• Greater functional envelope from the basic body powered device
• It offers reduced weight from the myoelectric device
• It offers the greater grip force like the myoelectric
• The harness system is reduced
• The initial costs and maintenance

 

 

Components & Devices

A typical below-elbow prosthesis includes a voluntary opening split hook, a friction wrist, a double-walled plastic laminate socket, a flexible elbow hinge, a single-control cable system, a biceps or triceps cuff, and a figure-of-8 harness.

An above-elbow prosthesis is similar but substitutes an internal-locking elbow for the flexible elbow hinge, uses a dual control cable instead of a single control, and does not have a biceps or triceps cuff

The major function of the hand that a prosthesis tries to replicate is grip. The 5 different types of grips are as follows:

• Precision grip (ie, pincher grip): The pad of the thumb and index finger are in apposition to pick up or pinch a small object (eg, small bead, grain of rice).
• Tripod grip (ie, palmar grip, 3-jaw chuck pinch): The pad of the thumb is against pads of index and middle finger.
• Lateral grip: The pad of the thumb is in apposition to the lateral aspect of the index finger to manipulate a small object (eg, turning a key in a lock).
• Hook power grip: The distal interphalangeal joint and proximal interphalangeal joint are flexed with the thumb extended (eg, carrying a briefcase by the handle).
• Spherical grip: Tips of fingers and thumb are flexed (eg, screwing in a light bulb or opening a doorknob).

Terminal devices generally are broken down into 2 categories: Passive and Active.

Passive terminal devices

The main advantage of a passive terminal device is its cosmetic appearance. With newer advances in materials and design, a device that is virtually indistinguishable from the native hand can be manufactured. However, passive terminal devices usually are less functional and more expensive than active terminal devices.

Active terminal devices

Active terminal devices usually are more functional than cosmetic; however, in the near future, active devices that are equally cosmetic and functional should be available. Active devices can be broken down into 2 main categories: hooks and prosthetic hands with cable or myoelectric-based devices. A prosthetic hand usually is bulkier and heavier than a hook, but it is more cosmetically pleasing.

A prosthetic hand can be powered with a cable or myoelectricity. With the myoelectric device, the patient can initiate palmar tip grasp by contracting residual forearm flexors and can release by contracting residual extensors.

The main mechanisms for opening or closing an active device are as follows:

Voluntary opening mechanism: With a voluntary opening mechanism, the terminal device is closed at rest. The patient uses the proximal muscles to open a hook-based device against the resistive force of rubber bands or cables. Relaxation of the proximal muscles allows the terminal device to close around the desired object. In a myoelectric device, contraction of the proximal muscles activates the electric motor. The voluntary opening mechanism is more common than the voluntary closing mechanism.

Voluntary closing mechanism: With a voluntary closing mechanism, the terminal device is open at rest. The patient uses the residual forearm flexors to grasp the desired object. This type of mechanism usually is heavier and less durable than a voluntary opening mechanism.

Wrist, Elbow, Shoulder, and Forequarter Units

Wrist Units

The wrist unit functions as an attachment for the terminal device and can be positioned manually or myoelectrically. The wrist unit can be a quick disconnect unit, a locking unit, or a wrist flexion unit.
Quick disconnect wrist unit
This style is configured to allow easy swapping of terminal devices with specialized functions.
Locking wrist unit
Wrist units with a locking capacity prevent rotation during grasping and lifting.
Wrist flexion unit
In a patient with bilateral amputations, a wrist flexion unit can be placed on the longer residual limb (regardless of premorbid hand dominance) to allow midline activities such as shaving or manipulating buttons.

Elbow Units

Elbow units are chosen based on the level of the amputation and the amount of residual function.
Flexible elbow unit
When the patient has sufficient voluntary pronation and supination as well as elbow flexion and extension, such as in a wrist disarticulation or a long transradial amputation, a flexible elbow hinge usually works well.
Rigid elbow unit
When a patient can achieve little or inadequate pronation and supination but does have adequate native elbow flexion, such as in a short transradial amputation, a rigid elbow hinge provides additional stability.

Shoulder and forequarter units

When an amputation is required at the shoulder or forequarter level, function is very difficult to restore. This is due to a combination of the weight of the prosthetic components, as well as the increased energy expenditure necessary to operate the prosthesis. For this reason, some individuals with this level of amputation choose a purely cosmetic prosthesis to improve body image and the fit of their clothes.