Scapula bone – General Features, Ligaments and Ossification

Scapula bone – General Features, Ligaments and Ossification

The scapula is a thin bone placed on the postero­lateral aspect of the thoracic cage. The scapula has two surfaces, three borders, three angles, and three processes.

General Features :

Surfaces :

1. The costal surface or subscapular fossa is concave and is directed medially and forwards. It 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 overhead abduction of the arm.

2. The dorsal 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.

Borders:

1. The superior border is thin and shorter. Near the root of the coracoid process it presents the suprascapular notch.

2. The lateral border is thick. At the upper end it presents the infraglenoid tubercle.

3. The medial border is thin. It extends from the superior angle to the inferior angle.

Angles :

The superior angle is covered by the trapezius.

The inferior angle is covered by the latissimus dorsi. It moves forwards round the chest when the arm is abducted.

The lateral or glenoid angle is broad and bears the glenoid cavity or fossa, which is directed forwards, laterally and slightly upwards.

Processes :

1. The spine or spinous process is a triangular plate of bone with three borders and two surfaces. It divides the dorsal surface of the scapula into the supraspinous and infra- spinous fossae. Its posterior border is called the crest of the spine. The crest has upper and lower lips.

2. The acromion has two borders, medial and lateral; two surfaces, superior and inferior; and a facet for the clavicle (Fig. 2.7).

3. The coracoid process is directed forwards and slightly laterally.

Side Determination :

1. The lateral or glenoid angle is large and bears the glenoid cavity.

2. 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.

3. The thickest lateral border runs from the glenoid cavity above to the inferior angle below.

Particular Features Muscles :

1. The multipennate subscapulars arise from the medial two-thirds of the subscapular fossa.

2. The supraspinatus arises from the medial two- thirds of the supraspinous fossa including the upper surface of the spine.

3. The infraspinatus arises from the medial two- thirds of the infraspinous fossa, including the lower surface of the spine.

4. The deltoid arises from the lower border of the crest of the spine and from the lateral border of the acromion. The acromial fibres are multipennate.

5. The trapezius is inserted into the upper border of the crest of the spine and into the medial border of the acromion.

6. The serratus anterior is inserted along the medial border of the costal surface: One digitation from the superior angle to the root of spine, two digitations to the medial border, and five digitations to the inferior angle.

7. The long head of the biceps brachii arises from the supraglenoid tubercle; and the short head from the lateral part of the tip of the coracoid process.

8. The coracobrachialis arises from the medial part of the tip of the coracoid process.

9. The pectoralis minor is inserted into the medial border and superior surface of the coracoid process.

10. The long head of the triceps brachii arises from the infraglenoid tubercle.

11. The teres minor arises from the upper two- thirds of the rough strip on the dorsal surface along the lateral border.

12. The teres major arises from the lower one-third of the rough strip on the dorsal aspect of the lateral border.

13. The levator scapulae is inserted along the dorsal aspect of the medial border, from the superior angle up to the root of the spine.

14. The rhomboideus minor is inserted into the medial border (dorsal aspect) opposite the root of the spine.

15. The rhomboideus major is inserted into the medial border (dorsal aspect) between the root of the spine and the inferior angle.

16. The inferior belly of the omohyoid arises from the upper border near the suprascapular notch.

Ligaments:

1. The margin of the glenoid cavity gives attachment to the capsule of the shoulder joint and to the glenoidal labrum.

2. The margin of the facet on the medial aspect of the acromion gives attachment to the capsule of the acromioclavicular joint.

3. The coracoacromial ligament is attached (a) to the lateral border of the coracoid process, and (b) to the medial side of the tip of the acromion process.

4. The coracohumeral ligament is attached to the root of the coracoid process.

5. The coracoclavicular ligament is attached to the coracoid process: The trapezoid part on the superior aspect, and the conoid part near the root.

6. The suprascapular ligament bridges across the suprascapular notch and converts it into a foramen which transmits the suprascapular nerve. The suprascapular vessels lie above the ligament.

7. The spinoglenoid ligament bridges the spinoglenoid notch. The suprascapular vessels and nerve pass deep to it.

Ossification:

The scapula ossifies from one primary centre and seven secondary centres. The primary centre appears near the glenoid cavity during the eighth week of development. The first secondary centre appears in the middle of the coracoid process during the first year and fuses by the 15th year.

The subcoracoid centre appears in the root of the coracoid process during the 10th year and fuses by the 16th to 18th years. The other centres, including two for the acromion, one for the lower two-thirds of the margin of the glenoid cavity, one for the medial border and one for the inferior angle, appear at puberty and fuse by the 25th year.

The fact of practical importance is concerned with the acromion. If the two centres appearing for acromion fail to unite, it may be interpreted as a fracture on radiological examination. In such cases, a radiograph of the opposite acromion will mostly reveal similar failure of union.