Shoulder Anatomy
The shoulder is a ball and socket joint.
The most important
part of the shoulder is not the bones but the muscles around the shoulder that
help keep the ball centred in the socket.
Although I talk about the anatomy separately ie. the bones, the ligaments the
muscles etc.
The reason the shoulder functions so beautifully is due to the co ordinated fine
interaction of all the components.
Bones and joints
The shoulder girdle is made up of 3 bones the shoulder blade (scapula), the collar bone (clavicle) and upper arm bone (humerus).
The scapula has three main parts, the main socket of the ball and socket
joint called the
glenoid and two projections around the socket the
acromion at the top and the
coracoid projecting to the front.
The upper end of the humerus forms the ball of the ball and socket
joint.
The
clavicle works like a strut connecting the shoulder to the body.
There are two joints, the main ball and socket joint (Gleno-humeral
joint) and the small Acromioclavicular (ACJ) joint between the outer end
of the clavicle and the acromion.
The ball of the ball and socket joint
, the proximal humerus has a groove in which the long head of biceps
tendon runs, called the bicipital groove.
The groove separates two bumps the greater and
lesser tuberosities.
The tuberosities provide anchor points for the
tendons of the
rotator cuff muscles,
Ligaments
Ligaments connect bones to bones.
The ligaments around the shoulder also help stabilize the joints, both
the ACJ and the glenohumeral joint.
Ligaments around the ball and socket joint (glenohumeral joint)
The ligaments around the glenohumeral joint help keep the ball centred in the ball and socket joint and help
prevent the shoulder from dislocating.
Ligaments around ACJ
The clavicle is kept in place next to the acromion by the
acromioclavicular and coracoclavicular ligaments.
The acromioclavicular ligaments between the clavicle and the acromion.
The coracoclavicular ligaments between the coracoid process
on the scapula and the clavicle.
Other ligaments
The Coracoacromial ligament connects the coracoid to the acromion.
The
labrum is not a ligament per se but a special soft tissue buffer
that extends the surface of the glenoid and works a bit like a suction
cup, helping to keep the ball in the socket.
Muscles
It is best thinking of the muscles around the shoulder in 3 big groups.
The rotator cuff muscles, four small muscles providing fine
control.
The parascapular muscles, anchor the
scapula to the chest wall.
The deltoid muscle, the power horse of the shoulder.
Balanced and coordinated interplay between all these muscles is needed
to achieve smooth painless movement of the shoulder.
The rotator cuff
The rotator cuff is critical to shoulder function it is made up of 4
muscles, supraspinatus, subscapularis, infraspinatus and teres minor.
Supraspinatus is the muscle most commonly involved in problems around
the shoulder.
The parascapular muscles
Several muscles hold the scapula against the chest wall and rotate the
scapula when the arm is elevated above shoulder height.
If the scapula is not held steady then the arm cannot be used
effectively.
The deltoid
The deltoid is the power horse of the shoulder. To function efficiently it
needs the parascapular muscles to stabilize the scapula and the rotator
cuff muscles to provide fine control keeping the head centred in the
socket.
Nerves
Several nerves supply the shoulder and cross the shoulder.
The axillary nerve supplies the deltoid muscle and can be injured in
shoulder dislocations and fractures of the proximal humerus.
The suprascapular nerve supplies motor power to supraspinatus and
infraspinatus and receives a lot of sensory fibres from the shoulder..
