Properties of natural materials - Orthopaedic Biomaterials in Research and Practice - page 120

There have been a number of studies done to determine the optimum
design and use of cutting edges for tools for bone surgery. There is no
general agreement, perhaps reflecting the differences in behavior of
bone in vitro and in vivo , but some principles have emerged.

1. Cutting performance varies with edge design.

2. Sharp tools produce less heat than dull ones.

3. Increasing bearing pressure on tools increases heat production.

Thus, good surgical practice suggests selecting tools that work well
for the individual surgeon and maintaining them as sharp as possible to
diminish required bearing forces and heat production.

Comparative properties of tissues

It is useful to provide a brief overview of the comparison of the mechani-
cal properties of soft and hard tissues with some common prosthetic
materials. This is shown in Table 5.2.
Table 5.2 makes the problem of prosthetic replacement of tissues
quite clear. No prosthetic materials provide the unique combinations
of even these selected mechanical properties that are seen in tissues.
Thus, increasing interest is being shown in prosthetic materials that are
composites of two or more materials. These show promise of producing
properties closer to those of tissues, mimicking the way the mechanical
properties of tendon arise from those of two of its components, collagen
and elastin. Further consideration also needs to be given to the site speci-
ficity of the mechanical properties of tissue. For example, cortical bone
modulus is conventionally based on properties for long bone cortical
bone. However, the properties can diminish as the cortical bony tissue
transitions to a thin layer at the end of the long bone or at the cortical
rim of vertebra.

Table 5.2 mechanical properties of tissue and prosthetic materials

Tensile
modulus
(GPa)

Tensile
strength
(MPa)

Strain to
failure
(%)

Typical tissues
Articular cartilage (unrelaxed) 0.001-0.03 5-25 80-120

Tendon 1 55 8-10

Cancellous bone (ρ = 0.1 to 1.0 g/cm 3 ) 0.2-10 10-20 5-7

10-20 100-200 1-3

Cortical bone (ρ = 2.0 g/cm 3 )

Typical prosthetic materials
Ultrahigh-molecular-weight polyethylene 0.012 40 420-525

316L stainless steel 193 480 40

Aluminum oxide ceramic (compression) 350 4000 0.5-1