Biomedical Engineering Reference
In-Depth Information
other by projecting throughout the lamellae very thin and long cytoplasmic ex-
tentions along the tiny canals called canaliculi. It is through the canaliculi that
electro-chemical information flows and the nutrients and minerals reach all points
of the bone matrix. Besides the osteons, long bones contain in the periphery sub-
periosteal circumferential lamellae, spawned by the inner layer of the periosteum.
6.1.3.2 Cortical Bone Remodelling
The remodelling process in cortical bone is activated by growing factors, or
changing of load patterns or due to micro or macro damage of the bone tissue. The
initial step is the removal of the inactive osteons, by osteoclast activity. Afterwards
new lamellae are deposited in new concentric layers, from exterior to interior, until
a new osteon is formed. In mature bone the recurring remodelling process causes
the formation and destruction of many osteon generations. The new osteons are
organized by complete circumferential layers, i.e., a complete Haversian system,
this causes that some older osteons present portions of the outer border occupied by
the outer border of a new osteon. In some old osteons, this process have occurred so
many times that all it is left from the original lamellae are small portions of the
circumferential lamellae. These small portions are called interstitial lamellae.
Cortical bone is remodelled by bone cells lining the periosteal, the endosteal and the
Haversian canal surfaces. The periosteal surface is responsible for the bone width
growth. The endosteal activity determines the diameter of the medullary canal. The
combined activity of the periosteal and endosteal structures determines the thick-
ness of the bone cortex. The Haversian canal surface plays an important role in the
remodelling process of the bone and it is responsible for the cortex density.
6.1.3.3 Trabecular Bone
The trabecular bone is composed of mature lamellar bone and, in contrast with the
compact structure of cortical bone, it is a complex network of intersecting curved
plates and tubes. The osteocytes within each trabecula are concentrically oriented
and have a well-developed canalicular set of connections. In the case of a long
bone, the trabecular bone is typically located at the proximal ends, here the
arrangement of the trabeculae is relatively regular, reflecting the direction of the
principal mechanical stresses to which this kind of bone is subjected. Trabecular
bone constitutes 20 % of the skeletal bone mass, being the remaining 80 %
occupied by cortical bone, nevertheless due to the trabeluae vast surface the sur-
face-to-volume ratio is ten times higher than the cortical bone. It is because this
high specific surface that the metabolic activity of trabecular bone is nearly eight
times higher when compared with the cortical bone, which may help to explain why
metabolic bone diseases have a greater effect on trabecular bone than on cortical
bone.
