The Anatomy of the Trapezium

Wrist Bone Giving the Thumb Both Motion and Stability

In This Article

The trapezium bone is one of eight small bones which make up the wrist joint. The trapezium is the last bone in the row of wrist bones, located beneath the thumb joint. The trapezium bone along with the base of the thumb joint forms what is called a carpometacarpal joint, also known as a CMC joint. Due to the location of the trapezium below the thumb joint, the trapezium plays a role in both the stability and the movement of the thumb. The trapezium also touches two other bones in the wrist joint, the scaphoid and trapezoid bones.

Anatomy

The trapezium bone is a small oblong bone which plays a role in the stability of the carpal tunnel as well as the stability of the thumb joint. The shape and size of this bone and its location below the thumb cause the integrity of the trapezium to deteriorate over time. The trapezium is involved in many small motions over time due to how frequently most individuals use their thumbs.

The superior, or upper, surface of the trapezium is an irregular shape that is smooth and touches another wrist bone called the scaphoid. The inferior, or lower, surface of the trapezium is oval and resembles a saddle, which contributes to the joint being classified as a saddle joint.

The inferior surface touches the first metacarpal bone, also known as the thumb. The palmar, or front-facing, surface of the trapezium holds an attachment to the flexor carpi radialis tendon, the transverse carpal ligament, and muscles including opponens pollicis and flexor pollicis brevis. The dorsal, or rear-facing, surface of the trapezium assists in holding these ligaments and tendons in place. The lateral surface also assists in keeping these attachments in place.

One aspect of the medial surface of the trapezium touches another wrist bone called the trapezoid. The second aspect of the trapezium’s medial surface touches the second metacarpal, or the bone of the second finger.

There are minor anatomical variations related to the trapezium bone. It has been noted that females have smaller surfaces of the trapezium bone. This often does not cause any issues regarding functional use of the trapezium, however, it may impact healing time and risk of infection if the trapezium is fractured.

Function

The trapezium bone along with the nearby first metacarpal bone in the thumb forms a saddle joint. This type of joint allows the thumb to flex, extend, move side to side, in a circular motion, and reach across the palm to touch other fingers. The trapezium works along with the thumb to perform motions such as gripping, pinching, and other tasks involving the application of force.

Abductor pollicis longus and opponens pollicis both insert on the trapezium, allowing for the thumb to move side to side and reach across the palm to touch other fingers. Due to the insertion of several ligaments and tendons on the trapezium, this bone plays a large role in maintaining the structure and form of the hand. The radial artery, which is the blood supply for most of the hand and parts of the forearm, passes directly through the trapezium.

Associated Conditions

The trapezium is a bone which commonly experiences symptoms of osteoarthritis (OA), rheumatoid arthritis (RA), bursitis, and other inflammatory conditions. This inflammation is also frequently seen where the trapezium meets with the first metacarpal bone, called the trapeziometacarpal joint, also known as the TMC.

Some treatments for severe inflammation related to the trapezium include removal or fusion of the bone to relieve pain and improve function at the wrist. These same inflammatory conditions or symptoms can cause tendons and ligaments to loosen, which may allow the trapezium to fall out of place. This can cause mild or moderate pain at first. However, if left untreated or undiagnosed, this can easily lead to a trapezium fracture which must be treated appropriately. Ligaments and tendons which have become loose should be treated through strengthening programs by a physical therapist or an occupational therapist.

A trapezium fracture is diagnosed through radiography, then treated by setting the bones in the appropriate location, and placing a cast to allow for immobilization and healing. A fractured scaphoid bone, along with other small bones in the wrist, is treated similarly to a trapezium fracture.

Rehabilitation

A large part of rehabilitating a trapezium fracture involves maintaining motion during the immobilization period. This includes moving the other digits of the hand along with the forearm. Occupational therapy and physical therapy treatment during this phase may also consist of exercises and activities to allow for near independent completion of functional tasks despite the fracture.

Once the cast is removed, rehabilitation services focus on a gradual restorative motion protocol which starts with basic range of motion exercises to the wrist, fingers, and forearm. Pain management is also a large part of treatment at this point in the rehabilitation process. Pain relief may consist of heat or ice packs, menthol lotions, massage, and therapeutic ultrasound. Once pain is more tolerable and basic range of motion goals have been achieved, the protocol moves on to lightly resistive exercises followed by strengthening exercises to all areas of the hand.

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