Boutonnière Deformity (Central Slip Rupture)

What is it?

Central slip rupture allows lateral bands to sublux volarly, creating PIP flexion and DIP hyperextension (boutonnière).

Commonly affected: Index through little fingers; after PIP injury

How it develops

1. Central Slip RuptureForced PIP flexion or RA causes rupture
2. Lateral Band Volar MigrationLateral bands shift below PIP axis
3. PIP Flexion DeformityLateral bands now act as PIP flexors
4. DIP HyperextensionTraction on terminal tendon hyperextends DIP

Symptoms

• PIP flexion deformityLoss of PIP extension, flexion contracture
• DIP hyperextensionDIP bent backward
• Dorsal PIP painPain at central slip insertion
• Grip dysfunctionImpaired grip due to finger flexion
• PIP swellingDorsal PIP swelling acutely

How it is examined

• Elson's testPIP at 90° flexion; DIP rigid with resisted extension = central slip rupture
• PIP extension lagMeasure PIP active extension deficit
• DIP hyperextensionMeasure DIP hyperextension angle
• Dorsal PIP tendernessTenderness over dorsal base of middle phalanx

Imaging

Lateral X-ray — characteristic PIP flexion + DIP hyperextension; check dorsal avulsion fragment.

• PIP flexion posture
• DIP hyperextension
• Possible dorsal avulsion fragment
• Rule out joint subluxation

MRI confirms central slip rupture and lateral band position when uncertain.

• Central slip discontinuity
• Volar lateral band migration
• PIP edema/hematoma

When surgery is considered

Open injury, large avulsion, chronic deformity, failed conservative

Procedures that may be discussed

• Primary central slip repair
• Lateral band relocation
• Tendon reconstruction (chronic)

Outlook

Early splinting gives good results; chronic deformity has limited recovery.