JOINTS OF THE SHOULDER REGION
The joints of the shoulder region are of the synovial variety and to discuss synovial joints the following points should be kept in mind:
· The type of synovial joint.
· Bones and part of the bone that forms the joints
· Type of cartilage covering the articular surface.
· Attachment of fibrous capsule.
· Structures found within the capsules (Intracapsular structures).
· The attachment or lining of the synovial membrane.
· Extracapsular structures.
· Movement and muscle causing the movement.
· Blood and Nerve supply.
· Applied Anatomy.
Joints or the shoulder region include:
- Sternoclavicular joint.
- Acromioclavicular joint
- Shoulder joint
It is a synovial joint of the gliding type formed by lateral end of the clavicle and the medial margin of the acromion. The articular surface of the clavicle is convex while that of acromion is slightly concave. It is lined by a fibrocartilage. A tough fibrocapsule enclosed the joint cavity, it wrap around the articular margin of the joint. Within the capsule there is an articular disc which separates the articular surface. In most cases it does not divide the joint cavity into two, from the superior surface it extends downward. The acromioclavicular joint presents two ligaments;
Acromioclavilar ligament extends from the acromium to the clavicle. It is a very tough ligament. Coracoclavicular ligament extends from the lateral border of the coracoid process to the inferior border of the clavicle; it is strong and it is made up of two parts; trapezoid part with the shape of a trapezius and conoid part with the shape of an inverted cone. It holds into the scapular and the clavicle.
Movements that occur in this joint are passive and it is controlled by scapular rotation.
Blood supply: The joint is supplied by branches of the suprascapular artery, acromial branch of the lateral pectoral artery
Nerve supply: Suprascapular nerve, axillary Nerve, lateral branch of the supraclavicular Nerve.
It is prone to dislocation. It can involve the coracoclavicular ligament as a result the scapular will descend downwards while the acromium will ascend upwards. A simply test known as the piano keyboard sign is carried out to ascertain the dislocation of acromioclavicular joint. This is carried out by using the tip of the index finger to press down the tip of the acromium, this action reduces the pain immediately. The test is positive if as soon as the finger is released a sharp pain is felt.
SHOULDER JOINT (GLENOHUMERAL JOINT)
It is a synovial joint of the ball and socket variety. It permits a lot of movement. It is a multiaxial joint (3O freedom of movement) e.g. Abduction and adduction, circumduction, medial and lateral rotation making it a multiaxial joint. It sacrifices its bony stability for movement.
The hemispherical head of the humerus forms the ball which faces medially, upward and backward. The Socket is the shallow glenoid fossa or cavity and has tubercles above and below to give attachment to the long head of bicep and the long head of tricep respectively. The one above is the supraglenoid tubercle while below is the infraglenoid tubercle. The glenoid fossa is shallow and pear shaped.
The articular surface is lined by hyaline cartilage. The joint is enclosed by a tough but loose fibrous capsule which is attached medially to the margin of the glenoid fossa, it encloses the origin of the long head of biceps, and laterally it is attached to the anatomical neck of the humerus, inferomedial to the humerus it extends to enclosed upper 1cm of the shaft of the humerus. The long head of bicep is described as intracapsular but extrasynovial since it is not enclosed within the synovial cavity.
Intracapsular structure include: Glenoid labrum which is a fubrocartilage attach to the rim of the glenoid fossa increasing the concavity of the glenoid fossa and acting as a cushion to the head of the humerus; Long head of bicep which originated from the supraglenoid tubercle, traversing the cavity of the joint to pass through the intertubercular sulcus by passing beneath the transverse humeral ligament. It gives stability to the shoulder joint.
There are three glenohumeral ligaments, which extend from the superomedial margin of the glenoid fossa passing superiorly, medially and inferiorly. It is believed that these ligaments are the thickened part of the capsule.
The synovial membrane lines the internal surface of the capsule it also form a sleeve that enclosed the long head of biceps, it also lines the glenoid labrum and the part of the humerus that lies within, the joint cavity.
The sleeve also accompanies the long head of biceps into the tunnel created by the transverse humeral ligament and acts as a bursa.
The transverse humeral ligament bridges the gap between the greater and lesser tubercles of the humerus converting it into a tunnel through which the long head of biceps passes, this prevents it from springing out during humeral movement thereby helping in the stability of the joint.
A ligament attaching the head of humerus to the coracoid process known as the coracohumeral ligament also helps to stabilize the joint by preventing the downward dislocation of the humerus.
Within the intracapsular structure there are some bursae which include:
· Subscapularis bursa which protects the subcapularis tendon.
· The infraspinatus bursa which separates the muscle as it crosses the joint toward it’s insertion into the intermediate facet of the greater tubercle.
· Supraspinatus bursa which also separates the muscle from the joint as it crosses towards its insertion into the upper facet of the greater tubercle.
· Subacromial bursa is an extensive bursa which lies below the surpaspinatus and infraspinatus bursae. It also served as a cushion for the head of the humerus, as it extends above the greater and lesser tubercle beneath the deltoid muscle preventing the fleshy fibers of the deltoid from being irritated by the joint.
Another extracapsular structure is the rotator cuff muscles which include the subscapularis, supraspinatus, infraspinatus and teres minor. Their tendons blend towards their insertion and form a cuff which serve as accessory ligament of the shoulder joint.
The coracoacromial arch and the subacromial bursa provide a secondary socket for the head of the humerus which gives stability to the head and prevent upward dislocation of the humerus.
MOVEMENT OF THE SHOULDER JOINT.
FLEXION: Anterior fibers of deltoid, coracobrachialis, clavicular head of pectoralis major, tendon of long head of bicep.
EXTENSION: posterior fibers of deltoid.
ABDUCTION: Deltoid (Intermediate fiber) supraspinatus.
ADDUCTION: Pectoralis major, Teres major and latissimus dorsi.
LATERAL ROTATION: Teres minor and infraspinatus
MEDIAL ROTATION: Subcapularis, latissmus dorsi, Teres major and Pectoralis major.
CIRCUMDUCTION: This is a combination of the above six movements in sequence.
BLOOD SUPPLY: Posterior and Anterior circumflex humeral artery, suprascapular artery.
NERVE SUPPLY: Axillary Nerve, Suprascapular nerve, Lower and Upper subscapular nerve.
It is the most easily dislocated joint of upper limb. Dislocation is mainly downwards. Upward dislocation is limited because of the thoracoaromial arch.