Anatomy of the shoulder

Bones of the shoulder
The shoulder consists of the main joint and several minor joints, like the acromioclavicular joint or the joint between bladebone and the chest wall. The main joint is composed out of the humeral head and glenoid. The shoulder joint is a ball and socket joint. The amount of movement is enormous and one of the biggest of the human body. The shoulder joint can be moved in all three axis of space with nearly no limitations.
From the inside to the outside there are:
1. The bones ( interior structures )
2. The joint capsule
3. Ligaments
4. Muscles and tendons
5. Bursas
6. Skin ( outer region )
1. Bones of the shoulder:
The bones are the most interior units of the shoulder. The central units are the ballshaped humeral head, which is articulating with the glenoid. Both are covered by a smooth and ivory-colored layer - the cartilage. Humeral head and glenoid are forming the main shoulder joint. The glenoid is a part of the bladebone, also called "the scapula". The glenoid is small, flat and pear-shaped. Only one third of the humeral head has contact and is compassed by the glenoid. In other words: the ball-shaped humeral head is much bigger than the glenoid. Thus, the shoulder is a loose and mainly soft tissue controlled joint - in contrast to the hip joint, which is a predominantly bony balanced joint. These circumstances allow the shoulder to have an enormous range of motion but on the other hand the shoulder joint is susceptible for dislocations. To enlarge the articulating surface of the bony glenoid, it is surrounded by a cartilage structure, called "the labrum". It is a cartilaginous wall contributing to shoulder stability. The main shoulder joint might be destructed by arthritis, infections or circulatory disorders, resulting in a necrosis of the humeral head. The construction of the shoulder allows an enormous range of motion and an enormous strenghts - although being soft tissue balanced - more then the own body weight can be lifted with the shoulder joint.
Above the main shoulder joint there are two bony prominences. First: the acromion - the roof of the shoulder. It is a part of the boneblade. Second: above and in front, to the anterior side, of the main shoulder joint, there is the coracoid process, another structure belonging to the boneblade. Ligaments important for the stability of the clavicle and connecting to the acromion, are arising from it. The ligament connecting the coracoid process with the acromion is called the coracoacromial ligament. Acromion and the coracoacromial ligament are forming the shoulder roof - the humeral fornix. This arch is very important, because it is forming the tunnel of the rotator cuff. If theis tunnel is to narrow, e.g. due to spurs and so on, the cuff and tendon of the rotator cuff is impinging. This is called the impingement syndrome.
An underestimated bone is the bladebone, called scapula. The scapula is forming a side joint - the thoracoscapular joint. A joint between the undersurface of the bladebone and thoracic wall. A malfunction of this joint is important for multidirectional shoulder instabilties and the scapular winging as well as the painful scapular snapping.
Adhesions of the scapulothoracic joint are diurbing the shoulder function enormously. Problems between the bladebone and thoracic wall occur quite often and are not recognized.
Scapula and clavicula are the only bones connecting the arm with the human body. We are talking about a "floating shoulder" if both bones, scapula and clavicula, are broken.
The clavicula is a bone connecting the breast bone with with shoulder roof ( acromion ), which means connecting the arm with the body. Tha clavicula is s-shaped. Most common pathologic conditions are fractures of the clavicle and an arthritis at the lateral end of the clavicle. Another kind of frequently seen injury are dislocations of the ac joint - the joint, formed by the outer end of the clavicle and the shoulder roof - the acromion.
Using red green or red blue glasses you can see a 3D holography like picture here.
2. The joint capsule: it´s like a wall separating the inside of the shoulder joint from the surrounding soft tissue. The capsule arrises from the frontal and back side of the bladebone and it´s fibers are attaching to the humeral head. The shoulder joint capsule is a quite elastic structure, a wide and thin layer. Thsi construction is contrubuting to an enormous range of motion, which is for the shoulder joint typical. At the bottom the joint capsule is enlarged, which is called the "axillary recessus" or the "axillary pouch". If the axiallary recessus adheres a frozen shoulder is developing. On the other side the capsule can be to elastic, pathologically elastic, so that a multidirectional instability is present and people are suffering from it. All structures within the shoulder joint capsule are forming the interior space and structures outside the capsule, especially under the acromion ( shoulder roof ), are forming the "subacromial space". Producing synovial fluid is an important fuctnion of the shoulder joint capsule. The synovial fluid is important for a smooth sliding between the humeral head an the glenoid, while moving the shoulder. Synovial fluid is working similar to lube oil in an engine.
3. Ligaments: there ligaments reinforcing the shoulder joint capsule and ligaments running outside the main shoulder joint.
The so called "glenohumeral ligaments are reinforcing the shoulder joint capsule. Besides the posterior glenohumeral ligament most important are the the anterior inferior and anterior middle glenohumeral ligaments. Reinforcing the shoulder joint capsule they are contributing to the stability of the shoulder joint.
4. Muscles: there important muscles responsible for centering the ball of the humeral into the glenoid while moving the arm. The most important muscle, because demonstrating the most pathologic changes, is lying in front and on top of the shoulder joint capsule. This muscle is called the "supraspinatus muslce". The supraspinatus is affected by impingement problems, rotator cuff tears or calcific deposits most often. The subscapularis muscle is lying in front of the shoulder joint capsule. It´s a strong internal rotator. Subscapularis ruptures are rare. These rare subscapulris ruptures, if occurring, are making a lot of problems. The muscle lying on top and on the backside is called "infraspinatus" and the muscles lying completely on the rear side has the name "Teres minor". If massive ruptures are present the infraspinatus is affected nearly all the time. Ruptures of the teres minor are nearly never occurring.
All four muscles, supraspinatus, subscapularis, infraspinatus and teres minor are forming the rotator cuff. Taking a view from outside the shoulder the rotator cuff is not visible, it is covered by the strong deltoid muscle. The rotator cuff is responsible for a painless movement of the shoulder joint and it is stabilizing the shoulder. Defects and tears of the rotator cuff are a common clinical shoulder pathology. Most of the time a layman can not distinguish between a tendon and a muscle tear. Nearly always it is meant the same, the terms are used synonimous. The muscle consist of the red part and the white tendinous part, which is inserting into the bone of the humeral head. Because of the tendinous having a bad vascular supply and being under tension, most of the defects and tears are occuring in this region of the rotator cuff - near to the insertion.
Apart from it, there are two other important shoulder muscles:
1. The deltoid muscle: it is forming a cap an lying above the rotator cuff. Taking a view from outside the delta can be observed at the front, lateral and back side. It is forming the outer shape of the shoulder. The deltoid muscle has an important function turning the arm to the side ( abduction ). Scars of the deltoid after operations can be responsible for a nasty shoulder shape.
2. The biceps muscle: so called "popeye muscle". He is lying in front of the upper arm. Bending the elbow to a right angle, the biceps can be recognized best in front of the upper arm. If the biceps is affected by a disease, it is most of the time an infalmmation of the long head of the biceps tendon, lying in front of the shoulder. Ruptures of the upper or distal end of the biceps are also possible. The long head of the biceps is originating from the upper end of the glenoid. At this point the cartilaginous labrum and the biceps tendon are forming the upper wall / upper labrum or called biceps anchor. A region where SLAP lesions occur. Ruptures of the long head of the bicpes tendon occur at the tendons entrance into the biceps groove. Passing the biceps groove or biceps sulcus the tendon is fixed by the bony shape of this groove and by ligaments ( so called "coracohumeral ligaments" ), which are forming the "biceps pulley". The pulley might be damaged and the biceps can dislocate, casing pain or snapping in front of the shoulder.
The biceps and deltoid mucles can be recognized well in people performing bodybuilding.
Muscles which are not belonging to the shoulder muscles but which are lying near the shoulder are: the breast muscles ( pectoralis major and minor muscle ) and a back muscle called "latissimus dorsi". Both muscles are important for a shoulder surgeons, because they are used for muscle transplantations to cover massive rotaor cuff tears.


5. Bursas: it is the function of these structures to form a buffer and support sliding between the different soft tissue structures of the shoulder ( muscles, tendons ). It is the function of the bursas to protect soft tissue structure from wear and adhesion / stiffness. One of the most important bursas is lying between the acromion and humeral head / rotator cuff - the "subacromial bursa". In inflammed sucacromial bursa due to an impingement or rheumatoid arthritis will cause a lot of pain. If this bursal inflammation will last over weeks and month an adhesion of the soft tissue structures due to adhesion and a limited motion of the shoulder joint will develop. The normal size / hight of the subacromial bursa is about 1-2 millimeters, an inflammed bursa may show an enormous swelling, sometimes. Performing a shoulder arthroscopy the subacromial bursa is inspected regularly, inflammed bursas can be removed, impingement problems and subacromial spurs can be removed and rotator cuff tears be visualized and treated.



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