Abstract
Shoulder problems account for a significant percentage of musculoskeletal disorders that present to the primary care provider. The anatomy of the shoulder is complex, and its extreme range of motion, inherent instability, and multiple pain generators can create significant diagnostic challenges for the primary care provider. Diagnosis and appropriate treatment can easily be attained with knowledge of the relevant anatomy and proper correlation with historical information presented at the time of evaluation. A working knowledge of shoulder anatomy and biomechanics is essential to providing optimal care for patients with shoulder pain complaints.
Keywords
shoulder, shoulder anatomy, shoulder biomechanics, glenohumeral (GH) joint, acromioclavicular joint, rotator cuff
Key Concepts
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Shoulder problems account for a significant percentage of musculoskeletal disorders that present to the primary care provider.
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The anatomy of the shoulder complex with its extreme range of motion, inherent instability, and multiple pain generators can lead to significant diagnostic challenges for the primary care provider.
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Diagnosis and appropriate treatment can easily be attained with knowledge of the relevant anatomy and correlation history and physical examination.
Bones
Clavicle
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S -shaped bone with convex anterior border medially, concave anterior border laterally.
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Only bone to connect the upper extremity to the axial skeleton.
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Functions as a strut to maintain distance between shoulders by opposing gravity and muscle action.
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Stabilizes the glenohumeral (GH) joint and upper extremity anteriorly and protects the medial brachial plexus.
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Eighty percent of fractures occur at the medial and lateral junction secondary to inherent weakness in this area.
Scapula
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Flat, triangular bone composed of the body, spine, acromion, glenoid fossa, scapular neck, and coracoid process.
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Provides numerous sites for muscular attachment.
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Approximates with posterolateral aspect of thorax between the second and seventh ribs.
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Body
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Posterolateral aspect of thorax between the second and seventh ribs.
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Functions mainly to serve as site for muscle attachment.
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Spine
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Posterior crest that divides the scapula body and ends laterally at the flattened acromion.
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Separates scapula into supraspinous and infraspinous fossae.
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Acromion Process
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Lateral extension of scapular spine that articulates with distal end of the clavicle.
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Glenoid Fossa
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Inverted, comma-shaped, to articulate with humerus
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Slightly concave fossa covered with hyaline cartilage
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Has a larger radius of curvature than the humerus
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Scapular Neck
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Constriction of scapula medial to glenoid fossa
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Coracoid Process
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Hooked, bony projection off the anterior surface, medial to the scapular neck, which projects anteriorly and laterally
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Proximal Humerus
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Nearly spherical humeral head that is inclined 130 to 150 degrees in relation to shaft, with 20 to 30 degrees of retroversion
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One-third has articular cartilage directed medial, superior, and posterior.
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Articulates with the glenoid fossa of the scapula
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The greater and lesser tuberosities are sites for muscular insertion of the supraspinatus and subscapularis muscles, respectively.
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The intertubercular (bicipital) groove between tuberosities houses the long head of the bicep tendon.
Joints and Ligaments
Sternoclavicular Joint
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Only articulation between upper limb and axial skeleton.
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Joint has ball and socket qualities, although least stable of all major joints.
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Allows clavicle 35 degrees of elevation, 35 degrees of forward/backward motion, and 50 degrees of rotation.
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The costoclavicular, interclavicular, and sternoclavicular ligaments and the intra-articular fibrocartilage disc provide stability.
Acromioclavicular Joint
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Only articulation between clavicle and scapula.
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Transmits forces from extremity to axial skeleton and supports upper extremity, allowing for 40 degrees of rotation during arm elevation.
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Superior and inferior acromioclavicular ligaments control anterior and posterior stability, providing principal restraints to axial and posterior translation.
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Coracoclavicular ligaments include the lateral trapezoid and medial conoid ligaments, limiting anterior and posterior scapular rotation, respectively.
Glenohumeral Joint
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Greatest mobility of all joints, although stability sacrificed to maintain range of motion
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Stability conferred by interaction of bony, ligamentous, and muscle constraints
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0.3 mm of superior/inferior translation, 5 mm of anterior/posterior translation
Glenoid
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Bony base from lateral scapula
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Thicker peripheral articular cartilage allows for “concavity compression” (suction) effect to enhance stability.
Labrum
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Ring of dense fibrous triangular shaped tissue that deepens glenoid by 2.5 mm, with thickening of periphery to increase joint congruity and stability
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Attachment site of GH ligaments and long head of biceps tendon
Glenohumeral Capsule
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Twice the surface area of the humeral head, lined with synovium
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Limited joint fluid volume, resulting in negative intra-articular pressure, which assists with joint stability
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Acts as “check rein” for GH articulation when placed under tension near extremes of range of motion
Glenohumeral Ligaments
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Composed of the superior, medial, and inferior GH ligament
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Collagenous thickening of shoulder capsule, providing static stability to the GH joint
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Assists in controlling anterior/posterior and superior/inferior translation of the humerus
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Primary stabilizer at extremes of shoulder motion
Coracohumeral Ligament
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Static suspensory function for the humeral head and glenoid with arm at side
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Provides restraint to inferior translation and reinforces rotator cuff interval
Coracoacromial Ligament
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Contributes to the roof of the GH joint
Transverse Humeral Ligament
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Connects greater and lesser tuberosities
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Maintains long head of bicep tendon in the intertubercular groove
Scapulothoracic Articulation
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Provides for movement of scapula on chest wall, although not a true joint
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