So these are my disclosures, none of which are directly relevant to this talk. As an overview, I'm actually going to start with a brief case presentation and then go over the anatomy of the AC joint, common mechanism of injury, the classification of AC joint injuries and then diagnosis and management. So this was a 59-year-old right-hand dominant male who presented two days status post fall off a stool directly onto his left shoulder. He had immediate pain over the superior aspect of his shoulder and difficulty with shoulder range of motion. He works as a campus security officer. On exam, he had mild swelling over the AC joint and prominence of the distal clavicle. He had limited shoulder range of motion secondary to pain and was complaining of pain directly over the AC joint with cross-body adduction. These were his x-rays in clinic that day, which showed a type 2 AC separation. His initial plan was a steroid injection into the AC joint and then physical therapy focusing on scapular strengthening and McConnell taping. He returned about two weeks later and he did get some pain relief initially with the injection, but pain was now back to his baseline. He was still complaining of pain and limitation with shoulder range of motion. And so we ordered an MRI at that point to evaluate for rotator cuff pathology. When he came back following that visit though, he said he had substantial improvement in his shoulder range of motion and pain. He had been doing his home exercises and physical therapy and the MRI showed a tear of the AC ligaments, but the CC ligaments were intact and the cuff was intact. So at that point, the plan was just to continue with non-operative management. Interestingly though, he presented then in June 2021, so about two years later, complaining of a week of left shoulder pain. There was no trauma. He had pain, this time with overhead motion and reaching away from his body and difficulty sleeping. These were his x-rays in clinic that day. And I present this really just to show you the significant amount of calcification which happened in the AC ligaments as a result of the injury. He was not complaining at all of symptoms related to his AC joint when I saw him most recently. So the AC joint is a diarthrodial joint between the medial facet of the acromion and the lateral aspect of the clavicle. It allows for gliding movement and assists in shoulder abduction and flexion. It stabilizes the scapula in relation to the clavicle by means of this complex of ligaments and muscles. The clavicle rotates about 40 to 50 degrees with overhead motion and only 5 to 8 degrees of motion occurs at the AC joint. As the clavicle rotates upward, the scapula then rotates downward and AC joint motion decreases. We know there are two main ligaments that stabilize the AC joint. This is the AC ligament and the CC ligaments. In terms of the AC ligaments, this consists of a superior, inferior, anterior and posterior components. These are thickenings of the joint capsule. And very importantly, this is the major stabilizer against horizontal translation and posterior translational forces that are generated by scapular protraction. A study in AJSM in 2016 commented on the AC ligaments or described them as two separate bundles. There's the superior-posterior component which is higher quality in terms of structural composition. And the anterior-inferior component which is inconsistent. It's thin and narrow but serves a very important role in horizontal instability. The CC ligaments consist of the conoid and trapezoid ligaments. These confer vertical stability to the AC joint and work as restraints against scapular internal rotation. The trapezoid ligament is about 22 to 25 millimeters from the lateral end of the clavicle. It attaches to the undersurface of the clavicle and provides resistance to AC joint compression. The conoid ligament, on the other hand, is 42 to 47 millimeters from the lateral edge of the clavicle and is responsible for 60 percent of the anterior and superior clavicular displacement and rotation. The AC capsule and CC ligaments allowed controlled rotation of about 40 to 50 degrees for the acromion in relation to the lateral clavicle while preserving that centered motion. This is a really important characteristic of normal scapulothoracic motion. And importantly, when this rotation is disrupted, that leads to decreased function and poor clinical symptoms. So a study in AJSM in 2018 wanted to perform a detailed biomechanical evaluation of the specific capsular structures of the AC joint and determine their contribution to translational and rotational stability. In this study they used fresh frozen human cadaveric shoulders. They dissected the AC capsule and looked at posterior translation, rotation and displacement of the lateral clavicle in relation to the center of rotation. They found that with a posterior translational force, the specimens with a completely cut AC capsule demonstrated significant loss of resistance force against translational motion. Cutting 50 percent of the capsule reduced the resistance torque for all segments compared with the native state and all groups demonstrated a significant increase of motion in all directions when the AC capsule was cut. The authors therefore concluded that cutting the entire capsule reduced the resistance force to less than 25 percent during translational testing and less than 10 percent during rotational testing compared to the native state. They also noted that the anterior segments of the capsule provided the greatest stability under rotational loading and that the amplitude of the joint's motion significantly increased under rotational stress. In terms of classification of AC joint injuries, they range from minor sprains and subluxations to complete dislocations. They are often associated with other injuries to the shoulder joint and that's important to keep in mind. AC joint injuries were first classified by TOSI in 1963 and then the Rockwood classification system as we know it was described in 1984. So Rockwood Type 1 is a sprain of the AC joint without a complete tear of the AC or CC ligaments. Patients present with joint tenderness and sometimes swelling. There's no widening of the AC joint on X-ray. Type 2 injuries are a tear of the AC ligament and a sprain or partial tear of the CC ligaments. This does confer some vertical subluxation of the distal clavicle. Type 3 injuries are complete tears of both the AC and CC ligaments. This leads to 25-100% displacement of the distal clavicle compared to the contralateral side. Also very important to note is the distinction between Type 3A and 3B injuries which was described by the ISSACOS Upper Extremity Committee where 3A is defined as horizontally stable and 3B is horizontally unstable. Type 3B injuries are characterized by overriding of the distal part of the clavicle on AP radiographs with the cross-arm adduction view. Important to note, these often have substantial scapular dyskinesia that does not respond to PT. And these patients may therefore benefit from early operative fixation. Type 4 injuries are a posterior subluxation of the clavicle into the trapezius. Type 5, both the AC and CC ligaments are completely torn. And we see 100-300% displacement of the clavicle compared to the contralateral side. Type 6 injuries are rare. And this involves the distal clavicle being displaced inferiorly into the subcoracoid position. In terms of associated shoulder pathology, about 15-18% of patients with AC joint injuries have associated pathology, most commonly slap lesions and rotator cuff injuries. For the mechanism of injury, AC joint injuries comprise about 12% of all shoulder injuries and are 5-10 times more common in males. They most often occur in the first three decades of life and usually are the result of a direct blow to the shoulder while the arm is in an adducted position or a fall directly onto the adducted arm. This commonly occurs in sports including bicycling, skiing, football and soccer. In terms of diagnosis, certainly we want to suspect an AC joint injury in any patient who has shoulder trauma with pain in the vicinity of the AC joint when they have an injured upper extremity and they're holding it in an adducted and supported position to alleviate pain and then localized pain and swelling over the AC joint. The pain is often accentuated with abduction and cross-body adduction. We may see tenting of the skin with the distal clavicle in Type 3 or 5 injuries. Patients can have SC joint pain in Type 4 injuries. And then they may be complaining of pain in the neck or trapezius in a Type 5 or 6 injury. On exam there are a lot of different tests that we can do. So there's the cross-arm adduction test where the arm is elevated to 90 degrees and adducted across the chest with the elbow flexed to 90 degrees. Pacino's test evaluates for tenderness by applying pressure on the posterior aspect of the AC joint as demonstrated in that picture. O'Brien's test, we're very familiar with this, but this can be used to determine whether the pain is caused by injury to the AC joint or as a result of labral or biceps pathology. In this exam we apply a downward force with the arm elevated to 90 degrees and adducted 10 to 15 degrees. Pain that's referred to the AC joint signifies AC joint injury, whereas pain referred to the anterior aspect of the glenohumeral joint is indicative of labral or biceps injury. We can also use the AC resisted extension test where the shoulder and elbow are flexed to 90 degrees and the patient attempts to extend the elbow against resistance. And then finally, it's very important for us to evaluate for horizontal instability in terms of assessing posterior translation of the clavicle. We just use one hand to shift the clavicle and the other to maintain the position of the acromion. This is just a short video demonstrating horizontal instability of the clavicle and then adducting the arm across the body and the distal clavicle goes posteriorly. So we really have to keep that in mind because that changes the patient's course pretty significantly. In terms of imaging, we always start with standard x-rays of the shoulder. And then to specifically evaluate the AC joint, you can obtain the ZENCA view, which is most accurate. Bilateral ZENCA views can be used to visualize both AC joints and then it's easy to compare. And then just throwing out a little reminder in terms of OITE stuff, right. So for the ZENCA view, you use half the x-ray exposure that's used in the standard radiograph of the shoulder. For a few other comments on imaging, so the average distance between the superior aspect of the coracoid and the inferior aspect of the clavicle is about 1.1 to 1.3 centimeters. A 40% to 50% difference in the CC inner space between normal and affected shoulders indicates complete disruption of the CC ligaments. And then in Rockwood and Mattson in 1990, they note that documented complete disruption of CC ligaments with side to side CC inner space difference of 25%. A few other comments about horizontal instability, this is often missed. It can lead to chronic pain and functional limitations. It can present clinically with significant shoulder pain and disability. And it's difficult to diagnose using standard x-rays. So in terms of recommendations, using the ZENCA view, axillary lateral view can be helpful. Other views include the Alexander view, which is when the arm is in an adducted horizontal stress position and the scapula is anaverted. I have pictures of this to show. And then supine dynamic lateral view where the arm is abducted to 90 degrees in the scapular plane. So this demonstrates the Alexander view. And then a picture here with the supine dynamic axillary lateral view. So just really important to remember to evaluate for horizontal instability and obtain appropriate imaging. MRI can be used to directly assess the AC and CC ligaments. It may be helpful in identifying associated pathology. And in terms of management, for types 1 and 2, these are treated nonoperatively, usually with a sling. And anti-inflammatory medications, activity modification and physical therapy can be helpful. We often recommend that our patients avoid contact sports and heavy lifting until they're pain-free and have symmetric range of motion. A very recent study in AJSM, the purpose of this was to assess the long-term outcome after non-op therapy for type 1 and 2 injuries regarding functional and radiologic outcome. So they had a median follow-up of 85 months. The constant score of the injured shoulder was 88.6 versus 93.3. So the red dash is shown there. And in terms of radiologic outcomes, there were similar rates of degeneration, but more frequent osteolysis of the distal clavicle, ossification of the ligaments, as we saw with that case presented earlier, and deformity of the distal clavicle. They concluded that there are frequently radiographic changes, but long-term functional outcomes after Rockwood 1 and 2 injuries are usually pretty good. And there's clinically non-relevant functional differences between the injured and contralateral upper extremities. For type 3 injuries, management is controversial. To maximize function, some surgeons will advocate for acute fixation of type 3 AC joint injuries in young and active patients. And certainly in those unstable injuries, we may seriously think about that. We know optimal treatment for our young athletes can be quite different from our older patients. And so we have to take all of that into account. The study in 2017 wanted to compare the rate of recurrence and outcome scores of operative versus non-op treatment of patients with type 3 AC dislocation. They noted a rate of recurrence in the surgical group of 14%. But there was no statistically significant difference between conservative and operative management with regards to post-op arthritis and the persistence of pain. So they concluded that there's insufficient evidence to establish the effects of surgical versus non-op treatment on functional outcomes of patients with type 3 AC dislocation, and that we really need high-quality randomized controlled clinical trials to establish if there's a difference in functional outcomes. So based on the current literature, there's really no difference between operative and non-operative management of type 3 injuries. So it's recommended to start with non-operative management. And then surgical management should be considered in patients who have significant AC deformity, have tenting of the skin, they have persistent pain despite non-op management. And then in those patients with higher functional demand of the injured shoulder, we may consider operating earlier. A few quick comments about scapular dyskinesia. This is a clinically important consequence of non-operative management. It leads to an alteration of the normal position or motion of the scapula during coupled scapular humeral movements. The dyskinetic pattern actually falls into three different categories based on the prominence of the inframedial border of the scapula, the entire medial border, or the superior medial border. And then just as a quick reminder, 6-scapula syndrome refers to scapular malposition, inferior medial border prominence, coracoid pain and malposition, and dyskinesis of scapular movement. This study in 2009 wanted to evaluate whether scapular dyskinesis and eventually 6-scapula syndrome develop in patients with chronic type 3 AC dislocation. They hypothesized that altered AC joint anatomy may interfere with scapulothoracic rhythm. They found that 70% of patients had scapular dyskinesis with arms at rest, two-thirds had prominence of the inframedial border of the scapula, and 58% had 6-scapula syndrome. They concluded that chronic type 3 AC separation causes scapular dyskinesis in 70% of patients, and again 58% have 6-scapula syndrome. Dyskinesis might be due to loss of the AC joint as a stable fulcrum of the shoulder or due to superior shoulder pain. So in summary, the AC joint is absolutely critical in assisting in arm movement. Injury to the AC joint is often caused by a direct blow to the shoulder in contact sports. The Rockwood Classification System is used to determine the extent of injuries and helps with management. And then non-operative or operative treatment depends on the severity of the injury. I'd be happy to answer any questions towards the end of the presentation. Thank you so much for your attention.

AC joint injuries

management

non-operative treatment

Rockwood classification system

diagnosis