Acromioclavicular Joint

• Joint classification:

  Synovial plane joint
• Axis of movement:

  Three degrees of freedom available (about 3 axes)
• Articular Surfaces:

  Reciprocally shaped oval facets on:

  • Lateral end of the clavicle (slightly convex)

  • Anteromedial tip of the acromion process of scapula (slightly concave)

  • Both surfaces covered with fibrocartilage

  • Wedge shaped intra-articular disc
• Joint Capsule:

  Loose, fibrous capsule attaching to joint margins
• Superior and inferior acromioclavicular ligaments

  • Thickenings of the capsule above and below the joint
• Coracoclavicular ligament

  • 2 parts (Conoid and Trapezoid)

  • Anchor lateral end of clavicle to coracoid process to prevent upward displacement
• Coracoclavicular ligament (2 parts)

  Conoid:

  • Posteromedial, fan-shaped

  • Attaches from ‘elbow’ of coracoid process to conoid tubercle on inferior aspect of clavicle

  • Restricts forwards movement of scapula
• Coracoclavicular ligament (2 parts):

  Trapezoid:

  • Anterolateral, flat quadrilateral band

  • Stronger than the conoid ligament

  • Attaches from superior surface of coracoid process to trapezoid line on inferior aspect of clavicle (anterolateral to the conoid tubercle).

  • Restricts backwards movement of scapula
• Scapular Elevation and Depression:

  Elevation: scapula gliding upwards (shrugging)

  Depression: scapula gliding downwards

  Linear range of the scapula is about 10 to 12 cm up and down
• Scapular elevation:

  • Acromion glides superiorly

  • Movement: about a sagittal (AP) axis

  • ROM: up to 15 (total range elevation + depression)

  • Limited by: tension in coracoclavicular ligaments
• Scapular depression:

  • Acromion glides inferiorly

  • ROM: up to 15 (total range elevation + depression)

  • Limited by: coracoid process coming into contact with under surface of clavicle
• Scapular Protraction and Retraction:

  Lateral and medial movement of the shoulder blade around the chest wall

  Protraction = glenoid fossa moves more anterior (rounding shoulders)

  Retraction = glenoid fossa moves more lateral (pulling shoulders back)

  Total range of linear translation of the scapula around the chest wall is about 15 cm
• Scapular protraction:

  • Acromion glides anteriorly

  • ROM: Minimal

  • Limited by: posterior AC joint capsule and both coracoclavicular ligaments
• Scapular retraction

  • Acromion glides posteriorly

  • ROM: Minimal

  • Limited by: anterior AC joint capsule and trapezoid ligament
• Scapular lateral rotation:

  • Acromion swings upward and outward in relation to clavicle at AC joint (ie. lateral rotation of acromion)

  • Axis of movement through conoid ligament and AC joint

  • ROM: up to 30 (total range lateral and medial rotation)

  • Limited by: Both parts of the coracoclavicular ligament
• Scapular medial rotation:

  • Acromion medially rotates back to original position

  • ROM: up to 30 (total range lateral and medial rotation)

  • Limited by: Both parts of the coracoclavicular ligament
• Scapulohumeral rhythm:

  Coordinated rhythm between scapula and humerus during shoulder flexion and abduction

  Initial 25 - 30 degrees of flexion/abduction from GH joint alone

  Beyond this, scapular lateral rotation occurs in conjunction with GH movement (1:2 ratio) to permit full ROM