Front Matter

268 Canine Sports Medicine and Rehabilitation

Common internal moment deficiencies include
carpal hyperextension, tarsal valgus from shear­
ing injury, and Achilles mechanism injury. Here
the Achilles mechanism (internal moment) is
unable to resist tarsal flexion created by the flex­
ion moment acting against the tarsus
(Figure 11.1). By employing mechanical control
systems to the body, these devices stabilize
joints either statically (no range of motion) or
dynamically (range of motion possible). There
are three general types of control systems: (1)
three‐point corrective system (3PCS); (2) modi­
fied three‐point corrective system (M3PCS); and
(3) force coupling (FC). While each control sys­
tem is used for specific conditions, each system
facilitates a common mechanical goal of creat­
ing an external moment to support or stabilize
an affected anatomical joint or limb segment.

Three‐point corrective systems

An orthosis applies a 3PCS (Figure  11.2), the
most commonly used system, to stop, resist, or
guide rotation of two limb segments about the
shared point of rotation (joint). This system con­

sists of a corrective force (CRF) that is opposed

by the proximal counter force and the distal

counter force. The CRF is applied in direct oppo­

sition to the level and direction of joint instabil­

ity. The purpose of the CRF is to resist the

direction of instability presented by the defi­

cient limb segment or joint. For example, trau­

matic palmar fibrocartilage injury results in

carpal hyperextension in weight bearing. The

instability is in the sagittal plane directed cau­

dally at the carpus; the CRF for this injury

would be at the same level directed cranially.

The counter forces are presented on the oppo­

site side of the limb from the CRF location. The

proximal counter force is known as the anchor

force (AF) and the distal counter force is known

as the counter moment force (CMF). These

counter forces are positioned as far proximally

and distally as functionally possible from the

affected joint center to maximize the lever arm.

The sum of the two counter forces is equal to

the magnitude of force acting at the corrective

force location: CRF = AF + CMF. The equation

T = LF describes the relationship of lever arm to

torque where T equals torque, L equals lever

arm length, and F equals force. Increasing the

length of lever arm by applying the counter

force far away from the corrective force reduces

the magnitude of force at each counter force

location. Subsequently, the corrective force will

Tarsal flexion moment

Moment = Force * Distance

F

D M

D = Distance between

GRFV and the

anatomical joint

Figure 11.1 The relative position of the tarsus joint to
the ground reaction force vector (GRFV) is responsible for
the production of the tarsal flexion moment.

3 point corrective system

Anchor force (AF)

Counter moment force

(CMF)

Corrective force

(CRF)

Figure 11.2 Coaptation is required to produce a three‐

point corrective system to create an external moment

capable of resisting an unstable internal moment.