Pectoral Girdle
Pectoral Girdle
or Shoulder Girdle connects the free part of the upper limb with the
trunk. It consists of scapula and clavicle on each side. A series
of anatomical organization makes the girdle extremely mobile. The mobility of
scapula is essential for free movement of upper limb. The clavicle acts as a strut
which holds the scapula laterally allowing the upper limb to swing freely from
the side of the trunk.
Pectoral girdle
consists of three joints: i) Sternoclavicular joint, ii) Acromioclavicular
joint and iii) Glenohumeral joint. The scapula is connected to the clavicle
and humerus (bone of the arm) by acromioclavicular and glenohumeral joints
respectively. The upper limb and pectoral girdle are connected to the trunk (axial
skeleton) only through the sterno-clavicular joint. Scapula does not make any
anatomical joint with axial skeleton rather it is connected to the vertebral
column and thorax only by the muscles. This connection of scapula with the
thorax is sometimes referred as conceptual scapula-thoracic joint or physiological
scapula-thoracic joint. Thus the extremely mobile pectoral girdle is
supported and stabilized by the muscles that are attached to the ribs, sternum
and vertebrae.
Joints of Pectoral
Girdle
Sternoclavicular Joint
Type:
Saddle type of synovial joint.
Articulation:
The sternal end of the
clavicle articulates with the manubrium sterni and the first costal cartilage.
Capsule:
This surrounds the joint and is attached to the margins of the articular
surfaces.
Synovial membrane: This
lines the inside of capsule and extends to the margins of the cartilage
covering the articular surfaces.
Ligaments:
Anterior
and Posterior sternoclavicular ligaments reinforce the capsule from the
front and behind.
Interclavicular ligament strengthens the capsule
superiorly. It extends from sternal end of one clavicle to the sternal end of
the other.
Costoclavicular
ligament connects the inferior surface of sternal end of
clavicle to the 1st rib and its costal cartilage.
Articular disc:
It is a fibrocartilaginous disc which divides the interior of the joint into
two compartments. The peripheral margin of the disc is attached to the interior
of the capsule. It is also strongly attached to the superior margin of the
articular surface of the clavicle above and to the first costal cartilage below.
Movements:
It is very mobile to
allow movements of the pectoral girdle and upper limb. The movements that occur
in SC joint are: forward and backward movements and elevation
and depression. During full elevation of upper limb the clavicle is
raised to approximately a 600 angle.
Muscles producing the
movement:
Forward movement:
Serratus anterior
Backward movement:
trapezius and rhomboids
Elevation: trapezius,
sternocleidomastoid, levator scapulae and rhomboids
Depression: pectoralis
minor and subclavius
Nerve supply:
The supraclavicular nerve and the nerve to the subclavius muscle.
Blood supply:
by internal thoracic and suprascapular arteries.
Acromioclavicular Joint
Type:
Plane synovial joint
Articulation:
lateral end of clavicle articulates with the acromion of the scapula.
Capsule:
Capsuleis relatively loose and is attached to the margin of the articular
surfaces. From the capsule an incomplete wedge shaped fibrocartilaginous disc
projects into the joint cavity from the above.
Ligaments: Superior and inferior
acromioclavicular ligaments reinforces the capsule. The integrity of the
joint is maintained by the extrinsic ligament called coracoclavicular
ligament. It consists of a pair of strong bands that connect the coracoid
process of scapula to the clavicle. The coracoclavicular ligament consists of
two parts: conoid and trapezoid part.
The conoid ligament
is cone shaped with its apex directed downward and attached to the root of the
coracoid process and its base to the conoid tubercle on the undersurface of the
clavicle. The trapezoid ligament is extends laterally for its attachment
from the superior surface of coracoid process to the trapezoid line on the
inferior surface of the clavicle. The coracoclavicluar ligament is also
responsible for suspending the scapula and free limb from clavicular strut.
Movement:
A gliding movement takes place when the scapula rotates or when the clavicle is
elevated or depressed. The thoraco-appendicular muscles that attach to and move
the scapula cause the acromion to move on the clavicle.
Nerve supply:
Supraclavicular, lateral perctoral and axillary nerves
Blood Supply:
Suprascapular and thoracoacromial arteries
Glenohumeral Joint
Glenohumeral joint is also
called shoulder joint.
Type:
Ball and Socket synovial joint
Articulation:
occurs between the large rounded head of the humerus and a shallow glenoid
cavity of the scapula. The articular surfaces are lined by the hyaline
cartilage. The shallow glenoid cavity is deepened by a fibrocartilaginous
tissue called glenoid labrum. The cavity receives more than a third of
humeral head which is held in position by the tonus of the musculotendinous
rotator cuff muscles.
Capsule:
The fibrous capsule surrounds
the joint and is relatively thin and lax allowing the greater range of
movement. Superiorly it is strengthened by the fibrous slips from the rotator
cuff muscles. Inferiorly it is relatively weaker and is common site for
dislocation of the joint.
Attachment of the
capsule:
Medially –
it is attached to the margin of the glenoid cavity outside the labrum. It also
enclosed the origin of long head of biceps brachii.
Laterally –
it is attached to anatomical neck of humerus except at two places, a) upper end
of bicipital groove to allow the passage of tendon of long head of biceps
brachii and b) inferomedially, the line of attachment extends downward for
about 1 cm to the surgical neck.
Synovial Membrane:
it lines the inside of the capsule and reflects from it onto the glenoid labrum
and humerus as far as the articular margin of the head. It forms a tubular
sheath around the long tendon of biceps brachii. It passes through the
anterior wall of the capsule to form the subscapular bursa beneath the subscapularis
muscle.
Ligaments:
Glenohumeral
ligament
Coracohumeral
ligament
Transverse
humeral ligament
Coracoacromial
ligament (accessory ligament)
Glenoid
labrum
Glenohumeral
and coracohumeral ligaments are the thickening of joint capsule that
strengthens the anterior and superior aspects of the capsule respectively.
Glenohumeral ligaments
are three fibrous bands, evident only on the interior of the joint capsule. The
superior, middle and inferior bands of the glenohumeral ligaments are attached
medially to the superomedial margin of glenoid cavity and blends with glenoid
labrum. Laterally, all three bands radiate and are attached to the upper part
of lesser tubercle, lower part of lesser tubercle and lower part of anatomical
neck respectively.
The coracohumeral
ligament extends from the root of the coracoid process to anatomical
neck opposite the greater tubercle of the humerus.
The transverse
humeral ligament bridges the gap between the two tubercles and strengthens
the capsule.
Coracoacromial ligament
is an accessory ligament which along with the inferior aspects of acromion and
coroacoid process of scapula forms coracoacromial arch and protects the
superior aspect of the joint as it prevents the superior displacement of
humeral head. A subacromial bursa intervenes between the arch above and
the tendon of supraspinatus and greater tubercle below.
Glenoid labrum:
It is made up of
fibrocatilaginous tissue and helps deepening the glenoid cavity. It is attached
to the margin of the glenoid cavity except above from where the long head of
biceps brachii arises.
Movements at shoulder
joint:
The shoulder joint has
greater mobility. (Its stability has been compromised at the cost of the
mobility as compared to hip joint which has greater stability but has limited
range of movements). The freedom of movement is mainly due to the thin and lax
capsule and the large size of humeral head compared to the receiving socket
provided by the glenoid cavity. The glenohumeral joint produces movements
around three axes and permits flexion-extension, adduction-abduction,
medial-lateral rotation and circumduction.
Flexion and Extension:
Flexion and extension
take place right angle to the plane of the body of scapula around an axis that
passes through the humeral head and center of glenoid cavity. Flexion moves the
arm forward and medially and is about 900. Extension moves the arm
backward and laterally and is about 450.
Adduction-abduction:
Abduction and adduction
take place parallel to the plane of the body of scapula around an axis which
passes through the head of humerus and is parallel to the glenoid cavity. The
abduction carries the arm laterally and adduction medially. During adduction
the arm can be swung 450 across the front of the chest.
Mechanism of Abduction:
Abduction of the arm is
accomplished by movement at shoulder joint as well as by the rotation of
scapula at conceptual scapula-thoracic joint. The abduction is initiated
for the first 150 by supraspinatus muscle which also holds the head
of the humerus against the glenoid cavity. The later action then allows deltoid
to take over and complete the further abduction. The limb can be elevated by
1800 during abduction. Except for the first 30 degree of abduction
which occur due to movement at shoulder joint only, in every 150 elevation,
100 occurs at shoulder joint and 50 by rotation of
scapula at conceptual scapula-thoracic joint in the ration of 2:1. When the arm
is abducted to 900, the further elevation is prevented as the
articular surface is exhausted and the greater tubercle impinges on the lateral
edge of acromion. The further elevation is then accomplished by the lateral
rotation of humerus by 1800 which brings the greater tubercle
posteriorly thus providing more articular surface to continue elevation. From
1200-1800, the abduction is accomplished by the rotating
the scapula.
Medial and lateral
rotations:
The plane producing
these movements is a vertical axis passing through center of the humeral head
to the center of the capitulum. In a semi-flexed elbow, the medial rotation at
shoulder joint carries the hand medially and lateral rotation moves the hand
laterally.
Circumduction:
It is a combination of
above mentioned movements so that the lower end of the humerus defines the base
of the core and the humeral head forms apex of the cone.
Muscles producing the
movements at shoulder joint:
Flexion: Anterior
fibers of Deltoid, Pectoralis Major and Coracobrachialis (weak flexor)
Extension: Posterior
fibers of Deltoid, Latissimus Dorsi and Teres Major
Abduction:
Supraspinatus (initial 150) and Deltoid
Adduction: Pectoralis
Major, Latissimus Dorsi, Teres Major and Subscapularis
Medial Rotation:
Subscapularis, Latissimus dorsi, Teres Major, Pectoralis Major, Anterior Fibers
of Deltoid
Lateral Rotation:
Infraspinatus, Teres Minor and Posterior Fibers of Deltoid
Factors stabilizing the
shoulder joint:
As noted earlier, the
shoulder joint is relatively weaker joint owing to its thin and lax joint
capsule, greater mobility and shallow glenoid fossa to receive larger head of
the humerus.
Rotator cuff:
The tendons of small muscles namely subscapularis, supraspinatus, Infraspinatus
and teres minor on their way to insertion on humerus flatten and blend with
each other and with the fibrous capsule of shoulder joint. The tone of these
muscles support and strengthen the shoulder joint from the front, above and
from the behind.
Glenoid labrum:
this fibrocartilaginous tissue deepens the glenoid cavity.
Glenohumeral
and Coracohumeral ligaments support the joint from anterior and superior
aspects respectively.
The coracoacromial
arch prevents the upwards displacement of the joint. Similarly, the tendon
of long head of biceps brachii also holds the head of the humerus in
position during abduction. Similarly, the downward displacement of humerus
during abduction is prevented by the long head of triceps brachii.
Blood Supply:
Anterior circumflex
humeral artery, branch of third part of axillary artery
Posterior circumflex
humeral artery, branch of third of axillary artery
Branches of
suprascapular artery, branch of thyrocervical trunk of subclavian artery or
directly from subclavian artery
Nerve Supply:
Axillary nerve and
suprascapular nerve
REFERENCES:
Following resources are used while preparing this post (readers are strongly recommended to go through them for more details):
Gray's Anatomy
K. L. Moore's Clinically Oriented Anatomy
R. Snell's Clinical Anatomy