Objectives: The aim of the current study was to investigate the pattern of extensor pollicis brevis (EPB) insertion macroscopically and histologically using cadaveric thumbs, and to compare the incidence of different insertions with that of thumb boutonnière deformity in rheumatoid arthritis (RA) patients who required surgical reconstruction. Methods: We examined 103 thumbs of 58 adult cadavers with no evidence of RA, and reviewed the surgical records of 28 thumbs of 23 RA patients who underwent surgical reconstruction for thumb boutonnière deformity. The incidence of different insertion patterns of the cadaveric thumbs and the RA thumbs were compared using the Fisher’s exact test. Results: Macroscopic and histologic examination revealed that the insertion patterns of EPB could be divided into three groups: insertion into the base of the proximal phalanx (Type P1), integration of EPB into the dorsal fibrocartilage of the MCP joint (Type P2), and insertion into the distal phalanx (Type D). The incidence of Type D was significantly higher in RA patients with thumb boutonnière deformity (64%) than that in the non-RA cadavers (29%; P <.05). Conclusion: EPB is inserted into the distal phalanx more frequently in RA patients who require surgery for thumb boutonnière deformity than non-RA cadavers, suggesting an additional possible mechanism of this deformity.
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