Page 356 - Kinesiology of the musculoskeletal system foundations for physical rehabilitation

332 Section III Axial Skeleton
This relatively low pressure, combined with the hydrophilic
nature of the nucleus pulposus, attracts water into the disc.
As a result, the disc swells slightly when one is sleeping. When
one is awake and upright, however, weight bearing produces
compression forces across the vertebral endplates that push
water out of the disc.94 The natural cycle of swelling and
contraction of the disc produces on average a 1% diurnal
variation in overall body height.201 The daily variation has a
strong inverse relation to age. Karakida and colleagues used
magnetic resonance imaging (MRI) to measure the variation
in water content in the discs of a group of working persons
between the ages of 23 and 56 years old, with no medical
history of low-back pain.100 Remarkably, significant diurnal
variation in water content was found only in the discs of
persons younger than 35 years of age. These findings are con-
sistent with the fact that the water-retaining capacity of inter-
vertebral discs naturally declines with increasing age.6,14 The
relative dehydration is caused by the parallel, age-related
decline in the discs’ proteoglycan content.158,203
A relatively dehydrated nucleus pulposus exerts less hydro-
static pressure when compressed.23 Once relatively depressur-
ized, the disc may bulge outward when compressed, similar
to a “flat tire.” The older, degenerated intervertebral disc is
subsequently less able to uniformly cushion the vertebral
body and endplates against compressive loads.145 As a con­
sequence, disc degeneration increases with age and affects
most persons, to varying degrees, over 35 or 40 years of
age.78,100,158,164,206 A diagnosis of disc degeneration is most effec- times of immobilization, these connective tissues may become
tively made using MRI, typically based on a diminished signal
intensity of the T2-weighted image (indicative of reduced
water content), loss of distinction between the border of the
annulus fibrosus and the nucleus pulposus, nuclear bulging,
and loss of disc space.78,100,157,164 The MRI scan in Figure 9-38
shows diminished signal intensity between L4-L5 and L5-S1
along with nuclear bulging. Furthermore, a degenerated disc
may display circumferential, radial, and peripheral fissures
(clefts) within the annulus.78 According to Adams, these fis-
sures can often be observed even in young adolescent persons.6
Excessive degeneration may also be associated with complete
depressurization of the nucleus in conjunction with delami-
nation of the annular fibers and microfractures of the verte-
bral endplates.78 In some cases the internal disruption of the
annular fibers may lead to a herniation (prolapse) of the
nucleus pulposus (typically posteriorly toward or into the
spinal canal). Remarkably, a significant percentage of persons
with observable signs of disc degeneration on MRI remain
asymptomatic, without experiencing continued mechanical
deterioration or loss of function.95 The important topic of
disc degeneration, including disc herniation, is described in
more detail later in this chapter.
REGIONAL ANATOMY AND KINEMATICS
ACROSS THE VERTEBRAL COLUMN
This section describes the anatomy and the kinematics
throughout the various regions of the vertebral column. For
each region, a maximum expected range of motion will be
cited, assuming a starting, neutral position (Figure 9-39).82,106,121
The reported range of motion in the literature is highly vari-
able, reflecting differences in research design and differences
resulting from the gender, age, and activity level of the sub-
jects.198 Data also vary for active and passive movements,
L2
L3
L4
L5
S1
FIGURE 9-38. A midsagittal T2-weighted MRI scan of a 35-year-old
man with a history of recurrent low-back pain that is provoked by
prolonged or repeated lumbar flexion. Evidence of disc degeneration
is indicated by a diminished (darker) signal intensity in the nuclear
regions of L4-L5 and L5-S1. Posterior displacement or “bulging” of
the disc is also noticeable at the L4-L5 and L5-S1 junctions (arrows).
(Image courtesy Paul F. Beattie PT, PhD.)
means used to stabilize the subject, and the tools used to
measure the motion. Methods typically include the use of
goniometers (manual, electrical, or fiberoptic), flexible rulers,
or inclinometers or more sophisticated tools that employ
three-dimensional MRI, planar and biplanar radiography, vid-
eofluoroscopy, ultrasonography, and computerized analysis
using electromechanical, potentiometric, optical, or electro-
magnetic tracking systems.*
The connective tissues within the vertebral column play a
major role in limiting and therefore defining the normal
limits of motion across regions; selected examples are pro-
vided in Table 9-6. In cases of disease, trauma, or extended
abnormally stiff, thus interfering with normal kinematics.
Understanding the structures’ normal function is a prerequi-
site for the design of treatments aimed to increase interverte-
bral mobility.85
Introduction to Spinal Coupling
Movement performed within any given plane throughout the
vertebral column is usually associated with an automatic, and
usually imperceptible, movement in another plane. This kine-
matic phenomenon is called spinal coupling. Although spinal
coupling can involve both rotation and translation, more
clinical attention is paid to the rotational kinematics.
The mechanical explanations for the cause of most pur-
ported spinal coupling patterns are varied and typically
*References 19, 20, 108, 121, 197, 198, 222.