Bones of the Pectoral Girdle, Acromioclavicular & Sternoclavicular Joints

The Shoulder Girdle

• Evolutionary change in function of upper limb from one of locomotion to a tool for manipulation
• Upper Limb attached to trunk via Pectoral Girdle (compare with Pelvic Girdle for lower limb) consisting of two freely mobile bones 
• Clavicle
• Scapula
• Sole true articulation between Upper Limb and Trunk is via Sternoclavicular Joint 

The Clavicle

• Lies horizontally, acting as a strut to hold the Upper Limb away from the body
• Medial (sternal) end is convex anteriorly 
• Lateral (acromial) end is concave anteriorly
• Note identifiable and palpable bony points

The Scapula

• Large flat bone situated on the posterior thoracic wall
• Note the glenoid fossa (for the shoulder joint)
• Note subscapular fossa (on costal surface) and 
• Spine of scapula dividing the supraspinous & infraspinous fossae (on dorsal surface)
• Scapula and clavicle articulate via the lateral end of the clavicle and the acromion process (see facet)

The Humerus

• Typical long bone
• Comparable with femur
• Note anatomical and surgical neck of humerus and the Intertubercular sulcus

Joints of the Shoulder Girdle

• Note differences between pectoral and pelvic girdles - relate to respective function
• Pelvic girdle - rigid and very stable poor for mobility
• Pectoral girdle - movable giving increased range to the Upper Limb
• This is achieved by the presence of joints within the pectoral girdle and between the girdle and the axial skeleton
• The Sternoclavicular
• and the Acromioclavicular Joints

The Sternoclavicular Joint

• Sole true articulation between Upper Limb & Axial Skeleton - Synovial, saddle joint.
• Between sternal end of the clavicle - larger of the 2 surfaces (convex superiorly - inferiorly concave antero-posteriorly) and  sternal facet - on manubrium of sternum above facet for 1^st rib (reciprocally concavo-convex)
• All surfaces covered with fibrocartilage 
• Poor congruency of facets improved by intra-articular disc - attached firmly to clavicle above and to 1^st costal cartilage below
• Blends with, the capsule peripherally and thinnest centrally (occasionally perforated)...(angled at 45 degrees)

The SC Joint

• Functions of the disc:
• i) Improve congruency
• ii) Shock absorption and weight dissipation,
• iii) Provide stability to the S/C joint (prevents the clavicle from sliding superiorly and medially along the sternal, facet)
• Capsule - Attached to articular margins - Very strong except inferiorly
• Synovial Membrane - Lines the capsule in the 2 separate cavities
• Attached to articular margins and to the margins of the disc

Ligaments of the SCJ

• 4 ligaments to consider: 

• Anterior Sternoclavicular ligament - Thickening in anterior capsule
• from the superior and anterior part of the clavicle running obliquely to the anterior aspect of the manubrium (blends with the tendon of Sterno-cleido-mastoid) 
• Posterior Sternoclavicular Ligament - Weaker than anterior SCL
• Reinforces posterior capsule, mirroring the anterior SCL
• Interclavicular Ligament - As name suggests passes from clavicle to clavicle
• Reinforces both superior capsules
• Some fibres attach to the superior aspect of the manubrium as they pass over it.
• Costoclavicular Ligament
• • Very strong ligament made up of 2 layers which are separated by a bursa
  • Anterior and posterior fibres are orientated obliquely to each other
  • Inferiorly attach to the 1^st rib and adjacent part of costal cartilage
  • Superiorly attaches to the roughened area on the inferior aspect of the medial, clavicle
  • Functionally very important primarily limiting elevation, of the clavicle but also playing a role in, preventing anterior and posterior displacement

Movements of the SCJ

• Elevation and depression – describes movement of lateral end of clavicle
• Axis of motion through the costoclavicular ligament (see saw effect)
• Protraction and retraction - Axis passes vertically through costoclavicular ligament
• Axial rotation
• Axis is through the length of the clavicle
• Occurs passively as a result of rotation, of the scapula causing increased tension, in, coracoclavicular ligament
• (usually in conjunction with lateral rotation during arm elevation)

The Acromioclavicular Joint

• Synovial plane joint between lateral end of clavicle and tip of the acromion process
• Articular surfaces:
• Oval facets on respective bones
• Clavicular facet is usually flat but may be convex
• Acromial facet is reciprocally shaped
• Both surfaces are covered with fibrocartilage
• Capsule attaches to articular margins - strongest superiorly and lined with synovial membrane
• An intra-articular disc partly divides the joint - when present may extend down into the joint from the capsule above and is wedge shaped (Its function is to improve congruency of the joint surfaces)

Ligaments of the ACJ

• Thickenings in capsule are described as superior & inferior AC ligaments
• Fibres orientated parallel to capsular fibres - superior part is most important as it prevents superior displacement of 1ateral end of clavicle 
• Coracoclavicular Ligament:
• Very strong ligament uniting the lateral end of the clavicle to the coracoid process...
• Divided into...
• Conoid Ligament - Fan, shaped from root of coracoid process to conoid tubercle
• Trapezoid ligament - Stronger part - quadrilateral in shape from the roughened area on the superior surface of the coracoid to trapezoid line
• AC joint is prone to dislocation/trauma

The Coracoacromial Arch

• The Corocoid process, acromion and the coracoacromial ligament together form and arch known as the coracoacromial arch whose function is to spread the loading during upward movement of the humerus
• A number of structures pass under the coracoacromial arch and as such are prone to impingement during shoulder movements

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