Biomedical Engineering Reference
In-Depth Information
the best means, biologically and mechanically, to anchor these fibrous tissues to the
surrounding joint to restore full function to the patient.
In this chapter, background information about the structure, composition, and
function of tendons and ligaments is reviewed. Different types of injuries and the events
of native tissue healing are summarized, as well as current reconstruction techniques for
injured tissues. This basis will facilitate the understanding of current tissue engineering
approaches for fibrous tissues and their interfaces, reviewed in the second half of this
chapter. In particular, a discussion of the advantages and disadvantages of the cell
sources, scaffolds, and exogenous factors currently used for creating tissue-engineered
tendon/ligaments, as well as their interfaces with surrounding bone, is presented,
followed by an examination of the remaining challenges that must be overcome before
clinical adaptation of these strategies can be realized.
15.2 Physiology and Function
Successful tissue engineering and regenerative medicine methods are based on the
structure-function relationship of tendons and ligaments. Tendons and ligaments
are bands of connective tissue that transmit forces and facilitate joint movement
[ 7 , 8 ]. The tensile and compressive strength of these tissues is derived from the
hierarchical organization of the fiber structure and is a key aspect in tissue replace-
ment. Therefore, this section summarizes the structure-function relationship in
tendon/ligament tissue.
Tendons and ligaments are similar in biochemical structure, but differ in function.
The tendon is a fibrous band of connective tissue that connects muscle to bone.
Tendons transmit forces, in the form of muscle contractions, to bones, thereby
facilitating locomotion and joint stability [ 9 , 10 ]. Ligaments connect bones to adjacent
bones. They restrict and guide joint motion and provide joint stability. In contrast to
tendons, ligaments are pliable but not elastic [ 11 ]. The structure of these fibrous tissues
enables them to withstand large tensile loads generated by musculoskeletal
components during active motion along the longitudinal direction of the tissue [ 12 ].
15.2.1 Structure and Biochemical Composition
Tendons and ligaments have a hierarchical structure. Tropocollagen, a triple helix
molecule with two alpha-1 collagen chains and one alpha-2 collagen chain, is the
basic structural unit of fibrous tissues. These molecules self-assemble and crosslink
to form collagen microfibrils, the smallest unit in the fibrillar structure [ 12 ].
Microfibrils arrange end-to-end in bundles to form fibril structures. Crosslinking
of fibrils increases Young's modulus, increases tensile strength, and reduces strain
at failure [ 10 , 13 ]. Fibrils are arranged into units of fibers surrounded by the
endotenon or endoligament, a layer of loose connective tissue that interfaces with
blood vessels, lymphatics, and nerves [ 14 ].
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