Osteoarthritis is a common musculoskeletal disorder that can affect any joints in the human body. The pathophysiology of osteoarthritis is complex. It is known that osteoarthritis generally starts when there is an alteration in the biochemistry of the synovial fluids of the joint (Findlay, 2013). In healthy joints, the balance is maintained by drawing water in and out of the joint area, but in osteoarthritis this balance is affected (Roman-Blas & Herrero-Beaumont, 2013). There is a decrease in the amount of proteoglycan in this cartilage, which can then increase the amount of water in the synovial fluid, making it less effective (Roman-Blas & Herrero-Beaumont, 2013). This in turn leads to exposed pieces of bone and a decreased space between the joints, which allows the surfaces of the bone to grind together and wear down (Herrero-Beaumont et al., 2016). This can also lead to inflammation in some cases, although osteoarthritis is not categorized as an inflammatory disorder as such (Herrero-Beaumont et al., 2016).
The main signs and symptoms of osteoarthritis are related to this rubbing of the bone surfaces. A generalized burning or aching pain is generally the main symptom and is due to the lack of synovial fluid protecting the joints (Herrero-Beaumont et al., 2016). This can also lead to stiffness and an inability to move the affected areas, and in some cases the muscles and tendons of the joint can become stiff when a patient does not move the joint for long periods of times (Herrero-Beaumont et al., 2016). The most commonly affected joints are the larger ones, including the hips and knees, as these are the joints that bear the most weight in the human body (Herrero-Beaumont et al., 2016). Theoretically, however, any of the joints of the body can be affected, although cases in the shoulders and ankles are rare (Herrero-Beaumont et al., 2016).

References
• Findlay, D. M. (2013). Focus: Long overlooked: the role of subchondral bone in osteoarthritis pathophysiology and pain. Medicographia, 35, 221–227.
• Herrero-Beaumont, G., Roman-Blas, J. A., Bruyère, O., Cooper, C., Kanis, J., Maggi, S., … Reginster, J.-Y. (2016). Clinical settings in knee osteoarthritis: Pathophysiology guides treatment. Maturitas. Retrieved from http://www.sciencedirect.com/science/article/pii/S0378512216303048
• Roman-Blas, J. A., & Herrero-Beaumont, G. (2013). Osteoarthritis pathophysiology: similarities and dissimilarities with other rheumatological diseases and the role of subchondral bone. Medicographia, 35, 158–163.