Acromioclavicular instability is a result of singular or multiple injuries to the ligamentous apparatus of the acromioclavicular joint leading to a non-physiological translation of the clavicula in relation to the acromion.
Instability stems from injuries that are commonly sustained during sporting activities. Young, active, and male patients are most likely to suffer from instability of the acromioclavicular joint.
Patients suffer from pain, muscle fatigue and weakness of the affected shoulder and may also develop scapular dyskinesis.
Diagnosis is made clinically by combining special techniques to detect instability. Ultrasound and radiographic imaging might assist in diagnosing.
There is ambiguity regarding treatment of chronic unstable acromioclavicular joints. These are managed according to their respective Rockwood type and biological reconstruction makes for the most beneficial outcome.
AC-instability is a result of singular or multiple injuries to the ligamentous apparatus of the acromioclavicular joint leading to a non-physiological translation of the clavicula in relation to the acromion.
Acute (<3 weeks after trauma) or chronic (>3 weeks after trauma) instability commonly stems from prior injuries to the acromioclavicular joint [1]. In most cases, injuries are sustained during sporting activity, where a direct blow to the shoulder or a fall on the outstretched arm leads to a dislocation of the joint [1]. For more information about acute dislocations of the AC-joint, please visit acromioclavicular dislocation. Dislocations or subluxations lead to a sprain or complete disruption of the acromioclavicular- and coracoclavicular ligaments (conoid and trapezoid ligament) [2]. Atraumatic instability only plays a minor role, although there are cases of iatrogenic instability, especially after extensive surgery of acromioclavicular osteoarthritis with resection of more than 1cm of the distal clavicle.
Acromioclavicular injuries account for 12% of all shoulder injuries, with young, active males being affected the most [1].
Clinically, chronic instability might not be as conspicuous as acute dislocations. Patients complain about pain over the acromion especially during overhead work as well as muscle fatigue and reduced strength [1]. A study showed that prevalence of scapular dyskinesis and SICK scapula (scapular malposition, inferior medial border prominence, coracoid pain and malposition, and dyskinesis of scapular movement) might be as high as 70% and 58% respectively [2]. These patients suffer from persistent pain that can be referred to the coracoid, the posterior shoulder or even to paraspinal regions and lateral aspects of the arm [2].
When taking the patients history, special focus should lay on prior AC-joint injuries and the treatment of such, as chronic instability often is a result of injuries that have not been diagnosed or treated properly [1].
Diagnosis of chronic instability is often missed. However, there are several examination techniques, which in combination of the patient’s history were proven to be beneficial in diagnosing instability. For the radiographic Rockwood classification of acute acromioclavicular dislocation or subluxation, please visit acromioclavicular dislocation.
Cross-body adduction test
With the patient in either sitting or standing position, the examiner brings the shoulder as well as the elbow into 90° flexion. Subsequently, the elbow is moved horizontally across the patient’s chest, which in a positive result causes pain in the AC-joint [1].
O'Brien's test
With the patient in either sitting or standing position, the examiner brings the shoulder in 90° flexion and 10-15° adduction. The patient is instructed to fully internally rotate the shoulder and pronate in the elbow. The examiner subsequently applies downward force on the patient’s forearm as he/she presses against it. This is recreated, although this time the patient fully externally rotates the shoulder and supinates in the elbow. The test is positive if pain can be felt, especially if pain is alleviated in supination [1]. Since O’Brien’s test was originally created to diagnose SLAP-lesions, the positive findings can further be distinguished. If pain ‘sits deep in the shoulder’ it usually indicates a SLAP-lesion. If pain however is situated above or in the AC-joint and the pain is not fully relieved by supination one should lean towards pathologies of the AC-joint.
Paxinos test
Patient is sitting upright with the affected arm hanging loosely to his side. The examiner places his/her thumb on the posterolateral side of the acromion, while his/her middle and index finger are placed superior to the mid-segment of the clavicula. With the examiners thumb pressing anterosuperior and the index and middle finger applying inferior directed force, pain is increased in positive finding [1].
Testing for horizontal instability
Similarly, to the Paxinos test, both acromion and clavicula are shifted against each other. However, this time the clavicula is moved posteriorly as the acromion is held in position. Recreating the pain is seen as a positive sign [1].
By combining all these techniques, accuracy of clinically diagnosing chronic acromioclavicular instability was shown to increase a lot [1]. There is ambiguity regarding the visualisation of instability on radiographs. Some studies claim that dynamic views in supine position with the arm abducted 90° in the scapular plane are superior in detecting horizontal instability, whereas other sources claim that a modified Alexander view with the arm placed in an adducted horizontal stress position with the scapula anteverted has more informative value [1]. Yet, it is agreed upon that both MRI and CT should only be used in unclear cases [1].
Acute dislocations are treated according to their respective Rockwood classification, where types I-II are managed conservatively (e.g. physiotherapy, local injection), types IV-VI surgically and treatment of type III dislocations being a heavily discussed topic in literature [2]. These recommendations can be transferred to chronic cases, although literature is scarce on indications for surgery in patients with prolonged cases [2]. However, consensus was reached on surgery technique, where a biological reconstruction of both acromioclavicular and coracoclavicular ligaments were shown to be most beneficial in chronic cases [3].
In order to address persistent pain in long-suffering patients, scapular dyskinesis and SICK scapula should be treated as well. Rehabilitation programmes show improvement of symptoms within 6 weeks of training [2].
Overall, prognosis is favourable. Regardless of which technique was used during surgery, about 19% of patients have shown features of horizontal instability [1]. These patients also had significantly worse outcomes of disability in arm, shoulder, and hand [1].
Chronic instability might lead to osteoarthritis of the acromioclavicular joint [2]. Yet, this course of disease is mainly reported in Rockwood type I and II as other Rockwood types show a greater separation of acromion and clavicula [2].
Aliberti, Gianna M., Matthew J. Kraeutler, Jeffrey D. Trojan, and Mary K. Mulcahey. 2020. "Horizontal Instability Of The Acromioclavicular Joint: A Systematic Review". The American Journal Of Sports Medicine 48 (2): 504-510. doi:10.1177/0363546519831013.
Cisneros, Luis Natera, and Juan Sarasquete Reiriz. 2017. "Management Of Chronic Unstable Acromioclavicular Joint Injuries". Journal Of Orthopaedics And Traumatology 18 (4): 305-318. doi:10.1007/s10195-017-0452-0.
Rosso, Claudio, Frank Martetschläger, Maristella F. Saccomanno, Andreas Voss, Lucca Lacheta, Ana Catarina Ângelo, and Emmanuel Antonogiannakis et al. 2020. "High Degree Of Consensus Achieved Regarding Diagnosis And Treatment Of Acromioclavicular Joint Instability Among ESA-ESSKA Members". Knee Surgery, Sports Traumatology, Arthroscopy 29 (7): 2325-2332. doi:10.1007/s00167-020-06286-w.
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