Structural and mechanical evaluation of collagen fibrils from equine tendon: the effect of age, tendon zone and type

Abstract

Depending on their function, tendons are classified either as flexor or as extensor tendons. The superficial digital flexor tendon (SDFT) of the horse is an example of a flexor tendon. The SDFT is characterized by large extensibility, which enhances energy storage, important for locomotion due to decrease in energetic cost. In contrast, the common digital extensor tendon (CDET) of the horse, an extensor tendon (or position tendon), is less extensible than the SDFT. Mechanically, the SDFT has a lower elastic modulus and failure stress but higher failure strain along its long axis, compared to the CDET. In addition, structural as well as biochemical differences exist between the SDFT and the CDET. Although, the structure and mechanics at the tissue level of the SDFT and the CDET have been studied, little is known on the structural and mechanical properties of smaller hierarchical components of these tendons. These fascicular matrix and the interfascicular matrix and the components therein, most prominently the individual collagen fibrils. It is unknown how the mechanical properties of individual collagen fibrils are influenced by age and tendon type (i.e. SDFT and CDET). This thesis, provides an insight how mechanical parameters like the elastic modulus change with age in these two tendon types. Therefore longitudinal as well as transverse cuts of both tendon types (SDFT, CDET) from 3 and 18 year old horses were tested via microindentation on the AFM. Nanoindentation applied on individual collagen fibrils, the smallest structural component of a tendon, was subsequently carried out to find potential differences in their mechanical as well as swelling characteristics, according to age and type of the tendon.