There are three major types of behaviour characteristic of viscoelasticity. Firstly creep, this is increasing deformation under constant load in the elastic portion of the stress-strain curve (Sandrey 2000). So that with a constant load, collagen length will increase over time. (Huijbregts and Smith 1999).
There are 3 major regions of the stress-strain curve that are of significance:
I. Toe region:
As the load increases so does the recruitment of collagen fibres causing them to ‘uncrimp’. This occurs when collagen is stretched to approximately 2% of its original length and returns to normal length when the force is removed, thus it is within its physiological range. It is characterized by relatively low stiffness. There is a non-linear relationship on the stress-strain curve at this stage.
II. Linear region (Elastic phase):
As the collagen fibrils become gradually uncrimped, the fibril itself is being stretched. There now becomes a linear relationship between deformation and load, as the tissue becomes relatively stiffer. This occurs when collagen is stretched to 2-4% of its original length and returns to its original geometric shape. The tissue is said to be elastic.
III. Yield and Failure region (Plastic phase):
The continued increase in load past 4% causes microfailure to the fibrils and damage to cross-links. It results in a plateau effect on the curve: this point represents the ultimate tensile strength of the tendon and is termed the ‘yield point’. The yielding of fibres occurs when the deformation is approximately 4-10% of the resting length. Stiffness is reduced and the fibrils do not return to normal length on release, the tissue then becomes ‘viscous’. This is known as ‘plastic’ deformation.
Finally, complete failure occurs as the ligament/tendon ruptures. Obviously this is a non-physiological range and there would be an inability to support load or function (Huijbregts and Smith 1999, Jozsa and Kannus 1997, Reid 1992).
Collagen demonstrates various mechanical and physical properties in response to load and deformation to allow it to withstand high tensile stresses. The point between the elastic and plastic region is where gross integrity is disrupted.
Viscoelasticity
Tendons and ligament are viscoelastic materials and display sensitivity to different strain rates. Viscoelasticity indicates collagen’s property of demonstrating time dependent and variable elastic behaviour. Thus, the relationship between stress and strain is not constant but depends on the time of displacement or load.
There are three major types of behaviour characteristic of viscoelasticity. Firstly creep, this is increasing deformation under constant load in the elastic portion of the stress-strain curve (Sandrey 2000). So that with a constant load, collagen length will increase over time. (Huijbregts and Smith 1999).
The second behaviour characteristic is force relaxation. This can be defined as the decrease in the amount of force required to maintain a set amount of deformation over time (linear region of curve). This occurs because the tissue relaxes (stress relaxation).
The third characteristic is hysteresis or energy dissipation. This means that if a viscoelastic material is loaded and unloaded, the loading curve will not follow the loading curve. The difference between the two curves represents the amount of energy that is dissipated or lost during loading. It refers to the amount of relaxation the tissue demonstrates during single cycle of deformation and relaxation. It is therefore an indication of the viscous properties of the tissue.