Surface, Imaging and Procedural Anatomy

🧠 Applied anatomy — Surface, Imaging and Procedural Anatomy. Regional structures, boundaries, named neurovascular relationships, bedside localisation and selected clinical cases. Focus on what changes when a vessel, nerve or fascial compartment is injured.

Overview and anatomical principles

• Neck and thorax: Cricoid cartilage roughly aligns with C6 in adult, sternal angle with T4/T5 and second rib, and jugular notch with upper thoracic inlet. These are guides affected by posture and body habitus.
• Systematic tracing: Identify orientation, level and modality, then label bones, muscle compartments, vascular structures, viscera and potential spaces before naming pathology.
• Lumbar puncture: Below conus medullaris, usually L3–L4/L4–L5, through skin, supraspinous/interspinous ligaments, ligamentum flavum, epidural space and meninges to subarachnoid CSF.
• Peripheral nerve block: A nerve travels with variable vessels and surrounding fascia; anatomical knowledge of target, sensory territory and motor effects is needed for safe trained use.
• Thoracic CT: On axial imaging identify sternum, vertebral body, descending aorta and lungs before tracing mediastinal vessels. Aortic arch, carina and pulmonary hila define useful levels; image labels determine patient left and right.

From a physical sign to a structure

• Surface anatomy offers the first localisation hypothesis: palpable bony landmarks, pulses, tendons, organ edges and neurological distributions guide further examination.
• The radial pulse is located lateral at the wrist; the femoral artery is usually palpated midway between the anterior superior iliac spine and pubic symphysis, though exact surface relations vary.
• The apex beat commonly lies near the fifth left intercostal space at or medial to the midclavicular line in an adult, but body habitus and cardiac disease alter its position.

The best anatomical reasoning uses more than a single landmark. A localised neurological deficit should be tested against peripheral nerve, root, spinal cord, brainstem and cerebral possibilities. A mass should be assigned to a specific compartment and its relation to adjacent nerves, vessels and organs determined.

Systematic reading of cross-sectional imaging

• On thoracic CT, identify right and left, then vertebral bodies, aorta, vena cava, airways, heart chambers, pleura and lung segments.
• On abdominal CT, locate liver, spleen, kidneys, pancreas, portal system, aorta, inferior vena cava and bowel by anatomical level.
• On neuroimaging, identify grey-white differentiation, ventricles, basal ganglia, insula, brainstem, cerebellum and major vascular territories before focusing on an abnormality.

• Contrast phase matters: arteries, portal veins and renal parenchyma enhance differently.
• MRI sequences reveal different tissue properties, and a bright signal is not meaningful without knowing the sequence.
• Ultrasound offers real-time spatial information but requires correct probe orientation, adjustment and confirmation in orthogonal planes.

Procedure-oriented safety anatomy

• Pleural procedures: account for pleural recesses, diaphragm and nearby abdominal viscera; use image guidance and current local protocols.
• Lumbar puncture: confirm there is no clinical contraindication, assess neurological risks and enter at a lumbar level below the usual adult cord termination.
• Central venous access: ultrasound defines the target vein, adjacent artery and trajectory; sterile technique and complication monitoring remain essential.
• Arterial access: identify vessel course, surrounding nerves, potential collaterals and haemostasis requirements.
• Regional anaesthesia: nerve block planning must account for vessel proximity, fascial planes and anatomical variation.

Anatomy at different ages

• Infant proportions, growth plates and developmental variants alter landmarks and expected imaging appearances.
• In older adults, reduced muscle mass, altered posture and vascular tortuosity can change surface relationships; age-related brain atrophy also affects the mechanics of bridging veins.
• Anatomy should therefore be applied to the individual rather than treated as a fixed template.

Surface landmark interpretation

• Neck and thorax: Cricoid cartilage roughly aligns with C6 in adult, sternal angle with T4/T5 and second rib, and jugular notch with upper thoracic inlet. These are guides affected by posture and body habitus.
• Abdomen: Xiphisternum, costal margins, umbilicus, iliac crests, ASIS and pubic tubercles locate nine abdominal regions and guide descriptions of tenderness or masses.
• Limb vessels: Brachial pulse near medial biceps tendon in cubital fossa, radial at distal radial wrist, femoral midway between ASIS and pubic symphysis, posterior tibial behind medial malleolus; anatomy is variable.
• Neurological levels: Named dermatomes and myotomes are approximations; a full pattern incorporating muscle action, sensory modality, reflex and imaging is more accurate than one landmark.
• Landmark limitations: Obesity, kyphoscoliosis, pregnancy, postoperative change and tumours distort correspondence between skin marking and internal structures; examine, image and re-evaluate.

Approach to cross-sectional imaging

• Systematic tracing: Identify orientation, level and modality, then label bones, muscle compartments, vascular structures, viscera and potential spaces before naming pathology.
• Contrast phases: Non-contrast, arterial, portal venous and delayed studies emphasise different structures; interpreting vessel opacification requires knowing acquisition phase and expected flow.
• Planes: Axial sections reveal transverse relations; coronal views clarify cranio-caudal spread, while sagittal views are often best for spinal alignment and midline brain structures.
• Air and fluid: In normal anatomy, air is confined to airways and gut; unexpected free air, fluid or gas within fascial planes has different implications according to location and clinical context.
• Vascular maps: Arterial branches, venous confluences and lymphatic stations can be traced slice by slice. Never infer a variant vessel from a single still image when multiplanar imaging is available.

Common procedures: anatomical risk map

• Lumbar puncture: Below conus medullaris, usually L3–L4/L4–L5, through skin, supraspinous/interspinous ligaments, ligamentum flavum, epidural space and meninges to subarachnoid CSF. Check contraindications and site according to local protocol.
• Pleural procedures: Avoid intercostal neurovascular bundle, assess diaphragm and underlying liver/spleen and use ultrasound appropriately. Posterior and basal effusions can shift with posture.
• Central venous access: Jugular vein lies within carotid sheath lateral to carotid artery; pleura and other vessels lie nearby. Ultrasound-guided assessment of actual vessel course is essential.
• Femoral access: Common femoral artery over femoral head and vein medially are access targets; high puncture risks retroperitoneal bleeding and low puncture risks vascular complications.
• Abdominal paracentesis: Avoid inferior epigastric arteries and underlying bowel; ultrasound detects safest fluid pocket and accounts for adhesions or organ displacement.

Procedure-related neurological anatomy

• Peripheral nerve block: A nerve travels with variable vessels and surrounding fascia; anatomical knowledge of target, sensory territory and motor effects is needed for safe trained use.
• Intramuscular injection: Deltoid region contains axillary nerve and posterior circumflex vessels at risk if site is too distal; ventrogluteal approach reduces risk to sciatic nerve when appropriately performed.
• Urinary catheterisation: Male prostatic and membranous urethral regions are anatomically different from female short urethra; suspected urethral injury changes the safe assessment pathway.
• Chest and abdominal drains: Puncture paths cross multiple layers and can enter an unexpected cavity if body habitus or fluid shifts change the planned geometry.
• Stop rules: Unexpected pain, arterial blood, resistance or neurological deficit can signal incorrect placement or structural injury; follow supervised local escalation protocols rather than persisting through uncertainty.

Cross-sectional orientation and common bedside landmarks

• Thoracic CT: On axial imaging identify sternum, vertebral body, descending aorta and lungs before tracing mediastinal vessels. Aortic arch, carina and pulmonary hila define useful levels; image labels determine patient left and right.
• Abdominal CT: Find aorta/IVC, portal confluence behind pancreatic neck, renal hila and psoas before tracing ureters. Contrast phase changes appearance of arteries, veins and collecting systems, so a vessel may be inconspicuous on one acquisition.
• Pelvic imaging: Ureters enter pelvis over iliac vessels and track towards bladder; female uterine artery crosses anterior/superior to distal ureter. Bladder filling changes the relationships and visibility of adjacent pelvic organs.
• Cervical ultrasound: Distinguish carotid artery from internal jugular vein by compressibility, pulse and Doppler rather than position alone. Variant overlap is common; inadvertent arterial puncture remains possible with landmark-only approaches.
• Femoral landmarking: Femoral artery lies near midpoint between anterior superior iliac spine and pubic symphysis, with femoral nerve lateral and vein medial. Ultrasound is preferred where applicable and separates true common femoral region from more distal bifurcation.
• Spinal level: Iliac crest line is a rough guide near L4/L4–L5, but variation in position, body habitus and spinal anatomy makes this imprecise. Cross-sectional or ultrasound confirmation may be needed for difficult neuraxial access.
• Pleural access: Lung and pleural boundaries shift with respiration and position; ultrasound identifies fluid, diaphragm and nearby organs. Surface drawings should not be treated as a fixed safe map for needle passage.
• Abdominal wall vessels: Inferior epigastric arteries rise from external iliac behind rectus; they may be injured by trocars or drains. Imaging or direct visualisation is especially valuable with altered surgical anatomy.

Integrating anatomy into patient safety

• Imaging sequences: MRI T1 clarifies anatomy and marrow fat; T2 and fluid-sensitive sequences highlight oedema, cysts and many pathological processes. Interpretation depends on sequence parameters and contrast, not a universal bright/dark mnemonic.
• Vascular territory test: Match observed deficit to arterial distribution and structural pathway; for example, aphasia suggests dominant hemisphere networks, while isolated foot drop suggests selected root or peripheral nerve possibilities.
• Musculoskeletal examination: Identify tendon action, prime-mover muscle, nerve, root and adjacent joint. Test both active and passive range and compare bilaterally; pain, tendon injury and denervation each impair movement differently.
• Lymphatic staging: Anatomical nodal stations are relevant to breast, lung, gastrointestinal and head-and-neck malignancies. A palpable or enlarged node's significance depends on regional drainage and disease-specific staging rules.
• Contrast timing: Arterial-phase imaging highlights arterial tree and active haemorrhage, portal-venous phase characterises abdominal organs and venous drainage, and delayed phases may demonstrate collecting-system leaks.
• Point-of-care ultrasound: Can establish a fluid pocket, identify a vessel and show organ movement, but image quality and interpretation depend on training and equipment. It augments rather than removes anatomical risk.
• Consent and communication: Explain a procedural complication by reference to adjacent nerve, vessel and organ anatomy. Patient-specific anatomical variation should be reflected in procedural planning and documentation.
• Teaching retrieval strategy: For each image or bedside case, list three structures, two relations and one clinical consequence; then check an atlas or appropriate radiological study. This encourages mechanism-based retention rather than memorising labels without context.

High-yield anatomical relationships

Exam-style clinical localisation

💡 Clinical reasoning: Translate the site of a lesion into expected motor, sensory, vascular and visceral findings. Correlate with examination and imaging; anatomical variants are common.

1. Clinical scenario: During preparation for thoracentesis the surface landmark suggests a pleural recess, but ultrasound shows liver immediately beneath a small fluid pocket. What is the anatomical lesson?

   Anatomical explanation: Inferior borders of lung, pleura and diaphragm shift with breathing, posture, effusion size and patient variation. Static rib-number maps describe typical surface anatomy but cannot reliably define a safe needle path for an individual. Ultrasound demonstrates target fluid, organ movement and adjacent viscera in real time, reducing risk of liver, spleen or lung injury. The correct action is to reassess the site and follow current procedural guidance rather than insist on the remembered landmark.

2. Presentation: What additional information does ultrasound provide before a central venous puncture beyond locating the expected surface landmark?

   Anatomical reasoning: It shows the actual vein, artery, nearby structures, anatomical variants and their relationships in real time, helping select a safer needle path.

3. Presentation: A clinician prepares pleural aspiration based only on rib counting despite a large liver and elevated diaphragm. What is the anatomical error?

   Anatomical reasoning: Surface landmarks cannot guarantee lung, pleura and liver positions; ultrasound is needed to map safe pocket and needle path.

4. Presentation: A patient has a large retroperitoneal bleed after overly high femoral arterial puncture. Why can compression fail?

   Anatomical reasoning: Puncture proximal to inguinal ligament enters relatively non-compressible pelvic/retroperitoneal space rather than vessel over femoral head.

Core examination recall

• Thoracic CT: On axial imaging identify sternum, vertebral body, descending aorta and lungs before tracing mediastinal vessels.
• Imaging sequences: MRI T1 clarifies anatomy and marrow fat; T2 and fluid-sensitive sequences highlight oedema, cysts and many pathological processes.
• Femoral landmarking: Femoral artery lies near midpoint between anterior superior iliac spine and pubic symphysis, with femoral nerve lateral and vein medial.
• Contrast timing: Arterial-phase imaging highlights arterial tree and active haemorrhage, portal-venous phase characterises abdominal organs and venous drainage, and delayed phases may demonstrate collecting-system leaks.
• Abdominal wall vessels: Inferior epigastric arteries rise from external iliac behind rectus; they may be injured by trocars or drains.
• Teaching retrieval strategy: For each image or bedside case, list three structures, two relations and one clinical consequence; then check an atlas or appropriate radiological study.

Reference framework and further reading

• Bailey & Love’s Essential Clinical Anatomy (2019): Regional surface anatomy and imaging.
• Sinnatamby, Last’s Anatomy: Regional and Applied, 12th edition (2011): Regional clinical anatomy throughout.
• Review anatomical variations and relevant original imaging before clinical or surgical procedures. Publisher-owned diagrams and text are not reproduced.

Expanded original anatomy: structure-by-structure facts

Integrated from the original Makindo anatomy chapters (209–210). These lists extend the regional and clinical anatomy notes above; original chapter URLs remain available.

Surface Anatomy of the Thorax and Heart

• Sternal angle (angle of Louis) - articulation of manubrium and body of sternum; level of the 2nd costal cartilage and the T4/T5 disc. Marks: start and end of the aortic arch, tracheal bifurcation (carina), azygos vein entering the SVC, upper border of the pericardium, the thoracic duct crossing to the left. Count ribs from here.
• Heart borders: right border - right atrium (3rd to 6th right costal cartilages, just right of the sternum); left border - left ventricle (2nd left intercostal space to the apex); inferior border - mainly right ventricle.
• Apex beat - normally 5th left intercostal space, midclavicular line; displaced laterally in left ventricular dilatation.
• Auscultation areas: aortic - 2nd right intercostal space at the sternal edge; pulmonary - 2nd left ICS; tricuspid - 4th/5th left ICS at the lower left sternal edge; mitral - apex.
• Lungs and pleura: the pleura extends lower than the lungs. "6, 8, 10, 12" rule - lower lung border at the 6th rib (midclavicular), 8th (midaxillary), 10th (paravertebral); pleura two ribs lower (8th, 10th, 12th).
• Fissures: oblique fissure from T3 spinous process to the 6th costal cartilage; horizontal fissure (right) along the 4th costal cartilage.
• Lung apices rise about 3 cm above the medial third of the clavicle - pneumothorax risk with subclavian and supraclavicular procedures.

Surface Anatomy of the Abdomen

• Transpyloric plane (of Addison) - halfway between the jugular notch and the pubic symphysis, at L1: pylorus, neck of pancreas, duodenojejunal flexure, hila of kidneys, fundus of gallbladder (at the 9th costal cartilage), origin of the SMA, end of the spinal cord (L1–2).
• Subcostal plane (L3) and supracristal plane (top of the iliac crests, L4 - aortic bifurcation; landmark for lumbar puncture).
• McBurney's point - one-third of the way from the anterior superior iliac spine (ASIS) to the umbilicus - the usual position of the appendix base.
• Inguinal ligament - ASIS to the pubic tubercle. Mid-inguinal point (halfway between ASIS and the pubic symphysis) - femoral artery pulse. Midpoint of the inguinal ligament (halfway between ASIS and pubic tubercle) - deep inguinal ring.
• Hernias: inguinal hernias emerge above and medial to the pubic tubercle; femoral hernias below and lateral.
• Liver - upper border at the 5th intercostal space; lower edge at the right costal margin (palpable in children and thin adults). Spleen - under ribs 9–11 on the left; must enlarge about threefold to be palpable, and does so towards the right iliac fossa.
• Kidneys - T12–L3; the right is lower because of the liver; ballotable when enlarged.
• Murphy's sign - tenderness at the tip of the right 9th costal cartilage (gallbladder fundus) with arrest of inspiration.

Vertebral Levels to Remember

Dermatomes, Myotomes and Referred Pain

• Key dermatomes: C5 - lateral arm (regimental badge area); C6 - thumb and lateral forearm; C7 - middle finger; C8 - little finger; T1 - medial arm; T4 - nipple; T10 - umbilicus; L1 - inguinal region; L3 - knee; L4 - medial leg and big toe/medial malleolus; L5 - dorsum of foot and big toe; S1 - lateral foot and sole, little toe; S2–4 - perineum (saddle area).
• Myotomes and reflexes: biceps (C5–6), supinator (C6), triceps (C7), finger flexion (C8), small hand muscles (T1); knee jerk (L3–4), ankle dorsiflexion (L4–5), big toe extension (L5), ankle jerk and plantarflexion (S1).
• Referred pain - visceral afferents enter the same spinal segments as somatic afferents: heart → left arm and jaw (T1–4); diaphragm → shoulder tip (C4); appendicitis → periumbilical (T10) then right iliac fossa when parietal peritoneum is involved; testis → loin and umbilical region (T10); ureteric colic → loin to groin (T12–L2); hip → knee (obturator and femoral nerves).

Chest X-ray Anatomy

• Quality: projection (PA versus AP - AP magnifies the heart), rotation (spinous processes midway between the medial clavicle ends), inspiration (5–6 anterior ribs or 10 posterior ribs above the diaphragm), penetration (vertebral bodies just visible through the heart).
• Right heart border = right atrium; left heart border = left ventricle (and above it the left atrial appendage, pulmonary trunk and aortic knuckle).
• Cardiothoracic ratio <0.5 on a PA film.
• Silhouette sign - loss of a normal border locates disease: right heart border → right middle lobe; left heart border → lingula; hemidiaphragm → lower lobe; aortic knuckle → left upper lobe (apicoposterior segment).
• Hila - the left hilum is normally higher than the right (left pulmonary artery arches over the left main bronchus).
• Trachea - central (slightly right at the aortic knuckle); deviated away from tension pneumothorax and large effusion, towards collapse and pneumonectomy.
• Costophrenic angles - blunting indicates effusion (about 200 mL needed on an erect PA film).
• Lines and tubes (check on every CXR): endotracheal tube tip 5–7 cm above the carina (T2–4); central line tip at the cavoatrial junction / lower SVC; NG tube below the diaphragm, crossing the carina, bisecting it and deviating to the left (see NG tubes).

Abdominal and Pelvic Radiograph Anatomy

• Small bowel - central, valvulae conniventes cross the full width; dilated if >3 cm.
• Large bowel - peripheral, haustra cross part of the width; dilated if >6 cm (caecum >9 cm) - the 3-6-9 rule.
• Volvulus: sigmoid - "coffee bean" arising from the pelvis pointing to the right upper quadrant; caecal - pointing to the left upper quadrant.
• Pneumoperitoneum - free gas under the diaphragm on erect CXR; Rigler's sign (both sides of the bowel wall visible) on supine AXR.
• Calcification - renal tract stones (about 80% radio-opaque), gallstones (only about 10–15%), pancreatic calcification (chronic pancreatitis), aortic calcification (aneurysm).
• Pelvis and hip: Shenton's line (inferior border of the superior pubic ramus continuous with the medial femoral neck) - broken in displaced neck of femur fracture; look at both the AP pelvis and lateral hip.

Cross-sectional Anatomy on CT

• Orientation: axial images are viewed from the feet - the patient's right is on the viewer's left; anterior is at the top.
• Windows: soft tissue, lung, bone and brain windows - abnormalities can be invisible on the wrong window.
• CT head: grey and white matter differentiation (lost early in infarction), ventricles (hydrocephalus), sulci (effacement in raised ICP), midline shift, blood (hyperdense - white - acutely), extradural (lens-shaped, does not cross sutures; middle meningeal artery) versus subdural (crescent, crosses sutures, bridging veins).
• CT abdomen at L1: liver, IVC (right of the aorta), aorta, pancreas draped over the SMA/SMV (SMV to the right of the SMA - reversed in malrotation), kidneys, adrenals (inverted Y or V shapes).
• CTPA - contrast-filled pulmonary arteries; emboli are filling defects.
• Contrast phases: arterial (aneurysm, bleeding source), portal venous (most abdominal CT), delayed (urinary tract).

Venous Access and Arterial Sampling

• Peripheral cannulation - forearm veins (cephalic along the radial border, basilic along the ulnar border, median cubital in the antecubital fossa); avoid joints and the antecubital fossa for long-term cannulas; at the wrist avoid the radial side near the superficial radial nerve.
• Antecubital fossa contents (lateral to medial): biceps tendon, brachial artery, median nerve ("TAN" - Tendon, Artery, Nerve). The bicipital aponeurosis protects the artery and nerve under the median cubital vein.
• Arterial blood gas - radial artery at the wrist, lateral to flexor carpi radialis; perform a modified Allen's test (ulnar collateral circulation) before sampling; alternatives are the brachial or femoral arteries.
• Femoral triangle (lateral to medial): NAVY - femoral Nerve, Artery, Vein, Y-fronts (or empty space and lymphatics in the femoral canal). The vein lies medial to the palpable artery - the basis of femoral venous access.
• Internal jugular vein - lies lateral to the carotid artery within the carotid sheath, under the triangle formed by the two heads of sternocleidomastoid and the clavicle; ultrasound guidance is standard (NICE). Complications: carotid puncture, pneumothorax, air embolism (head-down position), arrhythmia, infection.
• Subclavian vein - lower infection risk but higher pneumothorax risk; non-compressible if bleeding occurs.

Pleural Aspiration and Chest Drains

• Safe triangle (BTS): anterior border of latissimus dorsi, lateral border of pectoralis major, and a horizontal line at the level of the 5th intercostal space (nipple level in men), with the apex just below the axilla.
• Insert just above the upper border of the rib below - the intercostal neurovascular bundle runs in the costal groove on the inferior border of each rib (from above down: vein, artery, nerve - VAN).
• Ultrasound guidance is recommended for all pleural procedures for fluid (BTS 2023).
• Large-bore (surgical) drains for trauma, haemothorax and empyema needing larger tubes; small-bore Seldinger drains for most effusions and pneumothoraces.
• Complications: organ injury (liver, spleen - insertion too low), intercostal vessel bleeding, re-expansion pulmonary oedema (drain no more than about 1.5 litres at a time), surgical emphysema, infection.
• Needle decompression of tension pneumothorax - 4th or 5th intercostal space, anterior to the midaxillary line in adults (ATLS) - more reliable than the 2nd ICS midclavicular line because the chest wall is thinner laterally.

Lumbar Puncture

• Position: lateral decubitus (knees to chest - opens interspinous spaces; measure opening pressure) or sitting.
• Level: L3/4 or L4/5 - the intercristal (Tuffier's) line across the top of the iliac crests passes through L4 or the L4/5 space. The adult spinal cord ends at L1–2, so these spaces lie below it in the cauda equina.
• Layers traversed: skin → subcutaneous fat → supraspinous ligament → interspinous ligament → ligamentum flavum (felt as a "give") → epidural space → dura → arachnoid → subarachnoid space (CSF).
• Opening pressure: normal 10–20 cmH₂O (raised in idiopathic intracranial hypertension, meningitis, venous sinus thrombosis).
• Contraindications: signs of raised intracranial pressure with mass effect (CT first if focal neurology, reduced GCS, papilloedema, immunosuppression, seizure), coagulopathy or anticoagulation, local infection.
• Post-dural puncture headache - worse on sitting or standing; reduced by atraumatic (pencil-point) needles; treat with fluids, analgesia, caffeine; epidural blood patch if persistent.

Airway Anatomy, Cricothyroidotomy and Intubation

• Larynx landmarks: thyroid cartilage (laryngeal prominence), cricothyroid membrane (soft gap between the thyroid and cricoid cartilages - site of emergency front-of-neck access), cricoid cartilage (the only complete ring; C6), trachea and thyroid isthmus over rings 2–4.
• Emergency front-of-neck access ("can't intubate, can't oxygenate" - DAS guidelines): scalpel–bougie–tube technique through the cricothyroid membrane.
• Intubation anatomy: epiglottis, vallecula (the curved laryngoscope blade tip sits here), vocal cords; the right main bronchus is wider, shorter and more vertical - endobronchial intubation and inhaled foreign bodies favour the right (especially the right lower lobe).
• Recurrent laryngeal nerve injury (thyroid surgery, mediastinal disease): hoarseness; bilateral injury → stridor.

Other Procedures - Catheters, NG Tubes, Injections and Nerve Blocks

• Male urethral catheterisation - the urethra is about 18–20 cm long, with two curves; the narrowest part is the external urethral meatus, and the membranous urethra (through the external sphincter) is the least distensible. Never inflate the balloon until urine flows (risk of urethral rupture); replace the foreskin (paraphimosis risk).
• Nasogastric tube - measure nose → earlobe → xiphisternum (NEX); confirm position with pH ≤5.5 of aspirate or a chest X-ray. Misplaced NG tube feeding is a never event. Correct CXR position: the tube follows the oesophagus, bisects the carina, crosses the diaphragm in the midline and its tip is seen below the left hemidiaphragm.
• Intramuscular injection sites: deltoid (2–3 finger-breadths below the acromion - avoid the axillary and radial nerves), ventrogluteal (preferred gluteal site - away from the sciatic nerve), vastus lateralis (anterolateral thigh - infants). The dorsogluteal (upper outer quadrant of the buttock) carries sciatic nerve risk.
• Joint aspiration of the knee - lateral (or medial) approach under the patella, at the junction of the upper and middle thirds, with the knee extended and relaxed.
• Digital (ring) block - digital nerves run on the palmar and dorsal aspects of each side of the finger; plain lidocaine is traditionally used, although adrenaline-containing solutions are now considered safe in healthy digits.
• Fascia iliaca block for hip fracture - local anaesthetic deep to the fascia iliaca, lateral to the femoral nerve; blocks femoral, lateral femoral cutaneous (and sometimes obturator) nerves.

Surface Landmarks at a Glance

Spot the Structure

• Which structure is at risk if a chest drain is placed just below a rib? → The intercostal vessels and nerve in the costal groove.
• A needle for lumbar puncture suddenly "gives" before CSF flows. → Passing through the ligamentum flavum (then the dura).
• Loss of the right heart border on a chest X-ray. → Right middle lobe consolidation or collapse.
• Tip of the NG tube lies in the right lower zone on CXR. → Misplaced in the right main bronchus - remove; do not feed.
• Pain in the right shoulder after laparoscopy. → Diaphragmatic irritation (C4 referred pain) from residual CO₂.

Exam-style Scenarios

• Scenario. A 70-year-old has a large left pleural effusion. Where should the drain be inserted, and what imaging is required?
  Answer: Under ultrasound guidance within the safe triangle (anterior border of latissimus dorsi, lateral border of pectoralis major, 5th intercostal space), passing over the upper border of the rib below to avoid the neurovascular bundle. Drain no more than about 1.5 L initially to avoid re-expansion pulmonary oedema.
• Scenario. During a cannula attempt in the antecubital fossa, a patient feels electric pain shooting into the thumb, index and middle fingers. What has happened?
  Answer: The needle has contacted the median nerve (medial to the brachial artery in the antecubital fossa). Withdraw immediately, reassure, document and use a different site.
• Scenario. A patient in traumatic arrest has suspected tension pneumothorax. Where should decompression be performed?
  Answer: 4th or 5th intercostal space just anterior to the midaxillary line (or finger thoracostomy at the same site in trauma) - the chest wall is thinner here than at the 2nd ICS midclavicular line.

Key Points

• The sternal angle (T4/5) is the master landmark of the thorax.
• Always go over the top of the rib below - the neurovascular bundle runs under each rib.
• LP at L3/4 or L4/5 - below the end of the cord.
• Femoral triangle: nerve, artery, vein (lateral to medial).
• Use ultrasound guidance for central lines and pleural procedures.
• Check NG tube position with pH ≤5.5 or CXR before any feed.

Guidelines and Further Reading

• British Thoracic Society: Clinical statement on pleural procedures (2023)
• Difficult Airway Society: Guidelines for unanticipated difficult intubation in adults
• NICE TA49: Ultrasound-guided central venous catheter placement
• NHS Improvement: Nasogastric tube misplacement - patient safety alerts
• Moore KL, Dalley AF. Clinically Oriented Anatomy
• Raftery AT. Applied Basic Science for Basic Surgical Training

Lumbar Puncture and Epidural Anatomy

• Site — L3/4 or L4/5 interspace (below the end of the cord at L1/2); Tuffier's (intercristal) line — between the top of the iliac crests — crosses the L4 spinous process or L4/5 interspace.
• Position — lateral (knees to chest — opens the interspaces; opening pressure measured) or sitting.
• Layers traversed (lumbar puncture) — skin → subcutaneous fat → supraspinous ligament → interspinous ligament → ligamentum flavum ("give") → epidural space (fat and venous plexus) → dura mater → arachnoid mater → subarachnoid space (CSF).
• Epidural — the needle stops in the epidural space (loss of resistance after the ligamentum flavum); spinal (intrathecal) — the needle passes through the dura and arachnoid into CSF.
• Dermatome levels for blocks — T4 (nipple) for caesarean section, T10 (umbilicus) for labour and lower limb, sacral for perineal surgery.
• Caudal epidural — through the sacral hiatus (children's analgesia).
• Contraindications to LP — raised ICP (risk of coning — tonsillar herniation), coagulopathy, local infection, spinal abnormality (relative).
• Post-dural puncture headache — CSF leak lowers intracranial pressure — postural headache (worse upright) — reduced by atraumatic (pencil-point) needles — blood patch if persistent.
• Normal CSF — clear; opening pressure 10–20 cmH₂O; cells under 5/mm³ (lymphocytes); protein 0.15–0.45 g/L; glucose over 60% of plasma glucose.