MECHANICS OF LOW TONE POSTURE AND SEATING SUPPORT DESIGNS
Influenced by Gravity, Pelvic Stability and the Alignment of Body Segments
A Theoretical Approach

Chapter 11
Topographic Delineation Principles for Backrests
- Midline Tri-Quadrants -

As with seat cushions, the fabrication process for a contoured backrest can progress in several distinct manners: as a construct of geometric shapes, a freehand sculpture, or a mechanical carving. A construct of geometric shapes is most often referred to as "custom contoured" or "made-to-measure". We have previously referred to this structured fabrication method as the "Topographic Delineation Fabrication Process" (TDFP). We'll focus on the principles of TDFP for the remainder of this discussion. Refer to Chapter 8 for additional details regarding theory.

In addition to structuring the external profile for a posture support system we must also delineate the form and function of its layers; beginning with the sub-base, the geometric profiles and finally, the contact interface layers. With TDFP the design pattern for a backrest is sectioned into a Tri-Quadrant grid; three rows bridged by four columns. Together they comprise twelve distinct segments within which specific features can be identified; relative to the corresponding anatomical structure it must support. These segments are not necessarily equal in size or shape but are mirrored left and right from mid-sagittal. Where anatomical asymmetry exists, the distortion will dictate the size, shape and location of the corresponding tri-quadrant segments.

In the following discussions we will only look at design specifications based on symmetrical support requirements. Once again our test subject is: 5' 7" tall, 170 pounds, presents with low tone and has no permanent physical distortions.

The Horizontal Tri-Quadrants

When viewed from the frontal plane, the bottom row of the Tri-quadrant is margined by the inferior edge of the backrest and superiorly by either the pelvic crest or the base of the lumbar spine. Since a custom backrest is designed to rest above the seat cushion, the actual location of its inferior edge will vary from project to project. In other words, for the best fit, we must know the height of the seat cushion's posterior edge before determining the starting point of the backrest.

Moving vertically, the second row of the Tri-quadrant is margined superiorly by the inferior angle of the scapulas or the rearward facing apex of a kyphotic spinal projection; whichever is lowest (whether normal or abnormal). The upper most segment of the Tri-Quadrant is margined superiorly relative to the height of the shoulders; or a specified distance above/below that landmark. On rare occasions, this final segment will include sufficient height to provide support for the head.

Collectively, a backrest can consist of one, two or three rows of horizontal support; dictated by the needs of the client. They are defined as the: Posterior Pelvic Support, Sub-Scapular Support and Posterior Shoulder Support regions. Longitudinally, the contact surface of these three regions displays their own angle of orientation and physical profile relative to the body segment and support requirements at that level. Additionally, the three horizontal regions can be built as one unit or hinged together as independent components, with height and angle adjustability. When hinged together the contact surface of the three articulating regions will most likely display fewer and less intimate anatomical delineations.

Posterior Pelvic Support
By previous instruction for fabricating a seat cushion, we've established significant controls for every direction of pelvic movement, except one - posterior pelvic tilt. The seat cushion can influence this movement to some degree (via the ischial recess and the buttock/thigh elevation) but it is the posterior pelvic support that provides the greatest counterforce to deter this sometimes inappropriate alignment. When we follow this line of thinking there is no such profile as a lumbar roll or lumbar support. Don't get me wrong, you can find that profile but it is actually created by the juncture between the posterior pelvic support and sub-scapular support. (More below)

Conventionally, in the sagittal plane the posterior pelvic support has a somewhat vertically oriented profile; anywhere from zero to 5 degree of posterior angulation. This profile will also represents the thickest portion of the entire backrest; except when using a bi-angular or tri-angular articulation. Make no mistake in understanding this key point, the angle and orientation created here will dictate the support angles for all other superincumbent segments and consequently dictate issues of stability and alignment all the way to the head and hands

Since the seat is always positioned underneath the backrest, the backrest sometimes looses some of its vertical length; based upon the profile along the rear edge of the seat. That is to say that when a seat cushion includes an ischial recess and/or buttock/thigh elevation there will be a corresponding reduction in length at the bottom margin of the backrest. By contrast, if the seat's rear quadrant has a planar profile, there is no loss of vertical length for the backrest. Note, that some manufacturers and custom fabricators cut the rear of the seat and the bottom of the backrest at complimentary diagonals. This production style sometimes produces another variable of length for both the seat and backrest. The precaution to consider with this production style is that it creates difficulties to later incorporate growth adjustments between the two components when building a system for the pediatric market.

The foam selection for this profile is typically medium density and extends to the full width of the backrest. When the two lower regions of the tri-quadrant are built as one unit the top margin of the posterior pelvic support profile will tapper to zero thickness just below the inferior margins of the scapulas.

In the case where the buttocks projects in a pronounced rearward direction the inferior margin of this profile will display a corresponding angulation. A coccyx or sacral relief is most often displayed on the rear margin of the seat cushion when extreme posterior pelvic tilt is present but when the pelvis displays extreme anterior tilt those profiles are reflected within the posterior pelvic support profile.

Sub-Scapular Support
Now it's time to change direction, in more ways than one...

Posterior Shoulder Support
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