Swan Neck Deformity

Introduction

Characterized by
- hyperextension of PIP
- flexion of DIP
Caused by
- lax volar plate
- imbalance of muscle forces on PIP (extension force > flexion force)
Injuries include
- MCP joint volar subluxation (rheumatoid arthritis)
- mallet finger
- FDS laceration
- intrinsic contracture
Seen in rheumatoid arthritis

Pathoanatomy

Primary lesion is lax volar plate that allows hyperextension of PIP. Causes include
- trauma
- generalized ligament laxity
- rheumatoid arthritis
Secondary lesion is imbalance of forces on the PIP joint (PIP extension forces that is greater than the PIP flexion force). Causes of this include
- mallet injury
  - leads to transfer of DIP extension force into PIP extension forces
- FDS rupture
  - leads to unopposed PIP extension combined with loss of integrity of the volar plate
- intrinsic contracture
  - tethering of the lateral (collateral) bands by the transverse retinacular ligament as a result of PIP hyperextension.
  - if the lateral (collateral) bands are tethered, excursion is restricted and the extension force is not transmitted to the terminal tendon, and is instead transmitted to the PIP joint
- MCP joint volar subluxation
  - caused by rheumatoid arthritis

Presentation

Imaging

Treatment

Nonoperative
- double ring splint
  - indications
    - can prevent hyperextension of PIP
Operative
- volar plate advancement and PIP balancing with central slip tenotomy
  - indications
    - progressive deformity
  - technique
    - address volar plate laxity with volar plate advancement
    - correct PIP joint muscles imbalances with either
      - FDS tenodesis indicated with FDS rupture
      - spiral oblique retinacular ligament reconstruction
      - central slip tenotomy (Fowler)