp and be active at work, in everyday life and recreation.
The Residual Limb
There are three major critical factors involved in the design and optimisation of trans humeral and elbow disarticulation prosthetics, including:
- Length of the bony lever arm
- Quality and nature of soft tissue coverage
- Shape and muscle tone of the residual limb
The above-elbow prosthesis consists of:
- A single plastic upper arm shell
- An elbow joint, usually with incorporated locking mechanism
- A plastic forearm
- A wrist joint
- A terminal device, either a hook or a hand.
A body-powered elbow unit consists of a simple hinge, with a half-dome-shaped plastic covering. To retain an elbow position two options are available, which take the form of either a friction or a locking mechanism. Such devices that function as locking mechanisms are operated using a knob located on the forearm or using a cable attached to the shoulder harness. To assist the user of a body-powered above elbow prosthesis in flexing the elbow, an elbow unit with a forearm lift assist is used, which acts as a spring mechanism that somewhat compensates for gravitational forces.
A wrist unit is used for:
- Attaching purposes
- Rotation purposes (passively with sound hand)
- Interchange between hands/hooks.
The function of the terminal device is to replace the grasp function of the hand.
Three types of terminal device are available:
- To grasp small objects
- To grasp, hold, carry, pull or push.
- To grasp large, round objects
- To grasp between thumb and first two fingers (three jaw chuck pattern)
- Entire passive, for cosmetic purposes only
As the term suggests, a body-powered prosthesis uses the patient’s own body power to activate motion of the artificial limb. Thus, where it concerns upper limb prosthetics, the body harness is a necessary and extremely important component. The harness transfers motion and forces from the residual limb, shoulder girdle and trunk directly to the prosthesis. In cases where there is a loss of an elbow, the harness is also fixed to the socket above the elbow stump. The higher the amputation level, the more difficult it becomes to control and fix the prosthesis. Nevertheless, a patient with a shoulder level amputation can still be fitted with a prosthetic.
A body harness can also be used to control elbow flexion and locking. In the case of a below elbow fitting, the harness activates the terminal device only.
A body-powered prosthesis together with natural muscle motion enables good functionality. The pressure of the harness on the body also gives the patient sensory feedback. However, the degree of sensory feedback depends on the fit of the prosthetic socket, the harness as well as the features of the prosthetic hand.
When fitting a body harness, no two people are the same. Hence, the fitting must be performed on an individual basis. Two patients with the same physical constitution may be different with respect to the motion and force in the shoulder girdle region as well as each residual limb is different.
To accommodate an above elbow prosthesis, two very different harness systems have been developed and proven in practice, enabling performance of the required functions consisting of activating the terminal device, below elbow flexing and independent locking or releasing of the elbow joint.
Basic functions of the harness system
The main function of the harness is to suspend the prosthesis to the residual limb. In so doing it should:
- Distribute load as much as possible
- Be well suited to the patient’s body structure
- Be stable in all normal positions of use
Upper limb prosthetic rehabilitation
Follow up consultation sessions are the most important aspect of the upper limb prosthetic rehabilitation process. Yet, most often, this process is neglected. There are three important aspects to consider following the initial prosthetic fitting:
- Maintenance of the socket fit, suspension and comfort of the patient, especially where this pertains to volume changes in the residual limb.
- Ongoing monitoring to ensure that the patient fully masters the functions of the prosthesis at home as well as in the work environment.
- Re-evaluation of socket style, harness design and component selection based on the experience of the individual.
There are numerous aspects to upper limb prosthetic rehabilitation that cannot be addressed until the patient has had a reasonable amount of time to assimilate the use of the new prosthetic. Questions need to be answered and new skills need to be mastered. In line with goals and aspirations, the fit, comfort and function of the prosthesis need to be maintained and optimized over time.
Successful long term use of an upper limb prosthesis depends primarily on its comfort and perceived value. Innovative design and careful custom adaptation of socket and harness principles, careful attention to follow up adjustments and prescription revisions based on changing needs are essential to successful prosthetic rehabilitation.