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Surface marking refers to the projection of internal anatomical structures onto the external surface of the body. Understanding these lines and points helps correlate clinical signs, radiological images, and procedural landmarks during examination or surgery
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Cervical Pleura:
Represented by a curved line forming a dome over the medial one-third of the clavicle, rising about 2.5 cm above it on both sides.
Anterior Margin (Costomediastinal Reflection):
Right side: From the sternoclavicular joint, it descends to the midpoint of the sternal angle, then continues vertically to the xiphisternal joint, slightly to the right of the xiphicostal angle.
Left side: Similar course until the fourth costal cartilage, then curves laterally and downward along the sternal margin to the sixth costal cartilage
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.Inferior Margin (Costodiaphragmatic Reflection):
The line passes laterally across:
8th rib at the midclavicular line,
10th rib at the midaxillary line,
12th rib at the lateral border of the sacrospinalis muscle,
and ends 2 cm lateral to the T12 spine.
Clinically, the pleura extends below the costal margin at the xiphicostal and costovertebral angles, making these regions prone to pleural injury during renal surgery
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.Posterior Margin:
Extends from a point 2 cm lateral to the T12 spine to 2 cm lateral to the C7 spine, where the costal pleura becomes mediastinal pleura.
Apex:
Lies 2.5 cm above the medial one-third of the clavicle, following the dome of the cervical pleura.
Anterior Border:
Right lung: Follows the same path as the right pleural reflection from the sternoclavicular joint to the xiphisternal joint.
Left lung: Similar until the fourth costal cartilage, then curves laterally for 3.5 cm from the sternal margin, forming the cardiac notch, before curving back to the sixth costal cartilage
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.Inferior Border:
Crosses the:
6th rib at the midclavicular line,
8th rib at the midaxillary line,
10th rib posteriorly at the lateral border of the erector spinae.
Posterior Border:
Extends from the apex (2.5 cm above clavicle) down to T10 vertebra in the mid-scapular line, slightly below the posterior pleural line.
Knowledge of these markings helps in procedures like pleural aspiration (done in the 8th–9th intercostal space, posterior axillary line).
The pleural recesses (costodiaphragmatic and costomediastinal) are key sites for fluid accumulation in pleural effusion.
Accurate surface marking prevents lung or pleura injury during interventions such as thoracentesis and biopsies.
The heart lies obliquely in the thoracic cavity, with two-thirds to the left of the midline and one-third to the right.
The surface marking is obtained by joining points over the thoracic wall corresponding to the heart’s margins and chambers.
1. Right Border:
Formed mainly by the right atrium.
Extends from the right 3rd costal cartilage (1 cm from the right margin of sternum) to the right 6th costal cartilage near the sternal margin.
2. Inferior Border:
Formed chiefly by the right ventricle, with a small contribution from the left ventricle.
Draw a line from the right 6th costal cartilage (sternal margin) to the left 5th intercostal space, 9 cm from the midline, corresponding to the apex beat.
3. Left Border:
Formed mainly by the left ventricle.
Extends from the apex to the left 2nd costal cartilage, 1.5 cm from the sternal margin.
4. Upper Border:
Formed by the right and left atrial appendages and the ascending aorta.
Draw a line from the left 2nd costal cartilage (1.5 cm from sternum) to the right 3rd costal cartilage (1 cm from sternum).
Clinical Note:
The area between the 4th and 6th costal cartilages on the left sternal margin represents the area of cardiac dullness on percussion.
The apex beat is normally felt in the left 5th intercostal space, 9 cm from the midline, just medial to the midclavicular line.
1. Ascending Aorta:
Begins behind the left 3rd costal cartilage, slightly left of the sternum.
Ascends upward to reach the level of the sternal angle (T4/T5).
2. Arch of Aorta:
From the sternal angle, curves posteriorly and to the left, reaching the left 2nd costal cartilage.
It then descends behind the left bronchus to continue as the descending thoracic aorta at the level of T4 vertebra.
The arch gives off three main branches (from right to left):
Brachiocephalic trunk
Left common carotid artery
Left subclavian artery
3. Pulmonary Trunk:
Begins at the upper border of the left 3rd costal cartilage, runs upward and backward.
Divides into right and left pulmonary arteries at the level of the sternal angle.
Lies anterior to the ascending aorta in its initial part.
4. Descending Thoracic Aorta:
Lies to the left of the vertebral column, continuing from the arch of aorta at T4, and descends to the aortic opening of the diaphragm at T12.
1. Superior Vena Cava (SVC):
Formed by the union of the right and left brachiocephalic veins behind the first right costal cartilage.
Descends vertically along the right border of the sternum.
Enters the right atrium at the level of the 3rd costal cartilage.
2. Inferior Vena Cava (IVC):
Enters the thorax through the central tendon of the diaphragm at the T8 vertebral level.
Opens into the lower part of the right atrium behind the right 6th costal cartilage.
3. Pulmonary Veins:
Each lung gives two veins — superior and inferior.
They open into the left atrium — superior veins lie anterior and inferior veins lie posterior.
Clinical Correlation:
Enlargement of the SVC or azygos vein may cause visible venous engorgement over the chest wall.
The SVC syndrome occurs due to compression by mediastinal tumors or enlarged lymph nodes, leading to venous congestion of the head, neck, and upper limbs.
The trachea is represented on the surface by a midline structure in the neck and upper thorax.
Begins at the lower border of the cricoid cartilage (C6) and descends to the sternal angle (T4/T5), where it divides into right and left main bronchi.
It lies anterior to the oesophagus and is slightly tilted to the right.
Surface Marking:
In the neck, it lies in the midline from C6 to the suprasternal notch.
In the thorax, it continues downward behind the manubrium sterni, ending at the sternal angle, where the carina corresponds to the site of bifurcation.
Clinical Relevance:
The trachea may deviate to one side in pneumothorax, pleural effusion, or collapse of lung.
Tracheostomy is usually done between the 2nd and 4th tracheal rings.
The carina is the most sensitive area for the cough reflex.
The right principal bronchus begins at the lower end of the trachea at the level of the sternal angle (T4/T5).
It passes downward, backward, and laterally to enter the right lung hilum opposite the T5 vertebra.
The right bronchus is shorter (2.5 cm), wider, and more vertical than the left, making it a common site for inhaled foreign bodies.
On surface projection, it runs behind the sternal end of the 3rd right costal cartilage to the root of the right lung.
Clinical Relevance:
Foreign bodies, aspirated food particles, or vomitus often lodge in the right bronchus due to its vertical course.
Bronchoscopic view shows the right upper lobe bronchus arising before the carina — a feature known as the eparterial bronchus.
The left principal bronchus also begins at the sternal angle and runs downward, backward, and laterally beneath the arch of aorta, entering the left lung opposite the T6 vertebra.
It is longer (5 cm), narrower, and more horizontal than the right bronchus.
On the body surface, it corresponds roughly to a line drawn from the sternal angle to the root of the left lung (near the 4th costal cartilage).
Clinical Relevance:
Because the left bronchus passes under the arch of aorta and in front of the oesophagus and descending aorta, it is often compressed in aortic aneurysm or enlarged lymph nodes.
The left bronchus has no eparterial branch; all its lobar bronchi arise below the pulmonary artery (hence, hyparterial).
The oesophagus begins at the lower border of the cricoid cartilage (C6), runs downward through the superior and posterior mediastina, and passes through the diaphragm at T10, ending in the stomach at T11.
On the body surface, it lies behind the trachea in the midline, curving slightly to the left as it descends.
The lower end lies a little to the left of the midline, behind the 7th costal cartilage near the xiphisternal joint.
Constrictions (clinically important for endoscopy):
At C6 – at the cricoid cartilage.
At T4/T5 – where crossed by the aortic arch and left main bronchus.
At T10 – where it passes through the diaphragm.
Clinical Relevance:
Barium swallow radiographs show these constrictions as natural narrowings.
Left atrial enlargement may produce a posterior indentation on the oesophagus.
In portal hypertension, veins at the lower end of the oesophagus form varices, which may rupture and cause haematemesis.
Tracheoesophageal fistula may occur congenitally, causing coughing during feeding in infants.
The thoracic duct begins in the abdomen at the cisterna chyli (around the lower border of T12) and enters the thorax through the aortic opening of the diaphragm.
It ascends between the aorta (on the left) and azygos vein (on the right), behind the oesophagus in the posterior mediastinum.
At the T5 vertebral level, it crosses from the right to the left side and ascends in the superior mediastinum along the left side of the oesophagus.
In the neck, it arches laterally and downward to open into the left venous angle — the junction of the left subclavian and left internal jugular veins.
Surface Projection:
A line drawn from the aortic opening (T12) upward along the vertebral column, crossing to the left at T5, and ending at the left supraclavicular region (2 cm above the clavicle) represents its surface marking.
Clinical Relevance:
Chylothorax — rupture of the thoracic duct leads to accumulation of lymph (chyle) in the pleural cavity.
Obstruction by tumors or fibrosis causes lymphatic congestion of the lower limbs and left upper limb.
The duct must be preserved during neck dissections and mediastinal surgeries to prevent chyle leak.
When you look at a standard posteroanterior (PA) chest X-ray, you must read it in an organized way. These are the key structures you’re expected to identify and comment on.
Nipples may be seen as round shadows overlying the lung fields in both sexes.
In females, the breast shadows overlap the lower zones of the lungs.
The amount of overlap depends on breast size and how pendulous they are.
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Why it matters:
Nipple shadows can mimic lung nodules if you don’t recognize them.
Breast tissue can partially obscure lower lung fields.
Vertebral bodies are faintly seen.
Posterior parts of the ribs are more clearly seen than anterior parts, because they are more calcified.
Ribs become wider and thinner as they pass forward.
Costotransverse joints are visible.
Costal cartilages are normally not seen unless calcified in older age.
Medial border of the scapula may project over the lung field if the patient hasn’t protracted the shoulders.
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Why it matters:
Always check for fractures, lytic lesions, or rib destruction.
Misplaced scapula can look like a lung opacity.
Seen as a vertical air-filled lucent column in the midline of the neck and upper thorax.
Lies in front of the upper thoracic vertebrae.
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Clinical note:
Shift of trachea suggests volume loss (pull) or mass/effusion (push).
Pulmonary collapse pulls trachea toward that side.
Tension pneumothorax pushes trachea away.
Seen as two dome-shaped opaque curvatures.
The right dome is normally slightly higher than the left (because of the liver).
The sharp angles between diaphragm and chest wall are the costophrenic angles (right and left).
Blunting of a costophrenic angle suggests pleural effusion.
Under the left diaphragm you may see gas in the stomach fundus;
Under the right dome is the smooth soft-tissue density of the liver.
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Lung fields appear radiolucent (dark) because they are air-filled.
The hila/root of each lung forms a denser shadow due to:
Main bronchi
Pulmonary arteries and veins
Bronchial vessels
Hilar lymph nodes
As you go peripherally, branching vascular markings taper and fade.
The smaller bronchi are usually not seen clearly on normal X-ray.
For description, each lung can be divided into 3 vertical zones on PA film:
Upper zone: From apex down to the level of the 2nd costal cartilage.
Middle zone: 2nd to 4th costal cartilage (includes the hilum).
Lower zone: 4th costal cartilage down to the diaphragm.
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Why this matters:
You never say "there’s an opacity in the right lung."
You say "there’s a rounded opacity in the right mid zone, medial aspect," etc. This is how cases are presented.
The mediastinum on X-ray is not one single organ. The “mediastinal shadow” is produced by the heart plus great vessels plus superimposed soft tissues.
The transverse diameter of the heart is normally about half the transverse diameter of the thoracic cage on PA film.
During deep inspiration, the heart appears slightly elongated and more tubular, because it descends.
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Borders of the mediastinum on PA film:
Right mediastinal border (from top to bottom) is formed by:
Right brachiocephalic vein
Superior vena cava
Right atrium
Inferior vena cava (at the lower end)
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Left mediastinal border (from top to bottom) is formed by:
Arch of aorta (seen as the classic “aortic knuckle”)
Left margin of the pulmonary trunk
Left atrial appendage (left auricle)
Left ventricle, which forms most of the lower left heart border
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Inferior mediastinal border merges with:
Diaphragm
Liver shadow on the right
Why it matters:
If the aortic knuckle is widened → think aortic aneurysm.
If the pulmonary trunk segment is prominent → think pulmonary hypertension.
If the right atrial border is bulging → think right atrial enlargement.
If the left ventricular border is displaced laterally and inferiorly → think left ventricular enlargement.
Soft tissues (breast, nipple shadows, neck)
Bones (ribs, clavicles, vertebrae, scapula)
Trachea and mediastinum (position and width)
Diaphragm and costophrenic angles
Lung fields (symmetry, opacities, lucencies, vascularity)
Gastric air bubble (left subdiaphragmatic)
This sequence is often expected in viva.
These numbers are extremely commonly asked.
Anteroposterior diameter of the thoracic inlet: ~5 cm
Transverse diameter of the thoracic inlet: ~10 cm
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Level of suprasternal (jugular) notch: opposite the body of T2 vertebra
Level of sternal angle (manubriosternal joint): at the intervertebral disc between T4 and T5
Also the level where:
Arch of aorta begins and ends
Trachea bifurcates
Azygos vein drains into SVC
2nd costal cartilage articulates with sternum
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Level of xiphisternal joint: opposite T9 vertebra
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Subcostal angle: Between the right and left 7th costal cartilages at their sternal attachments (essential landmark for upper abdominal exam).
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Vertebra prominens: Spinous process of C7 (palpable at base of neck).
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Scapular surface landmarks:
Superior angle of scapula → level of T2 spine
Root (medial end) of spine of scapula → level of T3 spine
Inferior angle of scapula → level of T7 spine
These are used to judge rib levels on the back.
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Length of oesophagus: ~25 cm total
Cervical part: ~4 cm
Thoracic part: ~20 cm
Abdominal part: ~1.25 cm
Begins at C6
Ends at stomach at T11
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Trachea:
Begins at C6
Length: ~10–15 cm
Bifurcation: upper border of T5 vertebra (≈ sternal angle in living adult)
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Principal bronchi lengths:
Right principal bronchus: ~2.5 cm
Left principal bronchus: ~5 cm
This explains why inhaled foreign bodies prefer the right side.
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These radiological checkpoints and numeric levels are exactly what examiners love:
"At what vertebral level does the trachea bifurcate?"
"What forms the right and left borders of the mediastinal shadow?"
"Why is the right hemidiaphragm higher than the left?"
"Which bronchus is more vertical and why is that clinically important?"
Memorizing these lets you answer fast and confidently in both theory and viva.
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