The sternum, also known as the breastbone, is a long flat bone located in the anterior median portion of the chest wall. It consists of three parts - the manubrium, body, and xiphoid process. The manubrium is the thickest and strongest part, while the body is longer and narrower. The xiphoid process is the smallest portion at the bottom of the sternum. Several muscles attach to the surfaces of the sternum, including the pectoralis major and diaphragm. The sternum provides attachments for ribs and costal cartilages. It is an important landmark for anatomical structures in the thorax such as the heart and great vessels.

1. OSTEOLOGY- STERNUM
By:
Phanindra Prasad Poudel
Assistant Professor
Department of Anatomy
MCOMS, Pokhara

2. STERNUM
• Sternum (breast bone) is an elongated flat bone.
• It lies in the anterior median part of the chest wall.
• It is about 17 cm long.

3. • Sternum consists of 3
parts:
1. Manubrium
(episternum)
2. Body
(mesosternum)
3. Xiphoid process
(metasternum)

4. • Sternum resembles a
dagger or a small sword
in shape.
• Its 3 parts- manubrium,
body and xiphoid process
represent the handle,
blade and point of the
sword respectively.

5. • Upper part of sternum is
broad and thick.
• Lower part is thin and
pointed.
• Anterior surface is slightly
rough and convex.
• Posterior surface is smooth
and slightly concave.

6. • Manubrium and body of sternum lie at an angle of 163° to each
other.
• Which increases slightly during inspiration and decreases during
expiration.
• Angle between long axis of manubrium and long axis of body of
sternum is about 17°.

7. Anatomical Position of Sternum:
• It is directed downwards and inclined slightly forward
with its rough convex surface facing anteriorly.
• Its broad end is directed upwards and lower pointed end
is directed downwards.

8. MANUBRIUM (EPISTERNUM):
• It is roughly quadrilateral in
shape.
• It lies opposite to the T4 & T5
vertebrae.
• It is the thickest and strongest
part of the sternum and presents
the following features:
• Two surfaces:
– anterior and
– posterior.
• Four borders:
– superior
– Inferior and
– lateral (right and left).

9. Anterior surface:
• Provides attachment to:
– sternal head of Sternocleidomastoid and
– Pectoralis major.

10. Posterior surface: It is smooth and forms the anterior
boundary of superior mediastinum.
Attachments and relations:
• Sternohyoid at the level of clavicular notch.
• Sternothyroid at the level of facet for 1st costal cartilage.
• Lower half is related to the arch of aorta.
• Upper half is related to 3 branches of arch of aorta, viz.
brachiocephalic artery, left common carotid artery, left
subclavian artery, and left brachiocephalic vein.

11. Upper border:
• It is thick, rounded and concave.
• It presents a notch called suprasternal notch
or jugular notch.
• It provides attachment to the interclavicular
ligament.
• Clavicular notch on either side of the
suprasternal notch articulates with the
clavicle to form sternoclavicular joint.

12. Lateral border:
• It presents two articular facets:
• Upper facet articulates with the
1st costal cartilage to form
primary cartilaginous joint.
• Lower demifacet along with
other demifacet in the body of
sternum articulates with the 2nd
costal cartilage.

13. Lower border:
• It articulates with the upper end of the body of sternum to form secondary
cartilaginous joint called manubriosternal joint.
• Manubrium makes a slight angle with the body at this junction called sternal
angle or angle of Louis.
• Lies at the level of lower border of T4.
• It is recognized by the presence of a transverse ridge on the anterior aspect of
the sternum.

14. II CLASS

15. BODY (MESOSTERNUM):
• It is longer, narrower and
thinner than the
manubrium.
• It is broadest at its lower
end.
• Upper end articulates with
the manubrium at the
sternal angle to form
manubriosternal joint.

16. • Lower end articulates with the
xiphoid process to form primary
cartilaginous xiphisternal joint.
• Anterior surface presents 3 faint
transverse ridges indicating the lines
of fusion of 4 small segments called
sternebrae.
• Anterior surface on each side gives
origin to the pectoralis major
muscle.
• Posterior surface is smooth and
slightly concave.
• Lower part of posterior surface gives
origin to sternocostalis muscle.

17. • On the right side of median plane,
posterior surface is related to pleura,
which separates it from the lung.
• On the left side of median plane,
upper half of the body is related to
the pleura and lower half to the
pericardium.

18. • Lateral border articulates with the 2nd–7th costal cartilages to
form synovial joints.
• 2nd costal cartilage articulates at the side of manubriosternal
junction and 7th costal cartilage articulates at the xiphisternal
junction.

19. XIPHOID PROCESS (METASTERNUM):
• It is the lowest and smallest part of the sternum.
• It varies greatly in size and shape.
• It may be bifid or perforated.
• Anterior surface provides insertion to the medial fibres of the rectus
abdominis.
• Posterior surface gives origin to the sternal fibres of the diaphragm.
• Tip provides attachment to the upper end of linea alba.

20. Muscles attached on the
anterior & posterior surfaces of
sternum:
• Muscles attached on the
anterior surface:
– Sternal head of
sternocleidomastoid
– Pectoralis major
– Rectus abdominis
• Muscles attached on the
posterior surface:
– Sternohyoid
– Sternothyroid
– Sternocostalis
– Diaphragm (sternal fibres)

21. N.B. Features of interest at the sternal angle:
• It can be felt as a transverse ridge on the sternum about 5 cm below
the suprasternal notch.
• Sternal angle is an important landmark for many anatomical events at
this level, these are:
• Articulation of second costal cartilage with sternum, hence this level
is used for counting the ribs.
• It lies at the level of intervertebral disc between T4 and T5 vertebrae.

22. • Horizontal plane passing through
this level separates superior
mediastinum from inferior
mediastinum.
• Ascending aorta ends at this level.
• Arch of aorta begins and ends at
this level.
• Descending aorta begins at this
level.

23. • Trachea bifurcates into right and left
principal bronchi at this level.
• Pulmonary trunk divides into right
and left pulmonary arteries at this
level.
• Upper border of heart lies at this
level.
• Azygos vein arches over the root of
right lung to end in the superior
vena cava.

24. Sternal puncture:
• Manubrium sterni is the preferred site for bone marrow aspiration because it is
subcutaneous and readily accessible.
• The bone marrow sample is required for hematological examination.
• A thick needle is inserted into the upper part of manubrium to avoid injury to arch of
aorta which lies behind the lower part.
• Sternal puncture is not advisable in children because in them the plates of compact
bone of sternum are very thin and if needle passes through the manubrium it will
damage the arch of aorta and its branches, leading to fatal hemorrhage.

25. Mid-sternotomy:
To gain access to the
mediastinum for:
• Surgical operations on
heart and great blood
vessels.
• Sternum is often divided
in the median plane
called midsternotomy.

26. Funnel chest (pectus excavatum):
• Chest is compressed
anteroposteriorly
• Sternum is pushed backward by
the overgrowth of the ribs.
• May compress the heart.
Pigeon chest (pectus carinatum):
• Chest is compressed from side-to-
side.
• Sternum projects forward and
downward like a keel of a boat.

27. Sternal fracture:
• It is common in automobile
accidents.
• When the driver’s chest is hit
against the steering wheel, the
sternum is often fractured at
the sternal angle.
• The backward displacement of
fractured fragments may
damage aorta, heart, or liver
and cause severe bleeding
which may prove fatal.

28. OSSIFICATION:
• Sternum develops from two vertical cartilaginous plates (sternal plates), which
fuse in the midline.
• It ossifies from six double centers.
– One for manubrium.
– Four for body.
– One for xiphoid process.

29. Appearance of ossification centers:
• Ossification centers appear in descending order for different parts of sternum as
follows:
• Manubrium: 5th month
• Body:
– First sternebra: 6th month
– Second sternebra: 7th month
– Third sternebra: 8th month
– Fourth sternebra: 9th month
• Xiphoid process: 3rd year

30. Fusion: Fusion occurs as follows:
• Fusion between sternal plates takes place from below upwards.
• It begins at puberty and completed by 25 years.
• Xiphoid process fuses with the body at the age of 40 years.
• Manubrium does not fuse with the body throughout life.

31. Sternal foramen and cleft sternum:
• Two sternal plates fuse in caudocranial direction.
• Sometimes sternebrae fail to fuse in the midline, as a result
defect occurs in the body of sternum in the form of sternal
foramen or cleft sternum.
• The cleft sternum is often associated with ectopia cordis.

32. ASSIGNMENT WITH IN 24 HOURS- MCQs
1. Sternal angle lies at which of the following vertebral level?
a. Upper border of T4
b. Lower border of T4
c. Upper border of T5
d. Lower border of T5
2. Pick up a muscle which is attached on the posterior
surface of the manubrium sterni.
a. Sternothyroid
b. Sternocleidomastoid
c. Pectoralis major
d. Pectoralis minor

33. 3. What is the time of completion of fusion between two
sternal plates of the body of sternum?
a. Puberty
b. 23 years
c. 24 years
d. 25 years
4. Which part of the sternum is commonly used for the bone
marrow aspiration?
a. Upper part of manubrium
b. Lower part of manubrium
c. Upper part of body of sternum
d. Lower part of body of sternum

34. HOME ASSIGNMENT:
1. Describe the features of different parts of sternum
with the help of well labeled diagrams?
2. Write about the ossification and clinical anatomy
related to the sternum.
THE END