Osteoarthritis of the Base of the Thumb
The are many types of arthritis. The most common type seen at the base joint of the thumb is osteoarthritis, often described as wear and tear of the joint cartilage. In a normal joint, the ends of the bones are covered with smooth cartilage, which serves as a cushion and ensures the bones move easily in relation to one other. When osteoarthritis erodes the cartilage, the bone ends rub against each other, creating pain.
The thumb is made of three bones. The base bone (metacarpal) rests on one small wrist bone (trapezium), creating the thumb CMC (carpometacarpal) joint. The shape of the bones making the CMC joint allows the thumb to move through a wide range of motion. This movement allows the thumb to grasp a large variety of object shapes and sizes, including very small objects that require pinching between the thumb and finger tips.
Osteoarthritis in the thumb CMC joint is common in older adults, occurring from 40 years of age upwards. Fractures or sprains to the joint can hasten the onset of osteoarthritis. More women than men experience symptoms.
The initial symptom of osteoarthritis of the thumb CMC joint is pain at the joint when using the hand. Common complaints are pain when opening jars, opening a car door, turning keys, etc.
Heavy and/or repetitive use of the hand or prolonged pinching directs stress to the osteoarthritic thumb CMC joint, provoking pain. Thumb strength decreases, and the base joint may begin to move out of its normal position. Continued use without treatment may lead to a complete dislocation of the thumb CMC joint. As the CMC joint deformity progresses, joint movement becomes increasingly restricted.
Physical examination of the thumb will reveal a bump at the thumb base if the CMC joint is subluxated or dislocated. Direct pressure over the joint will be painful and moving the thumb while pressing on the joint may cause a painful grating sensation, demonstrating the erosion of smooth joint surfaces.
As the osteoarthritis progresses, the CMC joint mobility decreases. It becomes difficult to move the thumb out, away from the palm and fingers. As the base joint moves further out of joint, the other thumb joints often collapse when manipulating objects. The most common collapse is called the zig-zag deformity, where the middle joint of the thumb goes backward, especially when pinching strongly.
Pain at the base of the thumb due to osteoarthitis can be controlled with a brace that limits the extremes of movement and stabilizes the thumb CMC joint when using the hand. If the osteoarthitis has progressed to create a severe deformity, surgical reconstruction may be required.
Image 1:Osteoarthritis of the base of the thumb (CMC-I)