The shoulder joint or Shoulder complex is unlike other joints of the body as it is comprised of 4 articulations: Glenohumeral Joint (ball and socket), Scapulothoracic Joint,Sternoclavicular joint and the Acromioclavicular Joint. It is a minefield region as it could be the area of referred pain from the Cervical spine (Neck).
the main joint of this complex is the Glenohumeral joint which is a ball and socket. The head of Humerus which is semispherical is convex in shape and is kept in close proximity to the concave Glenoid by the joint capsule, surrounding muscles and ligaments. The glenoid cavity is shallow compared to the head of the Humerus, for that the cavity is deepened by cartilage covering the rim of the cavity the Glenoid Labrum. Unlike the joints of the lower extremity which are designed to provide stability on the expense of mobility the joints of the upper extremity they sacrifice stability to provide mobility. That doesn’t mean that the joints of the upper extremity are not stable but their stability is determined by the structures surrounding the joints. For example, the Hip joint is a ball and socket joint same as the shoulder. The difference is that the bony structures of the hip are more stable as the Acetabulum is deep enough that the Head of Femur fits in it.
Stability of the joints is provided by passive and active mechanisms. The passive mechanism includes the joint capsule and the ligaments. The active mechanism is the action of the ROTATOR CUFF group (Supraspinatus, Infraspinatus, Teres Minor and Subscapularis muscles). The supraspinatus,infraspinatus and teres minor muscles originate from the posterior surface (back) of the scapula while the subscapularis muscle originates from the anterior (front) surface of the scapula. These four muscles that originate from the scapula play a major role in stability of the joint and in steering the motion of the head of humerus against the glenoid.
The capsule of the glenohumeral joint is a fibrous band covers the articular surfaces ( head of humerus and the glenoid ).
It contains a semi transparent fluid called “Synovial Fluid”. This fluid is responsible of nourishment, wastes transportation, lubrication and it helps in providing a smooth movement between the head of the humerus and the glenoid by reducing friction.
SCAPULOTHORACIC JOINT ( Scapulo = scapula which is the shoulder blade. Thoracic in referral to Thorax which is the chest wall).
It is a “pseudo joint” since the scapula doesn’t have a capsule or ligaments to attach the scapula to the chest wall. It is stabilized against the chest wall by muscles. Bulky muscles originating from the cervical and thoracic spine and from the chest wall attach to the scapula. The scapula also serve as the origin of muscles that are responsible of moving the upper extremity the ROTATOR CUFF group. The stability of the scapula which is provided by the muscles attached to it is very important since it influences the movement of the arm and hand in the space. As the arm abducts ( elevation of the hand on the side of the body) to 180 degrees 120 degrees takes place at the GHJ while 60 degrees of abduction is provided by the scapular upward rotation forming what is known as SCAPULOHUMERAL RHYTHM. This rotation of scapula is carried on by the upward rotation of the Acromiocalvicualr Joint ACJ and elevation of the Sternoclavicular Joint SCJ ( explained below).
Normally the resting position of the scapula is described as the following: medial border of scapula is 3 finger widths away from the spine, the superior angle is at the level of the second Thoracic vertebra, the spine of scapula is at the level of the 3rd thoracic vertebra and the inferior angle is at the level of 9th rib (between Thoracic vertebrae 8 & 9).
Movements of the scapula are elevation (moving upward), depression (moving downward), upward rotation (when the inferior angle rotates up), downward rotation ( when the inferior angle moves down), retraction ( movement of the medial border towards the spine) and protraction (movement of the medial border away from the spine).
Poorly placed scapula on the chest wall will compromise the biomechanics of the shoulder thus resulting in dysfunction and pain. Muscular imbalance is what causes the scapula to lose the normal resting position. This is clearly seen in patients with Winged scapula (scapula alata), retracted, protracted, depressed or elevated scapula.
ACROMIOCLAVICULAR JOINT (ACJ): ( acrom- = topmost).
It is the joint between the acromion ( the lateral expansion of the spine of scapula) and the clavicle (collar bone). The collar bone attaches the shoulder to the axial skeleton via the Sternoclavicular Ligament SCL. The joint is covered by the joint capsule and is supported by the superior and inferior Acromioclavicular Ligaments. The ACJ and SCJ play an important role in the upward rotation of the scapula during full abduction ( described in the scapulothoracic joint section).
The subacromial space is the space between the Acromion and the Head of Humerus. This space is vital as many structures run through it. Subacromial Bursa ( gel like structure situated between the bone and the tendon to ensure a smooth sliding of the tendon and minimizing friction), Supraspinatus tendon and long head of biceps are the “residents of this space”. If this space is compromised these structures get squeezed causing what is known as Impingement syndrome. (read more about it in the impingement syndrome section below).
The ACJ is a joint subjected to subluxation and dislocation when someone falls on outstretched hand, direct fall on the shoulder or after a direct blow to the shoulder. The degree of joint damage is determined by the damage of the joint capsule, ligaments and muscles attached to it. an x-ray can show if there is a subluxation or dislocation of the AC joint.
STERNOCLAVICULAR JOINT: SCJ
The SCJ is the joint attaching the medial end of the clavicle to the Manubrium Sterni ( the upper part of the sternum to which the ribs are attached in the middle of the chest). This joint attaches the shoulder to the axial skeleton and provides the axis necessary for the shoulder movements. As mentioned in the ACJ section The SCJ along with the ACJ help to provide the 60 degrees of the upward rotation of the scapula.