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Re: How short can we get?

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  • Re: How short can we get?

    Andre Rodacki asked about estimates of maximal compression deformation of
    the spine.

    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

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