Bipolar dislocation of the clavicle is rare, and surgeon’s treatment experience is limited, both diagnosis and surgery are challenging for surgeon. As this injury is frequently one part of polytrauma (brain trauma, rib fracture, hemothorax, pneumothorax, scapula fracture, or chest injury) , bipolar clavicle injury is usually initially missed or delayed diagnosed on plain radiography because the only indication of an abnormality may be slight widening of the AC joint [16,17]. CT scanning with three-dimensional reconstructions is most valuable to get an early diagnosis, to evaluate the precise displacement of each end of clavicle, and to make a preoperative planning [5,18]. The authors recommend that whenever one end of pathology clavicle is suspected, the whole length of clavicle including both sternoclavicular and acromioclavicular joint should be examined, in such cases, CT scan would be necessary.
The mechanism of bipolar clavicle injury is still not well-known，this injury is frequently a result of high-energy trauma, like a deforming force on the lateral aspect of the shoulder or a driving force squeezing the shoulders together combined with trunk torsion [8,10]. Two hypotheses have been advocated. One theory suggests two dislocations occur simultaneously, the trauma force on the shoulder is initially transformed into the elastic energy to the clavicle . When the external force disappears, the clavicle relaxes and returns to its normal shape, the energy continues to conduct on both sides of the clavicle, causing each clavicle end ligaments damage and subsequent dislocation of the acromioclavicular joint and sternoclavicular joint [7,19]. And the another one proposes that an initial dislocation of the sternoclavicular joint followed by subsequent dislocation of the acromioclavicular joint .
Four different patterns of bipolar injury of the clavicle had been reported: (1) dislocation of both ends of the clavicle, (2) dislocation of the sternoclavicular joint with distal clavicle end fracture, (3) dislocation of the acromioclavicular joint with medial clavicle end fracture, and (4) segmental fracture of the clavicle . For most floating clavicle, the medial end displaced anteriorly while the lateral end displaced superiorly or posteriorly (Rockwood type III or IV). Eni-Olotu and Hobbs  reported a case of inferior displacement of lateral end and superior displacement of the medial end. And, only a few of bipolar clavicle injury with posterior SC joint dislocation have been found [4,8,22,23]. Posterior SC joint dislocations and medial clavicle fractures are life-threatening injuries because of their potential to cause damage to retrosternal structures. In this study, we encountered 3 different injury patterns, and where we treated an extremely rare case of acromioclavicular joint dislocation combined with medial clavicle end fracture-dislocation (Figure 1), to the best of our knowledge, in the past few of years, only Lee et al. had reported one such case.
Owing to the rarity and limited experience of this injury, treatment remains controversial and challenging. In the early stage, most authors treated their patients nonoperatively with satisfactory results [2,3,7,20,24]. However, most patients sustained deformity，residual pain or instability . Sanders et al.  reported a group of six cases with both ends dislocations, all patients treated with conservative methods initially, 4 cases required additional surgical intervention because of continuing pain, and finally got good results after AC joint reconstructions. Also, Lee et al. found superior results in patients treated with surgical treatment. Thus, a consensus towards the fact that younger and active patients should be treated with surgical treatment, due to unacceptable pain, deformity, and shoulder function limitation if the anatomical reduction cannot be restored [4,10,21].
When bipolar clavicle injury is treated operatively, surgical approach to the acromioclavicular dislocation and lateral end fracture is well-described and standard procedure. Surgical options vary from internal fixation (Hook plate, Kirschner, pin) to ligaments reconstruction (such as Weaver-Dunn, coracoclavicular reconstruction) . However, there is no consensus on the standard treatment strategy for SC joint injury. Surgical treatment is challenging due to the proximity between SC joint and important retrosternal structures (trachea, esophagus, brachiocephalic veins, brachiocephalic artery, and brachial plexus). Many operative procedures have been described for surgical treatments, and each had its own merits and drawbacks, such as pins, Kirschner wires, T-plate fixation, medial clavicle resection, and ligament reconstruction [9,12,26]. Our previous study had showed clavicle hook plate to be a very feasible option for displaced medial clavicle fracture and SC joint dislocation with several advantages, such as minimal risk of damage to retrosternal structures, dynamic fixation without damage to SC joint cartilage surface, and improved fixation stability for comminuted medial clavicle fracture . In this study, we fixed each SC joint injuries with hook plate. Meanwhile, in a report of two cases of bipolar clavicle injury, Schemitsch et al.  fixed SC joint with clavicle hook plate, at the follow up, both patients got good outcomes. Our treatment outcome was consistent with theirs.
More, controversy still exists about the management sequence of bipolar clavicle injury. Schemitsch et al. recommended initially fixed the more displaced end. Whereas, in a study containing 6 operative cases and 5 conservative cases, Lee et al. fixed AC injuries with a hook plate firstly, and then open reduction and anterior SC ligament repair was performed after failed closed reduction of the medial end of the clavicle. Yurdakul et al. and Thyagarajan et al. fixed the sternoclavicular joint firstly in their reports. The authors experience was consistent with the above two studies, in this present study, once the SC joint was stabilized, the AC dislocation was found to be reducible passively, the reasons might be that the sternoclavicular joint is much more incongruous because the clavicular end is bulbous in shape and the clavicular notch of the sternum is curved. Thus, the authors presume that SC fixation is sufficient for some part of bipolar clavicle injury, especially for cases with AC dislocation, and AC fixation is not necessary unless residual instability existence.
In this study, the authors noted that a patient (Figure 2) who preoperatively presented SC dislocation associated with distal clavicle fracture sustained a slight acromioclavicular joint dislocation after a hook plating in SC joint and an anatomic plate osteosynthesis in distal clavicle, the reasons might be an inaccurate reduction of distal clavicle due to chronic injury or a hook plating under large stress. Meanwhile, the authors admit limitations of this study, such as small size of samples, lack of a control group, and short time of follow-up period. However, considering the rarity of bipolar clavicle injury, the limitations did not influence the results.