Biomedical Engineering Reference
In-Depth Information
140
160
130
140
120
120
110
100
100
Tibial bone mass (left axis)
Ulnar BMC (right axis)
90
80
0
20
40
60
80
100
360
380
Load cycles per day
2.10
The bone formation response to a mechanical stimulus fades
with increasing loading duration. Although bone mass in the ulna
of turkeys (open triangles)
154
and tibia of rats (closed circles)
155
increases with mechanical loading, the anabolic effect saturates
as the number of loading cycles increases. There is limited benefit
of additional loading cycles above approximately 40 cycles/day
(reproduced with permission of Elsevier from Burr
et al
.
235
).
instance, rests of a number of seconds between each consecutive load cycle
result in greater bone adaptation than if the same strain stimulus is introduced
in back-to-back cycles,
156, 157
whereas rests of a number of hours between
consecutive loading bouts results in greater bone adaptation than if the same
strain stimulus is introduced in back-to-back bouts.
158-160
2.5.3 Site-specificity of the adaptive response of
bone to mechanical loading
As the response of bone to mechanical loading is highly stimulus-specific,
it follows that its adaptive response is also highly site-specific. Only those
bones that are actually loaded undergo adaptation. This has most clearly
been shown in racquet-sport players in whom the playing or racquet arm has
significantly greater bone mass and size than in the contralateral, non-playing
arm.
37, 161-163
Yet, the site-specific nature of bone adaptation to mechanical
loading can be localized further than to the individual bone level. Long bones
are curved such that they bend when axially loaded. Bending results in the
exposure of different regions within the bone cross-section to different levels
of microstrain. Only those regions within the bone that experience sufficient
strain stimulus adapt. This can be clearly observed in the rodent ulna axial
compression model (Fig. 2.11). The site-specific depositing of new bone is
functionally important. It adds bone and increases bone strength where it is
