|Swan Neck Deformity|
|<<Lumbrical Plus Finger||Boutonniere Deformity>>|
The term swan neck derives from the shape of the finger when this deformity is present. The neck of the swan arches back (into extension) as one moves from the torso to the head. Finally, before reaching the head, the neck flexes forward to allow the swans head to sit upright and forward.
With this introduction, one should realize that the Swan Neck Deformity is one of the most common conditions treated by the hand surgeon. Unlike Boutonniere deformity , the swan neck can be caused by many different anatomic problems. These will be discussed along with some treatment principles.
The so called "double jointed" finger:
Many children and even some adults have the ability to cock back the joints of the finger in a way they term "double jointed". In fact, all fingers are triple jointed and the phenomenon that they are exhibiting is usually nothing more than the formation of a temporary, self-induced, swan-neck deformity. They do this by hyperextending their proximal interphalangeal joints and allowing the lateral bands of the extensor to sublux dorsally. This anatomic configuration: dorsally subluxed lateral bands, hyperextended PIP joint and flexed dip joint is the signature of all types of swan neck deformity.
The pathological basis of Swan-Neck Deformity
The normal three jointed finger is designed to allow for controlled motion at three joints (DIP, PIP, and MCP). Because of space considerations, the joints of the finger cannot contain separate flexors and extensors for all three joints. These and other constraints make it so that the controlled flexion and extension of all three joints in a finger must be accomplished with only three separate groups of muscles: Extrinsic extensors, extrinsic flexors, and intrinsics. The design of the extensor mechanism is so refined that these three systems of muscles are all that is needed to independently control each of the joints of a finger.
In the setting of swan-neck deformity. The conjoined lateral bands which are normally volar to the axis of rotation of the PIP joint, sublux uncontollably dorsal to that same axis. Instead of functioning normally as flexors of the PIP joint, they become extensors of that same joint. The route that the lateral bands normally take around the condyles of the proximal phalanx as they proceed to their terminal insertion is thereby shortened. The shortening of the route of the lateral bands serves to "slacken" them and the decreased tension is manifested at the DIP joint. At the DIP joint, there is not enough tension to keep the joint fully extended and so the DIP joint droops down into flexion completing the swan neck deformity.