The Effect of Aging on Skeletal Mechanoresponsiveness: Animal Studies - Skeletal Aging and Osteoporosis: Biomechanics and Mechanobiology - page 202

4.2 Four-Point Tibial Bending
Four-point bending of the rodent tibia, as described for the rat by Turner et al. [ 42 ]
and for the mouse by Akhter et al. [ 43 ], was perhaps the first non-invasive,
extrinsic loading approach used to study bone adaptation. With the animal
anesthetized, the hindlimb (tibia) is supported on its medial surface by two sup-
ports a certain distance apart. An extrinsic load transverse to the long axis of the
bone is applied at two points (between the supports) on the lateral surface. The
hindlimb is held in position by securing the foot. Bone between the two load points
is analyzed to study adaptation in response to loading. If the loading fixtures are
connected to a materials testing system, then the load magnitude, rate and number
of loading cycles can be precisely controlled. As with other extrinsic loading
models, a force-strain calibration should be done a priori to determine the local
strain magnitude at the site of interest. Among the drawbacks of this model are that
it loads only cortical bone, and that transverse loading of the tibia can result in
periosteal bone formation related to contact pressure rather than bone bending.
A ''sham bending'' load case (where the loading and support points are placed
directly opposite each other) can be used to correct for this effect, and endocortical
results appear to be unaffected by periosteal contact. Nonetheless, most investi-
gators no longer use this model because of the confounding effects of local contact
near the site of interest.
Turner et al. [ 44 , 45 ] used this model in separate studies to examine the
influence of age on bone adaptation and concluded that aging increases the loading
threshold needed to trigger bone formation. Endocortical lamellar bone formation
in mature rats (9 months) showed a dose response to loading for loads above 40 N
(peak periosteal strain *2000 le, 36 cycles/day, 2 weeks). In contrast, a much
higher load (64 N; *3100 le) was required to induce an increase in endocortical
lamellar bone formation in middle-aged/old (19 month) rats. Moreover, the bone
formation rate at the 64 N load was fivefold less for 19-month old rats compared
to 9-month rats. The lower indices were observed in spite of almost equal
endocortical strain engendered by loading in mature and middle-aged animals
(i.e., similar force-strain calibrations). Loading resulted in periosteal woven bone
formation in 100% of mature (9 month) rats but only 60% of middle-aged/old
(19 month) rats at load magnitudes greater than or equal to 40 N. Based on the
endocortical and periosteal results, it was concluded that increasing age reduces
the mechano-responsiveness of bone.
On the contrary, Kesavan et al. studied bone adaptation in response to four-point
bending in two different strains of mice (C57Bl/6, C3H/He) at different ages
(2, 4 and 8 months) and concluded that age does not influence bone response to
loading [ 46 ]. Compared to the non-loaded limb, loading increased total area, total
mineral content, endosteal perimeter and periosteal perimeter (determined by
diaphyseal pQCT) of the loaded limb in both young (2, 4 month) and mature
(8 month) mice of each strain, indicating that age did not influence bone adaptation
in this model. However, middle-aged/old mice were not included in this study.