Biomedical Engineering Reference
In-Depth Information
2.5.2 Factors affecting skeletal adaptation to
mechanical loading
strain magnitude is a key determinant of bone adaptation to a loading
stimulus. Bone formation is initiated above a certain threshold strain and
incremental increases in strain beyond the threshold result in further increases
in formation activity.
133, 137-139
Thus, increasing strain magnitude is one
step towards more effective application of mechanical forces to promote
osteogenesis. however, there are a number of other important features of a
loading stimulus that determine the adaptive response.
The cellular accommodation theory does not solely use strain magnitude
to predict the adaptive response to a mechanical stimulus, but rather it uses
strain stimulus.
140
The strain stimulus is the product of the strain magnitude
and loading frequency.
141
In order for mechanical loading to induce adaptation,
the load needs to be introduced dynamically. This follows teleologically given
the dynamic nature of bone loading during typical normal functional use.
Dynamic loading induces significantly greater adaptation than if the same
strain magnitudes are held statically,
142-144
indicating that bone adaptation
is not dependent on strain magnitude alone.
Features of dynamic loading that are important determinants of bone
adaptation are loading frequency and strain rate.
145, 146
Loading frequency
refers to the number of loading cycles per second, whereas strain rate is the
product of strain magnitude and loading frequency. Loading has no effect
on cortical bone formation unless it is applied at a frequency of 0.5 hz or
greater.
145
a positive relationship between loading frequency and cortical
bone formation exists beyond this threshold, with increasing frequency
generating progressively greater adaptation.
145-148
however, this relationship
may not persist when loading frequencies exceed 10 hz
149, 150
and may not
hold true for trabecular bone, which has shown variable adaptive responses
in response to increasing load frequencies.
151-153
Further features of a loading stimulus that influence the skeletal adaptive
response are the duration of the skeletal load and the length of rest between
loading bouts. extending the duration of skeletal loading does not yield
proportional increases in bone mass.
154, 155
as loading duration is increased,
the bone formation response tends to fade as the mechanosensitive cells
accommodate to the prevailing environment (Fig. 2.10). The decline in
adaptation with ongoing loading fits a logarithmic relationship, such that after
only 20 loading cycles bone has lost more than 95% of its mechanosensitivity.
This indicates that loading programs do not need to be long in order to induce
adaptation. similarly, it indicates that bone cells need to be able to resensitize
when given a period of rest between loading bouts in order to be responsive
to future loading bouts. This is indeed the case with resensitization occurring
in seconds to hours, depending on the nature of the loading stimulus. For
