Swan-Neck Deformity of the Thumb
Swan-Neck deformity of the thumb occurs when the thumb’s metacarpophalangeal (MP) joint, the joint where the thumb attaches to the hand, hyperextends too much. The ligament that usually prevents this is called the ‘volar plate’. Some people are born with loose joints, also known as being ‘double-jointed’; joints are usually loose because of lax volar plates. Other people tear the volar plate during an injury. However, probably the most common cause of MP joint volar plate laxity is basilar thumb arthritis. The basilar thumb joint is where most of thumb’s motion should occur. When the basilar joint becomes stiff and/or subluxed (partially out of joint) from arthritis, it no longer moves normally. The basilar thumb joint no longer has the motion to grasp large objects, such as bottles or cans. In order to grasp around large objects, the MP joint’s volar plate compensates by stretching out so that the MP joint hyperextends.
Regardless of why the MP joint’s volar plate is lax or injured, this problem is often cured by a simple, predictable outpatient surgery called a ‘volar capsulodesis’. The volar capsulodesis involves repairing / shortening the volar plate by placing a suture anchor (a device that looks like a harpoon-head with stitches attached) into the thumb’s metacarpal bone (the large bone of the thumb). The volar plate is then tied down to the metacarpal bone in a shortened position. After surgery, a cast is worn for 3-4 weeks. Computer use and typing is possible immediately, but heavy gripping and pinching should not be performed for 3-4 months after surgery. This is the same as following an LRTI for basilar joint arthritis (please see the Basilar Thumb Joint Arthritis section of the website), with which the volar capsulodesis is commonly performed.
If a basilar joint reconstruction (i.e. LRTI) is being performed and the thumb’s MP joint has 30˚ of hyperextension or more, a volar capsulodesis should be performed at the same time. If the volar capsulodesis is not performed, the MP joint’s collapse into extension during gripping and pinching places high stresses on the LRTI reconstruction, leading it a higher chance of instability with loss of grip and pinch strength. The MP joint volar capsulodesis is performed at the same time as the LRTI, doesn’t hurt much, and doesn’t lead to any extra time in a cast.
The take-home message is that thumb MP joint hyperextension laxity (swan-neck deformity) can be well-treated with a simple, quick outpatient procedure.