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Pathology of Boutonniere Injuries

- See:
        - Rheumatoid Boutonniere
        - Thumb Boutonnere Injuries

- Mechanism of Injury:
    - rupture of central slip directly from its bony insertion commonly results from closed blunt trauma (w/ or w/o fracture);
    - rheumatoid Arthritis
    - volar PIP dislocation

- Pathology:
    - lesion results from disruption of extensor central slip along w/ disrupiton of triangluar ligament (the later of which can occur either at the time of injury, or can occur w/ chronic attenuation;)
    - w/ central slip disruption, PIP joint may initially be capable of weak finger extension if the triangular ligament remains intact;
            - active extension is possible, since the triangular ligment holds holds the lateral bands in an anatomic position;

- Disrupted Triangular Ligament:
    - patients will not be able to actively extend PIP joint, but will be able to actively hold the PIP in extension, if it is first passively extended;
          - this occurs because the lateral bands will relocate into their native position w/ PIP extension, allowing pt to hold finger in extension;
    - if it is unclear whether the central slip is disrupted, then test PIP joint extension w/ the wrist and MP joints held in flexion;
          - this should relax the lateral bands;

- Triangular Ligament Intact:
    - the intrinsic contributions to the central slip are disrupted, and therefore the intrinsics transmit their actions to the DIP, causing hyper-extension;
    - later on, the triangular ligament will slowly attenuate, allowing the deforming forces of the volarly positioned intrinsics and transverse retinacular ligament, to pull the lateral bands into a volar direction, below axis of PIP joint;
            - subsequently, the lateral bands will act as flexors of the PIP joint, as well as acting as extensors of the DIP joint;
            - forced active extension hastens this process;
            - this accenuates the PIP flexion & DIP extension deformities;
    - in chronic cases, there is contrature of the transverse retinacular ligament, which converts a dynamic deformity to a fixed PIP contracture;

- MP Joint:
    - may show a hyperextension deformity, since w/o the normal insertion of the extensor slip to the PIP joint, all of the force is concentrated on the MP joint