1. MacEwen's Triangle (Suprameatal Triangle / Fossa Mastoidea)

Definition

Boundaries

Key Features

• Overlies the mastoid antrum laterally - the mastoid antrum lies approximately 12-15 mm deep to this triangle in adults
• Cribrose (cribriform) area: the triangle is perforated by multiple small vessels, giving it a sieve-like (cribrose) appearance - hence the alternative name
• The spine of Henle (suprameatal spine) is a small bony prominence at the posterosuperior rim of the canal; it lies within or just anterior to the triangle and acts as a useful landmark

Surgical Importance (Vishram Singh emphasis)

1. Surface marking for the mastoid antrum: The triangle serves as a guide for cortical mastoidectomy - the surgeon drills through this area to reach the antrum
2. Mastoiditis and subperiosteal abscess: The abscess from mastoiditis points and discharges through this triangle
3. Access for mastoid exploration: Used in the Schwartze mastoidectomy (cortical mastoidectomy) to gain access to the antrum and mastoid air cells
4. Depth to antrum: In an adult, the antrum lies about 12-15 mm deep to the triangle; in a newborn, it is essentially at the surface (only 2-4 mm deep)

Lateral surface of temporal bone - note the porous/cribriform area of MacEwen's triangle

2. Foramen Magnum

Definition

Boundaries

Shape and Size

• Oval/ovoid in shape (anteroposterior diameter > transverse diameter)
• Anteroposterior diameter: ~35 mm; Transverse diameter: ~30 mm

Structures Passing Through the Foramen Magnum

From Above Downward (Endocranial to Extracranial):

Relations

• Anteriorly: Clivus slopes upward from the anterior rim of the foramen
• Laterally: Occipital condyles (articulate with atlas C1), hypoglossal canals, jugular foramina
• Posteriorly: Internal occipital crest (attachment for falx cerebelli)

Clinical Significance (Vishram Singh emphasis)

1. Tonsillar herniation (coning): Raised intracranial pressure can push the cerebellar tonsils through the foramen magnum, compressing the brainstem - a life-threatening emergency
2. Foramen magnum syndrome: Space-occupying lesions at or near the foramen (meningioma, neurofibroma, metastases) produce a characteristic pattern of lower cranial nerve palsies + upper cervical cord compression
3. Arnold-Chiari malformation: Herniation of cerebellar tonsils and brainstem through the foramen
4. McRae line: A diagnostic line drawn across the foramen magnum (anterior to posterior tip); the tip of the odontoid should be below this line - if above, it indicates basilar invagination
5. Atlanto-occipital dislocation: Trauma can disrupt the osseoligamentous structures at this level

Posterior cranial fossa (Gray's Anatomy for Students, Fig. 8.27) - the foramen magnum is the large central opening, flanked by the hypoglossal canals, jugular foramina, and groove for sigmoid sinus

Quick Comparison Summary

• Triangle boundaries marked on the lateral temporal bone
• Depth to mastoid antrum (12-15 mm)
• Cribrose/cribriform area noted
• Surgical use for cortical mastoidectomy

• Skull base (inferior view) with the oval foramen in the center
• Four parts of occipital bone labeled around it
• Contents: medulla, meninges, vertebral arteries, spinal root CN XI
• McRae line clinical note

Cervical Rib Syndrome

Definition

Anatomy of the Problem

• Anteriorly: Scalenus anterior muscle
• Posteriorly: Scalenus medius muscle
• Inferiorly: First rib

Thieme Atlas of Anatomy - cervical rib with scalene muscles, brachial plexus, and subclavian artery

Who Gets It? (Epidemiology - Vishram Singh emphasis)

• Young women are the usual victims
• Symptoms typically appear at puberty due to sagging (drooping) of the shoulder girdle as body habitus changes
• Sometimes appear later in life due to progressive weakness of shoulder girdle muscles
• Only 10% of those with a cervical rib become symptomatic - symptoms only appear when shoulder girdle muscles weaken enough to let the artery and nerve trunk compress on the rib

Three Disease Entities Produced

1. Neurogenic (Most Common - ~95%)

2. Arterial (Vascular) (~5%)

• Post-stenotic dilatation - eddy currents form
• Platelet aggregation and thrombus formation
• Distal embolization - emboli travel distally to occlude digital vessels
• Ischaemic changes in the hand and fingers (black areas of skin = embolic infarcts)
• Hand pale on elevation, cyanotic on dependent position
• Radial pulse feeble or absent on affected side
• Pain in forearm worsened by exercise

3. Venous

• Axillary vein thrombosis (effort thrombosis / Paget-Schroetter syndrome)
• Oedema, venous distension, pain, cyanosis of the upper limb

Local Signs

• Hard, fixed lump in the lower part of the posterior triangle of the neck (the rib itself, palpable above the clavicle)
• Local pain and tenderness in the supraclavicular region
• Occasionally a pulsatile mass (dilated subclavian artery) in the supraclavicular fossa

Clinical Tests

Adson's Test (most important)

• Patient keeps arm at the side, hyperextends and rotates the neck to the affected side, takes a deep breath and holds it
• Positive: Radial pulse diminishes or disappears
• Mechanism: Tenses the scalene muscles, further narrowing the interscalene space
• Also useful for Scalenus anticus syndrome

Other Tests

• Costoclavicular compressive manoeuvre: Bring shoulders back and down - reproduces symptoms
• Hyperabduction manoeuvre (Wright's test): Maximum abduction of arm - pulse diminishes (for pectoralis minor compression)
• Roos sign: Heaviness and fatigue in hands after holding arms above head for 3 minutes

Causes of Thoracic Outlet Syndrome (TOS) - Differential Approach

1. Cervical rib (bony or fibrous band)
2. Scalenus anticus syndrome - pull of scalenus anterior alone compresses the bundle
3. Costoclavicular syndrome - narrowing between first rib and clavicle
4. Pectoralis minor syndrome (hyperabduction syndrome) - compression under coracoid
5. Wide first thoracic rib
6. Fracture of first rib or clavicle (callus formation)

Investigations

Differential Diagnosis (Vishram Singh / S Das)

Treatment

Conservative

• Physiotherapy to strengthen shoulder girdle muscles (often sufficient if symptoms are mild/neurogenic only)
• Postural correction
• Avoid carrying heavy loads on shoulders

Surgical (indicated for arterial/vascular TOS or failed conservative therapy)

• Excision of cervical rib (the definitive treatment)
• Scalenectomy (division of scalenus anterior) - for scalenus anticus syndrome
• First rib resection - via transaxillary or supraclavicular approach
• If thrombus present: embolectomy + anticoagulation + rib excision

Key Exam Points (Vishram Singh Style)

• Cervical rib articulates with C7 (not C6 or C8)
• Present in 1% of population; only 10% are symptomatic
• Young women most commonly affected; onset at puberty due to shoulder girdle sagging
• Compresses lower trunk of brachial plexus (C8, T1) - hence ulnar distribution of symptoms
• Fibrous band is equally important as bony rib - not visible on X-ray
• Adson's test - rotate head to the affected side (not away)
• Treatment = excision of rib + division of fibrous band

Thieme Atlas - thoracic outlet neurovascular relationships; left: scalene triangle; right: hyperabduction position

Part 1: Differences Between Male and Female Bony Pelvis

Thieme Atlas: female pelvis (red outline) superimposed on male pelvis (gray) - note the wider pelvic inlet and broader subpubic angle in the female

Comprehensive Comparison Table

Differences in the Sacrum (between male and female)

Key Points for Exams (Vishram Singh emphasis)

• Subpubic angle is the single most reliable sex indicator: V-shape (male) vs U-shape (female)
• Greater sciatic notch: right angle or more = female; less than right angle = male
• Pre-auricular sulcus: present and prominent in females (childbirth-related)
• Obturator foramen: oval in male, triangular in female
• Parturition pits: on the dorsal border of symphysis pubis in females who have given birth
• The female pelvis is WIDER but SHALLOWER; the male pelvis is NARROWER but DEEPER
• Sex can be determined with 95% accuracy from the pelvis alone

Thieme Atlas - female pelvis, superior view showing the wide oval inlet, pubic symphysis, ischial spines, and arcuate line

Part 2: Direct vs Indirect Inguinal Hernia

Hesselbach's (Inguinal) Triangle - The Key to Understanding

• Medially: Lateral border of rectus abdominis / rectus sheath
• Superolaterally: Inferior epigastric vessels
• Inferiorly: Inguinal ligament

Rule: Indirect hernia = LATERAL to inferior epigastric vessels; Direct hernia = MEDIAL to inferior epigastric vessels (within Hesselbach's triangle)

Comprehensive Comparison Table

Clinical Tests to Differentiate

1. Ring Occlusion Test (Zieman's Test / most important)

• Reduce the hernia; press thumb firmly over the deep inguinal ring (½ inch above the midpoint of the inguinal ligament, i.e., mid-inguinal point)
• Ask patient to cough
• Indirect hernia: controlled - does NOT appear (it comes through the deep ring which is occluded)
• Direct hernia: NOT controlled - bulge appears medial to the occluding thumb (it does not pass through the deep ring)

2. Invagination Test (Finger in Canal)

• Invaginate the scrotal skin and push the finger into the superficial inguinal ring toward the deep ring
• Ask patient to cough
• Direct hernia: impulse felt on the pulp of finger (hernia comes directly forward)
• Indirect hernia: impulse felt on the tip of finger (hernia comes down the canal from above)

Anatomical Diagram

Thieme Atlas - indirect inguinal hernia (top): exits lateral deep inguinal ring, within spermatic cord; direct inguinal hernia (middle): exits through Hesselbach's triangle, medial to cord; femoral hernia in female (bottom): passes below inguinal ligament

Summary Mnemonic

"MDs get Direct" - Middle-aged, Degenerative, Directly through the wall, Medial to epigastrics

"Young lads get Indirect" - Young age, processus vaginalis, Lateral to epigastrics, down Into scrotum

Comparison at a Glance