Is the annular ligament disrupted in Monteggia fracture–dislocations? A cadaveric mechanistic study

Background Monteggia fracture-dislocation is characterized by an ulnar fracture with radial head dislocation. The mechanisms of dislocation and annular ligament integrity remain unclear. This study established a cadaveric Monteggia fracture-dislocation model to investigate radial head dislocation and associated annular ligament injury. Methods Twelve upper limbs from six fresh-frozen cadavers (mean age at death, 83 years; one male, five female) were examined. Soft tissues were removed while preserving the bones, ligaments, capsules, forearm interosseous membrane, and biceps tendon insertions. The humeroulnar joint was fixed at 90° flexion, and the ulna was manually fractured to create a Bado type I (direct-blow) Monteggia fracture-dislocation model. The annular ligament, quadrate ligament, oblique cord (Weitbrecht ligament), and forearm interosseous membrane were evaluated. Results Ulnar fracture created forearm deformity, causing the radius to contact the triangular fulcrum. Continued manual loading caused the radius to pivot around the fulcrum. The annular ligament remained intact in 11 of 12 limbs; one specimen showed injury at the annular ligament–capsular junction. In 11 limbs, the radial head dislocated beneath the distal margin of the annular ligament. One quadrate ligament rupture occurred; the oblique cord and forearm interosseous membrane remained intact. Conclusion In this standardized cadaveric direct-blow model, anterior radial head dislocation occurred in 11 of 12 limbs, beneath the distal margin of an intact annular ligament. These findings demonstrate that annular ligament rupture is not required for radial head dislocation in the described experimental setting. This mechanism provides a possible anatomical explanation for difficult reduction in selected cases.