Introduction - Preventive Biomechanics - page 5

level but in deeper tissue regions, taking objective mechanical tissue stress and
strain measures into account, is revealed. Interface pressure results from

(inaccurate) pressure mapping are shown to be insufficient.
In Chap. 7 a possible approach to biomechanical design optimization is demon-
strated based on the B OSS -Procedure. The method is presented based on a ''manual''
approach using the example of a newly developed bedding system, Sect. 7.1 and, in
Sect. 7.2 , an (automated) approach is presented, based on a two-dimensional opti-
mization algorithm using the example of a seat cushion. With the bedding system the
difficulties arising in conjunction with interface pressure mapping as a validation and
optimization tool are demonstrated. Comparisons between B OSS -Model interface
pressure output and pressure mapping results are shown.
This volume is a preliminary summary in the form of a static snapshot, while
research and development are in permanent flow. Nevertheless, we hope to provide
and stimulate new approaches, ideas and motivation to the basic problem of
developing adequate body supports and therapeutic appliances for handicapped
and healthy subjects. This topic is not a new textbook about human biomechanics
and the ambitious reader is referred to the fundamental works of Y.C. F UNG .
Rather, this topic concentrates on the mechanical interaction between the human
body and body supports, whereby knowledge of the biomechanics of the human
body and in particular its material properties are necessary. At present, such
knowledge must be gained from in vivo investigation of material properties of
body tissues. Data derived from animals or ex vivo data are not sufficient!
An additional problem is the issue of biological variability in human anatomy
and human tissue properties during interaction between support and tissue. The
primary goal in this investigation was to establish systematic and objective
methods to realistically describe tissue interaction. Hopefully, the B OSS -Procedure
will allow evaluation of a more diverse population. An increased number of
subjects will allow statistical validation and correction of the data and may reveal
possible generalisation.
It is principally possible to evaluate the effects of tissue interaction in terms of
stress and strain at any position (element) of a B OSS -Model (depending on the
model quality). Not realized to date is a valid comparison of these quantities with
permissible ''thresholds'' and ''deformation limits'' (in common use with technical
materials and structures) where cell damage occurs. The critical thresholds of
reduced blood flow within blood vessels (32 mmHg) and critical pressures
obtained from measurements in the finger nail bed at the arteriolar end of a
capillary loop (12 mmHg), established in the 1930s by Trumble and Landis,
cannot be regarded as authoritative.
To establish acceptable limits, a first approach would be to correlate simulated
model quantities with human perception. This is difficult since subjective tactile
perception and sensation of pain occurs, leading to varying perceived limits.
Primary steps in this direction have been undertaken by asking subjects to report
perceived ''pain'' and/or ''discomfort'' when placed on various seat and bedding
systems, modelled with FEA. Due to confidentiality this has not been documented.
Whether a definitive border can be drawn at which tissue depth, discomfort or pain