Biomedical Engineering Reference
In-Depth Information
through acidic proteolytic digestion. Then osteoblasts produce bone matrix
and osteoblasts become entrapped and differentiate into osteocytes.
With age, bone density diminishes and architecture changes. The reduction
in density depends on a number of factors, including gender and anatomic site
and is called osteoporosis (Fig. 4.7). in the lumbar spine, direct measurements
have shown a decrease in trabecular bone density of approximately 50% from
ages 20 to 80 years. With decreasing density, the number and thickness of
the trabeculae decreases with preferential loss of vertical trabeculae, while
the size of the intertrabecular spaces increases.
4.3.2 Bone failure
Damage and repair of individual trabeculae occurs in a physiological process
throughout life and increase with age (Melvin, 1993). Small cracks within
individual trabeculae (so called microfractures or microcracks) can be
repaired by callus formation resulting in the formation of new woven bone
or lamellar bone around the original crack. These microcracks are formed
over time owing to the application of repetitive loading of low magnitude
but whose number of cycles induces crack propagation failure of trabeculae.
This damage process is particularly critical in osteoporotic bone where the
remodeling process is much lower and can therefore lead more easily to
rupture of individual trabeculae (Caler and Carter, 1989).
Four types of damage are found in bone: transverse cracks, shear bands,
parallel cracks and complete fractures, of which the first two are dominant
(Watchel and Keaveny, 1997). The formation of these small cracks induces
local damage which could be seen as insignificant. However, this local damage
(a)
(b)
4.7
Scanning electron microscopy pictures of (a) a normal spongy
bone and (b) a spongy bone with osteoporosis (from Martini, 1989).
