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As part of the thinking process for constructing a seat cushion we know that we can amend pressure distribution, body orientation and skeletal alignment by manipulating the surface topography and immersion qualities of a cushion. And, as part of this effort we set goals to intentionally control, circumvent, support and distort the shape of soft tissues. We do this based upon our knowledge of anatomy, physics and physical disability combined with an acute understanding of the potential negative consequences of our actions. However, one of the biggest mistakes made interpreting the appropriateness of a contoured or molded seating system lies with the assessor's inability to scrutinize the total construct of the structure, relative to the specific seating requirements of the consumer. Using a seating simulator or foam-in-place appliance can help to create shapes that envelop and provides postural stability as well as skin protection but do the intimately fitting contours of the finished product really address issues of faulty biomechanics? As seating specialist, can we quantify the effectiveness of sculpted shapes relative to a specific list of needs? ...In short; we can, through a uniform understanding of the individual and collective influence that geometric shapes have on obligatory sitting posture. The key to this entire discussion about the core elements of seat construction is that the basic principles to controlling pelvic stability, orientation, alignment and position, while at the same time insuring skin integrity, do not change no matter what physical profile you find in the superincumbent body structures. And, yes, it is possible to design a safe environment for skin integrity and postural stability within the construct of a cushion made exclusively with foam. Background The term "Topographic Delineations" can be interpreted in several ways: as individual structures in the landscape of a cushion (such as an ischial recess or posterior pelvic support), as distinct areas of the landscape (such as a quadrant or region) or as a specific cross sectional profile of the cushion (such as a single digitized outline or specified cross-sectional delineation). The point is that by delineating the topography of a cushion (any cushion) the observer can define in detail, the function, benefit and structural requirements of each convoluting shape. By contrast, the collective goal of the "Topographic Delineation Fabrication Process" is to create a predefined support surface environment that is predictive yet retains conformable and enveloping qualities. The cornerstone of this fabrication process is that it ensures ample pressure redistribution for the maintenance of tissue integrity. The dimensional specifications and geometric formulas for each of the Topographic Delineations described in Chapters 9, 10, 11 and 12 were first cataloged in 1983. The formulas which are based upon anthropometric extrapolations hold true no matter the size or lack of symmetry of the consumer or that of the finished product. They also remain unchanged whether the end product is hand laminated, robotically sculpted or manufactured using any Solid Freeform Fabrication technique. This consistency allows us to build a system to the specific anthropometric and orthopedic requirements of any wheelchair dependant person. The detailed proprietary specifications of these Topographic Delineations are however, not within the scope of the following discussions. We will instead speak of them in general terms. Also note that not all of the following are intended to be mixed and matched at will. It is the responsibility of the seating specialist to select, apply and evaluate the individual elements and appropriateness of the completed design. Foundations There are several foundations that must be discussed in more detail before we proceed to the specifics of the posterior quadrants. These foundations influence the weight, thickness and dynamic flexibility of the finished product. There are also several modifications not discussed here, such as the drop seat and split front seat because the core of current Rehab wheelchair designs has rendered them relatively obsolete. The Solid Base (Plinth) The lateral edges of the solid base are usually parallel except when accommodating excessively abducted thigh alignments. Sometimes the lateral profile includes notches to allow close-in placement of hip panels on a planar cushion. This too is considered an optional growth feature. The front edge the seat should be perpendicular to the thighs and reflect their transverse alignment. So, as the thighs are fixed more into abduction, the seat's front edge (at the popliteal fossa) will reflect the diverging alignments. The front edge will also reflect the existence of a leg length discrepancy or include a cutback to accommodate an extreme hip extension variance. Front edge corner notches are cutouts made into the plinth that accommodate the leg hangers of the wheelchair frame. This modification is required whenever the seat rails of the wheelchair are too short to accommodate the entire length of the cushion. There are other profiles that can be added to the solid base but there is only one that will not be apparent while looking solely at the plinth. It is the front edge undercut. It can very in length but its purpose is to provide accommodation for knee flexion contractures or provide room for the top edge of AFO's. Keep in mind that with the current design criteria established by Medicare/SADMERC(1) the plinth is considered redundant to the solid pans which are now a basic component to many wheelchair configurations; "There is no separate payment for a solid insert that is used with a seat or back cushion because a solid base is included in the allowance for a wheelchair seat or back cushion... There is no separate payment when this is used with other types of power wheelchairs (K0010, K0011, K0014) because those wheelchairs include a solid seat pan." Foam-Only Base The original foam-only base introduced by the Palo Alto REC was built into the VASIO-P cushion (Veterans Administration Seating Interface Orthosis) but later describe as a feature that could be fabricated for any type cushion. Soon after it quickly became a key feature for many manufacturers and cushion suppliers. Both designs exhibited a profile that complimented the sling upholstery of a folding wheelchair on its undersurface and a flat platform on its top surface for the cushion to rest on, without risk of bowing. Now, with a pan to rest on, the foam-only base consists of a half-inch thick layer of closed cell foam and serves as a lightweight substitute for a plywood base. The added advantage of the foam-only base is that it can be notched or modified further to accommodate any irregularities on the surface of the pan. A major drawback is that accessories, such as hip panels and flip down pommels, must now be attached directly to the pan; a sometimes tedious process that requires additional (non-billable) labor. Sub-Base Lamination Pressure redistribution and load deflection qualities at the sub-base can be placed anywhere within the four quadrants and can serve to encourage the cushion to flex or deflect for specific reasons: minimize pressure resistance, ease transfers, ease weight shifts for pressure redistribution or facilitate the shift of COG/COM for the performance of a task. The thickness of a sub-base lamination is dictated by the size and shape of the consumer, the shape of the cushion (planar versus contoured/molded) and intended goals of the seating specialist. See Chapter 8 for additional insight about foam altering and lamination techniques. The Structural Components of the Posterior Quadrants If we agree that the pelvis represents the cornerstone structure of all seating alignment, stability and equilibrium goals then it should also follow that the seat cushion is the foundation on which this cornerstone finds its sure footing. Looking further within the cushion structure, it is the posterior half of the seat that most defines whether pelvic orientation and stability can be maintained sufficiently to provide a consistent, reliable foundation for the alignment and postural maintenance of all other body segments. Of the 12 voluntary or involuntary movements possible for pelvic alignment in sitting (6 directional, 6 axial), the seat cushion can control, restrict or influence eleven. Of these, nine are restricted or controlled most significantly by the structures found within the posterior quadrants of the seat and without any assistance from the backrest. The tenth (posterior pelvic tilt) is controlled by the buttock/thigh elevation on the seat but more effectively controlled by the posterior pelvic support of the backrest. The eleventh (anterior pelvic tilt) is restricted by the profile at the anterior quadrants and/or chest strap/harness and/or lap tray. The twelfth (pelvic elevation) is not influenced in any way by the seat but rather a lap belt, ASIS bar or other anterior pelvic apparatus. To better understand (in a tangible sense) how a seat cushion can restrict or control the pelvis to such a degree, the landscape of a seat cushion is first delineated into quadrants. It is the topography and composition of each quadrant that will dictate the stability, control and orientation of the pelvic and lower limbs. This perspective may not be so obvious when observing symmetry in the posterior quadrants of a cushion however as we introduce asymmetric physical anomalies, especially in the anterior quadrants, it becomes very conspicuous. The following are anatomy supporting seat structures of the rear quadrants that control pelvic stability, equilibrium, orientation and stationary alignment while promoting tissue integrity and pressure displacement. As we speak of these structures keep in mind that it is the rear quadrants of a cushion that dictates the height of the head above the floor. More specifically, head height is dictated by the suspended height of the ischial tuberosities. In a contoured or molded seat cushion the area below the ischial tuberosities is usually the thinnest part of the cushion yet thick enough (sub-base lamination) to prevent bottoming out as the cushion runs through its load deflection lifecycle. Growth Tail Here's a side note: some wheelchair designs (such as the Quickie, Iris) force the inclusion of a growth tail like feature at the rear of a seat cushion when the push handles are aligned into an open seat-to-back angle. The current frame design has a forward facing adjustment dial that projects above the seat rails when the push handles are tilted to an open angle. Another work-around is to use spacers to elevate the cushion above the raised metal obstruction but that may not be practical in all scenarios. Ischial Cutout NOTE: The 3" to 4" cushion thickness requirement for ischial suspension is consistent with the combination of ischial depth in sitting (for the average adult) of approximately 2 1/2" and typical foam compression of a 4-pound density, 16" X 16" seat cushion. Pre-ischial bar Ischial Recess 14mm Hg at the coccyx After writing a grant, the REC, in conjunction with the VAMC in Palo Alto, CA, embarked upon a four year study of several contoured cushion designs targeted towards SCI consumers and based on the concept of "selective repartitioning". The key component of the design was later defined as the Ischial Recess and today has become an integral design component for many commercial products. The structure of the ischial recess can be shaped in two manners: with or without a perineal trough. Without the trough the recess projects from the rear of the cushion forward to just anterior of the peri-ischial profile. The shape is mostly rectangular, akin to the ischial cutout but with sloped and radius edges to smooth out pressure transitions. With the perineal trough the recess projects forward with an elliptical profile (akin to a commode seat) to an area just forward of the genitals. The perineal trough is intended to promote aeration to the pelvic floor, allow the evacuation of moisture and reduce pressure on the perineum and peri-ischial structures. Again, the thickness of the recess is dictated by the decision of how much ischial pressure relief (suspension) and pelvic registry is required. As part of the research in Palo Alto cantilever postings were placed anterior to the trochanters to further reduce pressure readings on the ischial tuberosities. However, during trials it was demonstrated that the anterior postings had to be used with caution as one of the drawbacks is that an unopposed rise, anterior to the trochanters, can have a negative fulcrum effect and actually increase the load to the ischii. To balance out this consequence another more pronounced set of postings were sculpted into the cushion profile at each posterior corner to support the posterolateral aspects of the buttocks and later defined as the buttock thigh elevation. These combined changes became significant features for the success of the project. However, the ischial recess is not just for ischial suspension and pressure redistribution. It also functions to provide consistent registry and orientation for the pelvis. In fact, that is its primary purpose in pediatrics where the potential for tissue trauma is not of principle concern but symmetrical skeletal development is. NOTE: The three previously mentioned Topographic Delineations are intended for use with a pelvis that can achieve midline registry in the frontal plane and does not have fixed Y-axis rotation (right or left pelvic obliquity). Transverse rotation (Z-axis), on the other hand, can be easily accommodated for through detailed refinements. Each of the above also encourage external hip rotation through the combination of ischial immersion, increased support pressure to the posterior thighs and improved tibial alignment (vertical). Of the nine pelvic movements controlled, restricted or influenced at the posterior quadrants of the seat the ischial cutout and ischial recess are effective with eight. Pelvic Obliquity Contour Let"s first clarify that there are two distinctly different types of obliquity which sometimes combine to add even more complexity to achieving the goals of tissue integrity, accommodation and support. They are: axial obliquity in the frontal plane and axial obliquity in the transverse plane. In the frontal plane the elevated hip may be a result of a hip extension contracture and require a seat profile with a foam wedge that is higher at the rear to support the buttocks but diminishes anteriorly to support the affected thigh at or below the level of the opposite knee. Transverse obliquity is usually accompanied by unilateral hip extension and hip dislocation/subluxation on the contralateral side and consequential leg length discrepancy. With the exception of the high side femoral angle and contralateral trochanter depression there is no single formula that can blindly represent these abnormalities. Only through a body mold can the profile be accurately measured and replicated via hand lamination, foam-in-box construction or digital carving. In any case, the resulting cushion profile will reflect asymmetry in all four quadrants. (See also my concerns listed in Chapter 6) With the presence of pelvic obliquity the distinct anatomical profiles that can be recognized and measured are: the high-side femoral angle, and location and depth of the low-side ischium and trochanter indentations. Additionally, the goal for pelvic obliquity support should not be to align the knees to equal elevation but to provide maximum stability and pressure displacement within the full topography of the cushion. Buttock/Thigh Elevation (Recess) Unlike hip panels, the height of the B/T elevation rarely exceeds two inches when it accompanies an ischial recess, anti-thrust wedge or pelvic obliquity contour. The reason is that thigh control is largely achieved by allowing the pelvis to drop below the pre-ischial shelf of the cushion, which in turn increases containment of the femurs. Any additional height required near the lateral aspect of the knees can be achieved by adding built-in knee or hip adductor guides superior to the B/T elevation (see Chapter 10). This combination may require a rigid enforcement on the lateral aspect of the cushion for those who exhibit strong abduction. The B/T elevation can be constructed with full symmetry or shaped to compliment transverse pelvic obliquity. It can also be constructed in a half length profile to facilitate lateral transfers or to provide the thighs more freedom for lateral movement and/or abducted positioning. Finally, the B/T elevation can exhibit a trochanter relief for robust or dislocated hips. Here are a couple of points to keep in mind about the B/T elevation; it may not be compatible with a body jacket or other spinal orthosis (TLSO) because their lower posterior trim lines may conflict and push the occupant anteriorly; the B/T elevation will affect the overall vertical length and vertical placement of the backrest; the B/T elevation should not be used with hip/pelvic flexion angles of less than 65º as it may conflict with the pelvis' ability to position into posterior tilt. Buttock-Well For the wheelchair dependant consumer sitting is a lifelong requirement for every aspect of living. In sitting, the alignment of the pelvis is the cornerstone that dictates the posture for the rest of the body. Also, whether the consumer is or is not vulnerable to pressure ulcers the repartitioning of ischemic pressure away from the ischii, coccyx and sacrum is paramount for long-term comfort and upper body stability. The need to include air cells or gel bladders in custom seating designs is quite likely an indication that the shape and structural composition of the cushion are flawed to begin with. Understanding why, when and how a predefined topographic delineation works can make the difference between recurring success and repeated trial and error. Balance, stability, equilibrium and poise are all valued elements of not only good posture but functional posture as well. It is the shape of the rear quadrants of a seat cushion that dictates the shape of every other profile of a seating system and ultimately the posture of functional living. I cannot stress enough that as a seating specialist, how you delineate the shapes of the rear quadrants lays the groundwork for every other task you encounter with complex seating designs. Understand this and the art of custom seating manufacturing begins to look a bit more like science. © Copyright August 15, 2005 - CUSHMAKER.com .
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