Osteology of the Shoulder Notes
Shoulder Osteology
Overview of Shoulder Joints
The shoulder complex includes four primary joints:
Glenohumeral (GH) joint
Scapulothoracic joint
Acromioclavicular joint
Sternoclavicular joint
The glenohumeral joint is formed by the humerus and the glenoid fossa of the scapula. The clavicle is situated on the anterior side, and the anterior border of the scapula is also visible.
Scapula
Posterior View
The spine of the scapula runs from the medial root to the lateral aspect of the acromion. This spine divides the posterior scapula into:
Supraspinatus fossa: Located above the spine, housing the supraspinatus muscle.
Infraspinatus fossa: Located below the spine, housing the infraspinatus muscle.
Muscle attachments on the posterior scapula include:
Teres minor and teres major: Attach to the lateral border.
Rhomboids major and minor, and levator scapula: Attach to the medial border.
Anterior View
The anterior aspect of the scapula features the:
Subscapularis fossa: Housing the subscapularis muscle.
Serratus anterior attachment: Located on the medial anterior border.
Coracoid Process
A bony protrusion superior to the glenoid fossa, serving as the attachment site for the short head of the biceps. The long head of the biceps attaches superior to the glenoid fossa.
Glenohumeral Joint
Humerus
The humerus consists of:
Shaft
Neck
Head: The articular surface that articulates with the glenoid fossa.
Greater and lesser tubercles: Bony prominences serving as attachment sites for muscles, including rotator cuff muscles (supraspinatus, infraspinatus, and subscapularis).
Glenoid Fossa
The articular surface of the glenoid fossa covers only about 1/4 of the humeral head's articular surface. This size difference allows for a wide range of motion but also less stability.
Glenohumeral Ligaments
Three fibrous bands located inside the joint capsule that radiate laterally and inferiorly from the labrum and glenoid. They inhibit anterior translation of the humeral head.
Coracohumeral Ligament
Runs from the coracoid process to the greater tubercle of the humerus. It envelops the supraspinatus and subscapularis tendons, providing additional stabilization within the subacromial space.
Transverse Humeral Ligament
Runs from the greater to the lesser tubercle and holds the tendon of the biceps brachii in place during glenohumeral joint movement.
Coracoclavicular Ligament
Composed of the trapezoid and conoid ligaments, it prevents superior subluxation of the clavicle during GH joint movements.
Coracoacromial Ligament
Forms a protective arch over the humeral head, preventing superior displacement from the glenoid. It also forms the superior aspect of the subacromial space.
Scapular Orientation
The scapula is positioned anteriorly from the frontal plane at an angle of approximately to degrees. This is best viewed from a superior perspective. This natural positioning of the scapula is referred to as the scapular plane.
Resting Position of the Humerus
At rest, the glenoid fossa faces about degrees anterior to the frontal plane, defining the scapular plane. The humerus naturally elevates in this plane. In anatomical position, the humeral head is directed medially, superiorly, and posteriorly due to its retroversion, aligning it with the glenoid fossa. The normal resting position of the humerus involves some degree of internal rotation.
Subacromial Space
The subacromial space is located under the coracoacromial ligament and above the humeral head. It contains:
Supraspinatus muscle and tendon
Subacromial bursa
The subacromial bursa is situated between the acromion, coracoacromial ligament, and deltoid superiorly, and the supraspinatus tendon and joint capsule of the GH joint inferiorly. Its function is to facilitate movement of the supraspinatus tendon under the coracoacromial arch and of the deltoid over the joint capsule and greater tubercle. This space is a closed anatomical compartment and can become inflamed (subacromial bursitis).