Glenohumeral (shoulder) joint

Location: At the shoulder, between the head of the humerus and the glenoid cavity on the lateral angle of the scapula, under the acromion and the coraco-acromial arch.

Bones taking part

• Scapula (glenoid cavity)
• Humerus (head)

Articulating surfaces

• The rounded head of the humerus, covered with hyaline cartilage
• The shallow, pear-shaped glenoid cavity of the scapula, covered with hyaline cartilage and deepened by the fibrocartilaginous glenoid labrum
• The humeral head is much larger than the glenoid cavity, so only part of it is in contact with the glenoid at any time.

Structural

Synovial joint (ball-and-socket type).

Functional

Diarthrosis (freely movable).

Synovial subtype

Ball-and-socket (spheroidal).

Axes

Multiaxial: movement occurs about three axes (flexion-extension, abduction-adduction, medial-lateral rotation).

Capsule & cavity

A loose fibrous capsule. Medially it attaches to the margin of the glenoid cavity outside the labrum, and above to the supraglenoid tubercle so that it encloses the origin of the long head of biceps; laterally it attaches to the anatomical neck of the humerus, except inferomedially, where it extends a little way down onto the shaft. It is loose enough to let the bones be pulled apart a little, and its inferior part, which hangs as a fold (axillary recess) when the arm is by the side, is the weakest. It has openings for the tendon of the long head of biceps and for communication with the subscapular bursa. The rotator cuff tendons blend with and reinforce it.

• Places the hand in a very wide range of positions in space.
• Transmits forces between the upper limb and the trunk, through the scapula.
• Provides a mobile base for elbow, forearm and hand function.

Stability vs mobility: Mobility is gained at the expense of stability. The bones contribute little because the glenoid is small and shallow. Stability comes mostly from the rotator cuff muscles (supraspinatus, infraspinatus, teres minor, subscapularis), whose tendons fuse with the capsule and hold the head in the glenoid; then the labrum, the capsule and its ligaments, the long head of biceps, and the coraco-acromial arch above. The inferior side has no rotator cuff support, so it is the weakest.

• Dislocation of the shoulder is common, and is usually anterior (the humeral head comes to lie in front of and below the glenoid) because the inferior and anterior capsule is weakest.
• The axillary nerve passes close to the inferior capsule and can be injured in dislocation or in a fracture of the surgical neck of the humerus, weakening deltoid.
• Rotator cuff tears, most often of the supraspinatus tendon, cause pain and weakness when raising the arm.
• Adhesive capsulitis ('frozen shoulder') is a painful stiffening of the capsule that limits movement in all directions.
• Tears of the glenoid labrum can cause pain, catching and instability.

• Classify it fully: synovial, ball-and-socket, multiaxial.
• Explain the stability-mobility trade-off and name the rotator cuff as the chief stabiliser.
• Describe the capsule attachments and why the inferior part is weakest.
• Name the intracapsular tendon: long head of biceps (inside the capsule, outside the synovial membrane).
• Distinguish glenohumeral movements from scapular movements; know that full overhead elevation needs scapular rotation.
• Know that anterior dislocation is the commonest type and the axillary nerve is at risk.

• Saying ligaments are the main stabilisers. At the shoulder, the rotator cuff muscles are.
• Calling shoulder shrugging a glenohumeral movement; it is elevation of the scapula.
• Attaching the capsule laterally to the surgical neck; it attaches to the anatomical neck.
• Saying the long head of biceps tendon lies inside the synovial cavity; it is inside the capsule but enclosed in its own synovial sleeve.
• Forgetting that the inferior part of the capsule is the weakest and the usual route of dislocation.