Traumatic rotator cuff tendon tears in young patients are unusual and they are commonly associated with high-energy trauma. The entrapment of a rotator cuff tendon in the glenohumeral (GH) joint is even rarer and this malady is prone to be missed. We experienced a case of a pseudo-paralyzed shoulder after trauma. The cause turned out to be interposition of the rotator cuff tendons in the GH joint. We present the imaging of this rare condition with the operative correlation. We also speculate on the possible trauma mechanism from the evidence of the concomitant injuries and the image findings.

A 17-year-old male cyclist had accidentally fallen directly onto his face and right side in a farmland area. He presented to our emergency department with extensive swelling and ecchymosis of the face, right shoulder and elbow. The shoulder and elbow were too painful to move. The shoulder had diffuse swelling and tenderness, and any attempt at passive motion was resisted. The neurological examination, including the sensory status of the deltoid area, was difficult to assess due to the patient's enormous pain. An X-ray of the right shoulder revealed a widened joint space and an upward-migrated humeral head (Fig. 1A). Right olecranon and facial bone fractures were also found. According to the local physical findings and X-ray of the right shoulder, traumatic rotator cuff tendon tear was highly suspected by the consulting orthopedic surgeon. The magnetic resonance images (MRI) confirmed this suspicion. It revealed an interposition of the ruptured rotator cuff tendons, including the supraspinatus tendon and the infraspinatus tendon (Fig. 1B, C). Nevertheless, the patient has no subjective sense of shoulder dislocation during the trauma.

An inverted U-shaped skin incision was made during the operation, which allowed both an anterolateral and a posterolateral approach to the GH joint simultaneously. The operative findings included complete tears of the supraspinatus and infraspinatus tendons and these tendons were interposed in the GH joint. Besides, parts of the torn infraspinatus tendon were interposed into the axillary pouch. Avulsion of the transverse ligament of the bicipital groove leading to dislocation of the biceps long head tendon was also noted. However, the biceps long head tendon remained intact. It took effort to relocate the interposed tendons due to their tight incarceration. We then repaired the supraspinatus tendon and infraspinatus tendon. The dislocated biceps tendon was also reduced.

The rehabilitation program included arm sling immobilization for four weeks, followed by assisted passive range of motion (ROM) exercise for four weeks and then active ROM exercise. The patient regained a normal shoulder function, muscle strength and active ROM at eight months postoperatively.

Traumatic shoulder dislocation is a common injury, but irreducible dislocation is a rare complication. The cause can be of bony or soft tissue origin (1-8), and an interposed biceps tendon is thought to be the most common soft tissue block in the GH joint (4). Regarding traumatic rotator cuff interposition, the English medical literature contains only rare cases of this (1-4, 7, 8). Ten patients have been presented and most of them (8 of 10 had sequela of shoulder dislocation, which resulted in persistent subluxation after a closed reduction maneuver (Table 1). Only two other cases of rotator cuff interposition without recognizable GH dislocation have been reported.

To the best our knowledge, the traumatic mechanism causing the interposition has not been previously described in patients without recognizable GH dislocation. Because of the coexisting fractures and image findings, we inferred the possible mechanism in this presented case. We can speculate the injury was initiated by violent adduction and internal rotation of the humerus while falling onto the ground with a flexed elbow (Fig. 1D). It resulted in tear of the external rotator tendon, infraspinatus tendon and dislocation of the biceps long head. The following axial loading on the olecranon led to "superior subluxation" of the humeral head, which is evidenced by the MRI of the "kissing lesion" and bony contusions over the anterior humeral head and acromion (Fig. 1E, F). The supraspinatus tendon was avulsed during the forceful superior migration of the humeral head in the sustained adduction position. At the same time, the avulsed rotator cuff tendons were interposed into the GH joint by the bowstring tension. Due to the blockage by the rotator cuff tendons, the humeral head could not be completely spontaneously repositioned by ligamentotaxis.

The early detection of rotator cuff interposition is not easy since the radiological findings are subtle. Thus, the rate of delayed diagnosis was reported to be up to 70% (7 of 10) (Table 1). Widening of the GH joint on the plain radiograph would be the significant clue on the imaging study, though it might be erroneously explained by distension of the shoulder joint due to hemarthrosis. CT with soft tissue windows or MRI would aid in making the proper diagnosis.

In a review of the literature, we also noted that the supraspinatus and infraspinatus tend to tear and interpose concurrently, except for one patient in Walch's series (Table 1). This phenomenon might be explained by the "rotator cable" concept (9, 10). Since these two tendons are connected together by the cable-like structure on the insertion site of the humerus, they would be avulsed together during trauma, and the tendons are large enough to block the spontaneous reduction of the humeral head.

In conclusion, traumatic interposition of rotator cuff tendons without recognizable glenohumeral dislocation is a rare but challenging injury to properly diagnose and treat. We should have a high degree of suspicion when the shoulder X-ray shows subtle findings of a widened joint space. CT with soft tissue windows or MRI should be performed to confirm the diagnosis. Better results can be expected with early recognition and then timely surgical treatment.