Osteoarthritis is a wear-and-tear arthritis. It is a condition in which the knee cartilage wears away (cartilage is the natural cushioning that separates bones in a joint). When the cartilage gets worn out, the bones of the joint rub more closely against each other because of reduced shock absorption. This causes pain, swelling, stiffness in the knee and can also reduce its range of motion. Ageing is the most common cause of osteoarthritis with risk of developing it starting after the age of 45. It can affect younger individuals as well. Women are more likely to be affected with osteoarthritis compared to men. 

Osteoarthritis affects the medial side of the knee (inner side) in the vast majority of cases. Although OA can affect the lateral side of the knee as well, medial OA is more common than lateral OA. When the cartilage on the medial side of the knee gets worn out, the joint space between the medial side of the bones of the knee reduces. This can throw the entire limb out of alignment, since your ankle, knee and hip need to be in vertical alignment for good posture and normal gait. 

Podiapro designs custom insoles to load the unaffected side of the knee in order to reduce the load on the affected side. This can bring some pain relief and give some support to the knee.