Isolated anterior radial head dislocations are extremely rare ^[1] with almost all of them being associated with a hyperextension-hyperpronation injury. Anteromedial radial head dislocations are further rare with infrequent association with bony fractures or ligamentous injuries. As a distinct entity, there are nine case excerpts in the literature describing the association of anteromedial radial head dislocation with an impeding musculotendinous sling of biceps or brachialis. Impeding sling was missed and not identified over multiple attempts of open reduction in three of these cases, whereas closed reduction failed to reduce all of them. Veenstra et al. ^[2] and Sasaki et al. ^[3] described similar cases where the adhered and obstructing tendons needed resection and reattaching owing to their delayed presentation. In this case report, we present a delayed presenting case of an anteromedial radial head dislocation with a hidden brachialis tendon sling, which was treated with open reduction without tendon resection or reattachment.

A 7-year-old boy sustained a hyperextension-hyper pronation injury to his right elbow due to a fall on outstretched hand while playing. The traumatic elbow was treated primarily by an indigenous bonesetter, with the child presenting to our referral hospital 8 weeks after his injury. Clinical examination revealed persistent elbow swelling with warm – shiny overlying skin. There was a restricted flexion-extension (20°-70°) and pronosupination (±40°) arc of movement (Fig. 1).

Serum alkaline phosphatase was thrice the reference range confirming the clinical diagnosis of active heterotrophic ossification (HO). X-ray revealed persistent isolated anterior radial head dislocation with features of HO in the anterior joint capsule (Fig. 2). Computed tomography scan confirmed the anteromedial direction of the radial head with heterotrophic bone anteriorly (Fig. 3).  The child was finally taken up for open reduction 4 months following the injury post-resolution of the active HO. Under anesthesia, there was a soft restriction of the passive elbow flexion and extension with no restriction in the pronosupination. Boyd’s interval was utilized for the exposure of the radiocapitellar joint with the patient positioned in the lateral decubitus. The anconeus was elevated from the ulna and the lateral ulnar collateral ligament was detached from the supinator crest. The radial head appeared dislocated anteriorly with an entrapped posteriorly subluxated annular ligament. Repositioning of the annular ligament failed to reduce the radial head spontaneously. However, the radial head could be forcefully reduced in position with elbow hyperflexion only to redislocate with the slightest extension.  Distal extension of the approach elevating the supinator from the proximal radius revealed a musculotendinous sling posterolaterally, just distal to the cartilage-covered radial head (Fig. 4). The tendon appeared to have subluxated behind the radial head with no direct attachment over the radial neck. As tested using blunt artery forceps, its tautness worsened with elbow extension and was unaffected with pronosupination. Further, dissection confirmed the tendon’s brachialis origin, which was repositioned anteriorly using a blunt freer elevator. The repositioning of the tendon spontaneously reduced the radiocapitellar joint which was clinically stable in the position of supination. Due to its instability in pronation, a radiocapitellar wire was inserted keeping the elbow in 90° of flexion and complete supination. The wire was removed after 4 weeks with a gradual resumption in the elbow range of movement. At 18 months following surgery, the elbow showed excellent functional outcome with a Mayo Elbow score of 95 with arestricted flexion of up to 110° and  pronation of up to 50° (Fig. 5). Radiographs showed maintained radiocapitellar joint reduction (Fig. 6).

Isolated anterior radial head dislocations or fracture dislocations are rare [4, 5, 6, 7, 8, 9, 10]. The direction of  radial head dislocation, anterolateral, or anteromedial could give an idea about the possible impediments and success of closed reduction maneuvers. Armstrong et al. reported the first case of irreducible anteromedial radial head dislocation in a 7-year-old child, where the biceps tendon dislocated posterolaterally and prevented reduction ^[11]. Ozan et al. summarized the broad range of interposing factors in irreducible anteromedial dislocations ranging from the joint capsule, tendon, or annular ligament ^[6]. Nine different reports are present in the literature describing a similar occurrence of irreducible anteromedial radial head dislocation, all having the rare interposition of biceps or brachialis tendon ^[12] [Tables 1 and 2].

Cates et al. pointed out the intraoperative distinguishing features between biceps and brachialis tendon slings during such an injury. The relative proximal location around the neck, presence of muscle fibers along the tendon, and the absence of insertion over the radial neck could help distinguish the brachialis tendon from the biceps ^[4]. Tarallo et al. described a “sling effect” due to “spring preloading,” where the forearm elastically springs from supination to neutral rotation in presence of a biceps tendon loop. Besides, a pre-operative non-contrast magnetic resonance imaging (MRI) can be valuable in detecting the musculotendinous sling and injuries to collateral ligaments commonly encountered with such an injury ^[5]. Almost all the cases reported in the literature had a hyperextension force vector. The majority of these cases (7/11) had an associated radial head fracture [1, 4, 5, 7, 8, 9, 10] and half of them had a medial epicondyle fracture ^[3] or medial collateral ligament (MCL) tear [4, 5, 8, 13] suggesting a possible valgus injury. We hypothesize a probable sequence of events leading to posterolateral subluxation of the superficial brachialis tendon suspending the radial head. The first step would comprise of supine-hyperextension disrupting the anterior radiocapitellar capsule and/or annular ligament. This would be followed by a valgus moment causing MCL sprain or radial head fracture bringing the radial head medial to the superficial fibers of the brachialis tendon. The last event would comprise pronation and elbow flexion dislocating the tendon behind the radiocapitellar joint forming a dynamic sling impeding the reduction (Fig. 7). A similar mechanism was proposed by Upasani et al. who demonstrated the occurrence in a cadaveric model ^[8]. Extended lateral [3, 4, 7, 8, 9, 10, 11, 13] or Boyd’s approaches ^[14] have been used for treating such injuries. Lateral (extensor split approach) preserves the integrity of lateral collateral ligament whereas Boyd’s approach is advantageous in providing an access to ulnar osteotomy if deemed necessary. Careful distal exposure with the elbow in pronation prevents inadvertent injury to the posterior interosseous nerve. The tendon could be reduced anteriorly over the radiocapitellar joint using a blunt freer elevator holding the elbow in hyperflexion. Seldomly an adhered tendon could be resected and re-attached as in two neglected cases (10 weeks and 4-years-old) [2, 3]. Associated injuries to the annular ligament and collateral ligaments could render such cases unstable in pronation necessitating post-operative immobilization and/or radiocapitellar wire. HO could often occur following this injury [1, 6]. The severity of soft-tissue injury and multiple failed attempts of closed reduction could be the possible attributing causes.

Importance of the direction of dislocation has not been previously emphasized in the literature. Although this injury is rare, awareness of the possible tendon imposition is important to plan the management of such a dislocation. A pre-operative MRI to rule out the impeding soft-tissue structure should be done to diagnose such a case preoperatively in presence of the anteromedial direction of dislocation. Avoiding unnecessary closed reduction attempts, gentle handling of the soft tissues, and low-dose radiation/chemoprophylaxis could reduce the incidence of HO in such cases. Although age, time since injury and occurrence of HO were the poor prognostic indicators in the literature, majority of these cases including ours showed excellent post-operative function.

The main learning point from this case report was to identify anteromedial isolated radial head dislocation as a special entity that is commonly associated with an impeding musculotendinous sling of the biceps or brachialis. Pre-operative identification using MRI, early open reduction, and HO prophylaxis can produce excellent functional outcomes in this rare pattern of injury.