Simple Elbow Dislocation

Posterolateral Rotatory Mechanism

This mechanism of dislocation was described by Dr. O'Driscoll as occurring in three sequential stages; depending on the energy dissipated at the time of injury, the elbow may progress to stage I, stage II, or stage III ( Figs. 35.1 and 35.2 ).

According to this mechanism, as the elbow angulates in excessive valgus and the forearm rotates posterolaterally with respect to the humerus, the first structure disrupted is the lateral collateral ligament complex (stage I). If the disruption ends here, the elbow is quite stable in flexion and even into full extension in the absence of a posterolateral rotatory stress. However, incomplete healing of the lateral collateral ligament complex may lead to chronic posterolateral rotatory instability.

Stage II involves further disruption of the posterior capsular structures with incomplete dislocation of the elbow. The typical radiographic finding of this type of injury is one in which the coronoid appears to be perched on the trochlea ( Fig. 35.3 ).

As the severity of the injury increases, the medial structures are disrupted. In stage IIIa, the ligamentous injury involves the posterior band of the medial collateral ligament; the critically important anterior band of the ulnar collateral ligament is still intact in this mechanism of injury. Tearing of the posterior band allows frank dislocation of the elbow. However, following reduction, because the anterior band of the ulnar collateral ligament is intact, the elbow may remain reasonably stable. In the final stage, IIIb, the anterior band of the medial collateral ligament is also involved. Because of this severe soft tissue disruption, the elbow, even after reduction, is typically unstable to varus and valgus stress.

This pattern of instability and ligamentous disruption has been confirmed in cadaver studies. In one study, 12/13 elbows could be dislocated posteriorly without sectioning the anterior band of the medial collateral ligament. Furthermore, in all 13 elbows the coronoid could be perched on the trochlea with the release of only the lateral collateral alignment complex and the lateral portion of the capsule. In these cases, the elbow was stable to valgus stress following reduction.In addition, the posterolateral rotatory mechanism of elbow instability has been documented in a number of clinical series. In nearly all series of simple elbow dislocations that require surgical management, repair or reconstruction of the lateral collateral ligament (and in some cases the extensor mass) is adequate to restore stability to the elbow.

Alternative Mechanisms

Several articles published since 2012 have investigated whether the model of instability just described, involving initiation of injury on the lateral side of the elbow, is the only mechanism involved. Rhyou et al. have described an entirely different mechanism of dislocation, with support from MRI studies and intraoperative findings. In their initial study, which was based on MRI data, they documented a more severe and tensile injury to the medial side, putatively due to a valgus stress. There was contusion to the radial head and capitellum. The lateral collateral ligament complex was “stripped” from the lateral cortex, with less severe injury. These authors postulated that simple dislocations begin with disruption of the medial collateral ligament. In their series, medial initiation of dislocation was suspected in 11 patients, whereas lateral initiation was seen in only two patients. Another study on MRIs after simple elbow dislocation from a different group indicated a more frequent and higher level of injury to the medial collateral ligament, giving more support to the idea that initiation of dislocation may begin medially in some instances.

Support for this mechanism came in a subsequent article evaluating a treatment algorithm for surgical management of patients with persistent instability after simple elbow dislocation. Patients taken to the operating room were first evaluated for joint line opening with valgus stress (no firm end point or instability); if there was evidence of medial-sided instability, a medial-sided repair was performed first. Following repair of the ulnar collateral ligament and flexor pronator, the investigators repeated stability testing and repaired laterally only when unstable. They began laterally only in the absence of medial-sided instability. Using this algorithm, 8 patients required a lateral repair only, 12 patients underwent repair of both sides, and 3 patients underwent a medial repair only. A subset of patients with subluxation postoperatively (so called “drop sign,” which was most common in patients who had undergone an isolated lateral repair and a nerve block) were examined as well. At most recent follow-up, there were excellent clinical results and no patient had persistent instability, even those with a postoperative drop sign.

Another interesting study analyzed the mechanisms of simple elbow dislocation using video clips published on YouTube. In a review of 62 dislocations, the most common loading forces identified were extension, valgus, and pronation. This would appear to lead to further clinical evidence that a subset of dislocations, and perhaps the majority, are initiated by medial soft tissue disruption.

Whereas data continue to emerge on the mechanism and patterns of simple elbow dislocation, these studies into the mechanisms and methods of treatment of simple elbow dislocation suggest that there are more than one mechanism and pattern of instability for simple elbow dislocation. It is likely that both mechanisms occur. Further research, including comparative studies of surgical algorithms, will allow us to surgically address these issues in a more optimal way, while limiting surgical morbidity.

Assessment

The initial assessment of the patient with an elbow dislocation includes a careful and complete evaluation of the injured limb and a survey for other associated injuries. In the setting of a low-energy fall, or even an athletic injury, associated injuries are rare. In the setting of a polytraumatized patient, a careful primary and secondary survey should be completed, as associated injuries are common.

Next, a neurovascular exam of the affected arm is important. Documenting the function of median, radial, and ulnar nerves should be completed. Palpating pulses can allow assessment of arterial injuries. Pulses should be compared with the contralateral extremity, as collateral flow can allow reconstitution of pulses even with a major arterial injury in the arm. An assessment of soft tissues for open or impending open injuries is required.

Radiographs of the elbow should be completed to assess for associated bony injuries. Radiographs can help to aid in reduction by better assessment of the direction and degree of displacement ( Fig. 35.4 ). As discussed in Chapter 36 , assessment of fractures prior to reduction efforts is important for documentation in the medical record. Advanced imaging studies are seldom needed, but they have been used to investigate the pathoanatomy of simple elbow dislocations ( Fig. 35.5 ).