Bones - Biomechanics of the Human Body - page 123

difficult to reach the compressive breaking stress of 1,100 N/cm 2 but if,
incautiously, this force is applied in an area smaller than 0.59 cm 2 , the pressure
can reach the threshold of fracture.
But, what can happen to the vertebrae that have compressive strength 5.8 times
smaller than that of an intervertebral disc, according to Yamada's measurement?

7.7 Pressure on the Vertebrae

Vertebrae have an external thin coverage of compact bone and the internal part is
made of a spongy material called the trabecular. At the ends of the long bones such
as the femur and the humerus, trabecular bones are found predominantly. The
central long part is made of a compact bone which has a kind of channel in the
middle containing the marrow. As trabecular bone is relatively more flexible than
compact bone, it can absorb more energy when large magnitude forces act on it
during activities such as walking or running or even jumping. However, in terms of
strength to fracture, they differ greatly from bone of the compact type.
The limit of fracture of a vertebra measured as compressive strength by Yamada
is 87.9 times and 5.8 times smaller than the compressive strength of compact bone
(human femur) and intervertebral discs, respectively, as can be seen in Table 7.3 .
Thus, according to Yamada's data, if the intervertebral discs of the cervical region
have enough strength to support the weight of the body during the headstand
posture,

the cervical vertebrae do not, since their compressive strength is

190 N/cm 2 . If the area of the vertebra is about 3 cm 2 and it has
to support a weight of 652 N, the stress at the vertebra will be 217 N/cm 2 .
We would like to remember again that since Yamada's bone data have been
obtained with bones alone, outside the body and without ligaments or muscles, this
can be one of the reasons that, in fact, no cervical vertebra breaks when a person
executes the headstand posture with care.

10 7 Pa 0.19 ¼

7.8 Shear Stress in the Lumbosacral Intervertebral Disc

The spinal column of a normal person standing erect is not straight, when it is
observed from the side. Its curvature, defined by seven cervical vertebrae, twelve
thoracic and five lumbar, is called cervical lordosis, dorsal kyphosis, and lumbar
lordosis, respectively, and is shown in Fig. 7.6 .
The curvature of lumbar lordosis is determined by the lumbosacral angle, that is,
the angle between the horizontal and the top surface of the sacrum. This angle is
30 , for a normal person standing erect. An anomalous curvature of the lumbar
lordosis can be one of the reasons for low back pain and let us see why. Figure 7.7
shows the lumbar region of the spinal column of a person standing erect.