Dr KS Dhillon FRCS, LLM
Introduction
The true incidence acromioclavicular (AC) injuries is not known because many with such injuries do not seek medical treatment. About 12% of shoulder dislocations involve the AC joint. The treatment of AC joint injury remains controversial although good long term results can be seen in patients treated conservatively. There is an increasing tendency to treat higher grade injuries surgically.
Injuries to the AC joint can predispose the joint to osteoarthritis (OA). OA of the AC joint is a common cause of shoulder pain. Radiological changes of OA in the AC joint do not however correlate with symptoms. Symptomatic OA of the AC joint can be effectively treated with excision of the lateral end of the clavicle.
Anatomy of AC joint
The acromioclavicular (AC) joint is a synovial arthrodial joint with an intra-articular disc, where the oval lateral end of the clavicle articulates with an imperfect incongruent facet of the acromion. Although it is a small joint it supports the shoulder girdle and the upper limb (1).
The joint stability is provided superiorly by reinforcement of the capsule with a strong acromioclavicular ligament and inferiorly the capsule and a weak inferior acromioclavicular ligament. These ligaments provide about 68% of joint stability to superior translation of the clavicle. Additional stability is provided by the coracoacromial and the coracoclavicular (conoid and trapezoid) ligaments (1).
The clavicle rotates about 45 degrees on its own axis. Most of this motion occurs at the sternoclavicular joint and only about 5 to 8 degrees of motion occurs at the sternoclavicular joint (2).
Imaging anatomy of the AC joint
The AC joint is best viewed in an AP projection with a 10-15 degree cephalic angulation (Zanca view). In this view the clavicle is projected off the spine of the scapula (3).
The AC joint space is measured as an average of the cranial and caudal joint space measurements. It is usually less than 5mm. In males the average is about 3.3 plus minus 0.8 and in females the average is about 2.9 plus minus 0.8. In both males and females, the joint space reduces with age and space of less than 0.5 mm can be found. A joint space of more than 7 mm in males and more than 6 mm in females is pathological irrespective of age. There is no significant difference between the left and the right (4).
The coracoclavicular distance is usually less than 11-13mm and the right and left difference is usually less than 5mm (5).
Classification of AC joint injuries
The most commonly used classification for AC joint injuries is the one by Rockwood et al (6):
Type I. Partial tear of acromioclavicular ligaments. X-rays usually normal. Pain, swelling and local tenderness at the AC joint often present. Cross arm test is positive.
Type II. Complete tear of the acromioclavicular ligaments. Plain X-rays usually normal. Some widening of the AC joint space may be present. The inferior border of the clavicle is not elevated beyond the superior border of the acromion. Pain, swelling, tenderness and deformity at the AC joint is usually present.
Type III. Clinically obvious deformity is seen. X-rays show vertical translation with the inferior border of the clavicle elevated beyond the superior border of the acromion.
Type IV. Posterior displacement of the lateral end of the clavicle into or through the trapezius muscle occurs. An axial view will show the posterior displacement of the lateral end of the clavicle.
Type V. Disruption of the deltotrapezius fascia occurs which allows the lateral end of the clavicle to lie under the skin. X-rays show marked displacement of the clavicle beyond the medial end of the acromion.
Type VI. A very rare injury. The lateral end of the clavicle is displaced inferiorly and lies below the acromion and the coracoid (as well as the conjoint tendon).
Treatment of AC joint injuries
The treatment of AC joint injuries remains controversial. Although in recent years more surgeons are opting for surgical treatment of severe AC joint injuries, the outcome of conservative treatment is good on long term follow up. Rawes and Dias (7) reviewed 30 patients, who were treated conservatively for dislocation of the AC joint, at an average of 12.5 years after the injury. They found good outcome in all except one patient. X-rays showed persistent dislocation in 17 patients and in 13 the dislocation improved to a subluxation. Atrophy of lateral end of clavicle (similar to resection of lateral end of clavicle) was seen in completely dislocated joints.
Randomised prospective controlled trials also show good outcome of conservative treatment for dislocation of the AC joint (8 and 9). However, the current trend for the treatment of type I and type II injuries is conservative with symptomatic relief provided by ice, arm sling and analgesics. Heavy lifting and sports are usually avoided for 8 to 12 weeks (10).
The current consensus on the treatment of type III injuries remains that of a conservative approach because the prognosis after conservative treatment is excellent (11, 12 and13). However, patients with type III injuries who continue have symptoms after 3 months may be considered for surgical treatment (10). Some surgeons prefer surgical treatment in patients with type III injury who are heavy labourers and overhead athletes (14).
Type VI injuries are very rare and most of the reported cases are case reports where the patients have been treated surgically (15, 16 and 17). In medically fit patients who have type IV and V injuries the treatment is usually surgical (10).
Many surgical procedures have been described for treatment of AC joint injuries and no one procedure has been shown to be superior to the other. Procedures for acute injuries include reduction and stabilization of the joint with reconstruction of the ligaments. For delayed surgery an excision of lateral end of the clavicle is usually carried out in addition to the reduction, stabilization and ligament reconstruction (10).
Osteoarthritis (OA) of the AC joint
Primary OA of the AC joint is less common than that of the knee or the hip but is more common than OA of the glenohumeral joint. The criteria used to define OA of the AC joint is different in different studies hence the true incidence of primary OA of the AC joint remains unknown (1). Degeneration of the AC joint begins in the second decade of life and progresses with age and by the age of 50 almost all joints will have degenerative changes (18). Radiological reduction of AC joint space occurs with age and by age 60 a joint space of 0.5 mm or less is not abnormal (19). Peterson CJ (20) found age related degenerative changes in the AC joints in cadaveric dissections. After the age of 70, degenerative changes were very common. In one study MR imaging of the shoulder in asymptomatic individuals showed OA of the AC joint in 75% of the individuals (21). Despite the high prevalence of morphological and radiological degenerative changes in the AC joint, the presence of symptomatic OA is relatively rare (22).
The true prevalence of post-traumatic OA of the AC joint is unknown. There appears to be no studies in literature which have traced the natural history of OA of the AC joint. Eight to 42% of patients with type I and type II injuries can be symptomatic on long term follow up (1). Radiological changes are commonly seen in patients with type I and type II injuries but these changes do not correlate with symptoms (23). OA is less likely in higher grade injuries due to separation of bone fragments. However degenerative changes have been observed at surgery in these patient but it is uncertain if the degeneration is the cause of symptoms (10).
Treatment of post-traumatic OA
Conservative treatment for symptomatic OA includes, rest if there is acute exacerbation of symptoms, NSAIDs, activity modification in the form of avoidance of repetitive overhead and cross body activities (24).
Physical therapy in the form of cryotherapy, heat, ultrasound, stretching and mobilisation of the shoulder girdle may also be useful (25).
Significant improvement in pain and function can be seen with steroid injection when other treatments have failed. Short term improvement can be seen in about 93% of the patients (26). The effectiveness of steroid injections reduces with time but long term benefit can be seen up to 5 years (27). These injections can be repeated every 3 to 4 months if all other conservative treatment fails (28).
Conservative treatment is usually tried for at least 6 months before surgical option is considered.
Surgical treatment is usually carried out when all conservative treatment has failed to relieve symptoms which are affecting day to day activities (29). Distal (lateral) clavicle resection is the surgical option for OA of the AC joint and for clavicular osteolysis. This procedure involves the excision of 1 to 2 cm of the distal clavicle by an open or an arthroscopic approach (29). The optimal amount of distal clavicle which should be resected remains unknown (1).
Rabalais and McCarty (30) did a systematic review of the literature for articles on surgical treatment of symptomatic AC joints and they found that most of the reports were level III and level IV evidence. The articles all supported the use of distal clavicle resection for treatment of symptomatic AC joints. They found more ‘good and excellent’ results with arthroscopic surgery but the level of evidence was low. They also found that a ‘simple distal clavicle resection may have worse outcomes when performed after preceding trauma’. This probably reflects the uncertainty of the cause of pain in many of these patients. The presence of OA changes on x-rays may be present in asymptomatic patients. Instability of the AC joint has to be excluded and a definitive diagnosis must be established before surgery is contemplated (31).
Pensak et al (32) in another systematic review of literature comparing arthroscopic versus open resection of the distal clavicle for AC joint pathology found that patients undergoing arthroscopic resection returned to activities faster with similar long term results as the open technic. They also found a poor outcome in patients who had post-traumatic AC instability and in Workman Compensation patients.
AC joint surgery can be associated with complications some of which include, inadequate resection of the clavicle, diagnostic error, joint instability, weakness of the shoulder and infection (29). Inadequate resection of the clavicle can cause persistent pain and this is seen more often with the arthroscopic technique and less with the open technique. Diagnostic errors can be reduced by obtaining a complete resolution of symptoms with intra-articular lignocaine injection. Joint instability is usually caused by injury to the superior acromioclavicular ligament and over resection of the clavicle (29). Other complications such as heterotopic ossification, fracture and suprascapular nerve injury have been described.
Conclusion
The AC joint is a small imperfectly congruent arthrodial which supports the shoulder girdle and the upper limb. Though very little movements occur at the joint, injuries to the joint are not uncommon and the joint is not an uncommon cause of shoulder pain.
The long term outcome of conservative treatment of AC joint injuries is good. Despite a good outcome there is an increasing trend toward surgical treatment of these injuries especially for higher grade injuries.
OA of the AC joint though common is often asymptomatic. The radiological changes in the joint do not correlate with symptoms. Most patients with OA of the AC joint can be treated conservatively. Conservative treatment should be tried for at least 6 months. In the event of failure of conservative treatment and when a definite relationship can be established between the radiological changes in the AC joint and the patient’s symptoms, then excision of the distal end of the clavicle will produce a good outcome. The excision can be done arthroscopically or by an open technique with equally good long term outcome, though with the arthroscopic resection patient can return to activities earlier.
References
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