Engineering Fibrous Tissues and Their Interfaces with Bone - Structural Interfaces and Attachments in Biology - page 318

15.2.5 Mechanical Properties

Tendon and ligaments are viscoelastic tissues that exhibit time-dependent creep and
stress relaxation rates. This is most likely caused by the interactions between water
and the components of the ECM [ 14 ]. Below 4% strain, tendons exhibit elastic
behavior, meaning that the tissue returns to its original length after strain release.
Above 4% strain, loading causes microscopic and macroscopic failure [ 47 ]. The
stress-strain curve depicts the viscoelastic behavior of a fibrous tissue (Fig. 15.2 ).
The toe region (1.5-3.0% strain) results from gradual straightening of the crimped
areas of the collagen fiber [ 10 , 21 ]. The crimp pattern allows the tissue to respond
rapidly to sudden strain loads and acts as a shock absorber to prevent damage from
excessive loading [ 9 , 48 ]. As strain increases, the tendon or ligament exhibits a
linear elastic response to stress. In this region, fibers have become parallel and lose
all crimp appearance. The slope created due to the linear stress-strain response is
referred to as the Young's modulus and ranges from 1 to 2 GPa [ 8 , 21 ]. At the end of
this region, microfractures of collagen fibers occur. This yield point ranges from
5-7% strain [ 10 , 21 ]. Beyond the elastic yield point, macroscopic failures of
collagen fibers occur in an unpredictable manner in the failure region. Tearing of
the fibers is observed around 12 - 15% strain and results in rupture of the tissue.
Failure strength of fibrous tissues ranges from 50-150 MPa [ 21 ]. Through animal
experimentation, it has been shown that tensile strength, elastic stiffness (Young's
modulus), and weight of the tendon can increase in response to greater physical
activity [ 8 ].

15.3 Injury and Healing

Understanding the types of fibrous tissue injuries and general stages of the healing
process is significant because different injuries may require varied tissue engineer-
ing approaches that may be employed at specific times during healing to best restore
the tissue. The following sections will provide an overview of the classifications of
tendon and ligament injuries, the natural healing process, and the current recon-
struction procedures and their limitations.

15.3.1 Injuries

Injuries are classified as acute or chronic and direct or indirect [ 49 ]. Direct injuries
are caused by non-penetrating injuries from accidents resulting in rupture or
tearing. On the other hand, indirect injuries are often the result of acute tensile
load and chronic overuse and demonstrate repetitive microtrauma [ 3 , 23 ]. This
class of injury is often referred to as tendinosis or tendinopathy in tendons [ 50 ].