There are two types of classification:
Primary osteoarthritis – has no clear initiating courses.
Secondary osteoarthritis – can be directly related to an initial cause and are classified as traumatic, anatomic, metabolic, and inflammatory.
In articular cartilage there are inter-fibre cross links which stabilise the collagen tissue in the extra-cellular matrix. Enzymes that originate in cartilage cells (chondrocytes) can cause cleavage of these collagen cross-links. For a great diagram of the extracellular matrix check out http://jcs.biologists.org/content/123/24/4195
Also inside a joint there are proteoglycans (which are proteins with chains) that make up the extra cellular matrix of the articular cartilage. One of the the main types of proteoglycan is aggrecan. This molecule gives cartilage the ability to resist compressive loads by osmosis. Breaks in collagen cross-links cause aggrecan molecules to expand by binding with more water.
Changes in hydration are sensed by the cartilage cells and this alters the metabolism of the cells. There is a reduced rate of synthesis of aggrecan, and an increase the breakdown of aggrecan, causing a net loss (of aggrecan) from the extra cellular matrix. This changes the biomechanical properties of the extracelluar matrix and the tissue can no longer with stand compressive loads. For more detail refer to http://www.ncbi.nlm.nih.gov/pubmed/7919526
This inability to not take the load can lead to pain and swelling. Pain and swelling causes altered signalling to the brain, and the muscles around joints can actually stop working correctly due to to altered signals coming back down, to the joint from the brain. Fear of movement and altered movement can occur without conscious realisation.
Be guided by an experienced physiotherapist who can see what is happening to your movement and can develop strategies to improve pain and give appropriate exercises depending on the stage of your osteoarthritis.
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