Properties of natural materials - Orthopaedic Biomaterials in Research and Practice - page 116

periosteal membrane. The bone is well organized with a preferred direc-
tion for collagen fibers lying roughly along the axis of each spicule. The
spicules are organized into a spongy network, which in places condenses
into plates connected by spicular struts, as in the patella, or into a closed-
cell sponge, as in the diploe.
Cancellous bone, unlike fiber bone and callus, is an organized load-
bearing material that appears to follow Wolff's law. Its presence is asso-
ciated with the following structural requirements:
1. The need to provide a transition of direction of stress ( example :
the transition between the neck and the diaphyseal cortices of the
proximal femur)
2. The need to distribute concentrated loads, such as associated with
tendon and ligament insertion ( example : the greater trochanter of
the femur)
3. The need to resist impulsive loads ( example : the calcaneus)
Cancellous bone is anisotropic in terms of both mechanical proper-
ties and morphometry. It is stiffer (has higher tensile and compressive
moduli) but fails at a lower strain when loaded parallel to the predomi-
nant spicular direction than when loaded in other directions. On first
examination of sections of cancellous bone, there does not appear to
be such defined axes, but they are revealed by closer study. Figure 5.10
shows typical directions or trajectories in the proximal femur, in a plane
passing distal-proximal and bisecting the shaft and neck.
Both the degree of anisotropy and the relation between the elastic proper-
ties and the density of bone vary substantially with skeletal site and function.
Trabecular bone from the pelvis and lumbar spine are relatively isotropic
compared to sites such as the calcaneus and proximal tibia (Figure 5.11).

0.35

Pelvis
Calcaneus
Femoral neck
Iliac crest
L2
Proximal tibia

0.3

0.25

0.2

0.15

0.1

0.05

0

1 1.2 1.4 1.6 1.8 2

Degree of anisotropy

FIGUre 5.11 degree of anisotropy of bone at various anatomic
sites. (adapted from day, J.s., Phd dissertation, erasmus university,
rotterdam, the netherlands, 2005.)