By Dr. Avi Chezar, shoulder surgeonAnatomy of the shoulder
The shoulder is one of the most complex systems in the human body, and it allows a very wide range of movement: lifting the arm, rotating it, bringing it towards the body and away from it, and the complex movements needed in daily activities such as dressing, combing your hair, carrying objects or playing sport. The shoulder is made up of three bones – the shoulder blade (scapula), the collarbone (clavicle) and the upper arm bone (humerus) – and three joints that work together to produce smooth movement. The main joint is the glenohumeral joint, a ball-and-socket joint formed by the relatively large head of the humerus and, in contrast, the glenoid socket of the shoulder blade, which is shallow and small. This difference allows great mobility but also makes the joint less stable, so it depends heavily on the stability of the muscles and tendons around it, especially the rotator cuff tendons.
Alongside the glenohumeral joint there are two other important joints: the acromioclavicular joint (AC joint), between the acromion and the collarbone, and the sternoclavicular joint (SC joint), between the collarbone and the breastbone. These two joints allow the whole shoulder girdle to move over the chest wall, which widens the overall range of movement of the arm.
The bone structure includes several clinically important areas: the acromion – the "roof of the shoulder", which is involved in almost every shoulder problem; the coracoid – a projection at the front that anchors ligaments and muscles; and the glenoid – the socket in which the head of the humerus sits. The upper arm bone itself has two main tuberosities: the greater tubercle, to which the supraspinatus, infraspinatus and teres minor tendons attach, and the lesser tubercle, to which the subscapularis muscle attaches. Between the tuberosities runs the bicipital groove, in which the tendon of the long head of the biceps glides; it is considered an integral part of the shoulder's stabilizing mechanism.
Important muscles
The shoulder has an extensive system of muscles, but the most important clinically are the rotator cuff muscles – a group of muscles that stabilizes the head of the humerus within the glenoid during movement. This is a delicate mechanism: on the one hand the shoulder needs to be very stable, and on the other it needs to allow a large range of movement. That is why any small disturbance in these tendons – inflammation, wear or a tear – causes significant pain.
The rotator cuff muscles are:
- Supraspinatus – the most important muscle for moving the arm away from the body at the start of the movement (0–15 degrees). Clinically it is the most vulnerable muscle.
- Infraspinatus and teres minor – responsible for external rotation.
- Subscapularis – the main muscle for internal rotation.
The biceps tendon (long head) runs inside the joint itself and enters the bicipital groove. It stabilizes the head of the humerus from above, which is why pain in the "biceps muscle" is sometimes actually pain coming from the joint.
Another important muscle is the deltoid, the large muscle that wraps around the shoulder and provides most of the power for moving the arm away from the body above 15 degrees. When the supraspinatus is injured, the deltoid takes most of the load, but in advanced cases of a full tear its function is limited.
Bursae – the shoulder's "friction pads"
To allow smooth movement between the tendons and the bones, there are bursae – small fluid-filled sacs. The main and most sensitive bursa is the subacromial/subdeltoid bursa, located between the acromion and the supraspinatus and deltoid tendons. Inflammation of this bursa is one of the common causes of shoulder pain, especially when moving the arm away from the body.
Another bursa, the subscapularis bursa, lies under the subscapularis muscle; it communicates with the joint and can be involved in inflammation or irritation.
Ligaments and joint stability
Although the shoulder is a very mobile joint, it needs stabilizing ligaments. The main ligaments are:
- The glenohumeral ligaments (superior, middle and inferior) – reinforce the front and lower part of the joint.
- Coracohumeral ligament – supports from above.
- Transverse humeral ligament – holds the biceps tendon in its groove.
That said, it is important to understand that most of the shoulder's stability does not come from the ligaments but from the muscles, which is why any injury to the rotator cuff immediately affects shoulder function.
Key points to understand
The shoulder is a complex system that relies mainly on muscles and tendons. Most shoulder pain is not dangerous, but it needs the right treatment because it is linked to wear, inflammation or overload. Because the shoulder is stable mainly thanks to its muscles, physiotherapy is a central part of recovery in almost every situation – inflammation, a partial tear, or after a dislocation. In certain situations, when there is a large tear or recurrent dislocations, surgery can be a solution, but most people improve without surgery.
The 6 most common shoulder problems
1) Shoulder impingement syndrome
What is it?
A condition in which the shoulder tendons and bursa (mainly the supraspinatus tendon of the rotator cuff) are "squeezed" under the roof of the shoulder (the acromion) every time the arm is lifted.
How does it feel?
- Pain at the side of the shoulder, mainly when lifting the arm to the side or overhead
- Pain at night, especially when lying on the shoulder
- Difficulty with daily activities such as putting on a shirt, lifting a bag or combing your hair
Why does it happen?
- Repeated load (physical work, sport, lifting the arm overhead)
- Wear and age
- Sometimes small bony changes under the acromion
General treatment in brief
Relative rest, anti-inflammatory medication, physiotherapy to strengthen the shoulder's stabilizing muscles, and in some cases a subacromial injection.
2) Subacromial bursitis
What is it?
The bursa is a "fluid cushion" that reduces friction between the tendons and the bone. When it becomes inflamed and swollen, any movement of the shoulder can hurt.
How does it feel?
- Sharp pain when lifting the arm
- Pain when lying on the shoulder
- Sometimes a "burning" feeling or a dull ache at the side of the upper arm
Why does it happen?
- Increased friction around the acromion
- Secondary to impingement, rotator cuff syndrome or frozen shoulder
- Repeated load or minor trauma
General treatment in brief
Rest, ice in the early stages, anti-inflammatory medication, physiotherapy; sometimes a steroid injection into the bursa.
3) Rotator cuff tear
What is it?
The rotator cuff is a group of tendons and muscles that stabilizes the head of the humerus within the shoulder socket. A tear can be partial or full, and the tendon most often affected is usually the supraspinatus.
How does it feel?
- Pain in the shoulder and upper arm that comes on gradually
- Difficulty with daily activities such as combing your hair or putting on a shirt
- Weakness and pain when lifting the arm or rotating the shoulder
- Difficulty starting to move the arm away from the body
- Sometimes a click with movement
Why does it happen?
- Degenerative tear / wear with age (the most common): usually seen in people over 40
- Trauma (a fall, lifting a heavy object)
- Repeated load (sport, overhead work)
General treatment in brief
In most cases treatment starts conservatively (physiotherapy, painkillers, adjusting activity). In full tears, especially in young and active patients, or when there is no improvement after a few months, surgical repair is considered.
4) Anterior shoulder dislocation
What is it?
A condition in which the head of the humerus "pops" forward out of the socket (the glenoid). This is the most common type of shoulder dislocation.
How does it feel?
- Very sharp, severe pain
- The arm is "stuck" in a certain position and is difficult or impossible to move
- Sometimes a visible deformity of the shape of the shoulder
Why does it happen?
- A fall on an outstretched hand
- A sports injury or direct trauma
- After a first dislocation, the joint may be less stable and prone to recurrent dislocations
General treatment in brief
Putting the shoulder back in place (closed reduction, by an experienced doctor), rest and recovery in a sling, and then physiotherapy to strengthen stability. In cases of recurrent dislocations or structural damage (labrum, bone), surgery is sometimes needed.
5) Long head of biceps tendinopathy
What is it?
Inflammation (tendinitis) and/or chronic degeneration (tendinosis) of the biceps tendon, usually affecting the proximal long head near its origin at the upper rim of the glenoid, at the front of the upper arm. This tendon is sensitive to wear and irritation; if untreated, it can progress to a tear of the tendon.
How does it feel?
- Pain at the front of the shoulder
- Tenderness at one finger's point over the groove at the front of the upper arm
- Pain when lifting a weight, rotating the arm, or raising the arm forward
Why does it happen?
- Repeated load (gym training, weightlifting, sport)
- In combination with impingement or other shoulder problems
- Wear with age
General treatment in brief
Adjusting activity, anti-inflammatory medication, physiotherapy, and in resistant cases a targeted injection; more rarely, surgery (tenotomy/tenodesis).
6) Frozen shoulder (adhesive capsulitis)
What is it?
Frozen shoulder is a condition in which the shoulder capsule (the envelope of the joint) becomes inflamed, thickened and fibrotic. The capsule stiffens and contracts, causing severe restriction of movement and sometimes also significant pain.
Symptoms
It is characterized by pain and gradual stiffness of the shoulder joint at first, followed by significant restriction of the range of movement – especially moving the arm away from the body and external rotation.
Is it dangerous?
No. It is an unpleasant condition, but not a dangerous one.
Why does it happen?
People after prolonged lack of movement (after an injury, surgery or immobilization), systemic illnesses (such as diabetes), and sometimes for no clear reason.
The stages of frozen shoulder
- Stage 1 – freezing: pain that keeps increasing and a gradual decrease in range of movement.
- Stage 2 – frozen: less pain, but severe and constant restriction of shoulder movement.
- Stage 3 – thawing: gradual improvement in range of movement until an almost full return.
Treatment
In the frozen stage: treatment for relief only – heat, painkillers, avoiding painful movements. In the thawing stage: starting physiotherapy to improve the range.
In 95% of cases there is full recovery without the need for surgery. Surgery is considered only in rare cases with particularly prolonged restriction (mainly in people with diabetes).
A professional 3D anatomical model
View a detailed interactive model of shoulder anatomy – bones, muscles, nerves and arteries. Rotate, zoom in and explore every anatomical detail.
Open the interactive model MSD Manuals model
Tip: both models offer professional interactive viewing. BioDigital specializes in detailed anatomical display, and MSD Manuals offers extensive medical information.
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Why interactive models?
Understanding the three-dimensional structure of the shoulder is essential for understanding injuries and treatments. Interactive models let you see exactly how each part connects and interacts with the others, which helps you better understand the diagnosis and the treatment offered.
Further reading from leading medical sources
For readers who want to go deeper, here are reliable sources from the American Academy of Orthopaedic Surgeons (AAOS – OrthoInfo):
- Shoulder Pain and Common Shoulder Problems — OrthoInfo (AAOS)An overview of the anatomy of the shoulder joint and the common problems that arise from its structure
- Common Shoulder Injuries — OrthoInfo (AAOS)Common shoulder injuries – the structure of the joint and how injuries happen
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