- 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 ofi mmobilization 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.