Fibrous Connective Tissues

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It could be said that Musculoskeletal Medicine is the study of disorders of fibrous connective tissues. Fibrous connective tissues are those tissues that are designed to withstand mechanical deformation.

Structural Overview

Deformation comes in two principal forms, tension and compression. However there are also shear forces. To withstand tension the body forms "strings" in the form of collagens. To withstand compression the body develops a fluid "mush" in the form of proteoglycans.

The fibrous connective tissues exist in a spectrum of composition. The more a tissue is required to withstand tension the more collagen it will have, while the more it is required to withstand compression the more proteoglycans it will have.

Tendons (principally collagen) -> ligaments (principally collagen plus 0.5% proteoglycans) -> fascia (mixed collagen and proteoglycans) -> cartilage (principally proteoglycans plus some collagen) -> bone (essentially cartilage that has been doped by calcium)

These various components are synthesised by cells. Fibroblasts manufacture the components of tensile tissues. Chondroblasts produce the components of cartilage. Osteoblasts form the components of bone. These various cells can transform into one another for example ligaments can become cartilaginous and then osseous.

Collagen

File:Tendon hierarchical organisation.jpg

The hierarchical organisation of tendons

The hierarchical organisation of collagen molecules (tropocollagen) which self-assemble in a quarter-staggered array into microfibrils to form collagen fibrils with characteristic spacing.

Structure

The primary component of collagen is a polypeptide. The secondary structure is a helix. These helices are arranged into a tertiary structure called a triple helix. Those components are aggregated into a quaternary structures in increasing complexity: microfibrils -> fibrils -> fibres -> bundles -> fascicles

The primary functional unit of collagen is tropocollagen, which consists of a triple helix of polypeptide chains of around 1000 amino acid residues each. Tropocollagen measures around 3000 Å long by 15 Å wide.

Tropocollagen fibres aggregate end-to-end and side-to-side in a semi-overlapping pattern. The side-to-side tropocollagen fibres are joined by cross-links that occur during the normal maturation process. There is also end-to-end bonding.

One of the sources of resistance to tension is the end-to-end bonding between the polypeptide chains. Otherwise this triple helical structure when subjected to tension has the spaces between the helices taken up a little as it flattens out without being disrupted. However there is an electrical repulsion resisting this deformation.

Collagen is a very long-lived molecule with a slow rate of turnover. With normal ageing, there is an accumulation of cross-linking, which increases the stiffness and resistance to stretch.

Biomechanics

Typical stress–strain curve and schematization of the behaviour of the collagen fibres for tendons. Typical ranges of stress and strain are indicated on the x and y axes.

Typical stress–strain curve and schematization of the behaviour of the collagen fibres for ligaments.

With stretching of a collagen fibre, the fibre resists elongation due to a resistance force from the chemical bonds between the collagen fibrils, between tropocollagen molecules, between collagen fibres, and between collagen fibres and proteoglycans.

The biomechanics of collagen can be understood through the stress-strain curve. First, two key definitions:

• Stress = the applied elongating force, measured in units of force (newtons)
• Strain = the extent to which a fibre is elongated, measured as the fractional or percentage increase in length relative to the initial length. A fibre of length L₀ when stretched to a new length L₁ undergoes a strain of L₁/L₀ x 100%

The stress-strain curve therefore represents the behaviour of a collagen fibre when it is subjected to a force (stress) that ventures to elongate the fibre (strain).

There are three regions of the stress-strain curve

1. Toe Region: This is where the crimp is being removed from the collagen fibre with the application of stress. Crimp refers to the resting state of collagen fibres being buckled and wavy. Deformation here requires little energy due to no resistance from major chemical bonds, but not much deformation occurs at this point. The wavy fibres and transformed into straight fibres at the end of this region.
2. Linear Region: The second, linear region is the steep slope along the middle of the curve. Here the stress stretches the collagen fibre longitudinally after the crimp has been removed. The bonds within and between the collagen fibrils and tropocollagen molecules are being strained and there is some micro-failure. A large amount of stress is needed for only a small amount of elongation.
3. Macroscopic Failure: The last part of the curve is once macro-failure has occurred, with substantial numbers of bonds being irreversibly broken. Elongation occurs with ever decreasing amounts of stress. Collagen fibrils start to be strained and broken somewhere after 3% and 4% of elongation of the fibre. About 4% is the maximum a fibre can sustain without risking microscopic damage.

Tendons have specific load transfer functions, and the toe region of their stress-strain curve is short (2-5%), and is similar with each tendon in the body. Ligaments on the other hand allow different ranges of motion in different joints, and have wider ranges of strain for the toe region depending on the anatomical site (ACL is 4%, spine ligaments are 10-40%).

Clinical examination only evaluates no further than just beyond the toe phase.