AC Joint Sprains

AC joint injuries involve damage to ligaments connecting the clavicle to the acromion. These range from sprains to complete separations. The joint is superficial and vulnerable to direct trauma.

The acromioclavicular joint functions as a critical link in the kinetic chain transferring forces between your arm and axial skeleton. Despite its small size, this joint must accommodate substantial loads during upper extremity activities while allowing the scapula to rotate and translate relative to the clavicle. The AC joint's stability depends on both intrinsic capsular ligaments and extrinsic coracoclavicular ligaments (conoid and trapezoid), with each system serving distinct mechanical functions.

The extracapsular coracoclavicular ligaments provide primary vertical and compressive stability to the AC joint. These ligaments, connecting the coracoid process of the scapula to the inferior surface of the clavicle, must resist the downward pull of gravity on your arm. When you hold a weight at your side, the scapula wants to rotate downward and separate from the clavicle. The coracoclavicular ligaments prevent this separation, experiencing tensile loads proportional to arm weight. Cadaveric studies show these ligaments act as the primary restraint to vertical and superior displacement, explaining why high-grade AC separations involve coracoclavicular ligament rupture.

The capsular ligaments surrounding the AC joint itself provide stability in the anteroposterior and horizontal planes. During arm movements requiring scapular protraction or retraction, these ligaments resist horizontal translation of the clavicle relative to the acromion. Biomechanical studies of bench press technique show that wide grip positioning increases anteroposterior shear across the AC joint, stressing the anterior and posterior joint capsule. This explains why bench pressing, particularly with poor technique, frequently aggravates AC joint pathology.

Direct trauma mechanisms create the highest risk for acute AC joint injury. When you fall directly onto the point of your shoulder with your arm adducted to your side, the ground reaction force drives the acromion inferiorly while your torso's inertia maintains clavicle position. This creates a separation force that first damages the AC joint capsule (Grade I injury), then the AC ligaments (Grade II), and finally the coracoclavicular ligaments (Grade III or higher). Higher-energy impacts cause progressively more severe ligamentous failure, with the exact injury grade depending on force magnitude and direction.

Overhead activities create repetitive microtrauma to the AC joint through compressive and shear loading. Each time you raise your arm overhead, your scapula must rotate upward approximately 60 degrees. This rotation compresses the AC joint surfaces together while creating anteroposterior shear as the acromion translates relative to the clavicle. Athletes performing hundreds of overhead repetitions daily, such as swimmers, volleyball players, and CrossFit athletes, accumulate thousands of compression cycles weekly. Studies tracking overhead athletes show progressive AC joint degeneration correlating directly with training volume, with joint space narrowing visible on X-rays after 5-7 years of high-volume training.

CrossFit training and bench press variations create specific mechanical stresses on the AC joint. During wide-grip bench press, when your hands are positioned outside shoulder width, the AC joint experiences higher compressive forces as the clavicle and acromion are forced together. Biomechanical analysis of bench press technique shows that wide-grip positioning increases AC joint compression compared to a narrower grip, while also increasing shear forces during the eccentric lowering phase. This combination of compression and shear explains why powerlifters and CrossFit athletes show disproportionately high rates of AC joint pathology.

Degenerative AC joint arthritis develops through cumulative mechanical stress over decades. Unlike acute traumatic injuries, degenerative changes result from repeated low-to-moderate loads that gradually erode joint cartilage. Each arm elevation or cross-body movement creates small amounts of cartilage wear. Over years, this cumulative microtrauma leads to cartilage thinning, subchondral bone exposure, and osteophyte formation. Research using high-resolution CT imaging demonstrates that AC joint degeneration shows dose-response relationship with lifetime overhead activity, with manual laborers and overhead athletes showing degeneration 10-15 years earlier than sedentary individuals.

Scapular dyskinesis alters AC joint loading patterns by disrupting normal scapulohumeral rhythm. When your scapula doesn't move properly during arm elevation due to muscle imbalances or previous injuries, the AC joint must compensate by allowing greater translation or experiencing abnormal compression patterns. Studies of scapular kinematics show that individuals with scapular dyskinesis place altered loads on the AC joint during overhead reaching compared to those with normal scapular mechanics. This altered loading accelerates degenerative changes and increases acute injury risk during traumatic events.