Biology Reference
In-Depth Information
FIGURE 14.1
Image comparing bone cross-sections. The top row depicts male femora and the bottom row
depicts female femora from CT scans of individuals from the William M. Bass Donated Skeletal Collection.
the predominant force affecting the epiphyses, but also affects the midshaft cross-sectional
area (
Frost, 1993; Eckstein et al., 2002
). For example, if a bone undergoes extreme axial
loading, the bone will accommodate by increasing in cross-sectional area. Lastly, shear forces
are those that act on the bone in two opposing directions.
Greater bending strength in a certain direction suggests that the bone is loaded more in
that direction. Many studies have investigated changing activity patterns reflected in the
ratio of maximum (I
max
) to minimum (I
min
) bending strength in the femoral midshaft. It
has been suggested that a high I
max
/I
min
ratio (or shape index) correlates strongly with
greater levels of activity, especially over rough terrain (
Lovejoy et al., 1976; Ruff and Hayes,
1983a, 1984
). Dividing the maximum moment of inertia by the minimum (I
max
/I
min
) gives
a unitless "shape" variable. In the femur, a high shape index reflects more anteroposterior
(AP) elongation. If equal to one, the cross-section is more circular; if less than one, the
bone cross-section is elongated in the mediolateral (ML) direction (see
Figure 14.1
). Different
types of activities with multiple forces simultaneously (torsion, bending, shear, and compres-
sion) can complicate the interpretation of a bone's shape, but it is important to keep in mind
that the shape of bone can potentially reflect specific activities. The study of functional
morphology in skeletal biology explores this relationship between the structure and function
of the bones.
FUNCTIONAL MORPHOLOGY
Wolff's Law
The skeleton serves many purposes. It acts as a support system for other organs, it
provides levers for action, and it supports the weight of the organism while withstanding
forces during locomotion and impact (
Schmidt-Nielsen, 1984
). Due to the fact that bone is
plastic, bone will adapt and model or remodel itself as necessary according to the forces
applied. Roux first made the observation in 1881 that bone trabeculae appear to follow
engineering principles, a finding later supported by Wolff in 1892. Both researchers recog-
nized a principle of “functional adaptation” in bones, where a bone will reinforce itself along
