The Shoulder – The Most Movable and Complex Joint
Almost everyone has a shoulder problem at some point in their lives.
Most people think it wouldn’t happen to them, that it only happens to tennis or cricket players, or Olympic swimmers.
Many people try to “play tough” – hoping the pain will go away-only to have a small problem become a debilitating handicap.
People who engage in repetitive overhead motions (such as swimming, playing tennis, washing windows, painting or hanging wallpaper) are more prone to shoulder pain.
Low frequency vibration, repetitive work tasks, heavy work load, driving for long periods, sleep disturbance, and even smoking and caffeine consumption can predispose and contribute to shoulder pain and injuries.
Stress and psychological factors like job dissatisfaction and high mental workload, can increase the risk of shoulder problems as well.
With age, the prevalence of shoulder injuries increase. Women have a higher incidence of shoulder complaints.
Today, we see even children complaining of shoulder and neck pain. Several factors are contributing to this problem: poor posture, poor biomechanics while playing computer games, back and shoulder strain from carrying heavy backpacks.
Anatomy of the Shoulder
The shoulder is a very complex joint. It allows the upper extremity to rotate up to 180 degrees in three different planes, enabling the arm to perform a versatile range of activities.
This mobility comes at a cost: it leaves the shoulder vulnerable to overuse and prone to injury, and one reason why it is the most difficult and complicated joint in the body to rehabilitate.
The shoulder joint consists of a large ball and smaller socket, held in place with a series of bands called ligaments and tendons.
Ligaments attach bone to bone while tendons attach muscle to bone.
Bones and Joints
The shoulder is composed of four bones.
The clavicle is commonly known as the collar bone. The scapula is also known as the shoulder blade; the acromion is the part of the scapula that forms a bony roof above the rotator cuff, tendons and bursa. The sternum is often referred to as the breast bone, and the humerus is the upper bone of the arm.
Joints where bones come together, are surrounded by soft tissue, which includes ligaments, tendons, and bursas. There are several joints/articulations of the shoulder:
Acromioclavicular (AC)– This joint is formed by the acromion and the clavicle. Mainly it is active with shrugging movements.
Glenohumeral (GH) – The combination of the upper bone and the outside area of the scapula makes up this joint, which is responsible for most of the movements of the shoulder. Shoulder dislocation always refers to this joint.
Sternoclavicular (SC)– This joint is composed of the clavicle and the sternum and primarily operates during shrugs, although part of its function is to stabilise the shoulder girdle.
Scapulothoracic (ST)– This is not really a movable joint but serves as a base for muscles to be secured to.
Ligaments and Tendons
Ligaments are soft tissue structures that connect bones to bones.
A joint capsule is a watertight sac that surrounds a joint. In the shoulder, the joint capsule is formed by a group of ligaments that connect the humerus to the glenoid. These ligaments are the main source of stability for the shoulder. They help hold the shoulder in place and keep it from dislocating.
Ligaments attach the clavicle to the acromion in the AC joint. Two ligaments connect the clavicle to the scapula by attaching to the coracoid process, a bony knob that sticks out of the scapula in the front of the shoulder.
To compensate for the shallow socket, the shoulder joint has a cuff of cartilage called a labrum, that forms a deeper cup for the end of the arm bone (humerus) to move within.
This cuff of cartilage makes the shoulder joint much more stable, and allows for a very wide range of movements.
The labrum is also where the biceps tendon attaches to the glenoid.
Tendons are much like ligaments, except that tendons attach muscles to bones. Muscles move the bones by pulling on the tendons.The biceps tendon runs from the biceps muscle, across the front of the shoulder, to the glenoid.
At the very top of the glenoid, the biceps tendon attaches to the bone and actually becomes part of the labrum. This connection can be a source of problems when the biceps tendon is damaged and pulls away from its attachment to the glenoid.
The rotator cuff is an anatomical term given to the group of muscles and their tendons that act to stabilise the shoulder. It is composed of the tendons and muscles that hold the head of the humerus (ball) in the glenoid fossa (socket).
The following make up the major muscles of the shoulder:
Supraspinatus abducts the arm (lifts your arm from your side to above your head). The supraspinatus tendon is the most commonly involved tendon in rotator cuff injuries.
Infraspinatus -The main function of this muscle is to externally rotate the arm and stabilize the shoulder joint.
Subscapularis It is the muscle that resists motion of the humeral head in the forwards direction. It functions primarily to rotate the arm inward.
Teres minor muscle rotates the arm laterally and assists in bringing it toward the body. As it draws the upper arm bone (humerus) up, it strengthens the shoulder joint.
Each of the four rotator cuff muscles originates on the scapula and their tendons attach to the top of the humerus, helping to form the joint capsule.
The sac surrounding the joint is called bursa. A fluid- filled bursa is usually found between bones and tendons to help decrease friction during normal joint use. It provides lubrication to the joint.
Cartilage is the pad between joints, providing cushion to the joint.
The main nerves that travel into the arm and run through the axilla under the shoulder are: the radial nerve, the ulnar nerve, and the median nerve. These nerves carry the signals from the brain to the muscles that move the arm. The nerves also carry signals back to the brain about sensations such as touch, pain, and temperature.
Several factors can increase the likelihood of shoulder injuries:
- poor posture
- muscle imbalance
- neck, upper back and shoulders stiffness and tightness
- muscle weakness (especially rotator cuff muscles)
- excessive or inappropriate training or activity
- inadequate warm-up
- poor core stability
- inadequate rehabilitation following a previous injury
- shoulder joint hyper mobility
Neck and upper back (thoracic spine) are very important in the rehabilitation of shoulder pain and injury.
Neck or spine dysfunction can not only refer pain directly to your shoulder, but it can affect nerve function and supplying to your muscles causing weakness and altered movement patterns.
In the next couple of weeks we will be discussing common shoulder injuries and their management. We will give you some strategies to prevent such injuries and exercises to strengthen your shoulder.