Properties of natural materials - Orthopaedic Biomaterials in Research and Practice - page 119

Compression

Tension

Torsion

FIGUre 5.13 (See color insert.) fracture of cortical bone.

osteonal segments. Finally, cortical bone tested in torsion around its lon-
gitudinal fiber axis produces the expected spiral failure. However, there
are frequently vertical splits at the ends of the fracture plane, and the
angle of the plane to the axis is nearer to 30°-35° than the expected value
of 45°, again reflecting the fibrous nature of bone.
Defects such as screw holes produce significant stress concentration
in bone, as in other materials, with the minimum concentration factor
K approximately equal to 2. However, unlike inorganic materials, bone
shows a considerable ability to remodel. This can remove the stress con-
centration effect of a screw hole in 6-8 weeks in vivo (in dogs) if, for
instance, the hole is plugged with silicone rubber. The same effect can
be produced if a rigid screw is used and left in place during mechanical
testing; however, removing the screw just before testing reintroduces the
stress concentration.
Screws anchored in bone may be pulled out directly, producing a
shear failure at the points of the screw threads. The strength of screw
retention varies somewhat, depending on screw design and diameter, but
is 350-400 N/mm of bone thickness in cortical bone and 10-25 N/mm
of bone thickness in healthy cancellous bone.
Orthopaedic surgery is sometimes likened to carpentry, since many
of the tools of the two disciplines are the same: chisels, rasps, twist drills,
saws, and so on. This similarity arises from the “machining” properties
of bone, which greatly resemble those of a partly seasoned (dried), mod-
erately strong hard wood, such as yellow birch or elm. Bone can be made
to form a “chip” and can be split, sometimes inadvertently. However,
bone is different from wood in two significant respects.

1. The production of heat during machining must be actively avoided.
Temperature rises of as little as 4°C can produce cell death,
whereas larger rises (>10°C) can produce irreversible mechanical
property changes.

2. The calcium hydroxyapatite content of bone is highly abrasive and
rapidly dulls cutting edges of surgical tools.