Injuries to the Acromioclavicular (AC) Joint

By Edward Fink, M.D.

From the elite pro to the casual rider, falls or crashes sometimes occur in spite of our levels of skill and diligence. However, if you do fall and sustain an injury to the acromioclavicular (AC) joint, don’t despair. The overwhelming majority of riders recover fully without surgery and are back in the saddle happily riding within six weeks.

What is the AC joint?

The acromioclavicular joint is not a typical joint that enables motion and movement of an extremity. Rather, its purpose, along with surrounding muscles, is to attach the shoulder to the rest of the body. It does so by providing a point of attachment of the collarbone (clavicle) to the shoulder blade (scapula) at a bony prominence called the acromion; hence, the name acromioclavicular joint.

Two major ligaments, the acromioclavicular ligament and the coracoclavicular ligament, stabilize the joint by holding the clavicle and acromion together. Injuries to one or both of these ligaments produce varying degrees of joint instability and displacement.

How does an AC joint injury occur?

Injuries to the AC joint typically occur from a fall from a bicycle. With the forward momentum of the fall, the shoulder is usually the first body part striking the ground. The magnitude and the direction of the force usually determine the type and severity of the injury.

If the full body weight lands on the shoulder during the fall, a clavicle fracture will typically occur. With weight and force directed more to the outside the shoulder, AC joint injuries are more commonly seen. With these injuries, the downward pressure on the shoulder and scapula stresses the stabilizing ligaments of the AC joint as the clavicle remains relatively immobile. If these forces were to continue, the ligaments would fully rupture and separation would occur between the acromion and the clavicle.

How is an AC joint injury diagnosed?

A fall onto the shoulder oftentimes produces significant pain, and medical attention should be sought. A practitioner will usually inquire about the injury, how it occurred, and will then perform a physical examination.

The clavicle and the acromion are located directly beneath the skin and are easily palpable, enabling doctors to diagnose AC joint injuries with little difficulty. While infrequent in occurrence, injuries to nerves or blood vessels around the neck and shoulder can have dire consequences and they should be evaluated as well.

Descriptive confusion

You may hear other terms erroneously used to describe injuries to the AC joint. They are sometimes called “shoulder separations” even though the shoulder is not directly involved. “Shoulder dislocations” occur when the humeral head fully displaces from its socket and produces a completely different injury.

What are the different levels of AC joint injuries?

Injuries to the AC joint are usually graded according to the degree of disruption of the ligaments, with greater forces producing more significant injury. The injuries typically follow this pattern:

Grade 1: The acromioclavicular ligament is strained but the coracoclavicular ligament is uninjured. There is no joint instability, simply pain with examination by touch over the injured ligament at the acromioclavicular joint.

Grade 2: As more force is imparted, the acromioclavicular ligament tears. The coracoclavicular ligament may be strained yet it is intact. Physical examination may show some subtle increased motion at the joint, though not significant.

Grade 3: With increasing force, both the acromioclavicular and coracoclavicular ligaments tear, creating instability at the AC joint. With no suspensory ligaments, the shoulder drops below the clavicle, which gives the impression that the clavicle has actually been displaced upward.

Grades 4, 5, and 6: These describe relatively rare conditions where there is a grade 3 rupture of both ligaments. However, with continued forces, the clavicle becomes displaced in abnormal positions around the shoulder and neck.

If you fall on your shoulder

Pain with palpation over the acromioclavicular joint is a constant feature of all AC joint injuries. With grade 1 or 2 injuries, there is usually no accompanying joint instability. In a grade 3 injury, with rupture of both ligaments, the clavicle will be “high-riding” compared to the acromion, as the weight of the arm carries the shoulder blade and acromion lower than the clavicle.

The degree of separation between the acromion and the clavicle can be as much as several inches. At times, immediately after an injury, the muscles around the shoulder fall into spasm, preventing the dropping of the shoulder. The degree of height discrepancy between the clavicle and acromion may not be fully apparent in that case.

An x-ray of the shoulder may be ordered while a weight is suspended from the wrist on the affected side. In this manner, the muscles are allowed to relax, and the separation deformity is accentuated, allowing documentation of the degree of injury.

What is the best treatment for AC joint injuries?

For almost all grades of AC joint injury, the treatment is usually conservative, without surgical intervention. For grades 1 and 2, the joint is not significantly disrupted and immobilization of the shoulder in a sling for one to two weeks usually allows early ligament healing and pain relief. You may need up to six weeks of healing before being safely able to return to riding a bicycle.

In grade 3 injuries, although the two ligaments are torn and the joint may be significantly disrupted, no attempt is made to reconstruct the ligaments or restore the joint to its normal architecture. Recommended treatment is much like in grades 1 and 2, with a period of immobilization followed by movement to restore shoulder motion and strength.

After time for healing, individuals with grade 3 injuries usually have no deficits in strength, movement or stability, nor do they have pain, even while significant displacement between the clavicle and the acromion is present. The clavicle may remain more prominent on top of the shoulder.

The surrounding muscles provide strength and muscle tone to stabilize the shoulder. Surgical attempts to reconstruct the ligaments and restore alignment to the joint are not performed as they can actually produce significant joint degenerative changes and pain.

For grades 4, 5, and 6, surgery is usually recommended to effectively create a “grade 3” injury by manipulating the clavicle out of its abnormal location.

Infrequently, the AC joint can heal with partial displacement of the acromion in relation to the clavicle. The joint is partially disrupted and over time, painful arthritis can occur due to the abnormal joint surface relationship. In this setting, surgery may be required to relieve the pain by removing the end of the clavicle, yet this occurs in a very small percentage of individuals.

---

Premium Member Edward Fink is an orthopedic surgeon in Arizona.