Properties of natural materials - Orthopaedic Biomaterials in Research and Practice - page 115

Because of this immature structure, it is essentially an isotropic mate-
rial, with equal properties in all directions.
Callus Callus, the mineralized tissue that forms in healing fractures
and produces a natural “splinting” or reinforcing effect to the broken
bone ends, is similar in organization, composition, and properties to
primitive fiber bone (Figure 5.10). It is very poorly bonded to preexist-
ing bone and, thus, its contribution to fracture stiffness and strength is
difficult to determine, even in in vitro testing. To add to the complex-
ity of quantifying the mechanical properties of callus tissue, there is
variability due to its heterogeneous nature. Multiple tissue types exist
within the callus, namely, granulation tissue, chondroid tissue, and
woven bone. Animal studies, such as from rat fracture healing experi-
ments, suggest that the variability can range by over three orders of
magnitude, from approximately 0.6 MPa to 1000 MPa. The nature of
the callus tissue will depend on the phase of fracture healing. During
the first phase, the inflammatory response leads to the formation of
a hematoma and granulation tissue. Then, a soft callus consisting of
cartilaginous or chondroid tissue is formed in the second phase, typi-
cally starting 2 weeks after the fracture begins to heal. In the third
phase (between 6 and 12 weeks), the soft callus ossifies to form a hard
or bony callus consisting primarily of woven bone tissue. Finally, the
woven bone is remodeled into lamellar bone tissue (between 12 and 16
weeks). At any given time, a heterogeneous mixture of these tissues can
be found within the fracture callus.
Cancellous bone Next in order of strength and organization is can-
cellous or spongy bone. This is found in the endochondral areas of long
bones, supporting the subchondral cortical plates, in the central areas of
smaller bones loaded in complex fashion, such as the metacarpals, and
as the diploe between the tables of the calvarium. It consists of spicules
or trabeculae of fairly acellular bone surrounded by a highly cellular

Cortical
(subchondral)

Epiphyseal
scar

Fiber
(callus)

Healing
fracture

Fiber
(callus)

Cancellous

Cortical
(shaft)

FIGUre 5.10 (See color insert.) microstructural elements of bone.