Bennett Fracture

Synonyms
Partial intra-articular bacse of the thumb fracture, Bennett's fracture dislocation

INTRODUCTION

Definition

A base of the thumb metacarpal fracture dislocation that is partially intra-articular. 

Epidemiology

About 80% of thumb fractures involve the metacarpal base.

Mechanism

These fractures are caused by axial force applied to the thumb in flexion. 

Patoanatomy

  • There is an an oblique intraarticular metacarpal fracture (the palmar beak fragment) which remains attached to the palmar beak ligament.
  • The fracture starts at ulnar base of metacarpal bone. 
  • The volar fracture fragment remains attached to CMC by volar anterior oblique ligament. This ligament anchors volar lip of metacarpal to tubercle of the trapezium. This means that a small volar lip fragment will remain attached to anterior oblique ligament which is attached to trapezium.
  • The distal metacarpal fragment (containing most of articular surface) is displaced proximally, radially, and dorsally by an APL-pull. 
  • The displaced metacarpal is also rotated in supination by the pull of APL.
  • Metacarpal head is also displaced into palm by pull of ADP.
  • Due to the force of the abductor pollicis longus and adductor pollicis the the metacarpal shaft is displaced dorsally and radially.

CLINICAL FEATURES

Symptoms

The patient has acute pain at the base of thumb. 

Signs

Inspection: Swelling and hematoma.
Palpation: Tenderness to palpation at the CMC joint.
Motion: Painful range of motion.

DIAGNOSIS

X-ray

Projections: AP, lateral and oblique.
  • True AP of thumb (Robert’s view): The arm is in full pronation with dorsum of thumb on cassette. 
  • True lateral of thumb: The hand is pronated 30° and beam angled 15° distally.
Findings:
  • There is a oblique fracture line with a triangluar fragment at ulnar base of metacarpal bone.
  • The triangular fragment remains attached to trapezium with proximal displacement of the metacarpal bone. 

TREATMENT

Overview

Usually treated with surgically because of its notorious instability. 

Nonoperative Treatment

Indication: Dislocation < 1 mm.
Method: Reduction and casting. If the fracture is older than 3 weeks and there is no motion at fracture site, there is no need for reduction.
  • Reduction: Performed with longitudinal traction on end of thumb, in addition to abduction and extension of thumb metacarpal. The thumb can then be pronated to bring it into opposition with non-displaced palmar fragment.
  • Casting: Thumb spica cast.

Operative Treatment

Closed reduction and Percutaneous pin fixation
Indication: Dislocation > 1 mm. The beak of the fragment should involve less than 50% of the palmar slope of the metacarpal and the concave dome of the metacarpal should be maintained.
Method: K wires are used to maintain reduction by stabilizing the first metacarpal to trapezium or second metacarpal bone. Small volar lip fragment should not be pinned.
Postoperativ care: Thumb spica cast for 4-6 weeks.

Open reduction internal fixation
Indication: Dislocation > 1 mm.
  • If reduction is not possible
Method: ORIF with AO cortical screw.
  • Approach: Volar approach of Gedda and Moberg.
    • The dorsal sensory branches of radial nerve are identified and protected.
    • Thenar muscles are reflected volarly and a longitudinal capsulotomy is made.
    • The radial artery is protected and retracted ulnarly.
    • The fracture is visualized and reduced by traction of first metacarpal.
  • Fixation: The fracture reduction is provisionally secured with a K wire and fixed with screws.

COMPLICATIONS AND PROGNOSIS

Complications

Posttraumatic arthirtis: The exact incidence is unclear. There is a higher risk with highly comminuted intra-articular fractures, major step off and multiple small fragments.
Other complications: Malunion
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References