20-Bio-B8 Applied Optics_Photonics · December 2013
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format: National Exams, December 2013 — 04-Bio-B8 Rehabilitation Engineering. Three hours, open book, non-communicating calculator permitted. Seven questions of equal value (20 marks each); five constitute a complete paper and only the first five appearing in the answer book are marked. All seven are solved here as a complete study resource. Every question is an essay/design question (block-diagram assistive-technology system design, or descriptive explanation).
Reference texts (the books a candidate should have reviewed for this subject):
Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.
(i) Why seating is a problem, and the anatomy at risk. Loss of sensation below the level of injury removes the normal protective reflex of shifting weight when discomfort builds, and loss of voluntary motor control removes the ability to act on that reflex even if pain were felt; the individual is therefore immobile, seated, and unable to self-relieve pressure for hours at a time, while impaired circulation and, often, reduced tissue nutrition/muscle bulk over bony prominences make the tissue itself less tolerant of sustained loading. The areas of the seated anatomy requiring special attention are exactly the bony prominences that bear load in sitting: the ischial tuberosities (the primary weight-bearing points), the sacrum and coccyx (loaded in a slumped/posterior-pelvic-tilt posture), the greater trochanters (loaded in side-lying or with pelvic obliquity), and the heels/malleoli on the footrest. Spasticity and asymmetric muscle tone can also progressively distort the pelvis and spine (windswept hip deformity, scoliosis), which then concentrates load unevenly and compounds the pressure-injury risk over time.
(ii) Biomechanical principles. Pressure-ulcer risk follows an inverse pressure–time tolerance relationship (the Reswick & Rogers curve): tissue can tolerate high interface pressure only briefly, and much lower pressure only for a sustained period, so both the peak pressure AND the duration of loading must be controlled together, not pressure alone — a cushion is judged not just on its lowest peak pressure but on how well it distributes load so no single area stays loaded near its short-duration tolerance for long. Shear force (relative sliding between skin and support surface, e.g. as the pelvis slides forward in the seat) and its associated deep-tissue distortion compound the pure-pressure effect and must also be minimized. Postural stability — keeping the body's centre of mass over its base of support, and maintaining a neutral, symmetric pelvis as the foundation for trunk alignment — is the second governing principle, since poor postural support both increases shear (sliding to regain balance) and concentrates load asymmetrically.
(iii) Seating technologies presently available. Options span a spectrum of complexity and cost: planar or contoured foam cushions (lowest cost, adequate for lower-risk users with good sensation); viscoelastic (gel or gel/foam hybrid) cushions, which conform to distribute load over a larger area; segmented air-cell cushions (e.g., ROHO-type), which equalize pressure across interconnected air cells and can be dynamically adjusted; alternating-pressure (actively cycling) cushions for the highest-risk users, which periodically off-load different zones automatically; and fully custom-moulded seating systems (a positive cast of the individual's own seated shape, foamed into a rigid or semi-rigid moulded support), used when fixed skeletal deformity (scoliosis, windswept hips) means no off-the-shelf cushion can distribute load acceptably.
(iv) Modern pressure-measurement tools. Interface-pressure mapping systems — a thin, flexible mat containing a grid of capacitive or resistive (force-sensitive resistor) sensor cells, read out electronically and displayed as a colour-coded pressure map in real time — are now the standard clinical tool for evaluating and fitting a seating system: the clinician can directly see peak-pressure "hot spots," compare cushions objectively, and coach the individual on effective weight-shifts while watching the map update live. Adjunct tools include shear-sensing mats (measuring the horizontal/tangential component of loading, which plain pressure mapping misses) and, in research settings, sub-surface tissue-deformation imaging (ultrasound or MRI) to assess deep-tissue strain beneath the visible skin surface.
