Properties of natural materials - Orthopaedic Biomaterials in Research and Practice - page 121

effects of aging As individuals age, the mechanical properties of their tissues change.

This is a variable process with physiologic age, even in normal individu-
als lagging or leading chronologic age by up to 20%. Although all mate-
rial properties change, with tissues becoming less stiff, less ductile, and
weaker, the most pronounced effect is that of loss of strength. Strength
properties in general reach a peak near the age of 20 years in humans and
then decrease thereafter. An overall or composite “aging rate” has been
calculated at 0.5% strength loss per year (Yamada 1970). Mean effects
for various tissues are given in Table 5.3. Another way of thinking about
the effects of aging is that ultimate stress, ultimate strain, and energy
absorption decrease by 5%, 9%, and 12% per decade, respectively, from
the age 20 to 100 years. In terms of trabecular bone architecture, after
60 years of age, there is significant decline in bone volume fraction and
trabecular thinning, with a change in the microarchitecture of cancel-
lous bone from plate-like to rod-like structures. As a result, the degree of
anisotropy increases with age. Cortical bone also becomes more brittle
and fractures with less energy with aging.

effects of disease Joint disease may set in as individuals age, from overuse, or progressive

wear and tear. Rheumatoid arthritis is an immune-mediated chronic,
inflammatory disease that can damage and destroy the joint and is
manifested through inflammatory synovitis. Animal studies have shown
that the mechanical properties of arthritic femurs are impaired, having
lower modulus, yield stress, and ultimate stress, and resultantly making
bone more prone to fracture. The deregulation of the immune system
disturbs the balance of biochemical bone markers and the arrange-
ment of collagen, therefore affecting bone metabolism and possibly

Table 5.3 age effects on strength of tissues (as proportion of values
for age 20 -29)

Age (years)

Tissue 30-39 50-59 70-79

Costal cartilage
Tension 0.93 0.56 0.29

Compression 0.97 0.73 0.73

Tendon—tension 1.0 1.0 0.80

Cancellous bone (vertebral body—compression) 1.0 0.78 0.55

Cortical bone
Tension 0.98 0.76 0.70

Compression 0.95 0.95 —

Musculoskeletal system (average) 0.97 0.85 0.77

Source: Adapted from Yamada, H., Ratios for age changes in the mechanical
properties of human organs and tissues. In Evans, F.G. (ed), Strength of
Biological Materials , Williams & Wilkins, Baltimore, pp. 255-271,
1970.