Healthcare and Medicine Reference
In-Depth Information
Inside and around the bone is a sparse but active com-
munity of two types of osteocytes: the osteoblasts and
the osteoclasts. Each are sent forth with simple com-
mandments: osteoblasts lay down new bone; osteoclasts
clean up old bone. Osteoblasts are allowed to lay down
new bone anywhere they like - as long as it is within
the periosteum. The osteoclasts may eat of any bone,
except those parts that are piezo-electrically charged
(mechanically stressed). 2 0 Allow the cells to operate
freely under these rules over time, and a femoral head
is produced that is both specifically designed to resist
individual forces coming through it, but also capable of
changing (given some reaction time) to meet new forces
when they are consistently applied.
This mechanism explains how dancers' feet get
tougher bones during a summer dance camp: the
increased dancing creates increased forces which create
increased piezo-electric charges which reduce the ability
of the osteoclasts to remove bone while the osteoblasts
carry on laying it down - and the result is denser bone.
This is also part of the explanation for why exercise is
helpful to those with incipient osteoporosis: the forces
created by the increased stress on the tissues serve to
discourage the osteoclastic uptake. The reverse process
operates in the astronauts and cosmonauts deprived of
the force of gravity to create the pressure charge through
the bones: the osteoclasts have a field day and the
returning heroes must be helped off their ship in wheel-
chairs until their bones become less porous.
This extraordinary ability to respond to demand
accounts for the wide variety in joint shapes across the
human spectrum, despite the consistent pictures aver-
aged into most anatomy textbooks. A recent study
detailed distinct differences in the structure of the sub-
talar joint. 2 1 Smaller differences can be observed over the
entire body. In Figure 1.11 A we see a 'normal' thoracic
vertebra. However, in Figure 1.11B, we can see the body
distorted as pressure creates a demand for remodeling
under Wolff's Law, 2 2 and hypertrophic spurs forming as
the periosteum is pulled away by excess strains from the
surrounding connective tissues and muscles (see also
Ch. 3 on heel spurs). A non-union fracture can often be
reversed by creating a current flow across the break,
reproducing the normal piezo-electric flow, through
which the collagen orients itself and begins the process
of bridging the gap, to be followed by the calcium salts
and full healing. 23 - 2 4
This same process of response occurs across the entire
extracellular fibrous network, not just inside the bones.
We can imagine a person who develops, for whatever
reason (e.g. shortsightedness, depression, imitation, or
injury) a common 'slump': the head goes forward, the
chest falls, the back rounds (Fig. 1.12). The head, a
minimum of one-seventh of the body weight in most
adults, must be restrained from falling further forward
by some muscles in the back. These muscles must remain
in isometric/eccentric contraction (eccentric loading)
for every one of this person's waking hours.
Muscles are designed to contract and relax in succes-
sion, but these particular muscles are now under a con-
stant strain, a strain that robs them of their full ability,
and facilitates the development of trigger points. The
strain also creates a piezo-electric charge that runs
through the fascia within and around the muscle (and
often beyond in both directions along the myofascial
Fig. 1.12 When body segments are pulled out of place and
muscles are required to maintain static positions - either
stretched/contracted ('locked long') or shortened/contracted
(locked short') - then we see increased fascial bonding and
thixotropy of the surrounding intercellular matrix (ECM),
Fig. 1.11 Even bones will alter their shape within certain limits,
adding and subtracting bone mass, in response to the mechanical
forces around them. (Reproduced with kind permission from
Oschman 2000.)
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