Why does a joint hurt?

In healthy joints, bones that are adjacent to each other are covered by a thin layer off cartilage. Cartilage consists of cells in a water-binding gel that acts as a shock absorber. There is also a viscous fluid known as synovial fluid in the joint that acts as a lubricant. Both the synovial fluid and the actual cartilage contain glucosamine and hyaluronic acid. The smooth surface of the cartilage distributes the load across the joints and, together with the synovial fluid, reduces the friction on movement. A constant process is in progress in the cartilage with new formation of cartilage to replace the breakdown that occurs naturally. Equilibrium normally prevails, but in osteoarthritis an imbalance arises in the formation of new cartilage and more cartilage is broken down. The cells try to repair the damage but eventually are unable to form the new cartilage needed. The cartilage is broken down, the surface becomes dull and uneven and eventually the cartilage may disappear completely so that the bone ends come into contact with each other and pain occurs. At the same time more structures in the joint may be attacked, such as the joint capsule, ligaments and underlying bone
Research shows that if hyaluronic acid is supplied by injection or as a dietary supplement, the inflammation in the joint is reduced, while lubrication and nutrient exchange is improved.


Below references to independent research in the area:

Hyaluronic acid helps keep the joints healthy and mobile.

Pain and stiffness in the joints is often due to inflammation in one or more structures in the joint cartilage. Joint inflammation (osteoarthritis) develops slowly and gradually leads to breakdown of the articular cartilage, thickening of the joint capsule, inflamed synovial fluid and bone deposits in and around the joint. On loading, the inflammation gives rise to pain, stiffness and reduced mobility.
The synovial fluid consists of hyaluronic acid, a sugar molecule that gives the fluid a viscous consistency. The properties of hyaluronic acid contribute to shock absorption and lubrication of the joint, and play an important role in transporting nutrients and waste products to and from the synovial fluid and cartilage.
When inflammation occurs in the joint, the hyaluronic acid is broken down and therefore cannot contribute to lubrication, shock absorption and nutrient exchange in the same way, so that the cartilage is exposed to further wear and breakdown.
The effect of oral hyaluronic acid on patients with knee osteoarthritis  has recently been shown (Nelson F.R. et al 2014). A brief summary follows here:
The study is based on 40 patients aged 50-75, all of whom had daily knee pain and diagnosed osteoarthritis in the joint. Half the patients took hyaluronic acid daily for 3 months and half took a placebo daily. Inflammatory enzyme was measured in the synovial fluid and serum, and the patients were asked to assess their pain and mobility.
The result shows that the patients who had taken oral hyaluronic acid had statistically significantly less pain and improved mobility after 3 months. A statistically significant decrease in circulating breakdown enzyme could also be measured in the serum and synovial fluid of the treated patients compared with the patients who took placebo tablets.
Nelson, F.R., Zvirbulis, R.A., Zonca, B., li, K.W., Turner, S.M., Pasierb, M., Wilton,P., Martinez-Puig,D and Wu,W. The effect of an oral preparation containing hyaluronic acid (Oralvisc®) on obese knee osteoarthritis patients determined by pain, function, bradykinin, leptin, inflammatory cytokines, and heavy water analyses. Reumatol Int, 05, 2014

Effect of oral ingestion of hyaluronic acid on joint problems and pain

Effect of oral ingestion of hyaluronic acid on the skin