BOUTENNIERE DEFORMITY

40 yrs female with inability to extend 3rd finger of left hand following trauma.

Frontal and lateral projections of 3rd finger of left hand showed flexion deformity of proximal interphalangeal joint and extension deformity of distal interphalangeal joint. moderate soft tissue swelling around the proximal phalanx. No evidence of fracture/ avulsion fragment.

High resolution sonography of 3rd finger of left hand revealed rupture of central slip of extensor digitorum tendon at its insersion onto the middle phalanx (1st image. see normal central slip of other finger for comparision). The two lateral slips are intact(2nd image). Axial images of normal and abnormal fingers showing edema at the site of disruption with absent tendon fibres and widely seperated lateral slips(3rd image).

Anatomy:

·        The extensor tendons to the four fingers pass across their respective metacarpophalangeal joints.

·        The deeper fibres of the tendons form the posterior capsule of the joints.

·        The bulk of each tendon passes freely across each joint. 

·        The tendons then broaden out and divide into three slips on the dorsal aspect of each proximal phalanx.

·        The central slip passes on to the base of the middle phalanx. 

·        The lateral slips diverge around the central slip and receive strong attachments from the interossei and lumbricals forming the extensor expansion.

Boutonnière (“button hole”) deformity:

·        Injury to the central tendon results in an imbalance in the extensor mechanism. 

·        Flexor digitorum superficialis is unopposed and it flexes the proximal interphalangeal joint.

·        This forces the head of the proximal phalanx between the lateral bands.

·        With time, the triangular ligament ruptures causing the lateral bands to displace volar to the axis of motion of the proximal interphalangeal joint and become flexors of the joint. 

·        The extensor hood retracts proximally, causing extension at the metacarpophalangeal and distal interphalangeal joints.

·        This results in  flexion of the proximal interphalangeal joint and hyperextension of the distal interphalangeal and metacarpophalangeal joints, known as the boutonnière (“button hole”) deformity.

·        Types of injury:

1.     Avulsion fracture of middle phalanx

2.     Partial rupture with stretching of central slip

3.     Complete rupture with seperation

Reference : Ultrasound imaging of finger tendons at the bedside in the emergency department, Southern Cross University ePublications@SCU.