Acromioclavicular joint injury
Illustration of the ligaments of the acromioclavicular joint undergoing injury


Acromio-clavicular (AC) joint injury is a common pathology of the shoulder consisting in the sprain or tear of one or more ligaments that hold the acromion and clavicle together. It is also referred to as shoulder separation.

A healthy shoulder with intact ligaments around the acromioclavicular joint and the rotator cuff


The AC joint is located at the junction between the acromion on the upper scapula and the clavicle or collarbone and is supported by multiple ligaments. The distal end of the clavicle is fixed at the AC joint by the capsule and its ligaments. The clavicle has an attachment, the coraco-clavicular ligament, which consists of two components, the conoid and the trapezoid ligaments that provide horizontal stability to the AC joint. Depending on the severity of the AC injury, one or more ligaments can be damaged or ruptured. This causes sprains or separations of the AC joint. In addition to ligament injury, a fracture of the distal clavicle and/or acromion can be associated with this pathology.

Type 1 to 3 are the most common acromioclavicular joint injuries


The injuries to the AC joint are graded according to the severity of damage caused to the ligaments, as well as the presence of joint displacement and deformity.

Type 1: mild sprain of the AC capsule and coraco-clavicular ligament.

Type 2: tear of the AC joint capsule and a sprain of the coraco-clavicular ligaments. In some cases one of these ligaments, either the conoid or the trapezoid ligament is torn.

Type 3: complete tear of the AC joint capsule and two components of the coraco-clavicular ligament. The latter results in the evident bump on the shoulder.

Type 4, 5 and 6: less common severe injuries with high degree displacement of the distal clavicula in different directions (not shown).

Left: Distal radius intraarticular, displaced fracture; Middle: Older distal radius fracture with callus formation; Right: Distal radius and ulna fracture, extraarticular and displaced

No 2.

Intra-articular fracture extends to the wrist joint (or articulation)

Extra-articular fracture is located outside of the wrist joint

Open fracture when bone fragments perforate the skin

Comminuted fracture when the bone breaks into multiple fragments

Non-displaced when the anatomical alignment of the bone is maintained or displaced when the bone fragments move apart.

Melone’s classification describes the characteristics of intra-articular fractures of the radius:

i    Stable fracture

ii    Unstable "die-punch"

iii   "Spike" fracture

iv Split fracture

v   Explosion injuries

Acetabular fracture of the pelvis

Acetabular fractures

These fractures are divided into:

Anterior pillar (not weight bearing part of joint)

Posterior pillar (often associated with dislocation of the hip including the weight bearing part of joint)


Comminuted involving both column type 

Sacral / coccygeal fractures

The sacrum is a triangular-shaped bone formed by 5 fused vertebrae, which provide a posterior wall to the pelvic ring. At each side of the sacrum, the ala structures articulate with the ilium bones forming the sacro-iliac joints. Sacral fractures are usually parallel to the spine and can involve the ala. Less frequently sacral fractures may display an “H” shape, including a transversal fracture uniting both sides of the sacrum. Three zones are described where sacral fractures can occur that are along vertical lines relative to the alignment of the foramina. Sacral fractures may result in sacral instability and require treatment via sacroplasty (injection of bone glue into the fracture). Surgery is necessary in case of associated neurological symptoms.

Fractures of the coccyx involve the tailbone, the terminal portion of the spine situated below the sacrum formed by 3 to 5 fused vertebrae. Coccyx fractures occur when falling on a seated position. They are more common in elderly women and seldom require surgical treatment.

Falls are the main cause for a an acromioclavicular joint injury


The most common mechanisms for an AC joint injury are a fall or a blow directly onto the upper shoulder (acromion). This includes a fall onto an outstretched hand and a downward force applied to the shoulder, pushing down the scapula.

Riding a motorcycle increases the risk of a fall and shoulder injuries

Risk factors

In athletes, injuries to the AC joint are the most frequent type of shoulder injury followed by shoulder dislocation. Usually men between 20 and 40 years are affected by this pathology almost exclusively with an incomplete ligament tear. Sports exposing to the risk for his pathology include:

Contact sports

Horse riding


Automobile driving



Patient showing symptoms of an acromioclavicular joint injury


The most common symptoms of an AC joint injury are:

Shoulder tenderness and swelling

Increasing pain particularly over the AC joint

Appearance of a bruise and/or protrusion of the distal clavicle (Type 3)

Pain while exercising (weight lift)

Pain when sleeping onto the injured shoulder

Popping or catching noises with movement

Limited shoulder range of movement when raising the arm (“cross body adduction test”).

Shoulder palpation is part of the examination to diagnose the pathology


A detailed medical history is discussed including past and present injuries of the shoulder, and description of ongoing symptoms. Medical examination is usually sufficient to diagnose an AC joint injury. This involves palpation of the shoulder, measurement of movement restrictions and evaluation of pain intensity. X-rays are performed when AC joint injury is suspected and to exclude a clavicle or other bone fractures. Different types of imaging planes are used for this diagnosis: Panorama X-rays of both shoulders: The patient is asked to hold a weight keeping the arms straight while doing X-rays. Lateral view of the shoulders with the arms resting along the body to detect any anatomical differences. MRI is only employed with recurrent discomfort in older patients to establish the possibility of a rotator cuff tear or presence of arthritic changes of the AC joint.


Temporary immobilisation is beneficial for an acromioclavicular joint injury

Nonoperative treatment

Conservative treatment is recommended for uncomplicated Type 1, 2 and possibly Type 3 AC injuries beginning with a period of rest using a sling, administration of NSAIDs, if necessary intra-articular steroid injection, and application of ice pads to control pain and swelling. Fractures in the AC area are usually treated with a sling unless bone misplacement is present. Physical therapy is critical for those patients with restricted range of movement caused by a mild AC injury/sprain.

With a clavicle fracture surgery is required to fix the bone with a plate and screws

Surgical treatment

Surgery is usually advised in Type 3 and more severe types of AC injuries, especially if accompanied by bone fractures. Different forms of materials are available to fix the clavicle (soft wires, screws, non-resolvable sutures) to immobilise the AC joint and allow the ligaments to heal. The screws are usually removed after 6-8 weeks. Reconstruction of ruptured AC ligaments is achieved using local tissue or exogenous material (tendon graft).


Approximately four weeks post-surgery when the ligaments have healed, physiotherapy can begin with passive exercises performed by a therapist. Strenuous exercises are recommended 6 to 8 weeks post-surgery to restore the range of movement and strength of shoulder muscles. Healing of AC joint injury may require a long time and it is essential to keep the muscles toned and the joint flexible at all times to prevent secondary injuries. Although most patients heal well after an AC injury, some complications may arise. Pain may remain a recurrent problem. If the alignment of the bones is altered there is a high risk of permanent deformity, impaired function and development of AC joint arthritis.


If balance is precarious it is convenient to use the handrail to prevent falls


The main preventative strategy for an AC joint injury and its associated complications is to avoid falls in the elderly and high-energy impacts in the younger population. The following measures are recommended:

Regular exercise

Use of a walker if balance is poor

Use of handrails when walking on stairs

Injury prevention in automobile (airbags)

Shoulder taping and padding when returning to sport practice

Prevention of recurrent AC joint injuries relies on the early diagnosis of an initial injury. Following such pathology, returning to sport activities is only recommended when swelling and pain have subsided, and when normal range of movement has been achieved. This can take a few weeks to months. An educational program with a trained physiotherapist informs patients on how to approach specific movements to avoid future AC joint injuries.