Causes / typical injury mechanism: A Monteggia fracture is an uncommon type of forearm fracture consisting of a radial head dislocation and a proximal ulna fracture.

The mechanism of injury involves direct trauma to the forearm or falling onto an outstretched hand, resulting in a proximal ulna fracture. Force generated by this fracture transmits through the interosseous membrane and disrupts the annular and quadrate ligaments that connect the radius and ulna at the proximal radioulnar joint. This causes radial head dislocation. Due to this tethering effect of the interosseous membrane, the direction of the radial head dislocation is correlated with proximal ulna fracture angulation. Reduction of the proximal ulna fracture restores anatomic alignment at the proximal radioulnar joint as this mechanism leaves most of the interosseous ligament intact. Delayed diagnosis may result in elbow deformity and loss of motion, particularly forearm pronation and supination.

Complications:

• Monteggia fractures, particularly Bado type 2 fractures, pose risk to the posterior interosseous nerve (PIN), a motor nerve that innervates all the extrinsic wrist extensors except the extensor carpi radialis longus. Traction of the nerve may result in wrist radial deviation and finger drop. This typically resolves in 9-12 weeks.
• Posttraumatic elbow stiffness from immobilization is seen more commonly in adults.
• Proximal radioulnar synostosis seen in these fractures will limit forearm pronation and supination.
• Malunion or nonunion.
• Ulnohumeral arthritis and elbow instability when there is a concomitant coronoid process fracture.
• Compartment syndrome.

Classic history and presentation: The typical patient has had trauma to the elbow. The elbow is usually painful, swollen, and tender with limited range of motion.

Prevalence: Monteggia fractures account for between 1% and 2% of forearm fractures. They are more common in children than in adults.

Grade / classification system:

Bado classification

• Type 1: anterior dislocation of the radial head and proximal ulna fracture
  • Most common type in children
  • Direct trauma to posterior elbow
• Type 2: posterior dislocation of the radial head and proximal ulna fracture
  • Most common type in adults
  • Axial force in semiflexed elbow
• Type 3: lateral dislocation and proximal ulna fracture
  • Varus force on extended elbow
• Type 4: anterior dislocation of the radial head with both radius and ulna fractures, typically at the same level
  • Mechanism of injury likely similar to type 1, least common of all types

Jupiter classification further describes the ulna fracture in Bado type 2 fractures.

• Type 2A: coronoid process and olecranon
• Type 2B: distal to coronoid process at the junction of the metaphysis and diaphysis
• Type 2C: diaphyseal
• Type 2D: complex fracture extending from olecranon to the diaphysis