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Most literature addressing balanced sitting posture for the wheelchair occupant describe that body weight should be evenly distributed under both hips and thighs, the lower legs slightly extended and the feet firmly planted on the footplates. Clinicians and researchers also describe or illustrate that the pelvis should be firmly placed against the backrest and the pelvis aligned to a relatively neutral position with the thighs and knees facing forward and supported at a level plane, equal to the height of the trochanters. There is almost never any discussion of the body in the frontal plane except in reference to sitting anomalies such as scoliosis, pelvic obliquity, contractures etc. The implication being, it is understood that midline symmetry in the frontal plane is a universal objective. In the early days of the seating and mobility industry there was substantial research and discussion on shear forces; specifically how these forces impacted overall skin integrity and pelvis migration in the sagittal plane. For example, a study in 1962 demonstrated that, in sitting, as much as 70% of body weight is borne by only 8% of the buttock/thigh surface area (1). In contemporary literature, however, the focus has shifted away from ischemic ulcer prevention to postural alignment, with particular attention given to the pelvis and superior structures. With our focus redirected, there is an implication that everything is now clear about body alignment and skin management from the trochanters and below. I happen to disagree with that assumption as it pertains complex and low-tone sitting posture. (Before we go any further, let me clarify that for the duration of these chapters my solutions will be based upon structured cushion designs laminated with varying densities of visco-elastic foam. Chapter 8 provides a very clear description of the principles for this process. It is important to note that irregardless of what happens either above or below the pelvis; the pelvis and surrounding soft tissue of the buttocks must be held by a stable yet forgiving support surface. This minimal criteria must be present no matter what other accommodating features must also become incorporated into the design.) In the custom seating arena, the inability to prevent pelvic migration remains a principle flaw with most cushion designs. And of course, if we can't control the pelvis we can't prevent horizontal sheer or achieve optimal postural alignment, orientation or function. In fact, until the tri-axial orientation of the pelvis is moderated in all twelve directions of movement we can't secure either equilibrium or postural stability to any other segments of the body. One of the key components to controlling pelvic movement or migration is the peri-ischial elevation. This closely cropped, horseshoe shaped platform with ischial depression (trough) can be created in various forms: ischial cutout, pre-ischial bar with cutout, ischial recess or a unilateral, pelvic obliquity modification. Unlike the anti-thrust wedge, these structures divert ischemic pressure away from the ischial tuberosities, provide medial/lateral and transverse pelvic stability, restrict transverse axial migration and help limit all sheer forces to the entire pelvic floor. When complimented with a posterior pelvic support, the combined structure forms the greater portion of the mechanism necessary for tri-plane and tri-axial pelvic stability. The depth of an ischial cutout/recess or height of the peri-ischial elevation is dictated by the amount of ischial pressure we wish to displace to another region of the body. For our theoretical subject the offset is 1.5" minimum. The maximum is 4" for total ischial suspension. Also, the deeper the recess the more that ischemic pressure is transferred to higher tolerant areas of the sub-trochanteric shelf and posterior thighs. The upshot of this delineated offset is increased control over forward migration and prevention of sheer. For those living in the United States, Medicare (with the help of SADMERC) was very keen to recognize the significance of at least a couple of these features when it mandated that the "minimal structural features" of a positioning seat cushion, skin protection cushion and a custom fabricated seat cushion should include "a pre-ischial bar and...Two lateral pelvic supports...intended to maintain the pelvis in a centered position in the seat and/or provide lateral stability to the pelvis...". (2) In reference to the buttocks and thighs, keep in mind when working with spheres; any contact below the apex of a curve is an opportunity to defuse ischemic pressure at vulnerable bony landmarks. On the other hand, more contact may also mean less freedom of movement. Whether that's good or bad depends on functional capability of your client and your/their list of goals. Moving downward to the thigh and hip flexion angles, rather than just level, their placement will actually depend on the magnitude of control you wish to introduce to prevent or allow forward pelvic migration. To answer that question you need to project how much or what form of movement you perceive as necessary or what magnitude of "perturbation" (3, 4) (outside force) you anticipate your client will be subjected to. For example: frequent travel over door jams, cracks in a sidewalk or terrain with a lateral slant could be a problem for a low-tone client. Consider also vibrations created on an unpaved surface while being pushed in a mobility base that has no type of dampening mechanism. The vertical and lateral force placed upon the lower body (via perturbation) is further compounded by the superincumbent weight and disposition of the upper body. In general, however, the higher you raise the hip angle via the cushion profile the more resistance you create against anterior pelvic migration. Also affecting this decision of lower body postural angles may be the ISO 16840-4 project that intends to test the crashworthiness of all of wheelchair seating and accessory components. Returning our attention back to the frontal plane, the thighs should be positioned forward facing and femurs aligned parallel to each other with zero abduction. This serves several functions: to promote aeration of the perineum (5), and to maintain external rotation of the trochanters which in turn helps suspend the ischial tuberosities. With that in mind, a medial thigh support (pommel) is an excellent accompaniment to assist with these goals. The height of the pommel is dictated by the amount of abduction control and pressure distribution is required. Excessive external rotation of the thighs should not be of big concern with a physical profile of little or no tone. The rationale being that a by-product of the ischial recess is the transfer of increased pressure to the posterior thighs. This combined with appropriate foot placement on the footplates should provide enough friction to hold the thighs in stable alignment. However, I routinely add a slight perimeter elevation to the seat cushions as a precaution against all forms of perturbation, whether anticipated or not. Further down and beyond the limits of the seat cushion, the lower limbs should be placed directly below the thighs, with the tibias aligned vertically as viewed in the frontal plane. The lower legs should also be slightly extended with the feet resting upon the footplates or leg troughs so that the ankles are supported neutrally in both the sagittal and frontal planes. The feet and ankles should be allowed to assume a natural anatomical toe-out of 3º. Swearingen also demonstrated in 1962 that for an average male sitting on a planar surface, as much as 18% of body weight can be supported through the use of footplates (1). However that percentage decreases as residual body weight is transferred to the thighs as a result of lower limb atrophy or amputation. Foot deformities may also preclude the ability for effective weight bearing. No matter why non-weight bearing of the feet occurs, the result is that the thighs (and posterior pelvic region) must share a higher responsibility for the tasks of unloading the ischii and providing stability to the pelvis. Again, the ischial recess is a key structure for accomplishing this task. Finally, if there is no weight bearing on the feet then leg troughs or calf panels serve primarily as protective zones and flexion limiters for all structures of the lower legs. That concludes our synopsis of the positioning goals for the lower half of the body. We should now have a stable platform on which to build postural equilibrium and stability for the remaining segments of the upper body. This has been a lot to cover for one session but it represents only a cursory glimpse into the world of custom seating and posture. In Chapter 2 we'll take a huge leap in thinking and offer a common language from the world of gravity and physics. Chapters 6, 7, 8 and 9 provide more perspectives regarding pelvic positioning as it relates to deformities, fabrication principles, and upper body posture. © Copyright August 15, 2005- CUSHMAKER.com
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