Page 385 - Handbook of Properties of Textile and Technical Fibres

358 Handbook of Properties of Textile and Technical Fibres

fibroblast necrosis (Provenzano et al., 2002a,b), and altered collagen fiber orientation.
At the macroscopic level collagen fiber failure in tendon has a distinct morphological
character.
Statistical analyses indicate that the onset of subfiber failure occurs at a strain of
about 5% in a ligament that is below the threshold for structural damage (Provenzano
et al., 2002a,b). Cellular damage induced by ligament sprains occurs at strains signif-
icantly below failure strains (Provenzano et al., 2002a,b). Subfiber failure appears to be
associated with altered collagen fiber rotation during tissue extension. While tissue
remodeling and synthesis of collagen types I and III can occur at subfailure strains
through fibroblast mediated processes, the pre-subfailure strengths of ligaments are
never regained and permanent joint laxity occurs (Provenzano et al., 2002a,b,
2005). Subfiber failure is first observed in thin diameter collagen fibrils followed by
failure of the larger diameter collagen fibrils (Yahia et al., 1990). Ker (2008) has
reviewed the macroscopic fracture mechanics of tendon. When a tendon is notched
laterally and loaded in tension longitudinally, the crack opens up and the tip becomes
curved. Since the ratio of the shear modulus to that of the tensile modulus of tendon is
3
about 10 , the crack propagates longitudinally and leads to a mode of failure called
“interdigitation.” This failure surface is characterized by the presence of numerous
collagen fibrils that one by one tear at different lengths and morphologically look
like series of fingers projecting across the failed tendon ends. Ker et al. (2000) reported
a correlation between stress-in-life (physiological operating stress levels) and resis-
tance to fatigue damage leading to the conclusion that all tendons are equally likely
to experience damage independent of their normal operating stresses. Ker (Ker,
2008; Ker et al., 2000) further hypothesized that damage of tendon during normal
loading acts to trigger tendon repair processes and that tendon damage and failure
are limited by the weakness of the attached muscle.

11.9 Nondestructive methods for studying mechanical
behavior of collagen fibers and tissues

The ability to monitor the mechanical properties of collagen and ECMs in vivo is an
important measurement needed for early diagnosis of disease and the ability to follow
disease progression. The ability of physicians to “palpate” changes in the properties of
tissues associated with tumors and calcification suggests that there are major changes
in the structure and properties of collagen and ECMs during disease processes. It is
essential that clinicians be able to assess the changes at the collagen fibril and fiber
levels of structure to accurately diagnose and treat diseases such as cancer. Several
new methods have been developed to try and discern these changes early in the disease
process. It is essential that these methods be validated so that the properties measured
have some meaning.
There are several fairly new methods that have been evaluated in the literature to
study the mechanical properties of tissues in vivo such as magnetic resonance elastog-
raphy (MRE), ultrasound elastography (UE), optical coherence tomography (OCT),