AMPUTATION LEVELS AND PROSTHETIC PRINCIPLES
The shoulder provides the center of the radius of the functional
sphere of the upper limb. The elbow acts as the caliper to position the
hand at a workable distance from that center to perform tasks. People
normally perform multiple joint segment tasks simultaneously. Upper limb
prostheses perform these same tasks sequentially; thus, limb length and
joint salvage are directly correlated with functional outcome. Motion at
the retained joints is essential to maximize that function. Residual limb
length is valuable for both prosthetic socket suspension and providing the
lever arm necessary to drive the prosthesis through space.
Limb salvage is more critical in the upper limb than the lower limb,
because sensation is critical to upper extremity function. An insensate
prosthesis provides less function than a partially sensate, partially
functional salvaged limb. This fact is in contradistinction to the retained
lower limb in which function is not as dependent on sensation.
When upper limb amputation is necessary, prosthetic fitting should be
initiated as soon as possible, even before the wound is healed. Outcomes of
prosthetic limb usage vary from 70% to 85% when prosthetic fitting is
initiated within 30 days of amputation, as opposed to less than 50% when
prosthetic fitting occurs late.
Myoelectric prostheses are promising, but are slow to perform tasks.
Increased speed and power requires increased weight for the motor and
battery pack. These prostheses appear to be most successful in the
midlength transradial amputee, in whom only the terminal device needs to be
Hand Amputation Surgical reconstruction to obtain prehension can be
accomplished with pollicization, ray transposition, central ray resection,
or toe-to-hand transfer. Functional static partial hand prostheses can
provide a stable post for opposition from remaining digits or the palm.
Cosmetic partial hand prostheses may be psychologically beneficial because
they retain body image.
Wrist Disarticulation Wrist disarticulation has two advantages over
transradial amputation: (1) preservation of more forearm rotation due to
preservation of the distal radioulnar joint, and (2) improved prosthetic
suspension due to the flare of the distal radius. However, wrist
disarticulation provides challenges to the prosthetist that may outweigh
its benefits. Cosmetically, the prosthetic limb will be longer than the
contralateral remaining limb, and, if myoelectric componentry is used, the
motor and battery cannot be hidden within the prosthetic shank.
Transradial Amputation High levels of function can be obtained at
this level of amputation (Fig. 1). Forearm rotation and strength are
directly related to the length of the residual limb, with the optimum
length being at the junction of the middle and distal thirds of the
forearm. At this level, the soft-tissue envelope can be constructed with
adequate muscle myoplasty or myodesis, and the components of a myoelectric
prosthesis can be hiddenÂ within the prosthetic shank. Because function at
this level is accomplished prosthetically only by opening and closing the
terminal device, elbow joint function is essential. When the residual
forearm is so short as to preclude an adequate lever arm for driving the
prosthesis through space, supracondylar suspension (the Munster socket) and
step-up hinges can be used to augment function.