Motions of the Thumb

 

The motions of the thumb are complex and are often difficult to visualize, as multiple joints and planes are involved. The motions are crucial to the overall function of the hand, with amputation of the thumb resulting in 40% impairment. Amputation has also been cited as causing 22% disability of the whole person. In cases of thumb hypoplasia and carpometacarpal joint instability, thumb amputation and index finger pollicization is recommended. Because of the value of the thumb, reimplantation of the amputated thumb at any level is recommended. The main motions of the thumb include flexion, extension, abduction, adduction, and opposition. Valued as 50-60 percent of overall thumb function, opposition is one of the most critical motions. Abduction and adduction of the thumb occur mainly at the carpometacarpal (CMC) joint. Abduction is described as the motion of the thumb away from the second metacarpal, whereas adduction is the opposite of the motion. These motions are defined in two planes: palmar and dorsal. In radial abduction-adduction, the thumb moves parallel to the radius and the palm of the hand. A smaller range of motion is generally seen in the radial plane. Alternatively, in palmar abduction-adduction the thumb follows a plane perpendicular to the palm of the hand. Clinically, these motions are not typically measured directly but are assessed in the motion of opposition. Flexion and extension can occur at all three joints of the thumb, including the CMC, MCP, and IP joints. These motions at the CMC joint are often not measured, as they are challenging to quantify. The average flexion observed at the MCP joint is approximately 50 degrees, although this may be limited in some individuals to less than 30 degrees. Extension is not usually seen at the MCP joint. Flexion at the IP joint approaches 80 degrees on average. Typically, the IP joint is capable of approximately five degrees of extension. Opposition is a very important motion and, as mentioned, 50-60% of overall thumb function. It is comprised of several integrated motions: palmar abduction that transitions to adduction at the CMC joint, rotation at the CMC joint, and flexion at the CMC, MCP, and IP joints. The composite motion involves crossing the thumb over the thumb of the hand toward the base of the little finger. Complete opposition of the thumb may be defined in two ways: by the tip of the thumb touching the base of the fifth digit and by the tip of the thumb touching the tip of the little finger. Therefore, deficiencies in opposition can be quantified by measuring the distance from the tip of the thumb to the base of the little finger or from the tip of the thumb to the tip of the little finger. Alternatively, the degree of opposition may be determined by measuring from the flexor crease of the IP joint to the distal palmar crease over the third metacarpal. A normal span is at least 8 cm. opposition may also be assessed by systematically touching the tip of the thumb to various parts of the hand, a clinical test outlined by Kapandji.

Author
Nabil Ebraheim, MD

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