The scapula, also known as the shoulder blade is a triangular, flat bone of shoulder girdle that articulates with the head of the humerus at the glenohumeral joint which is popularly called as the shoulder joint, and with the lateral end of the clavicle at the acromioclavicular joint. In doing so, the scapula connects the upper limb to the trunk. Following diagram would explain the relation of the scapula to the thorax.
Scapula Anatomy Details
Scapula has got two surfaces, three borders, three angles and three processes. Scapula serves as a site for attachment for many important muscles around the shoulder.
The Surfaces of Scapula
Scapula has got two surfaces – costal surface that abuts the thorax and dorsal surface which can be felt when we palpate the bone behind.
The Costal surface
The costal surface of the scapula is concave and is directed medially and forwards.
A concave depression over most of its surface is called the subscapular fossa. The subscapularis muscle, one of the rotator cuff muscles, originates from this side.
Coracoid process is a hook-like projection originating from the superolateral surface of the costal scapula which lies just underneath the clavicle.
The costal surface is marked by three longitudinal ridges. Another thick ridge adjoins the lateral border. This part of the bone is almost rod-like. It acts as a lever for the action of the serratus anterior in the overhead abduction of the arm
The Dorsal Surface
This surface gives attachment to the spine of the scapula which divides the surface into a smaller supraspinous fossa and a larger infraspinous fossa. The two fossae are connected by the spinoglenoid notch, situated lateral to the root of the spine.
Acromion is the projection of the spine that arches over the glenohumeral joint and articulates with the clavicle.
Borders, Angles, and Processes of Scapula
Scapula has three borders. three angles and three processes. [ See diagram for location]
Lateral border of the scapula is thick and presents the infraglenoid tubercle at the upper end.
The superior angle is covered by the trapezius. The inferior angle is covered by the latissimus dorsi and moves forward around the chest when the arm is abducted. The lateral angle is broad and bears the glenoid cavity which is directed forwards, laterally and slightly upwards. Glenoid cavity articulates with the head of the humerus to form shoulder joint.
The spinous process or spine of scapula is a triangular plate of bone with three borders and two surfaces. It divides the dorsal surface of the scapula into the supraspinous and infraspinous fossae. Its posterior border is called the crest of the spine. The crest has upper and lower lips.
Coracoids process is directed forwards and slightly laterally and plays important role in shoulder stabilization by providing attachments to ligaments and muscles
How to Determine Side of Scapula?
- The lateral angle is large and bears the glenoid cavity.
- The dorsal surface is convex and is divided by the triangular spine into the supraspinous and infraspinous fossae.
- The costal surface is concave to fit on the convex chest wall.
- The lateral thickest border runs from the glenoid cavity above to the inferior angle below.
Muscles and Ligaments of Scapula
Various muscles and ligaments attach on scapula. These stabilize and move the bone. Movement of scapula helps in movements of the shoulder joint.
Muscles of Scapula
It arises from the medial two-thirds of the subscapular fossa.
Serratus anterior muscle is inserted along the medial border of the costal surface-one digitations to the superior angle, two digitations to the medial border, and five digitations to the inferior angle.
It arises from the medial two-thirds of the supraspinous fossa (including the upper surface of the spine).
arises from the medial two-thirds of the infraspinous fossa (including the lower surface of the spine).
Deltoid muscle arises from the lower border of the crest of the spine and from the lateral border of the acromion.
Trapezius muscle is inserted into the upper border of the crest of the spine and into the medial border of the acromion.
The long head of the biceps brachii arises from the supraglenoid tubercle. The short head from the lateral part of the tip of the coracoid process.
The coracobrachialis arises from the medial part of the tip of the coracoid process.
The long head of the triceps arises from the infraglenoid tubercle.
Ligaments of Scapula
The margin of the glenoid cavity gives attachment to the capsule of the shoulder joint and to the glenoid labrum.
The margin of the facet on the medial aspect of the acromion gives attachment to the capsule of the acromioclavicular joint.
The coracoacromial ligament extends between the coracoid process and the acromion. It is attached to the lateral border of the coracoid process, and the medial side of the tip of the acromion process.
This ligament protects the head of the humerus from dislocating during motion.
It is attached to the root of the coracoid process.
It is attached to the coracoid process. The trapezoid part attaches to the superior aspect and the conoid part near the root.
It bridges the spino-glenoid notch. The suprascapular vessels and nerve pass deep to it.
Clinical Significance of Scapula
Winging of Scapula
Scapula needs to be kept close to the posterior chest wall by muscle forces. A break in forces as in paralysis of the serratus anterior cause ‘winging’ of the scapula. The medial border of the bone becomes unduly prominent, and the arm cannot be abducted.
Winging may also be caused by trapezius muscle.
Scapula bone is frequently injured in trauma
It is a developmental anomaly called where the medial border is concave.
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