Coronoid Process Fracture: Regan-Morrey Classification and Biomechanical Stability

The coronoid process of the ulna is the primary anterior bony stabilizer of the humeroulnar joint, functioning as a rigid anatomical buttress that prevents posterior subluxation or dislocation of the forearm bones beneath the distal humerus. In addition to providing anterior bony resistance, the coronoid serves as the critical insertion site for the anterior joint capsule, the anterior bundle of the ulnar collateral ligament (inserting onto the sublime tubercle), and the brachialis muscle tendon. Fractures of the coronoid rarely occur in isolation; instead, they represent the bony signature of severe ligamentous injury and rotational instability. Accurately classifying coronoid fractures using the classic Regan-Morrey system and the contemporary O’Driscoll classification is fundamental for restoring joint congruency and structuring safe post-injury rehabilitation.

High-Velocity Road Crashes, Scaffold Collapses, and Farm Equipment Trauma

In rural and semi-urban India, coronoid process fractures are frequently observed in agricultural workers and industrial laborers following heavy mechanical trauma. Operating agricultural thresher machines, falling from elevated scaffolding at building sites, or being thrown from speeding motorized three-wheelers generates high-energy axial loading through the flexed elbow. In domestic environments, falling backward onto an extended hand from a stool while cleaning ceiling fans or changing light bulbs drives the distal humerus downward, shearing off the coronoid tip as the forearm subluxates posteriorly. Without anatomical reconstruction and precise early rehabilitation, patients develop recurrent subluxation sensations whenever they push up from a chair or reach forward.

Regan-Morrey Types, O’Driscoll Anteromedial Fractures, and Instability

In 1989, Regan and Morrey classified coronoid fractures based on the vertical height of the fractured fragment relative to the entire coronoid process: Type I involves an avulsion fracture of the tip (less than twenty-five percent of coronoid height); Type II involves a single or comminuted fragment encompassing up to fifty percent of coronoid height; and Type III involves a large fracture extending through more than fifty percent of the coronoid base, frequently destabilizing the humeroulnar joint completely. In 2003, O’Driscoll recognized that fracture morphology depends heavily on the vector of traumatic force. In varus posteromedial rotatory instability (VPMRI), the anteromedial facet of the coronoid is sheared off, compromising the sublime tubercle and causing catastrophic posteromedial instability even when the fracture fragment appears deceptively small on lateral radiographs.

Post-Fixation Protection, Extension Blocking, and Isometric Activation

Because the coronoid buttress prevents posterior displacement of the ulna, the joint is mechanically most vulnerable in extension, where the distal humerus slides forward over the deficient coronoid slope. Following surgical repair, which utilizes suture lassos for tip fractures or low-profile plates and screws for larger basal and anteromedial fragments, the elbow is protected in a hinged brace locked with an extension block at thirty degrees of flexion. Active and active-assisted flexion within the functional arc (thirty to one hundred and thirty degrees) is encouraged from post-op week two. Concurrently, isometric activation of the biceps and brachialis is performed with the elbow flexed at ninety degrees to draw the ulna anteriorly against the trochlea, reinforcing joint reduction.

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