UNLABELLED: Acute boutonniere deformity is a heterogeneous clinical entity that has traditionally been attributed to rupture of the central slip of the extensor mechanism, with secondary palmar migration of the lateral bands. The temporal relationship between central slip detachment and lateral band displacement remains incompletely defined. We report a clinical case in which a patient presented with an immediate, actively irreducible boutonniere deformity caused by an uncommon injury pattern, characterized by longitudinal disruption of a single lateral band and rupture of the ipsilateral collateral ligament, with the central slip largely intact. The deformity could not be reduced by closed means and required surgical treatment.Motivated by this observation, we performed an exploratory anatomical study on 16 cadaveric fingers. Isolated lesions of the central slip or of a single lateral band did not, in this model, produce an actively irreducible boutonniere deformity. By contrast, combined injuries consisting of a longitudinal lesion of one lateral band and rupture of the ipsilateral collateral ligament consistently resulted in immediate palmar displacement of the lateral band below the axis of the proximal interphalangeal (PIP) joint, irrespective of central slip integrity.These exploratory findings suggest that, alongside the classical mechanism of central slip rupture, an under-recognized injury pattern combining longitudinal lateral band disruption and ipsilateral collateral ligament failure may produce an immediate, actively irreducible boutonniere deformity, with potential implications for early diagnosis and treatment. Further clinical validation is required.
LEVEL OF EVIDENCE: IV.
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