Trans-tibial alignment
Normal bench alignment

Noelle Lannon, Canada
This article is the first in a series on prosthetic alignment.

"Alignment" refers to the spatial relationship between the prosthetic socket and foot. The main purpose of alignment is to position the prosthetic socket with respect to the foot so that undesirable patterns of force applied to the residual limb are avoided. A second purpose is to produce a normal pattern of gait.

Concept of moments
A moment is defined as "the tendency of a force to create rotation about a certain point". In prosthetic alignment it may be assumed that this point is located roughly in the geometric center of the prosthetic socket ("center of socket"). The socket may be considered to have a tendency to rotate about this point, and this rotation is resisted by the force applied by the residual limb (stump).

The prosthesis is subject to downward forces applied by the residuum from above, and by the reaction force from the ground below. If the downward force applied by the stump, and upward acting ground reaction force (GRF) are acting in the same line (collinear), there is no tendency for the socket to change its angular relationship with respect to the residual limb (rotate about the theoretical center of socket). If they are not collinear, there will be a tendency for the socket to change its relationship with respect to the stump (rotate). This tendency is best resisted with a good fitting total contact socket, and is influenced by the alignment of the prosthesis.

This described tendency of the socket to rotate about the limb in turn creates forces that act on the stump. Forces acting on the stump can be controlled so that relatively higher forces (pressure) will be applied to the areas where they are best accommodated and are most effective. These forces may also be reduced in pressure sensitive areas.

Bench alignment
Taking the time to properly bench align a prosthesis contributes to a smooth, energy efficient gait pattern. This includes controlled knee flexion after heel strike, smooth rollover with limited recurvatum (hyperextension), and heel off prior to initial contact on the normal (other) foot.

In the sagittal plane, proper anteroposterior (AP) positioning of the socket with regard to the foot will result in even weight distribution between the heel and toe portion of the foot statically.
For a trans-tibial prosthesis, a plumb line (gravitational line 90° to the ground) should fall through the center of socket (it may be easier to mark this with an erasable marker), slightly anterior to where the ankle joint axis would be, and through the weight bearing area of the foot between the middle of the weight bearing surface of the heel, and the metatarsal heads (shown in Figures 1 and 2). When this is done make sure the appropriate heel height is approximated using a wood block or other device to lift the foot. Although some texts differ, generally five degrees of socket flexion is incorporated into an initial bench set-up (Figure 2). If the patient has a knee flexion contracture, this five degrees should be added to the degree of contracture present. For example, if the patient has a contracture of 10 degrees, the socket should initially be set in 15 degrees of flexion. This initial flexion, or socket tilt, will help assist better loading in the socket and helps create a smoother gait pattern. This flexion in the socket positions the limb in a natural midstance position and helps reduce hyperextension tendencies during gait.



Fig. 2: standard flexion angle of the socket in the sagittal plane
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