Just wanted to add a bit of detail to this thread, it reiterates and lends support to Kevins comments and looks at a parallel issue.
I am trying to improve my interpretive reasoning of in shoe and pressure mat data currently and reading around the subject with reference to gaining a deeper understanding of the tissue material characteristics, the mechanical responses of our anatomy to stress during stance and gait and how the measurements we can take can be informative or misleading.
One thing I have noticed using in shoe pressure measurement (ISPM) for evaluating foot orthoses design for DM related metatarsal head overload is that effective reduction of ground reaction force by foot orthoses does not necessarily eliminate damage to associated skin which commonly is attributed compression stress.
I am curious to know if others have noticed this. For example I did ISPM study last week on a patient with loss of protective sensation, and prior history of neuropathic ulceration. The comparative IPSM data showed a reduction of ground reaction force at the lesion site from 2X normal values unprotected to zero in terms of peak pressures and force/time integrals with foot orthoses use. In terms of design goals the foot orthoses worked perfectly however using these foot orthoses with appropriate foot-wear he develops significant keratoma and pre-ulcerative changes which to date are only managed with debridement at two weeks interval.
A couple of extracts from studies below explore possible explanations for this and the link between ISPM values and bone stress/strain using finite element modeling which seems to give us best available picture currently.
1 FROM
Three-dimensional finite element analysis of the foot during standing--a material sensitivity study.
Source:
Journal of Biomechanics [0021-9290] Cheung yr:2005 vol:38 iss:5 pg:1045 -54
Knowledge on the effect of soft tissue compliance or
other structural characteristics on the stress distribution
of the plantar foot surface and bony structures is
essential to achieve an appropriate individualised treatment
strategy such as an orthotic design.
The pressure distributions between the foot and
different supports were measured experimentally with
the use of in-shoe pressure sensors and pedobarograph
(Cavanagh et al., 1987; Lavery et al., 1997; Patil et al.,
2002; Raspovic et al., 2000; Lord and Hosein, 2000;
Lord et al., 1986). Due to the difficulties and lack of
better technology for the experimental measurement, the
load transfer mechanism and internal stress states within
the soft tissues and the bony structures were not well
addressed.
In order to supplement these experiments, researchers
have turned to computational methods. The finite
element (FE) analysis has been an adjunct to experimental
approach to predict the load distribution
between the foot and different supports, which offer
additional information such as the internal stresses/
strains of the ankle–foot complex.
A number of foot models have been developed based on certain assumptions
such as simplified geometry, limited relative joint
movement, ignorance of certain ligamentous structures
and simplified material properties (Chen et al., 2001;
Gefen, 2000; Gefen, 2003; Jacob and Patil, 1999;
Kitagawa et al., 2000; Nakamura et al., 1981). The
models developed by Jacob and Patil (1999) and Gefen
(2003) have been employed to investigate the biomechanical
effects of soft tissue stiffeningin the diabetic feet.
Their models predicted that the peak plantar pressure
was found to increase with soft tissue stiffness but with
minimal effect on the bony structures. Gefen (2003)
further speculated that the development of diabeticfoot-
related infection and injury was more likely
initiated by micro-damage of tissue from intensified
stress in the deeper subcutaneous layers rather than the
skin surface.
It has been shown in the literature that FE models can
contribute in familiarizing the effects of thickness and
stiffness of plantar soft tissue on plantar pressure
distribution (Gefen, 2003; Jacob and Patil, 1999;
Lemmon et al., 1997). A detailed model of the human
foot and ankle, incorporating realist ic geometrical
properties of both bony and soft tissue components is
needed to provide a more realistic representation of the
foot and the supporting condition s, in order to enhance
the understandingof the ankle–foot biomechanics
(Camacho et al., 2002; Kirby, 2001).
For the sake of convergence of solution and
Minimizing computational efforts, most of the linearly
elastic FE foot models reported so far (Chen et al., 2001;
Chu et al., 1995; Jacob and Patil, 1999) assigned
relatively stiff mechanical properties for soft tissue,
where the Young’s moduli were selected as being 1MPa
or larger. These values of Young’s moduli are much
larger than those obtained from in vivo experimental
measurements of plantar soft tissue, ranging from 0.05
to 0.3MPa under strains of 10–35% (Gefen et al.,
2001b; Zhenget al., 2000). For FE models usinga
nonlinear material model for plantar soft tissue (Gefen
et al., 2000; Gefen, 2003; Nakamura et al., 1981;
Lemmon et al., 1997), the adopted stress–strain behaviour
varied as a result of the intrinsic variation of
individual tissue, measurement techniques and environment.
The stress–strain response of plantar soft tissue
was often obtained from either indentation or compression
test of in vivo or cadaveric specimens (Gefen et al.,
2001a; Klaesner et al., 2002; Lemmon et al., 1997;
Nakamura et al., 1981; Miller-Younget al., 2002).
In the literature, there is still a lack of material sensitivity study
to quantify the effects of soft tissue stiffening on plantar
pressure distribution usinga geometrical accurate threedimensional
(3D) foot model.
The objective of this study was to develop a
comprehensive FE model of the foot and ankle, using
3D actual geometry of both skeletal and soft tissues
components and to investigate the effect of soft tissue
stiffness on the plantar pressure distributions and the
internal load transfer between bony structures.
2 FROM
Surg Radiol Anat. 2007 Oct;29(7):561-7. Epub 2007 Jul 10
Clinical significance of musculoskeletal finite element model of the second and the fifth foot ray with metatarsal cavities and calcaneal sinus.
Because of high incidence among runners and military personnel,
plantar fasciitis and metatarsal stress fractures are
commonly assumed to be caused by repetitive microtrauma
of the corresponding tissues [1, 16]. The results in microtrauma
are associated with not only the conditions of
stresses and strains of plantar longitudinal arch [20, 21], but
also the distributions of plantar pressures. While techniques
for plantar pressure measurements are well developed,
direct measurement of the internal stress/strain is diYcult
[4]. In the past three decades, two-dimensional (2D) Wnite
element (FE) models [6, 13, 15] and 3D FE models [4, 7] of
the foot have been reported to predict stresses/strains of
metatarsals and plantar fascia. In these models, homogeneous
medium for foot bones and nonlinear material characteristics
for plantar soft tissues were assumed; however,
the architecture of intrinsic muscles, metatarsal cavities and
calcaneus sinus have almost not accounted for. Therefore,
an accurate FE model of plantar arches, incorporating more
realistic geometrical and material properties of musculoskeletal
components is needed to enhance realistic understanding
of podiatric biomechanics of metatarsal stress
fractures and plantar fasciitis [12, 22].
The goal of this study was to develop anatomically
detailed FE models of the second and the Wfth foot ray
using experimental dataset of Virtual Chinese Human
(VCH) “female No 1”, which incorporated four bone media
accounting for metatarsal cavities and calcaneal sinus, as
well as muscles and ligaments in plantar arch. Then, internal
stresses/strains of the detailed models were compared
with that of the simpliWed models without metatarsal cavities
and calcaneal sinus during standing posture.
Musculoskeletal FE models of the second and Wfth foot ray
with metatarsal cavities and calcaneal sinus have capability
to predict stresses/strains concentration on the shaft and
basis of metatarsals, bone trabecula around metatarsal cavities,
and plantar fascia insertion of calcaneus during standing
posture. Moreover, the predicted tension/compression
stress Xows are geometrically similar with the tension/compression
trabecular architectures in sagittal sections of
metatarsal and calcaneus.
With more accurate and realistic
FE predictions, podiatric biomechanics will contribute
more extensive and valuable insight into metatarsal stress
fractures and plantar fasciitis.
Hope this adds something helpful
Cheers
Martin
The St. James Foot Clinic
1749 Portage Ave.
Winnipeg
Manitoba
R3J 0E6
Phone [204] 837 FOOT (3668)
Fax [204] 774 9918
www.winnipegfootclinic.com
Foot type and plantar pressures
Linear Mode
