Along with a more common thumb deformity caused by osteoarthritis and rheumatoid arthritis, thumb deformities can also be a result of thenar wasting, caused by median nerve injury or compression.
What causes the deformity?
The median nerve innervates the opponens pollicis (OP) and abductor pollicis brevis (APB). These two muscles make up most of the bulk of the thenar eminence and allow the thumb to perform palmar abduction (thumb opening away from the palm) and rotation for pad to pad prehension.
Pressure on, or a laceration of the median nerve, inhibits its ability to innervate OP and APB resulting in weakness and thenar wasting. While its possible to identify this deformity based on presentation, a history of many years of carpal tunnel syndrome or a median nerve laceration are other reasons to look for this deformity.
What is the deformity?
Diminished or lost innervation of the thenar muscles produces a flattening of the thenar eminence. Because OP and APB are responsible for opposition and palmar abduction, these motions become significantly weakened or absent resulting in an adducted thumb posture and ultimately the loss of web space length. The functional result is the degradation of fine motor skills involving tip to tip or pad to pad pinch and diminished ability to grip large objects when the ability to open the thumb away from the hand is lost.
How to Treat it?
A splint can be used for to counteract the loss of both pinch and grip function and provide a significant aid to both fine and gross motor function.
As noted, with the loss of opposition and palmar abduction/opening of the thumb, many functional tasks can become difficult. A splint that positions and holds the thumb in palmar abduction and allows opposition to the index and middle digits is an important option to consider for increasing functional use of the hand. A short opponens splint such as the ThumSaver MP or FIX Comfort Thumb Brace is a good option to consider for daytime use to allow for increased use of the hand.
It is important to pick a splint design that allows the most functional use, is comfortable with the loss of the muscle bulk in the thenar region, and takes into consideration any accompanying change in sensation in the hand.
Secondly, because the thumb assumes an adducted position, there may be a tightening or loss of web space. It is important to consider a splint for night during sleep to maintain the length of the soft tissue in the thenar web space. This splint should be sturdy, but comfortable enough to sleep in and maintain the web space at its optimal length. A custom fabricated orthosis may be called for to retain or restore the web space.
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