Front Matter

270 Canine Sports Medicine and Rehabilitation

Force coupling control system

Force coupling allows both orthoses and pros­
theses to guide and control motion about and
within a joint (or motion segment) without lim­
iting the range of motion desired at the joint.

Orthosis force coupling

In orthopedic force coupling, two bones are
linked together with a shared joint axis. An
example is the femur and crus united by the sti­
fle. The stifle joint allows osteokinematic
motion (flexion and extension) as well as
arthrokinematic motion (movement within the
joint such as rolling, spinning, and sliding
translation over the joint surface).
A force coupling is defined as two lever
arms joined at a pivot point with two forces
applied at either end of each lever arm
causing the lever arms to rotate about an
imaginary axis in the center of each lever
arm (Figure 11.5A). Thus, there are two force
couples, one for each lever arm. To obtain bal­
ance in this system, each force couple is of

equal magnitude and they are linked about

a  mechanical hinge between the two lever

arms. In this scenario, the action of the

couples about the hinge is completely dictated

by the physical limitations of the hinge.

An orthosis force coupling control system

allows osteokinematic motion to persist ( flexion

and extension rotation) while controlling

arthokinematics at the joint level. The effect is

to create osteokinematic rotation without trans­

lational displacement where translation means

roll, spin, and slide over the joint surface. An

example of displacement within a joint is the

arthrokinematic instability resulting from CCL

insufficiency. The tibia is translated cranially

and internally (cranial drawer and internal

rotation).

The use of an orthosis to control CCL insuffi­

ciency instability is the most common applica­

tion of orthosis force coupling. This orthosis

typically consists of two shell segments that

communicate with the femur and crural bones,

respectively, and are joined together with a

mechanical hinge aligned with the anatomic

hinge (stifle joint).

For the cranial cruciate stifle orthosis to effect

control it must create a force couple that resists

cranial tibial thrust and internal tibial rotation,

preserves normal arthokinematics, enables nor­

mal osteokinematics, and ensures device sus­

pension. Importantly, a 3PCS is not able to

control arthokinematics and preserve the nor­

mal osteokinematics of a cranial cruciate‐defi­

cient stifle joint.

Figure  11.5B illustrates the combined four

points of contact, two on the femur encourag­

ing a cranially directed distal femur rotational

force at the stifle, and two points of contact act­

ing on the tibia encouraging a caudally directed

force at the stifle. Because a static point of con­

tact is unable to harness the power of force cou­

pling, the system requires a dynamic force to

counteract cranial tibial thrust. Dynamic stabili­

zation motion is created through muscle con­

traction, creating expansion during both

eccentric and concentric contraction. The con­

traction of muscles in  the pelvic limb during

stance phase of the gait creates a caudally

directed force that is communicated through

the orthosis straps and shell to the proximal

cranial aspect of the tibia.

Modied 3 point corrective system

Anchor force (AF)

Corrective force

(CRF)

Counter moment force
(CMF)

Figure 11.4 When the desired corrective force (CRF)
location is obscured by a bony structure, the CRF can be
split into proximal and distal components placed
superior and inferior to the bony structure.