Classification
- Type: Synovial plane (gliding)
- Degrees of freedom: 3 (gliding with rotation — allows the scapula to tilt, rotate, and wing relative to the clavicle)
- Region: Shoulder complex (along with the sternoclavicular, glenohumeral, and scapulothoracic articulations)
Articular Surfaces
- Lateral end of the clavicle (flat to slightly convex): A small oval facet on the distal clavicle. Covered with fibrocartilage. The clavicular facet faces laterally and slightly inferiorly.
- Medial facet of the acromion (flat to slightly concave): A matching small facet on the anteromedial acromion. Also covered with fibrocartilage.
- Intra-articular disc: A fibrocartilaginous disc (meniscoid) is present in many AC joints but is highly variable — it may be a complete disc, partial disc, or absent. The disc degenerates early (often by the fourth decade) and is frequently absent in older adults. When present, it improves congruence and absorbs compressive forces.
- Joint inclination: The AC joint surfaces vary considerably in orientation between individuals — from nearly vertical to significantly undercut. This variation partly explains why some individuals are more susceptible to AC separation.
Movements and ROM
| Movement |
Normal ROM |
Plane |
Mechanism |
| Anterior-posterior glide |
5–8 mm |
Sagittal |
The clavicle glides anteriorly and posteriorly on the acromion during protraction/retraction |
| Rotation (scapula on clavicle) |
~20° |
Multiple |
The scapula rotates relative to the clavicle during arm elevation — allows upward scapular rotation |
| Tilt |
Variable |
Sagittal |
Anterior and posterior scapular tilt relative to the clavicle |
AC motion is subtle but essential. The AC joint allows the scapula to adjust its position relative to the clavicle during arm elevation. Without AC mobility, the scapula cannot fully upwardly rotate, limiting overhead reach.
Capsular Pattern
Pain at extremes of range, especially full horizontal adduction and full elevation
The AC joint does not have a clearly defined proportional capsular pattern like the GH joint. Capsular restriction presents as pain at end-range horizontal adduction (cross-body reach) and end-range elevation.
Resting Position
- Arm resting at the side
- Capsule and ligaments in neutral tension
Close-Packed Position
- 90° arm abduction
- Ligaments taut, joint surfaces maximally congruent
End-Feels
| Movement |
Normal End-Feel |
Type |
| Horizontal adduction |
Capsular (firm) |
AC ligaments and posterior capsule stretched; may also include tissue approximation |
| AP glide |
Capsular (firm) |
AC ligaments provide end-range resistance |
Ligaments
Acromioclavicular Ligament
- Attachments: Superior and inferior surfaces of the AC joint — capsular thickenings bridging the acromion and lateral clavicle
- Function: The superior AC ligament is the primary restraint against posterior translation and small amounts of superior displacement. Resists horizontal (anteroposterior) displacement.
- Injury mechanism: Direct fall onto the point of the shoulder (acromion driven inferiorly while the clavicle is held by the trapezius). This is the classic AC separation mechanism. Grade I and II sprains primarily involve the AC ligament.
- Grade classification: Grade I — AC ligament sprain, intact CC ligaments, no displacement; Grade II — AC ligament torn, CC ligaments sprained but intact, mild superior clavicle prominence; Grade III — both AC and CC ligaments torn, obvious superior clavicle prominence ("step deformity")
- Assessment test: Cross-body adduction (horizontal adduction stress test) — pain at the AC joint is specific for AC pathology. AC shear test (squeeze test). Palpation of the AC joint line for tenderness and step deformity.
- Condition link: AC joint sprain/separation (Rockwood classification Types I–VI)
Coracoclavicular (CC) Ligament
- Attachments: Two components — conoid ligament (medial, cone-shaped): coracoid process → conoid tubercle on the inferior clavicle; trapezoid ligament (lateral, flat): coracoid process → trapezoid line on the inferior clavicle
- Function: The primary structural stabilizer of the AC joint. The CC ligament suspends the scapula from the clavicle. The conoid resists superior displacement and anterior rotation; the trapezoid resists axial compression and lateral displacement. Together they prevent the scapula from dropping away from the clavicle.
- Injury mechanism: Same mechanism as AC ligament but with greater force — the acromion is driven far enough inferiorly to rupture the CC ligament (Grade III and above). Complete CC disruption produces the classic "piano key sign" where the lateral clavicle can be depressed and springs back up.
- Assessment test: Piano key sign (depress the elevated lateral clavicle — it springs back up when released). Weighted radiograph (clavicle displaces superiorly under load when CC ligaments are torn).
- Condition link: AC separation Grade III+ (Rockwood Types III–VI)
Coracoacromial Ligament
- Attachments: Coracoid process → anterior acromion
- Function: Not a true AC joint ligament but functionally related — forms the coracoacromial arch that prevents superior humeral head migration. Does not stabilize the AC joint itself but is clinically relevant in subacromial impingement.
- Condition link: conditions/subacromial-impingement
Mobilization Techniques
Hands-on instruction is required. The descriptions below provide clinical reference detail for understanding and supervised practice. They are not a substitute for instructor-led technique training. Correct hand placement, force dosage, and tissue response interpretation require hands-on coaching and feedback.
Convex-Concave Rule at the AC Joint
The AC joint is essentially a plane joint with flat-to-slightly-curved surfaces. The convex-concave rule is less directly applicable than at ball-and-socket joints. Mobilization is performed by applying direct translatory glides in the restricted direction.
General Contraindications (All AC Mobilizations)
- Absolute: AC joint instability or separation (Grade II+ with laxity), acute fracture (distal clavicle or acromion), active joint infection, malignancy
- Relative: Acute AC joint sprain (Grade I — mobilize gently only after acute phase), OA with significant osteophyte formation, osteoporosis
Anterior-Posterior AC Glide
Purpose: Restores AP accessory motion at the AC joint. Indicated when horizontal adduction is limited by AC joint stiffness. Also useful for AC joint OA with stiffness.
Patient position:
- Seated or supine
- Arm at the side or supported in slight abduction
Hand placement:
- Stabilizing hand: Cups the acromion posteriorly, stabilizing the scapula. Alternatively, the clinician's hand wraps over the shoulder with the fingers posterior to the acromion.
- Mobilizing hand: Thumb pad contacts the anterior surface of the lateral clavicle, just medial to the AC joint line. Force directed posteriorly.
Technique execution:
- Apply a slow, sustained or oscillatory force directed posteriorly on the clavicle while the acromion is stabilized
- Grade I–II: Small oscillations for pain modulation — appropriate in early OA or post-injury stiffness
- Grade III–IV: Oscillations into the end-range posterior resistance. The posterior capsule and AC ligament provide the barrier.
- Duration: 20–30 seconds per set, 2–3 sets
- Reassess horizontal adduction between sets
Indications:
- Decreased AP accessory motion at the AC joint on joint play testing (compare bilaterally)
- Painful and limited cross-body reach (horizontal adduction) with AC joint line tenderness
- AC joint OA with stiffness
Technique notes:
- Common error: Mobilizing the entire shoulder girdle rather than isolating AC motion. Stabilize the acromion firmly.
- Common error: Pressing on the AC joint itself rather than on the clavicle — joint line pressure is painful and not therapeutic.
- Reassessment: Re-test horizontal adduction (cross-body reach). Pain-free improvement confirms AC involvement.
Inferior AC Glide
Purpose: Restores inferior accessory motion. Indicated when arm elevation is limited by AC stiffness and the SC joint has been ruled out as the restriction.
Patient position:
- Seated or supine
- Arm at the side
Hand placement:
- Stabilizing hand: Placed on the superior scapula/acromion region
- Mobilizing hand: Thumb or thenar eminence contacts the superior surface of the lateral clavicle, just medial to the AC joint line. Force directed inferiorly (caudally).
Technique execution:
- Apply a slow, sustained or oscillatory force directed inferiorly
- Grade I–II: Gentle oscillations for assessment and pain modulation
- Grade III: Oscillations into the inferior barrier — the inferior capsule and CC ligaments provide resistance
- Duration: 20–30 seconds per set, 2–3 sets
Indications:
- Decreased inferior accessory motion at the AC joint
- Elevation limitation that does not fully respond to GH and SC mobilization
- AC joint OA with superior clavicle prominence and stiffness
Technique notes:
- Important: Ensure the CC ligaments are intact before applying inferior force. If a Grade II+ separation is present, inferior mobilization could worsen the instability.
- Reassessment: Re-test arm elevation. AC joint contribution to elevation loss is typically modest (5–10°).
Muscles Crossing This Joint
No muscles cross the AC joint directly. The following muscles attach near the AC joint and affect its mechanics:
Conditions Affecting This Joint
- AC joint separation/sprain — Rockwood Types I–VI; the most common traumatic AC condition; classified by degree of AC and CC ligament disruption
- conditions/osteoarthritis — AC joint OA is extremely common, especially in weight lifters and manual laborers; may present as a painful arc at end-range elevation or painful cross-body reach
- conditions/subacromial-impingement — AC osteophytes projecting inferiorly can narrow the subacromial space, contributing to mechanical impingement
- Distal clavicle osteolysis — erosion of the lateral clavicle from repetitive microtrauma (bench press, overhead sports); pain with cross-body adduction
Clinical Notes
- AC joint pathology mimics GH pathology. Both produce shoulder pain with elevation and horizontal adduction. The differentiating maneuver is the cross-body adduction test — pain localized to the AC joint line (not deep in the shoulder) is specific for AC pathology. Palpation of the AC joint line is also highly informative.
- AC joint degeneration begins early. The intra-articular disc degenerates by the fourth decade, and radiographic OA is common by the fifth decade. Many asymptomatic individuals have significant AC joint degeneration on imaging.
- Osteophytes from AC OA can cause impingement. Inferior osteophytes projecting into the subacromial space mechanically narrow the space and contribute to rotator cuff impingement. When impingement does not respond to conservative treatment, inferior AC osteophytes should be considered.
- The CC ligaments are the key stabilizers, not the AC ligaments. In assessment, the AC ligaments resist horizontal motion while the CC ligaments resist vertical displacement. The "piano key sign" (depressing the lateral clavicle and watching it spring back) indicates CC disruption — this is a Grade III+ injury.
Key Takeaways
- The CC ligament (conoid + trapezoid) is the primary stabilizer — its disruption produces the classic step deformity and piano key sign of Grade III+ AC separation.
- Cross-body adduction (horizontal adduction) is the key differentiating test — AC joint line pain during this maneuver distinguishes AC pathology from GH pathology.
- AC joint OA is common and can contribute to subacromial impingement via inferior osteophytes narrowing the subacromial space.
- Always assess the AC joint as part of the shoulder complex — it is frequently the overlooked source of persistent shoulder pain with overhead activities.
