Front Matter

Chapter 11 Veterinary Orthotics and Prosthetics 271

Prosthesis force coupling

A prosthesis communicates with a patient’s
residual limb (residuum) through shell and
strap contact to the body. During all phases of
gait, the patient is either using the residuum to
create prosthesis motion or to control prosthe­
sis motion. In prosthesis force coupling, the
connection between the residuum and the pros­
thesis can be thought of as a joint. At their inter­
section, the residuum and prosthesis are subject
to forces applied by virtue of the GRF vector
and the TBF vector, which shift in direction and
magnitude with stance and gaiting. In all these
events, the patient will present two points of
contact (proximally and distally) to the prosthe­
sis by utilizing their residual limb as a lever
arm (Figure  11.6A). The points of contact will
be presented in the same manner as the indi­
vidual orthosis force coupling segment where
the rotation about the lever arm is around an
imaginary central axis and the lever arm can
rotate in either direction (Figure 11.6B, C).

Prosthesis force coupling case example: subtotal

amputation via an antebrachiocarpal disarticulation

The patient’s distal residual limb segment

includes the radius and ulna. The prosthesis

provides socket communication with the

residuum and extends distally to the ground

recreating lost limb length. During swing

phase, the patient will advance the prosthesis

cranially with shoulder extension and elbow

flexion for ground clearance. As the patient

flexes the elbow joint, the residuum will con­

tact the prosthesis along the distal cranial and

proximal caudal aspect of the residuum creat­

ing a swing‐phase force couple within the

socket (Figure 11.7). During stance phase, the

patient will create elbow extension resisting

both (1) elbow flexion created from the GRF

vector and the resulting flexion moment; and

(2) the socket tendency to rotate caudally. The

patient will communicate a force couple along

the distal caudal and proximal cranial aspect

of the residuum (Figure 11.8).

Force coupling

(lever arms)

Force coupling

(dynamic force)

Cranial rotation

force applied to

femur

Cranial rotation

force applied to

femur

Caudal rotation

force applied to

tibia

Caudal rotation

force applied to

tibia

Mechanical hinge Mechanical hinge

(A) (B)

Figure 11.5 (A) A force couple is defined as two lever arms joined at a pivot point (mechanical hinge) with two forces
applied at either end of each lever arm. These forces cause each lever arm to rotate about an imaginary axis in the
center of each lever arm. A force couple control system allows coaptation to resist joint translation (arthokinematics)
while enabling sagittal plane range of motion to persist (osteokinematics). (B) A force couple control system requires
muscle contraction to create expansion force, producing dynamic force and enabling arthokinematic control while
allowing osteokinematic motion.