What anatomical mechanism contributes to swan neck deformity?

What anatomical mechanism contributes to swan neck deformity?

• A. Hyperextension of DIP with PIP Flexion
• B. Contraction of intrinsic muscles or tearing of volar plate
• C. Extensor Tendon Rupture at DIP
• D. Flexor Tendon Rupture

Answer: (B)

Swan neck deformity comes from an imbalance in the finger’s extensor mechanism around the proximal interphalangeal joint. Normally, the volar plate and the intrinsic muscles (lumbricals and interossei) keep the lateral bands aligned toward the palm, helping stabilize the PIP. When intrinsic muscles are chronically tight or the volar plate is torn, these lateral bands slip dorsally over the PIP. That dorsal displacement acts as a tether that hyperextends the PIP when the finger tries to extend. Meanwhile, the distal joint remains flexed because the flexor tendons continue to pull the fingertip inward, so you end up with PIP hyperextension and DIP flexion—the telltale swan neck posture. Other scenarios, like a rupture of the extensor tendon at the DIP or a rupture of the flexor tendons, produce different patterns and don’t create this PIP hyperextension with DIP flexion, which is why this mechanism best explains swan neck.

Swan neck deformity comes from an imbalance in the finger’s extensor mechanism around the proximal interphalangeal joint. Normally, the volar plate and the intrinsic muscles (lumbricals and interossei) keep the lateral bands aligned toward the palm, helping stabilize the PIP. When intrinsic muscles are chronically tight or the volar plate is torn, these lateral bands slip dorsally over the PIP. That dorsal displacement acts as a tether that hyperextends the PIP when the finger tries to extend. Meanwhile, the distal joint remains flexed because the flexor tendons continue to pull the fingertip inward, so you end up with PIP hyperextension and DIP flexion—the telltale swan neck posture. Other scenarios, like a rupture of the extensor tendon at the DIP or a rupture of the flexor tendons, produce different patterns and don’t create this PIP hyperextension with DIP flexion, which is why this mechanism best explains swan neck.