Page 355 - Kinesiology of the musculoskeletal system foundations for physical rehabilitation

Chapter 9 Axial Skeleton: Osteology and Arthrology 331
A 300
B applied compression more than a slow or light compres-
C relatively rigid at high loads.
FIGURE 9-36. The mechanism of force transmission through an
intervertebral disc. A, Compression force from body weight and
muscle contraction (straight arrows) raises the hydrostatic pressure in
the nucleus pulposus. In turn, the increased pressure elevates the
tension in the annular fibrosus (curved arrows). B, The increased
tension in the annulus inhibits radial expansion of the nucleus. The
rising nuclear pressure is also exerted upward and downward against
the vertebral endplates. C, The pressure within the disc is evenly
redistributed to several tissues as it is transmitted across the endplates transient changes in the shape of even the healthy disc. Sus-
to the adjacent vertebra. (Modified from Bogduk N: Clinical anatomy
of the lumbar spine, ed 4, New York, 2005, Churchill Livingstone.)
resisted by the tension created within the stretched rings of
collagen and elastin of the annulus fibrosus (see Figure 9-36,
B). Pressure within the entire disc is thus uniformly elevated
and transmitted evenly to the adjacent vertebra (see Figure
9-36, C). When the compressive force is removed from the
endplates, the stretched elastin and collagen fibers return to
their original preloaded length, ready for another compressive
force. This mechanism allows compressive forces to be shared
by multiple structures, thereby preventing a small spot of
high pressure on any single tissue. Because it has viscoelastic
properties, the intervertebral disc resists a fast or strongly
500 NachemsonDisc pressure normalized to standing (percent)
450 Wilke et al
400
350
250
200
150
100
50
0
FIGURE 9-37. A comparison between data from two intradiscal pres-
sure studies (see text). Each study measured in vivo pressures from
a lumbar nucleus pulposus in a 70-kg subject during common pos-
tures and activities. The pressures are normalized to standing. (Modi-
fied from Wilke H-J, Neef P, Caimi M, et al: New in vivo
measurements of pressures in the intervertebral disc in daily life,
Spine 24:755, 1999.)
sion.101 The disc therefore can be flexible at low loads and
In Vivo Pressure Measurements from the Nucleus Pulposus
In vivo studies have confirmed that pressure within the
nucleus pulposus in the lumbar region is relatively low at rest
in the supine position.13,140,219 Much larger disc pressures
occur from activities that combine forward bending and the
need for vigorous trunk muscle contraction. Intradiscal pres-
sures can rise to surprisingly high levels and can produce
tained flexion in the lumbar spine, for example, can reduce
the height of the disc slightly as water is slowly forced outward.
Sustained and full lumbar extension, in contrast, reduces the
pressure in the disc; this allows water to be reabsorbed into
the disc, thus reinflating it to its natural level.
In vivo data on pressure within the disc during movement
and changes in posture have greatly increased the understand-
ing of ways to reduce injury to the disc. Data produced by
two separate studies are compared in Figure 9-37.138,219 Both
studies reinforce three points: (1) disc pressures are large when
one holds a load in front of the body, especially when bending
forward; (2) lifting a load with knees flexed places less pressure
on the lumbar disc than does lifting a load with the knees
straight (the latter method typically generating more demands
on the back muscles); and (3) sitting in a forward-slouched
position produces greater disc pressure than sitting erect.
These points serve as the theoretic basis for many educational
programs designed for persons with disc degeneration, includ-
ing disc herniation.
Diurnal Fluctuations in the Water Content
within the Intervertebral Discs
When a healthy spine is unloaded, such as during bed rest,
the pressure within the nucleus pulposus is relatively low.138