Ulnar diaphyseal fractures combined with the anterior dislocation of the radial head were first described by Giovanni Monteggia in 1884 [1]. Subsequently, Louis Bado added three subtypes, and all are generally classified collectively as “Monteggia fractures” [2].

Associated with anterior dislocation of the radial head in Monteggia fractures, nerve palsy uncommonly occurs, and the posterior interosseous nerve (PIN) is mostly involved [3,4]. Although rare, anterior interosseous nerve (AIN) palsy has also been documented. Partial median nerve palsy is even more uncommon at higher levels and may present with sensory changes [5,6].

According to the Bado classification, radial head dislocation in Monteggia fractures mostly occurs in the anterior, posterior, or lateral directions [2]. Minor anteromedial dislocations are generally classified as anterior. Reports of severe anteromedial dislocations in isolated radial head dislocations are rare, especially in the context of Monteggia fractures [7,8].

Herein, we present an adult Monteggia fracture with incomplete median nerve palsy at a higher level than AIN accompanied by remarkable anteromedial dislocation of the radial head.

This study was approved by the Institutional Review Board of the Korea National Institute for Bioethics Policy (No. P01-202505-01-069). Written informed consent was obtained from the patient for publication of this report, including all clinical images.

A 24-year-old woman presented with an injury to her left arm sustained from a fall while riding a bicycle three hours prior. She reported sensory loss in the first and second digits, an inability to pronate and supinate the forearm, limited flexion and extension of the elbow, and impaired flexion of the first and second digits. Flexion strength was assessed as Medical Research Council grade 3 for the first digit and grade 2 for the second digit (Fig. 1). Radiographic examination revealed a fracture of the ulnar shaft at the midpoint of the diaphysis and dislocation of the radial head. However, the radial head dislocation was atypically anteromedial and was severely displaced medially up to the level of the coronoid fossa of the humerus, with a gap between the radial head and the humerus (Fig. 2). Considering the fracture location and the dislocated radial head, the authors suspected compression of the median nerve between the radial head and humerus. As the patient’s neuropathy symptoms worsened over time, emergency surgery was performed on a weekend evening without a preoperative MRI.

The surgery was performed under brachial plexus block anesthesia. The ulnar shaft was opened and fixed with plates and screws. Closed reduction of the radial head was attempted by supination and traction of the forearm while pressing on the radial head. Successful reduction was achieved after several attempts (Fig. 3). As the reduction was stable, open surgery for the radial head and the nerve did not proceed.

Postoperatively, there were no worsening neurologic symptoms, and sensory abnormalities in the first and second digits improved by the third week and were completely resolved by the eighth week. The reduction in flexion strength of the first and second digits also began to improve by the fourth week and fully recovered by the eighth week (Fig. 4). Owing to the rapid improvement in neurological symptoms, additional neurological tests such as nerve conduction studies were not performed. The patient underwent immobilization of the elbow for approximately 2 weeks after surgery, followed by rehabilitation therapy that included elbow movement. About two months later, the patient regained full active range of motion in the elbow and wrist.

The most commonly used classification for Monteggia fractures is the Bado classification; type 1 involves anterior dislocation of the radial head, type 2 involves posterior dislocation, type 3 involves lateral dislocation, and type 4 refers to fractures of both the ulna and radius with radial head dislocation in any direction [2]. Among these, type 1 (anterior dislocation) is the most prevalent. We believe that cases of mildly displaced radial head anteromedial dislocation have rarely occurred previously, but most of these cases are thought to have been classified as type 1 [7]. In cases like the present one, where the radial head is severely dislocated anteromedially, such a presentation appears to be exceptionally rare. However, the authors believe that although the dislocation in this case was severe, the anteromedial dislocation was a consequence of an anterior dislocation. Therefore, its mechanism is thought to be similar to that of a typical anterior dislocation; thus, it is classified as Bado type 1.

Two common mechanisms explain the pathogenesis of Bado type 1 Monteggia fractures. The first is the hyperpronation theory proposed by Evans in 1949 [9], and the second is the hyperextension theory described by Tompkins in 1971 [10]. The hyperpronation theory posits that, during a fall, an outstretched hand contacts the ground, and as the body twists further, excessive pronation occurs, leading to ulnar shaft fracture and radial head dislocation. Conversely, the hyperextension theory suggests that when an outstretched hand lands and hyperextends, the contraction of the biceps muscle during hyperextension causes the radial head to dislocate anteriorly, and the increased force results in an ulnar shaft fracture. In the present case, the spiral fracture pattern and severely medially dislocated radial head suggested a rotational force as the underlying mechanism. Therefore, we believe that the pathogenesis in this case corresponds to the hyperpronation theory. The atypical dislocation pattern, characterized by severe medial displacement, can be attributed to the patient falling from a bicycle traveling at a high speed. This suggests that radial head dislocation likely occurred prior to the ulnar shaft fracture, resulting in severe anteromedial dislocation, or alternatively, that the fracture occurred following the usual Bado type 1 mechanism, with additional rotational force caused by intense trauma.

Most of the nerve injuries reported in previous studies of Monteggia fractures are palsies of the PIN, particularly in cases classified as Bado type 1 [3,4]. This is attributed to the anatomical relationship whereby the PIN passes directly anterior to the radial head, making it susceptible to stretching during anterior radial head dislocation. However, reports of AIN palsy are rare, and previous studies suggest that the mechanism involves traction injury to the AIN caused by anterior dislocation of the fractured ulnar segment [5,6]. There has been a case report of a Bado type I Monteggia fracture in a 6-year-old girl, in which the main trunk of the median nerve was entrapped at the fracture site [11]. However, in this patient, neurological symptoms that were absent preoperatively developed postoperatively. Due to difficulty achieving closed reduction of the fracture during surgery, an open reduction of the ulna was performed to release the entrapped median nerve, and, because the radial head could not be successfully reduced closed, open reduction was also performed. Since the patient’s neurological symptoms cannot exclude the possibility of iatrogenic nerve injury or nerve injury caused by the radial head, it is insufficient to explain the median nerve palsy solely as a consequence of ulnar fracture. In contrast, median nerve palsy caused by remarkable anteromedial dislocation of the radial head has not been documented.

We believe that the partial median nerve palsy observed in this case differs from previously reported AIN palsies for three main reasons. First, radiographs show a visible space between the radial head and coronoid fossa. There has been a case report of an isolated radial head dislocation with severe anteromedial displacement, in which the radial head dislocated into the coronoid fossa but remained attached beneath the median nerve without neurological symptoms [8]. In contrast, the present case exhibited a similar severity of medial dislocation, but a space existed between the radial head and coronoid fossa, with the median nerve likely entrapped within this space (Fig. 5). Second, the median nerve paralysis was partial, and recovery was rapid. In this case, the motor and sensory deficit was observed only in the radial side fingers (the first and second digits), with no involvement of the ulnar side fingers (the third and fourth digits). Additionally, because the nerve injury was caused by blunt trauma, the recovery was faster compared to cases involving sharp ulnar fracture fragments. Third, the ulnar shaft fracture in this case occurred near the midshaft, which was distinctly more distal than that in other cases involving AIN palsy. Although AIN injury caused by fracture fragments at the midshaft is possible, the possibility of such an injury resulting in sensory nerve palsy is considered very low. Consequently, we believe that the partial median nerve palsy observed in this case resulted from neuropraxia due to nerve compression by the severely anteromedially dislocated radial head. It is also important to highlight that the neurological symptoms progressed during the clinical course and required prompt surgical treatment. However, owing to the progression of neurological symptoms, urgent surgical intervention is necessary. Additionally, because the procedure was performed over the weekend, a preoperative MRI could not be performed due to institutional constraints. Therefore, the precise anatomical relationship between the radial head and the median nerve could not be determined via MRI, nor could the exact location or extent of the nerve injury be identified through electrophysiological studies. In this case, the partial median nerve palsy is a conjecture based on clinical presentation and radiological findings, which constitutes a limitation of this study.

The current standard treatment for adult Monteggia fractures involves open reduction and plate fixation of the ulnar fracture, combined with closed reduction of the dislocated radial head [3,4]. Additionally, if ligament or nerve interposition prevents proper reduction of the radial head, open reduction may be attempted [3]. Reviewing previous studies, even in cases with neurological symptoms involving the PIN or AIN, most neuropraxic injuries tend to resolve with time [4-6]. Therefore, if surgical reduction of the ulnar fracture and radial head dislocation is successful, exploration of the nerve during the initial operation is generally deemed unnecessary.

This report presents a rare case of a Monteggia fracture with severe anteromedial dislocation of the radial head associated with partial median nerve palsy. Although Monteggia fracture patterns are highly variable, remarkable anteromedial dislocation of the radial head could result in median nerve palsy. With awareness of atypical nerve involvement, prompt and appropriate treatment is expected to lead to favorable outcomes in such cases.