Biomedical Engineering Reference
In-Depth Information
Collagen fiber orientation (CFO) has been measured by circularly polarized light microscopes as
one parameter to represent the collagen network and to predict post-yield energy of bone tissue.
84
Hence, the post-yield behavior revealed by ERT rats' bone tissue could partially stem from a
highly organized collagen fiber network. Though the information regarding collagen
orientation in the present study is not available in our previous rodent studies,
55,77
it has
been reported that dogs after one-year of intensive endurance running (40km/day) revealed
a higher organization of collagen fibers in bone tissues.
87
Such highly organized collagen
fibers seemed to be able to compensate for the 10% BMD decrease in running dogs. Thus, a
highly organized CFO would be expected in rodent studies. As mentioned above, rats
subjected to short-term freefall landing exercise (5 days, 10 or 30 times per day) from a
height of 40cm also showed enhanced post-yield energy of ulnae.
79
In that study, authors
tried to measure the CFO of cross-sectional ulnae. Unfortunately, no difference in CFO
between exercise and control groups was found. One major reason for this lack of significant
results could be species difference. That is, CFO analysis might not be sensitive enough to
detect biomaterial differences in rodents. To date, CFO-related analysis in bone tissue
specimens have all been obtained from big mammals,
88, 89
which have more mature
Haversian's systems and visible osteons. However, in smaller mammals (e.g. young adult
rodents), it is not possible to find Haversian's systems or complete osteon.
90, 91
Per our
observation, the organization of collagen fiber tends to be relatively irregular in rats and,
thus, CFO analysis might not be sensitive enough to predict post-yield material properties.
On the other hand, cross-links within collagen networks might be another contributor to
changes in tissue post-yield behaviors.
92
In an exercise-related study, Kohn and colleagues
verified that cortical toughness enhanced by a 21-day ERT could be correlated with the
overall maturity of collagen cross-links.
93
In addition to individually measure CFO or cross-links, the biomaterial properties (e.g.
tissue strength or tissue post-yield behaviors) might benefit from better integration between
collagen and its crosslinks. Related measurement methods are awaited and are worthy of
further study.
Finally, microdamage is another factor influencing tissue's post-yield behavior.
Accumulation of microdamages (or microcracks) would lead to a fragile bone with lower
capability in post-yield energy dissipation.
84, 94
However, such accumulated microdamages
seemed to be more related to aging. Also, as in CFO-related studies, microdamage studies
have been more frequently done in big mammals. Whether microdamage measurement can
be performed on exercise-related rodent study needs further verification.
6. Summary
Endurance running is a popular aerobic activity and typical training type. However, related
human studies reveal no significant benefits to bone health based on densitometric
measurement of bone mineral. On the other hand, animal studies apparently indicate that
ERT enhances biomaterial of bone tissue in a size-independent way. The effects of
endurance running on the organic bone matrix or other parameters, as well as their
relationship to mechanical properties of bone tissues, are worthy of further study.
7. Acknowledgments
This study was supported by a grant from the National Science Council: NSC 99-2410-H-006
-114 -MY2. Miss Jae Cody is appreciated for her editorial assistance.
