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unknown user
03-14-2001, 07:27 PM
Dear Subscribers:

Here is the summary of responses of my question, which was the following:



There is a considerable number of studies reporting changes in body height
(spinal shrinkage) during physical activity, different work-loads and
circadian variations (e.g., Althoff; Boocock; Reilly; Van Dieen, etc).
However, there is no reference to the maximal shrinkage a person could
withstand without injury.

One attractive way of doing this could be extrapolating the mechanical
behaviour of a single intervertebral disc examined in an "in vitro"
condition. In this case, it would be necessary to assume that all
intervertebral discs (i.e., cervical, thoracic, lumbar discs) behave in a
similar manner, i.e., they lose height proportionally to their initial height
(a normalised value e.g., X% of their unloaded/resting condition). So, by
replacing all intervertebral discs by one large disc (i.e., representing the
height of all intervertebral discs together) and knowing the maximal
deflection that occur within the elastic zone of the disc, changes in the
whole spine could be estimated if we assume that the intervertebral discs
constitute approximately 30-33% of the entire spine.

However, I found difficult to predict the maximal "theoretical" shrinkage
using the literature as reference. In most experiments, where the discs were
exposed to axial compressive loads, only the absolute change in disc height
was reported (e.g., Virgin, 1951; Kazarian, 1975). Perhaps, the reason for
this is because most studies preserve the intervertebral discs attached to
the adjacent vertebras (for clamping the specimens) and do not quantify the
disc height before testing (initial, unloaded condition).

Please, during this discussion disregard that all changes that may occur in
the appendicular skeleton and assume that all changes in the height of the
spine occurs in the intervertebral discs.

Any reference in the literature I may have missed that could clarify this?
Any comments?

I thank you in advance

__________
Nat Ordway wrote:

This is an interesting question. In the tests we've done on the lumbar
spine, we see a 10-15% decrease in disc height with an applied compressive
load of 1200N. So if we assume all discs behave the same, we could estimate
the maximal spine shrinkage to be 15% of 30% or approximately 5% of the
entire spine. One thing I was curious about was what you meant by "without
injury"? Do you mean trauma? Small annular fissures could develop under
lower loads, but not show up as pain until the future.

Nat Ordway
________
Edsko Hekman wrote sugegsting to have a look in Brinckman's work
"The article suggests that the amount of bending of the vertebral endplate is
important.

"Deformation of the Vertebral End Plate under Axial Loading of the Spine"
Brinckmann, P.; Frobin, W.; Hierholzer, E. and Horst, M.
Spine Volume 8,1983,pp 851-856

Regards,
Edsko Hekman

_______
Deric Wisleder wrote:

In my doctoral research at Penn State U., I loaded healthy college-aged
male subjects with 1 BW axial compression for ten minutes and measured
lumbar spine response to loading in sagittal MR images (Dr. Vladimir
Zatsiorsky, myself, and colleagues have 2 papers in review with 'Spine').

A spine segment was defined as the distance between vertebral centroids
(see Boos et al. 1996). The relaxed segment heights were 35.12.2 mm. Pure
compression of individual segments during loading (0.1 +- 0.6 mm) was
insignificant (n= ~60 segments, 10 subjects T12-S1); however, five
individual segment compressions were greater than or equal to 1.0
mm. These compression deformations were approximately 10% (possibly more
in a couple cases) of the resting disc height (comparing to disc heights
reported by Gilad and Nissan 1986).

Pure cumulative compression from T12 to S1 was small (0.80 +- 0.9 mm) but
significant. One subject (n=8 for this measure) compressed 2.1 mm from T12
to S1. Shortening of the chord from T12 to S1 (2.9 +- 1.8 mm) indicated
greater influence of bending than pure compression. Re-orientation of the
'lumbar chord' was also accounted for by determining the length (and change
in length, 3.9 +- 1.2 mm, n=8) in projection on the long body axis.

Thus, the lumbar spine shortened 1.85% {3.9 mm / (35.1 mm x 6 segments)}
comprised of bending >re-orientation > pure compression. The load was
moderate considering the range of physiological loading (576 N during
standing, Khoo et al. 1994; to more than 10,000 N for a dynamic sagittal
lift with 95 kg, McGill et al. 1995). I tried to maximize the compression
load in order to maximize the response (and especially pure disc
compression), but the load was limited by the subjects' endurance tolerance
to sustain the load while motionless in the MRI tube. The duration of
loading was ten minutes to allow for creep deformation followed by MRI
acquisition.

Broberg (1993) estimated that lumbar compression accounts for one third of
the total spine compression in upright activities. In my work, there was
no decreasing load gradient in the cephalic direction as in gravitational
loading; therefore, one might expect >8.0 mm of compression between T1 and
T12. That remains for further study. Broberg (1993) attributed 75% of
total spine compression over long periods to diurnal water exchange from
the disc (the other 25% attributed to visco-elastic response). The time
constant for that deformation is on the order of hours (2.25 hours in
vitro, Smeathers 1984), which will require substantially smaller loading by
a spine compression device.

I would propose that maximum spine shortening might be achieved (and
measured) by having subjects perform heavy labor (perhaps athletes lifting
heavy weights) followed by application of a compressive load in the MRI to
prevent relaxation of any compression deformation achieved. By imposing
the work bout late in the day, short and long-term components of
deformation could be accounted for.

Boos N, Wallin A, Aebi M, Boesch C (1996) A new magnetic resonance imaging
analysis method for the measurement of disc height variations. Spine 21
(5), 563-70

Broberg KB (1993) Slow deformation of intervertebral discs. J Biomech 26
(4-5), 501-12

Gilad I and Nissan M (1986) A study of vertebra and disc geometric
relations of the human cervical and lumbar spine. Spine 11 (2), 154-7

Khoo BC, Goh JC, Lee JM, Bose K (1994) A comparison of lumbosacral loads
during static and dynamic activities. Australas Phys Eng Sci Med 17 (2), 55-63

McGill SM, Sharratt MT, Seguin JP (1995) Loads on spinal tissues during
simultaneous lifting and ventilatory challenge. Ergonomics 38 (9), 1772-92

Smeathers JE (1984) Some time dependent properties of the intervertebral
joint when under compression. Eng Med 13 (2), 83-7


___________
Andre Rodacki
Department of Exercise and Sport Sciences
Manchester Metropolitan University
Hassal Road, Alsager, Staffordshire
United Kingdom
ST7 2HL

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