Swan Neck Deformity

Swan neck deformity is a finger deformity characterised by PIP joint hyperextension and DIP joint flexion — the mirror image of boutonnière deformity. It arises from any cause that produces relative FDS weakness, volar plate laxity at the PIP joint, or imbalance between intrinsic and extrinsic forces — allowing the PIP joint to hyperextend while the DIP joint (deprived of the terminal extensor contribution) falls into flexion. The deformity is common in rheumatoid arthritis and in conditions causing intrinsic tightness, but can also occur after PIP volar plate injury, mallet finger, or FDS laceration.

• Key anatomy: the volar plate at the PIP joint normally prevents PIP hyperextension; the FDS inserts at the base of the middle phalanx and is a PIP joint flexor — loss of FDS function allows the PIP to hyperextend; the intrinsic muscles (lumbrical and interosseous) extend the PIP joint via the extensor mechanism — in intrinsic tightness, the PIP is held in hyperextension; the terminal extensor tendon inserts at the distal phalanx and extends the DIP — in swan neck deformity, as the lateral bands migrate dorsally (opposite of boutonnière), the terminal extensor may become relatively lax at the DIP, allowing DIP flexion
• Causes: rheumatoid arthritis (most common — MCP joint synovitis causes FDS attenuation; PIP joint synovitis attenuates the volar plate; intrinsic muscle spasm from MCP synovitis causes intrinsic tightness → PIP hyperextension); mallet finger (untreated terminal extensor disruption — the imbalance at the DIP propagates proximally, causing FDS to pull the PIP into hyperextension); FDS laceration (loss of FDS flexion force at the PIP allows hyperextension); PIP volar plate injury (the volar plate is the primary restraint to PIP hyperextension; attenuation or rupture allows the PIP to hyperextend); intrinsic tightness (spasticity, cerebral palsy, post-traumatic intrinsic contracture)

• Bunnell intrinsic tightness test: the examiner holds the MCP joint in extension (which tightens the intrinsic muscles passing dorsal to the MCP joint) and then attempts to flex the PIP joint; if the intrinsics are tight, the PIP cannot flex in this position; the examiner then holds the MCP joint in flexion (which relaxes the intrinsics) and again attempts to flex the PIP — if PIP flexion improves with MCP flexion, intrinsic tightness is confirmed (Bunnell positive); this distinguishes intrinsic tightness from fixed joint or joint capsule contracture (where PIP cannot be flexed in any MCP position)
• The Bunnell test is central to distinguishing Nalebuff Type II (intrinsic tightness — Bunnell positive; PIP flexion better with MCP flexed) from Type III (fixed joint contracture — Bunnell negative; PIP limitation is the same regardless of MCP position)

• Figure-of-8 ring splint (oval-8 splint): a figure-of-8 thermoplastic or silver ring splint worn at the PIP joint allows PIP flexion while blocking PIP hyperextension; ideal for Nalebuff Type I flexible swan neck deformity; does not prevent PIP flexion but provides a physical block to hyperextension; allows activities of daily living while correcting the functional problem (inability to flex the PIP effectively due to the snapping into hyperextension); highly effective for Type I; custom-made by hand therapists
• FDS tenodesis: a volar surgical procedure in which the FDS tendon is used to create a restraint to PIP hyperextension; the FDS tendon is divided at its insertion and one slip is re-routed through a bony tunnel in the proximal phalanx (or around the A1 pulley) to create a volar tether at the PIP joint; this procedure reduces the tendency of the PIP to snap into hyperextension; indicated for Type I–II flexible deformities in appropriate patients when splinting fails; preserves PIP range of motion while preventing hyperextension
• Intrinsic release: for Nalebuff Type II (intrinsic tightness); the ulnar lateral band is released proximal to the PIP joint through a lateral approach; reduces the hyperextension force driving the PIP into swan neck posture; often combined with MCP joint arthroplasty in RA where MCP disease is the root cause
• Lateral band mobilisation: for Types I–III; the dorsally displaced lateral bands are mobilised to a more volar position, reducing the DIP flexion and the dorsal force on the PIP; can be performed through a lateral digital approach
• PIP arthrodesis / arthroplasty: for Nalebuff Type IV; as described for boutonnière — index/middle fused at ~35–40° flexion; ring/little — arthroplasty to preserve motion preferred over arthrodesis in RA

• Mallet finger leading to swan neck deformity: an untreated mallet finger (terminal extensor disruption at the DIP) allows the DIP to fall into flexion; the proximal extensor mechanism becomes imbalanced — the FDS is now unopposed at the PIP joint and the intrinsic contribution pulls the PIP into hyperextension; over time the volar plate attenuates and the PIP hyperextends permanently; prevention is by treating the mallet finger appropriately with DIP extension splinting for 6 weeks; established swan neck from an old mallet can be addressed by DIP arthrodesis (which corrects the terminal extensor imbalance) combined with volar plate reconstruction or FDS tenodesis at the PIP if the hyperextension is established

• Swan neck vs pseudo-swan neck: true swan neck has PIP hyperextension + DIP flexion as a result of intrinsic/extrinsic muscle imbalance or volar plate laxity; pseudo-swan neck may be seen in patients with generalised joint hypermobility who can hyperextend their PIP joints without any pathological cause; the distinction is clinical — pseudo-swan neck is correctable, painless, and not associated with any underlying pathology; it does not require treatment unless symptomatic
• The role of MCP joint disease in driving swan neck deformity in RA: in rheumatoid arthritis, MCP joint synovitis leads to stretching of the intrinsic muscles on the ulnar side of the finger; the intrinsics pass dorsal to the MCP axis and their tightening drives PIP hyperextension; correction of the MCP disease (MCP joint arthroplasty with realignment of the intrinsics) can significantly reduce or resolve the swan neck deformity at the PIP without direct PIP surgery; this demonstrates the importance of treating the underlying cause of the deformity (the MCP) rather than the secondary deformity (the PIP hyperextension)
• Snap phenomenon in Type I swan neck: in Nalebuff Type I flexible swan neck, the PIP joint can snap suddenly from a position of limited flexion into full hyperextension as the lateral bands jump dorsal to the PIP joint axis; this snapping is functionally disabling — the patient cannot initiate PIP flexion effectively once the PIP is in hyperextension; the figure-of-8 ring splint and FDS tenodesis both address this snap; the snap is the primary functional complaint in Type I swan neck and the main driver for treatment

• Swan neck deformity: PIP HYPEREXTENSION + DIP FLEXION; mirror of boutonnière (PIP flexion + DIP hyperextension); caused by volar plate laxity, FDS insufficiency, or intrinsic tightness
• Causes: rheumatoid arthritis (most common); untreated mallet finger; FDS laceration; PIP volar plate injury; intrinsic tightness; spasticity
• Nalebuff classification: Type I — flexible, full PIP flexion (ring splint); Type II — intrinsic tightness (Bunnell positive; intrinsic release + MCP arthroplasty); Type III — fixed, cartilage intact (splinting then capsulotomy + FDS tenodesis); Type IV — fixed + joint destruction (arthrodesis/arthroplasty)
• Bunnell intrinsic tightness test: PIP flexion easier with MCP flexed (relaxes intrinsics) than with MCP extended (tightens intrinsics) = intrinsic tightness = Nalebuff Type II
• Figure-of-8 ring splint: for Type I flexible swan neck; allows PIP flexion; blocks PIP hyperextension; prevents the snap into hyperextension; custom-made by hand therapist
• FDS tenodesis: surgical volar tether at PIP joint using one FDS slip; for Types I–II failing splinting; reduces PIP hyperextension while preserving flexion
• Mallet → swan neck: untreated mallet → DIP flexion → proximal extensor imbalance → PIP hyperextension → volar plate attenuation; treat mallet with DIP extension splint for 6 weeks to prevent swan neck
• Type I snap phenomenon: PIP snaps from slight flexion into full hyperextension; lateral bands jump dorsal to PIP axis; highly disabling; ring splint or FDS tenodesis corrects this
• RA swan neck: MCP synovitis → intrinsic tightness → drives PIP hyperextension; treat MCP disease (arthroplasty + intrinsic release) to correct PIP deformity without direct PIP surgery
• PIP arthrodesis fusion angles for swan neck: index/middle ~35–40°; ring/little prefer arthroplasty (motion more critical); same as boutonnière Stage III management