The human thumb is unique when compared to the digits of non-primate mammals because it is opposable, that is, it can bend forward to touch the other digits on the hand. This makes it possible to pinch the fingers together and easily grasp an object. It accounts for primates' ability to use tools so effectively. It also means that the joints of the thumb are subject to greater stresses which makes it more susceptible to injury and to arthritis. The value of the rather homely looking thumb is easily missed until thumb joint pain makes simple daily activities almost impossible.
The nature of how a thumb is used makes it vulnerable to physical trauma. It can be struck by an object, or bent or twisted. It is also susceptible to injuries caused by repetitive motion. Any of these injuries could cause a sprain, tendinitis or bursitis. A broken bone can effect the joint as well.
Osteoarthritis, the arthritis that most commonly affects the basal joint of the thumb, is caused when the smooth cartilage covering the ends of bones is slowly worn away as the bones of the joint rub against each other. When the bones are no longer protected, the friction from the rubbing of bone against bone causes damages. This commonly happens over time from normal use of the joint, but can be hastened by overuse.
A sprain occurs when the ligaments of the thumb are injured, usually when the thumb is forced to bend backward. A patient will experience pain when a sprained thumb is moved (especially backwards).
Tendons connect bones to muscles. Bursitis occurs when a tendon becomes inflamed. Movement becomes painful and may be accompanied by swelling. Tendons tend to become weaker with age and therefore tendonitis is less likely to occur in the young, although it is seen in those involved in repetitive tasks.
A bursa is a fluid-filled sac that separate muscles or tendons from the bones that they move over. Sliding over the bursa decreases friction. Bursitis occurs when the bursa becomes irritated or inflamed. Bending the thumb will be painful.
In the case of a sprain, tendonitis, or bursitis a physician can usually make a diagnosis from symptoms and physical examination. She may also order an x-ray to rule out a broken bone.
If a physician suspects that thumb joint pain is caused by arthritis, he will be looking for pain at the base of the thumb. There will be weakness when trying to grasp an object or pinch thumb to finger. Range of motion will be limited. The joint itself may show obvious swelling. When the physician moves the patient’s thumb, he may feel a grittiness as the bones move against each other over damaged cartilage. He may actually be able to hear a grinding noise.
In addition to the physical examination and discussion about symptoms, the patient will probably be sent for x-rays and/or any of several types of imaging scans. These will show the condition of the bones and tissues of the joint which will confirm (or not) the physician’s preliminary diagnosis of arthritis and give additional information about the severity of the condition.