 |
|
||
|
GRAVITY |
|||
| Gravity is the force that shapes every aspect of the world in which we live. So why wouldn’t it be a subject of discussion in seating system designs? Look closely and you’ll see that most rules and tricks of the trade put forward by the industry are an effort to either overcome the effect of gravity or to influence its consequences to the advantage of the client. Gravity plays a roll in all aspects of sitting posture no matter your physical status. If only by fatigue or the result of discomfort, everyone eventually attempts to find the most supportive and comfortable position for relaxation, which coincidentally requires the least amount of ongoing physical effort. This position of max comfort with minimal effort also allows active muscles and joints to periodically stretch and rest again. And, if after trying, things aren’t still to our liking, we can always stand up or lye down completely. However, for the person who is lifetime dependant upon a wheelchair, the prolonged affects of gravity combined with the inability to escape its force can create long-term physiologic and orthopedic harm. In this scenario, finding comfort is only one in a long list of concerns. Consider the consequence of ischemia and what the wheelchair dependant person requires to prevent its potentially damaging effects. To counter the skin compromising components of gravity, ischemia-producing pressure must be reduced to a safe level or relieved at risk reducing intervals. For an individual to independently perform a pressure relief the lifting or suspension force must be greater than the downward force. A one-handed or two-handed pressure relief requires enough strength to lift one’s body off the surface of the cushion. This lifting action extends the torso and reduces pressure against the soft tissues of the buttocks. With severely compromised strength in the upper extremities performing a forward lean and placing the head between the legs can accomplish a similar risk reducing result. Then of course, there’s that high tone technique called the “extensor trust” (Lombard’s Paradox). But, what about a person lacking sufficient muscle strength, coordination or cognitive ability to independently perform either of these exercises? What also happens to the structural integrity of the human body after sitting in a wheelchair hour-after-hour, day-after-day? Selectively relocating ischemic pressure away from vulnerable sites to areas that are better suited to tolerate compression and sheer stress works quite well for custom seat cushion designs. Features such as the ischial cutout, ischial recess, pre-ischial bar, and sub-trochanteric shelf modification play vital rolls to preserve tissue integrity and at the same time promote tri-plane pelvic stability. However for the backrest it is a different story altogether. This is where our knowledge of biomechanics, force mechanics, and counterbalances come together to play a part in our decision-making processes. In fact much of what we know about creating spinal stability in custom seating has been borrowed from the Prosthetics and Orthotics Industry. The molded seating systems from Pin Dot, Otto Bock, Cushmaker.com, Aspen Seating and others, as well as such developments as the seating simulator, MOSS (Modular Orthotic Seating System), MPI (Multiple Position Insert), foam-in-place and grid back systems all sprouted from or found their home at one point, in P&O laboratories. And why not, P&O is the same industry that for decades engineered and refined body jackets, suspension jackets, scoliosis bracing, the sitting orthosis, TBI brace (total body involved), the sitting socket, the VASIO-P cushion (Veteran’s Administration Seating Interface Orthosis for Paraplegia), the STS (spherical trunk support) pad, and many other posture influencing devices. In fact, at one point in the 1980’s the P&O industry had full command of every aspect of the custom seating fabrication industry. So what methods or techniques did they employ back then? The three-point-pressure system, total thoracic containment, lever mechanics, skin trauma management, and the principles of pressure transference. In other words, “accentuate the positive and eliminate the negative”, all in an effort to counter gravity. But here’s the bottom-line question. Do the principles of the P&O industry have the same degrees of application and success in today’s seating and mobility industry? I wonder? The wheelchair dependent consumer relies on their support and mobility platform for upwards of ten to sixteen hours per day. The physical demand and structural dynamics of having to stay put for such durations just aren’t the same when compared to the abled body. Even if applied correctly, the three-point-pressure system for spinal alignment works to a limited degree with the ambulatory population. It functions more for containment and as a delaying force once an idiopathic curve progresses beyond a certain degree. Even within the range that a scoliosis brace should provide benefit, it won’t, if the candidate is too obese or lacks sufficient muscle tone. We’ve all seen the illustrations of the three-point-pressure system straightening out a tree or a seated skeleton but why do we expect that this approach will have an equal effect as part of a custom seating system design? In upright sitting, its real benefit is to provide trunk stability, security and accommodation of deformity. We also know that the three-point-pressure system is most effective when the counter force is applied through anatomically sculpted and apical oriented structures. So, do planar surfaces with symmetrically placed and marginal points of contact provide the same degree of benefits? One advantage of a seated posture is that we can move the points of contact anywhere we want, starting from the floor of the pelvis all the way up to the neck or cranium. But we can’t apply the same amounts of pressure, we can’t lock the vertebral facets and we loose some of the de-rotational components necessary for scoliosis prevention. The two biggest shortcoming of its application in sitting is that: 1) Unlike a body jacket or scoliosis brace it becomes fixed into one location thus denying the occupant any freedom of thoracic movement and a substantially diminished range of motion for the upper extremities. 2) It only provides benefit so long as the consumer remains in the wheelchair. As mentioned previously, an agile person can move away from the forces created by the three-point-pressure system in order to achieve spinal distraction and pressure relief. However, that task is almost impossible if you have little or no muscle tone and are fixed in a static sitting posture for hours on end. So all in all our provisional knowledge and use of the three-point-pressure system isn’t enough to outline all the solutions to combat spinal mal-alignment and thoracic instability. Here again because we’re working with a different cause and effect scenario when the human skeleton with volatility is flexed and positioned into a seated posture. Another reason could be that the orientation and alignment of the foundation on which the thorax sits may itself be compromised. That is not to say that the three-point-pressure system isn’t useful. Unfortunately, and in general, its manner of application is only marginally understood and as a tool it provides only a fractional (second step) solution to very complex postural alignment problems. Fact is, you can’t talk about functional postural alignment or the three-point-pressure system without first defining what is postural balance or postural stability. Listen closely! There is a definable relationship between pelvic, hip and sacrolumbar angles combined with upper body gravity-center correlation between the pelvis, torso and head/neck complex, which when properly aligned create postural balance and non-motion stability. These baseline formulas help us characterize posture balance points and then fine-tune the results for each person, each orthopedic profile and each sitting scenario. But all of that, my friends, is a discussion is for another time. Thank goodness for the development of the tilt-in-space (TS) wheelchair frame. Look at how much has changed since it was finally introduced into the adult Rehab market in the late 80’s. Funny thing is that it had already been a part of the pediatric market for at least fifteen years. But finally, what our clients (and we) could not achieve in upright sitting is now possible in the tilted position (sometimes even independently); spinal distraction, ischial pressure relief, pressure transference, posture realignment, clothing adjustments and much more. Regrettably however, all of this excitement is short lived because once back into the functional upright seated position, the same problems reappear: vertical compression of the thorax, clinical kyphosis, spinal instability, compromised functional alignment, forward head and neck flexion, diminished respiration, etc. Unfortunately for the industry, what we find is that most of today’s research is skewed more towards the technology component of seating and not the human. It took us decades of trial and error to figure out that 90/90/90 isn’t natural under any circumstance and that a hip belt is less affective at 45 degrees. Now take a look at all the remaining theories we work with, and ask your self, how many were scientifically proven before they were translated into clinical guidelines? We need to find more ways to make incremental improvements in our profession with rationalizations based on science and not with just one-up or selectively screened case studies. For the short term we accept that spinal fusions and the tilt-in-space wheelchair frame provide imperfect but welcomed solutions to many of the postural alignment problems we face in custom seating. But in the end we still haven’t demonstrated mastery over that ever present, all influencing and function depriving force we call gravity.
|
|||
