Basilar Joint Arthritis (also referred to as Basal Joint Arthritis) is a condition that affects the joint at the base of the thumb. The basilar joint attaches to the wrist and is the most mobile joint in the hand. Basilar joint arthritis occurs when the cartilage, a smooth coating that covers the joint and allows painless and smooth movement, wears away. Basilar joint arthritis can affect both thumbs. As the disease progresses, Basilar Joint Arthritis can affect both thumbs with weakness and decreased flexibility.