The main joint of our upper limb is the shoulder joint which can be moved in various positions when looked at in a three-dimensional perspective. In order to be able to have these movements, many other components help in order to maintain a stable shoulder. In short, there is a complex interplay between the shoulder joint, other joints, muscles and ligaments that make the shoulder a complex and unique part of our body.
Anatomy of the Shoulder Complex
The Shoulder complex consists:
- The true joint called the Shoulder joint (Glenohumeral joint – GH)
- The Clavicular joint with the scapula (Acromioclavicular joint – AC)
- The Scapular joint with the body wall (Scapulothoracic joint – ST)
- The Clavicular joint with the breastbone sternum (Sternoclavicular joint – SC).
The shoulder joint (GH) is made of two main bones that articulate with each other forming the ball and socket joint. The ball of the arm bone(humerus) and the glenoid cavity of the shoulder blade(scapula) is articulated at the shoulder joint (GH joint). Similarly, on the inner chest, the clavicle articulates with sternum to form the SC joint while on the outer end towards the shoulder the clavicle articulates with the acromion process of the scapula bone to form the AC joint. Both GH, SC and AC are true joints with union by fibrous, cartilaginous or synovial tissues. Lastly the ST joint, while this is not a true bony joint, its muscular attachments create a shoulder joint complex.
The humeral head (ball) is about three times larger than the glenoid fossa. Actually, only 25 percent of the humeral head articulates with the glenoid fossa. Glenoid cavity (fossa) forms a very shallow socket as compared to the hip socket of the hip joint. Therefore, the humeral head articulates with a smaller open and shallow saucer- type of articulation, lacking stability in its own. However, it is with all the soft tissue structures both inside and outside the joint that are responsible for the overall stability of the arm during movements.
Soft tissue structures that support the Shoulder Joint
The important soft tissue structures are:
- Articular Cartilage
- Joint Capsule
A smooth, white tissue that covers the humeral head (ball) and the glenoid fossa to make it easier for the two bones to move at the joint. It allows the bones to glide over each other with very little friction.
Since the head(ball) of the upper arm bone is larger than the glenoid fossa, the articular cartilage forms a soft fibrous tissue rim called the labrum which surrounds the socket to help fit the head into it thus stabilizing the joint.
The socket can be divided into four regions namely anterior (front), posterior ( back), superior (the upper end near your head), and inferior (the lower end which is towards the elbow). Based on these regions the labrum is also called as superior, inferior, anterior and posterior labrum.
The shoulder joint capsule is a membranous sac that encloses the entire joint. The joint capsule of the shoulder is attached along the outside rim of the glenoid labrum of the glenoid cavity and attaches to the neck of the arm bone. The capsule by itself is quite loose and it is the surrounding reinforcement by the muscles, tendons, and ligaments that are largely responsible for keeping the shoulder joint stable.
In the shoulder, there is a group of ligaments that is responsible for the stability of the shoulder.
Glenohumeral Ligaments (GHL)
This ligament attaches from along the outer glenoid socket covering the joint to the upper part of the arm bone.
- Superior (upper) GHL
- Middle GHL
- Inferior (lower) GHL
Coraco-acromial Ligament (CAL)
This ligament attaches from the coracoid process to the acromion process of the shoulder blade (Scapula).
Coraco-clavicular Ligaments (CCL)
These two ligaments (trapezoid and conoid ligaments) attaches from the clavicle to the coracoid process of the scapula. This ligament can carry the load and is extremely strong. These tiny ligaments (with the AC joint) keep the stability between the scapula and the clavicle and thus keeping your shoulder ‘square’.
Transverse Humeral Ligament (THL)
This ligament protects the long head of biceps tendon muscle in the groove of the arm bone.
Muscles for the stability of the Shoulder Joint
Muscles of the shoulder connect the shoulder girdle, the clavicle and arm bone.
- Muscles that origin from the spine and attaches to scapula and/or clavicle
- Muscles that origin from the clavicle or scapula and/or body wall(ribs) to the top end of the humerus.
Trapezius, Levator scapulae, Rhomboids and Serratus Anterior
Originate from the base of the skull and/or spine and connect the scapula and clavicle to the trunk of the body.
- Trapezius forms cross-shaped web along the neck and run from the spinal column out to the shoulder blade and clavicle bone. It helps to shrug the shoulders.
- Rhomboids and levator scapulae are important muscles that join the shoulder blade to the spinal column helping the scapular movements.
- Serratus anterior muscle helps to stabilize the shoulder blade on the chest wall. When this muscle is weak, winging of the scapula occurs which is when the shoulder blade protrudes from the back.
Deltoid, Pectoralis major, Pectoralis minor, Latissimus dorsi, Teres major, Serratus Anterior
These arise from the clavicle and/or scapula and/or body wall and connect to the upper end of the arm (humerus) and anchor the shoulder joint to our body.
- Deltoid muscle is a muscle that is responsible for overhead activities. It helps to move the arm sideways up.
- Pectoralis major muscle like the deltoid is another powerful muscle which is the main muscle when doing push-ups. It originates from the front of the chest and collar bone and inserts on the upper part of the arm bone (humerus).
- Latissimus dorsi is another powerful muscle that together with the teres major muscle pulls the arm down to the side. We use this muscle when doing chin-ups.
What are the Shoulder blade movements?
The muscles of the shoulder complex work together to perform a particular action. The Scapula and arm bone move together in a pattern to perform a movement.
The movements of the Scauplo-thoracic joint includes,
- Depression – Downward arm and shoulder girdle movement
- Elevation – Upward arm and shoulder girdle movement
- Retraction – backward shoulder girdle movement
- Protraction – forward shoulder girdle movement
Rotator cuff muscles- small in size, big in importance
The four rotator cuff muscles are important for the stability and movements of the shoulder joint. They are,
- Teres minor
These muscles connect the shoulder blade (Scapula) to the arm bone (Humerus) supporting the entire shoulder joint during movements.
The major function of the four rotator cuff muscles is to work simultaneously with each other to allow the arm to move freely in numerous positions. They do all this while pulling the humeral head downward and inward within the glenoid fossa.
Movements at the shoulder joint
The main movemnts at the GH joint are:
- Internal and External rotation
- Supraspinatus assists with lifting the arm with the deltoid above the head (abduction). This is the most common muscle / tendon to tear in the shoulder.
- Subscapularis twists the arm behind (Internal rotation) the back.
- Infraspinatus and the teres minor twists the arm outwards(External rotation) and sideways from the body.
- Subscapularis assists with Deltoid, Biceps, coracobrachialis, Tere major to bring about shoulder forward flexion movement.
- Triceps, latissimus dorsi, pectoralis major, teres major brings the arm backwards (Extension).
Why is the Rotator cuff is so important?
In order to prevent upward dislocation of the arm or tear within the inner soft tissue structures like labrum and capsule of the shoulder, balanced rotator cuff strength and function are necessary. All the rotator cuff muscles work together stabilizing the humeral head within the glenoid while the larger muscles like the ltissimus dorsi, pectoralis major and deltoid produce the forces necessary for movements.
Common Injuries to the shoulder
- Broken collar bone (Clavicle)
- Dislocations of the shoulder
- Frozen shoulder (Adhesive capsulitis)
- Rotator cuff injury or strain (tendonitis or tendinopathy)
- Acromioclavicular joint sprain
- SLAP Tear (Superior Labrum Anterior Posterior tear)
- Bankart’s lesion (Anterior inferior Labral tear, sometimes a part of the genoid cavity bone is also broken)
Most injuries to the shoulder are due to sudden trauma or repetitive trauma to the soft tissues and bones. Some of the injuries occur because of improper exercise selection, faulty technique, lack of warm-up, lack of dynamic stretches, dehydration and many more. However, knowing the anatomy and functions of the joints and soft tissue structures of the shoulder complex not only gives you a better understanding of it but will possibly give you a prospective as to how important is their role in maintaing the stability of the shoulder.