Biomedical Engineering Reference
In-Depth Information
Interestingly, in the same study both young and old rats responded favorably to
jump training with only modest evidence of an age effect. Because the jump
training likely produced higher magnitudes of bone strain (and strain rate), their
results suggest that young bones are more responsive than old bones to low-strain
loading but if the strain exceeds an ''adaptation threshold'' both age groups are
responsive by a similar amount. On the other hand, Leppanen et al. [
30
] conducted
a very thorough evaluation of the effects of treadmill running in old rats and
reported that there were favorable bone responses. One notable difference between
these studies is the treadmill inclination angle (none reported for Umemura et al.,
versus 30 for Leppanen et al.). It is possible that making the animals run uphill
increases the bone strain sufficiently to exceed the putative threshold.
4 Extrinsic Loading Models
4.1 Functionally Isolated Avian Ulna
Rubin and Lanyon [
37
,
38
] greatly advanced the science of bone adaptation using
an animal model that allows for complete control of loading history during the
experimental period. In this model an 11 cm segment of the ulnar diaphysis of
turkeys is functionally isolated by removing the distal and proximal ends and
placing caps at each end, held in place with transverse pins through which loads
are applied. An external fixator prevents incidental loading of the bone segment.
Controlled static loading is applied by springs, while controlled dynamic loading is
applied using a materials testing machine. A strength of the model is that the
loading history is completely controllable while maintaining the bone in situ with
muscle, nerve and vascular attachments intact. Limitations include the drastic
change in normal loading environment (which is likely to be ''perceived'' by the
bone as a disuse state), and the invasive surgical procedure, which may produce
local or systemic responses that influence bone adaptation (e.g., a ''regional
acceleratory phenomenon''). Such non-voluntary, invasive loading models have
also been developed to study effect of loading on caudal (tail) vertebrae in rats [
39
]
and lumbar vertebrae in rabbits [
40
].
Related to aging, Rubin et al. observed an age-related decline in the response to
mechanical stimulus in male turkeys using the functionally isolated ulnar loading
model [
41
]. Compared to non-loaded control, 8 weeks of cyclic loading (3000 le,
300 cycles/day) increased the cortical area of the loaded bone in young adult
(mature; 1 year) but not old (3 year) animals (Table
3
). The greater cortical area in
the mature group was a result of greater periosteal area and lower endosteal area.
Also, compared to non-loaded controls the mineral apposition rate was greater in
the loaded bone of mature animals but not old animals. Based on these observa-
tions it was concluded that a loading stimulus that is ''clearly osteogenic in the
young adult skeleton is hardly acknowledged in older bone tissue'', indicating a
decline with age in the ability of bones to sense and respond to loading.
Search WWH ::
Custom Search