Swan neck deformity | Radiology Signs

Answer:

Swan neck deformity is a finger deformity characterized by hyperextension of the proximal interphalangeal (PIP) joint combined with flexion of the distal interphalangeal (DIP) joint. This deformity develops from damage to the extensor mechanism of the digit, specifically from laxity of the volar plate and tightness of the triangular ligament around the PIP joint. In rheumatoid arthritis, chronic inflammation weakens or ruptures the stabilizing ligaments and tendons of the finger joints, initiating this cascade of joint position abnormalities. The condition can also result from untreated mallet finger, traumatic injuries to the extensor tendon, or other inflammatory and neurological conditions including psoriatic arthritis, scleroderma, cerebral palsy, stroke, and Parkinson’s disease. Clinically, patients experience stiffness, snapping sensations, and progressive difficulty bending the affected finger, eventually impairing functional activities such as gripping or buttoning.

Why is it called so?:

The deformity is named for its characteristic appearance, which resembles the curved, extended neck of a swan. The distinctive S-shaped contour of the affected finger, with the middle joint bent backward and the end joint bent downward, mirrors the graceful curvature of a swan’s neck.

Pathophysiology:

Swan neck deformity develops through a sequential mechanical process. Damage to the extensor tendon attachment or weakness of the volar plate at the PIP joint initiates loss of extension at the distal phalanx. As the PIP joint extends, other tendons slide toward the back of the finger, progressively increasing PIP extension beyond normal limits. This abnormal positioning of tendons and the imbalance in forces across the PIP joint reduces the mechanical advantage of the tendons that extend the DIP joint. Consequently, the DIP joint becomes unable to extend and assumes a flexed position. The combination of PIP hyperextension and secondary DIP flexion creates the characteristic swan neck appearance. In rheumatoid arthritis specifically, chronic synovial inflammation causes progressive ligamentous laxity and potential tendon rupture, making the PIP joint increasingly unstable and perpetuating this deformity.

Alternative names:

No widely recognized alternative names for this specific sign exist in the medical literature.

Other associated named signs:

Mallet finger deformity, which represents isolated DIP flexion, is mechanistically related and can progress to Swan neck deformity if untreated.