Biomedical Engineering Reference
In-Depth Information
Tendons transmit force from the muscle to the bone and act as a buffer by absorbing
external forces to limit muscle damage. Tendons exhibit high mechanical strength, good
flexibility, and an optimal level of elasticity to perform their unique role. Tendons are visco-
elastic tissues that display stress relaxation and creep. The mechanical behavior of the
constituent collagen depends on the number and types of intramolecular and intermolecular
bonds.
Experiments have confirmed cell growth and function would be controlled locally through
physical distortion of the associated cells or through changes in cytoskeletal tension.
Moreover, experimental studies have demonstrated that cultured cells can be switched
between different fates including growth, differentiation, apoptosis, directional motility or
different stem cell lineages, by modulating cell shape.
(Barkhausen T et al. 2003) (Brown RA
et al. 1998) (Wang JH et al. 2004) (Schulze-Tanzil G et al. 2004)
Externally applied cyclic strain under in vitro conditions has enormous effects on various
functions of tenocytes, such as their metabolism, proliferation, orientation and matrix
deposition (Screen HRC et al. 2005) (Yamamoto E et al. 2005)
Kessler et al. (Kessler et al. 2001)
demonstrated that collagen fibres and tendon cells can be
oriented along the direction of the stress and can upregulate synthesis of tissue inhibitor
matrix metalloproteinases-1 and -3 as well as of collagen type I, the main component of
tendinous extracellular matrix.
It is known that cyclic strain can affect cell morphology and induce uniaxial cellular
alignment. It was observed that cyclic strain stimulation enhanced the cellular alignment
and changed the cellular shape.
Other experiments have demonstrated the beneficial effects of motion and mechanical
loading on tenocyte function. Repetitive motion increases DNA content and protein
synthesis in human tenocytes in culture.(Almekinders LC et al. 1995) (Sharma P and
Maffulli N. 2005)
Even fifteen minutes of cyclic biaxial mechanical strain applied to human
tenocytes, results in improved cellular proliferation.(Sharma P and Maffulli N. 2005)
(Zeichen J et al. 2000)
Moreover,
in vitro
cyclic strain allows an increased production of TGF-β, FGF and PDGF by
human tendon fibroblasts.( Bagnaninchi PO et al. 2007) (Slutek M et al. 2001)
Cyclic stretching of collagen type I matrix seeded with MSCs for 14 days (8 h/day) resulted
in the formation of a tendon-like matrix.
(Bagnaninchi PO et al. 2007) (Zeichen J et al. 2000)
Expression of collagen types I and III, fibrinonectin and elastin genes was found to have
increased when compared with nonstretched controls in which no ligament matrix was
found.
(Bagnaninchi PO et al. 2007) (Yang G et al. 2004)
The model reproduces
in vivo
tendon healing by preventing differentiation of tenocytes into
fibroblasts.
In animal experiments, mechanical stretching has improved the tensile strength, elastic
stiffness, weight and cross-sectional area of tendons.( Sharma P & Maffulli N 2005) (Kannus
P et al. 1992) (Kannus P et al. 1997) These effects result from an increase in collagen and
extracellular matrix network syntheses by tenocytes. Application of a cyclic load to
wounded avian flexor tendons results in the migration of epitendon cells into the wound.(
Sharma P and Maffulli N 2005) (Tanaka H et al. 1995)
Also, Qin et al. (Qin et al. 2005) found that cyclic strain promotes cell proliferation, matrix
deposition and increased collagen production. In another study, Juncosa-Melvin et al.
(Juncosa-Melvin et al. 2007) showed that the application of cyclic strain elevated the gene
expression levels of collagen type I. Finally, cyclic strain can enhance mechanical
