- Discussion:
- exhibit
stress /
strain behavior that is time-rate dependent, and varies w/ the material, that
is a function of the material's internal friction;
- visoelastic materials are stiffer and stronger at high
strain rates than at low
strain rates;
- for example in bone-ligament interface, slow rate of loading will result in
avulsion frx of bone but a fast rate of loading will cause ligament failure;
- hence, bone has a higher modulus of elasticity at higher strain rates;
- ligaments are viscoelastic - that is, their
stress-
strain behavior is time-rate dependent,
w/ elongation of ligament being more likely to occur with slower loading conditions;
- rate of loading affects ultimate load to failure.
-
isotropic materials:
- possess the same properties in all directions;
-
anisotropic materials:
- have mechanical properties that vary w/ orientation of the loading;
- General Ligament Tensile Properties:
- ligaments display visoelastic characteristics (time-rate dependent), with slow loading allowing
more ligament elongation);
- load elongation curve of bone ligament complex contains an initial low stiffness region (toe
region) followed by a high stiffness region;
- similar loading properties are found in the tendon;
- ligaments and tendons have a non linear,
strain stiffening structural response;
- this may result from the crimp pattern collagen fibrils which elongate when small tensile loads are applied;
- once the crimp pattern is unwound, larger forces are required to produce strain;
-
creep:
- ligaments also possess characteristic creep properties (progressive elongation or change in
strain w/ constant load over time);
-
stress relaxation:
- stress-relaxation properties (amount of stress measured in preloaded ligament decreases w/ time);
- for
ACL, stress relaxation has been noted to stabilize at 80 % of the initial
stress over time;
- this property is demonstrated in scoliosis correction in which peak distraction forces can be
reduced by 50% because of vertebral soft tissue creep;
- Effects of Immobilization:
- after immobilization, the resumption of joint motion leads to a slow reversal in the effects of
immobilization on the structural properties of the FMTC and FATC;
- these
ultimate load and energy absorbed at failure of these two
complexes reach 80% to 90% of control at one year;
- histologic evidence of new bone formation at ligament insertion reveals that time required to
return to normal is much longer than the immobilization period;
- in contrast, material properties of the MCL substance returns to normal after nine weeks of remobilization;
Viscoelastic properties of muscle-tendon units:
The biomechanical effects of stretching.
Taylor DC, Dalton JD, Seaber AV, et al:
Am J Sports Med 1990;18:300-309.
