The acromioclavicular (AC) joint is located at the top of the shoulder, connecting the collarbone (clavicle) and the highest point of the shoulder blade (acromion). Injuries to this area, often called a shoulder separation, typically result from a direct fall onto the shoulder. This trauma stretches or tears the stabilizing tissues, allowing the clavicle to shift out of alignment and create a noticeable protrusion or “bump.” The persistence of this visible deformity depends entirely on the severity of the underlying ligament damage.
Understanding AC Joint Anatomy and Bump Formation
The AC joint relies on two sets of ligaments for stability. The acromioclavicular (AC) ligaments connect the clavicle directly to the acromion. The second, stronger set, the coracoclavicular (CC) ligaments, anchors the clavicle to the coracoid process on the scapula. These CC ligaments provide vertical stability for the outer end of the clavicle.
An injury occurs when the force of impact disrupts these stabilizing structures. If the damage compromises the CC ligaments, the shoulder girdle drops due to gravity and muscle pull. Since the clavicle remains connected to the chest wall, it shifts upward, creating the visible step-off deformity or shoulder bump.
Injury Grading Determines Bump Persistence
The persistence of the AC joint bump is determined by the degree of ligament damage, categorized by a six-level grading system.
Grade I
Grade I injuries involve only a mild sprain or stretching of the AC ligaments, with no clavicle displacement. The bump seen in these instances is due to localized swelling and bruising, which resolves completely as inflammation subsides over a few weeks.
Grade II
Grade II injuries involve a complete tear of the AC ligaments, but the stronger CC ligaments are only stretched. This results in a slight, partial separation, causing a small bump that may diminish significantly but sometimes leaves a subtle residual prominence. The surrounding joint capsule scars down, providing enough stability to hold the joint near its proper position.
Grade III and Higher
Grade III injuries involve a complete tear of both the AC and CC ligaments. With the main vertical stabilizers fully compromised, the clavicle displaces significantly upward, resulting in a prominent, unmistakable bump. This bump is considered a permanent structural change because the torn ligaments cannot naturally reduce the displacement. More severe types (Grades IV through VI) involve greater displacement or displacement in unusual directions and universally result in a large, permanent bump without surgical intervention.
Non-Surgical Management and Recovery Timeline
The majority of AC joint separations (Grades I, II, and many Grade III injuries) are managed successfully without surgery. Initial treatment focuses on pain management and protecting the joint. This typically involves applying ice to minimize swelling and using a sling briefly to immobilize the arm.
Patients begin rehabilitation aimed at restoring movement and strength as soon as pain allows. Recovery timelines vary: Grade I injuries return to full activity within one to two weeks, and Grade II injuries take three to six weeks. Functional recovery for non-surgically treated Grade III injuries can be achieved within six to twelve weeks. The goal of this conservative approach is to regain pre-injury function, even if the structural bump remains visible.
When Surgical Reduction is Necessary
Surgical intervention is generally reserved for high-grade separations (Grades IV, V, and VI) due to the severity of the displacement, or for Grade III injuries that result in persistent, debilitating symptoms. If a Grade III injury causes continued pain, weakness, or nerve issues that interfere with a person’s work or athletic demands, an operation may be considered. For high-level athletes or individuals with physically demanding jobs, surgery is sometimes recommended upfront to ensure the best possible functional outcome.
The primary goal of the operation is functional restoration by stabilizing the clavicle back into its correct anatomical position. This is accomplished by reconstructing or repairing the torn CC ligaments, often using high-strength sutures or a tendon graft. While the surgery is performed to improve shoulder mechanics and alleviate pain, a secondary outcome is the cosmetic reduction of the visible bump. A successful surgical reconstruction offers the best chance to restore the shoulder’s original contour and long-term stability.