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 (‘double’) joints; these 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 volar plate laxity / thumb MP joint instability / swan-neck deformity 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 can no longer move normally. The thumb’s stress of reaching around large objects, such as bottles or cans, is then transferred to the MP joint. In order for the thumb to grasp these objectsm the MP joint has to stretch into extension to compensate for the fact that the basilar joint is not moving well. Over time, the MP joint’s volar plate stretches out, leading to hyperextension laxity.

Regardless of why the MP joint’s volar plate is lax or injured, this problem is often curable by a simple, predictable outpatient surgery called a volar capsulodesis. The volar capsulodesis involves repairing or 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.

If a basilar joint reconstruction (ie. LRTI; please see the Basilar Thumb Joint Arthritis section of the website) is being performed and the thumb’s MP joint has 30˚ of hyperextension or more, the volar capsulodesis must be performed at the same time. If the volar capsulodesis is not performed, the MP joint’s collapse into extension with 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.

So, if your MP joint has a swan-neck deformity or weakness due to laxity of the volar plate, it can be well-treated with a simple, quick outpatient procedure.

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