X-Ray Physics for Orthopaedic Surgeons
Air (black, -1000 HU): No absorption
Fat (dark grey, -100 HU): Low absorption
Soft tissue/water (grey, 0-80 HU): Moderate absorption
Bone (white, +400-1000 HU): High absorption via photoelectric effect
Metal (bright white, +3000 HU): Near-complete absorption
Key: Differential absorption between tissues creates image contrast — this is the fundamental principle of radiographic imaging
- X-rays are produced when high-energy electrons strike a tungsten anode, generating Bremsstrahlung (80-90%) and Characteristic radiation (10-20%).
- kVp controls beam penetration (quality): higher kVp = more penetrating beam but lower contrast.
- mAs controls photon quantity (exposure): higher mAs = more photons, higher dose, darker image.
- Inverse Square Law: Intensity is proportional to 1/distance squared. Doubling distance quarters dose.
- Bone appears white (radiopaque) due to high atomic number calcium absorbing more X-rays via the photoelectric effect.
- “Bremsstrahlung = 'braking radiation' = electron deceleration near nucleus = continuous energy spectrum.
- “Photoelectric effect dominates at lower kVp and high Z materials — responsible for bone vs soft tissue contrast.
- “Compton scatter dominates at higher kVp — creates image fog and is the main source of staff radiation exposure.
- “Lead apron attenuates approximately 95% of scatter radiation at diagnostic energies.
- “Paediatric patients require LOWER dose settings due to increased radiosensitivity and longer remaining lifespan.
Plain radiography physics is commonly examined in both written and viva formats. You must understand kVp vs mAs, the two mechanisms of X-ray production, the photoelectric effect vs Compton scatter distinction, and the inverse square law. Practical questions often focus on: why you choose specific views, how to optimise dose in pregnancy, and systematic interpretation to avoid missed fractures.
BEAMX-Ray Exposure Parameters
Hook:BEAM: the physics of every radiograph you request starts with these four concepts.
ABCSSystematic X-Ray Interpretation
Hook:Always Be Checking Systematically — satisfaction of search is the most dangerous error in radiograph interpretation.
TDSRadiation Protection Principles
Hook:Time, Distance, Shielding — the three pillars of ALARA that every orthopaedic surgeon must practise in theatre.
Overview
Plain radiography remains the first-line imaging modality in orthopaedic practice because it is fast, inexpensive, widely available, and provides excellent spatial resolution for cortical bone. Despite the proliferation of advanced cross-sectional imaging, the plain radiograph is the starting point for the vast majority of orthopaedic consultations and remains the backbone of fracture diagnosis, arthritis assessment, alignment evaluation, and postoperative surveillance.
The clinical value of a radiograph depends on three linked factors: the physics of image generation (understanding what creates the image), the technical parameters chosen (ensuring diagnostic quality with minimum dose), and the interpretive framework used by the reader (systematic review to avoid missed pathology). All three are examined at fellowship level.
Excellent cortical bone detail with high spatial resolution. Fast acquisition (seconds). Low cost globally. Gold standard for fracture diagnosis, alignment assessment, arthritis grading, and implant surveillance. Weight-bearing views provide functional information. Universally available in emergency departments and clinics.
Superimposition of 3D anatomy onto a 2D projection. Limited soft tissue contrast (cannot reliably evaluate ligaments, tendons, or cartilage). Projection-dependent — a fracture may be invisible on one view and obvious on another. Low sensitivity for occult fractures (10-20% of scaphoid fractures initially radiograph-negative). Ionising radiation exposure, though dose is low for extremity imaging.
Clinical Imaging
Imaging Atlas
X-Ray Production Physics
X-rays are a form of electromagnetic radiation with wavelengths between 0.01 and 10 nanometres, placing them between ultraviolet light and gamma rays on the electromagnetic spectrum. In medical imaging, X-rays are produced artificially in an X-ray tube through the interaction of high-energy electrons with a metal target.
X-Ray Tube Components
The X-ray tube is a vacuum-sealed glass or metal envelope containing two electrodes. The cathode is a coiled tungsten filament that emits electrons when heated to high temperatures through thermionic emission. The anode is a large rotating tungsten disc (chosen for its high atomic number of 74 and extremely high melting point of 3422 degrees Celsius) that serves as the electron target. A high voltage potential difference (measured in kilovolts peak, kVp) is applied across the tube, accelerating the cathode electrons toward the anode at tremendous speed.
When these accelerated electrons strike the tungsten anode, their kinetic energy is converted into two forms: heat (over 99% of the energy) and X-ray photons (less than 1%). The X-ray photons exit through a window in the tube housing and form the imaging beam. This extreme inefficiency is why anode rotation and heat dissipation are critical engineering challenges.
Two Mechanisms of X-Ray Production
| Feature | Bremsstrahlung (Braking) | Characteristic |
|---|---|---|
| Mechanism | Electron decelerates near the nucleus of a target atom, losing energy as an X-ray photon | Electron ejects an inner-shell electron from the target atom; outer-shell electron fills the vacancy, emitting a photon of specific energy |
| Energy spectrum | Continuous spectrum — photons of all energies from zero up to the maximum keV (which equals the kVp) | Discrete line spectrum — photons at specific energies determined by the binding energy differences between electron shells |
| Proportion of beam | 80-90% of the useful X-ray beam | 10-20% of the useful X-ray beam |
| Depends on | Atomic number of the target and the kinetic energy of the incident electron | Binding energies of the target atom electron shells (K-shell, L-shell) |
| Clinical relevance | Provides the majority of imaging photons across the diagnostic energy range | Contributes discrete peaks to the energy spectrum; tungsten K-edge characteristic X-rays have energies of 59 and 67 keV |
The word Bremsstrahlung is German for 'braking radiation.' In a viva, explain it as: 'When a fast-moving electron passes close to a tungsten nucleus, the strong electrostatic attraction decelerates the electron. The kinetic energy lost during this deceleration is emitted as an X-ray photon. Because the electron can pass at varying distances from the nucleus, losing different amounts of energy each time, a continuous spectrum of X-ray energies is produced.'
Technical Parameters
The two most important operator-controlled parameters are kilovoltage peak (kVp) and milliampere-seconds (mAs). Understanding their distinct effects on the X-ray beam is fundamental.
Kilovoltage peak (kVp) controls beam quality — the penetrating ability of the X-ray beam.
Higher kVp accelerates electrons to greater speed before they strike the anode, producing higher-energy (shorter-wavelength) X-ray photons that penetrate tissue more easily. However, this comes at the cost of reduced radiographic contrast because more photons pass through both bone and soft tissue without being absorbed, creating a more uniformly grey image.
| Region | Typical kVp | Rationale |
|---|---|---|
| Fingers and toes | 50-55 | Thin structures need low kVp for maximum contrast |
| Hand, wrist, forearm | 55-65 | Moderate soft tissue with fine bony detail required |
| Elbow, knee, ankle | 60-70 | Larger joints need moderate penetration |
| Shoulder, hip | 70-80 | Thick soft tissue envelope requires higher penetration |
| Spine (AP) | 75-85 | Dense overlapping structures need good penetration |
| Pelvis | 80-90 | Large body habitus and dense bone |
| Chest | 110-120 | High kVp reduces rib contrast to visualise lungs |
The key relationship is: increasing kVp by 15% has the same effect on image density as doubling the mAs, but with different contrast behaviour.
This is the answer expected in an exam context.
Image Formation and Contrast
Radiographic image contrast arises from the differential absorption of X-ray photons as they pass through tissues of varying composition and density. Two physical interactions dominate at diagnostic X-ray energies.
Photoelectric Effect
The photoelectric effect is the complete absorption of an incoming X-ray photon by an inner-shell electron of an atom. The photon's energy is entirely transferred to the electron, which is ejected from the atom (becoming a photoelectron). The vacancy is then filled by an outer-shell electron, sometimes releasing a characteristic X-ray.
The probability of the photoelectric effect is proportional to Z³/E³ — the cube of the atomic number divided by the cube of the photon energy. This means:
- High Z materials (bone, calcium Z=20) absorb dramatically more than low Z materials (soft tissue, water equivalent Z approximately 7.4)
- Lower kVp increases photoelectric absorption, producing higher contrast
- The photoelectric effect is responsible for the excellent bone-vs-soft tissue contrast in plain radiography
Compton Scatter
Compton scatter occurs when an X-ray photon interacts with a loosely bound outer-shell electron, transferring part of its energy and changing direction. The scattered photon continues in a different direction with reduced energy.
- Dominates at higher kVp (above approximately 30 keV in soft tissue)
- Independent of atomic number — depends only on electron density, which is relatively uniform across soft tissues
- Creates image fog — scattered photons that reach the detector carry no useful spatial information and reduce contrast
- Main source of occupational radiation — scattered photons travel in all directions, including toward staff
| Property | Photoelectric Effect | Compton Scatter |
|---|---|---|
| Type of interaction | Complete absorption of photon | Partial absorption with scattered photon |
| Depends on | Z cubed (atomic number) and 1/E cubed (photon energy) | Electron density only (approximately constant in soft tissue) |
| Dominates when | Low kVp, high Z material (bone, contrast agents) | Higher kVp, soft tissue (water-equivalent Z) |
| Effect on image | Creates useful contrast between tissues | Creates fog — degrades contrast |
| Effect on dose | All energy deposited locally in the patient | Partial energy deposited; scattered photon continues |
| Staff radiation | No contribution (photon is fully absorbed) | Main source of occupational radiation dose |
Controlling Scatter
Several practical measures reduce the degradation of image quality by Compton scatter:
- Collimation: Limiting the X-ray field to only the region of interest reduces the volume of tissue generating scatter. This also reduces patient dose.
- Anti-scatter grid: A grid of thin lead strips placed between the patient and detector absorbs obliquely travelling scattered photons while allowing primary beam photons (travelling in a straight line) to pass through. Grids improve contrast but require an increase in mAs (typically 2-4 times), increasing patient dose.
- Air gap technique: Increasing the distance between the patient and detector allows scattered photons to diverge and miss the detector. Used occasionally for lateral cervical spine and chest imaging.
Understanding this relationship is key for explaining why different technical settings are chosen for different body parts.
Systematic Interpretation
A systematic approach to radiograph interpretation is the single most important habit for avoiding missed pathology. Studies consistently show that an unsystematic 'gestalt' approach leads to satisfaction of search errors — where the discovery of one obvious abnormality causes the reader to stop looking for additional findings.
The ABCS Approach
| Step | What to Assess | Common Findings |
|---|---|---|
| A — Alignment | Overall bone axis, joint congruity, angular deformity, subluxation, rotational malalignment | Fracture displacement, joint dislocation, scoliosis, valgus or varus deformity |
| B — Bone | Cortical integrity (fracture lines), trabecular pattern, bone density (osteopenia), focal lytic or sclerotic lesions, periosteal reaction, bone size and shape | Fractures, tumours, infections (Brodie abscess), metabolic bone disease, Paget disease |
| C — Cartilage and Joints | Joint space width and symmetry, subchondral sclerosis, osteophytes, erosions, ankylosis, loose bodies, chondrocalcinosis | Osteoarthritis (asymmetric joint space narrowing), inflammatory arthritis (erosions), CPPD (chondrocalcinosis) |
| S — Soft Tissues | Swelling, effusion (fat pad signs), calcification, foreign bodies, gas (surgical emphysema), muscle wasting, soft tissue mass | Lipohemarthrosis (fat-fluid level on horizontal beam), posterior fat pad sign (elbow), quadriceps or Achilles calcification |
The Two-View Minimum
Every long bone and joint requires a minimum of two views at 90 degrees to each other. A fracture that is invisible on an AP view may be obvious on a lateral view. The classic example is the lateral condyle fracture in children, which can appear as a subtle fleck on the AP view but shows a large displaced fragment on the lateral. A single-view radiograph is considered an incomplete study and should prompt a request for the orthogonal view before clinical decisions are made.
View Selection by Region
| Region | Standard Views | Key Additional Views |
|---|---|---|
| Hand/Wrist | PA, True Lateral, Oblique | Scaphoid views (4 projections), Carpal tunnel, Roberts (thumb CMC) |
| Elbow | AP (full extension), True Lateral (90 degrees flexion) | Radial head views, Obliques, Greenspan view (coronoid) |
| Shoulder | AP (internal and external rotation), Axillary lateral | Scapular Y (outlet), West Point (anterior glenoid), Stryker notch (Hill-Sachs) |
| Cervical spine | AP, Lateral, Open mouth (odontoid peg) | Flexion/Extension (instability), Swimmer (C7-T1 junction) |
| Thoracolumbar spine | AP, Lateral | Cone-down lateral (pars), Obliques (scotty dog) |
| Pelvis | AP (standing if for arthritis) | Inlet, Outlet (pelvic ring), Judet obliques (acetabulum) |
| Hip | AP Pelvis, Lateral (cross-table or frog) | Dunn view (45 degrees for cam morphology), False profile (anterior coverage) |
| Knee | AP (weight-bearing), True Lateral | Rosenberg (45 degrees PA flexion weight-bearing), Sunrise/Merchant (patellofemoral), Long-leg alignment |
| Ankle | AP, Mortise (15-20 degrees internal rotation), Lateral | Weight-bearing AP (syndesmosis), Stress views (talar tilt, anterior drawer if ligament injury suspected) |
| Foot | AP (weight-bearing), Lateral (weight-bearing), Oblique | Sesamoid axial, Harris heel view (calcaneus), Meary angle assessment |
Understanding which views to request and why provides a significant advantage in viva examinations.
Differential of a Linear Lucency: Fracture vs Mimics
A common interpretive trap is over-calling or under-calling a lucent line. The following differential helps distinguish a true fracture from its radiographic mimics.
| Entity | Distinguishing Features | Resolves the Problem |
|---|---|---|
| True fracture | Lucency does not follow a known anatomical structure; cortical break/step; often with soft-tissue swelling or effusion | Orthogonal view, clinical correlation, repeat film at 10-14 days or CT/MRI |
| Nutrient foramen / vascular channel | Smooth corticated (sclerotic) margins, oblique constant course, no cortical break | Recognise typical site (e.g. distal femur, tibia); corticated margin confirms benign |
| Physis / accessory ossification centre (paediatric) | Symmetrical, smooth, sclerotic margins, expected location and age; CRITOE sequence at elbow | Contralateral comparison view, knowledge of ossification timetable |
| Mach band / Mach effect (optical illusion) | Apparent lucency where two bone edges overlap; not reproducible on other projection | Repeat or orthogonal view — line disappears as it is an optical artefact |
| Grid line / processing artefact | Linear, geometric, extends beyond bone into soft tissue or background | Repeat exposure; check grid alignment and processing |
| Bipartite/accessory bone (e.g. bipartite patella, os trigonum) | Smooth corticated cleft, typical location (superolateral patella), often bilateral | Corticated margins and bilaterality distinguish from acute fracture |
A true fracture line is typically non-corticated, does not respect anatomy, and is supported by a soft-tissue sign; a benign channel or accessory centre has smooth sclerotic (corticated) margins.
Specific Radiographic Signs
Certain radiographic signs are pathognomonic or highly suggestive of specific diagnoses and are commonly tested at fellowship level.
Fat Pad Signs
Fat pads are radiolucent structures that become visible or displaced when adjacent joints develop effusions. Their recognition provides indirect evidence of occult pathology.
Elbow Fat Pads: The posterior fat pad of the elbow is normally hidden within the olecranon fossa. An effusion pushes it posteriorly, creating a visible lucent triangle behind the distal humerus on the lateral view. The anterior fat pad (sail sign) is normally a thin lucent stripe; when elevated and sail-shaped, it indicates an intra-articular effusion. In the context of trauma with no visible fracture, a positive fat pad sign indicates an occult fracture (most commonly a radial head fracture in adults) and warrants further evaluation or immobilisation and follow-up imaging.
Knee Lipohemarthrosis: A horizontal-beam lateral radiograph of the knee may show a fat-fluid level within the suprapatellar pouch. This lipohemarthrosis occurs when an intra-articular fracture releases marrow fat into the joint, which layers above the blood due to lower density. This sign indicates an occult intra-articular fracture (most commonly tibial plateau) even when no fracture line is visible on standard views.
These signs are frequently used as viva image stations.
Radiation Safety in Practice
Every orthopaedic surgeon who requests or supervises radiographic examinations has a legal and ethical obligation to understand radiation safety. The guiding framework is the ALARA principle: As Low As Reasonably Achievable.
Dose Reference Values
| Examination | Effective Dose (mSv) | Equivalent Natural Background | Context |
|---|---|---|---|
| Extremity radiograph (hand, foot, ankle) | 0.001-0.01 | A few hours | Negligible dose — safe in virtually all circumstances including pregnancy |
| Chest radiograph (PA) | 0.02 | 2-3 days | Very low dose — the standard reference examination for dose comparison |
| Shoulder or elbow radiograph | 0.01-0.05 | 1-5 days | Low dose — no specific precautions needed |
| Hip or pelvis radiograph | 0.3-0.7 | 1-4 months | Moderate dose — relevant to gonadal shielding discussions |
| Lumbar spine series | 1.0-1.5 | 5-8 months | Higher dose — justify indication and minimise repeat exposures |
| CT abdomen/pelvis | 6-20 | 2-7 years | High dose — always require clear clinical justification |
Protection Principles
Minimise exposure duration. In fluoroscopy: use pulsed mode instead of continuous, use last-image-hold to review without ongoing radiation, and plan the procedure to minimise screening time. For plain radiography: avoid unnecessary repeat exposures by ensuring correct positioning before exposure.
Inverse Square Law: Doubling the distance from the radiation source reduces dose to one-quarter. Standing 2 metres from a fluoroscopy source instead of 1 metre reduces your exposure by 75%. In theatre, stand as far from the C-arm as practically possible during screening.
Lead apron (minimum 0.25mm Pb equivalent, preferably 0.5mm for wrap-around) is mandatory during fluoroscopy. Thyroid shield is strongly recommended. Lead glasses (0.75mm Pb equivalent) should be worn for high-volume fluoroscopy users. Gonadal shielding for patients of reproductive age when the gonads are within or near the primary beam and shielding will not obscure relevant anatomy.
Pregnancy Considerations: Fetal dose should be kept below 1 mGy throughout pregnancy. Extremity radiographs pose negligible fetal risk because the beam is remote from the uterus. Pelvic and abdominal imaging requires careful justification and optimisation. Crucially, lead shielding of the abdomen does NOT fully protect the fetus from internal scatter radiation within the patient's body — the only reliable dose reduction is to reduce the primary beam parameters and field size.
Image Quality Optimisation
Diagnostic image quality is a balance between four parameters, all of which can be optimised by understanding the underlying physics.
| Parameter | Definition | Optimised By | Degraded By |
|---|---|---|---|
| Contrast | Ability to distinguish adjacent structures of different density | Lower kVp (increases photoelectric absorption), collimation (reduces scatter), grid use | Higher kVp (more Compton scatter), large field size, patient obesity |
| Spatial Resolution | Ability to resolve fine structural detail | Small focal spot, short exposure time (reduces motion blur), short object-to-detector distance (OID) | Large focal spot, patient motion, magnification (long OID) |
| Noise (Quantum Mottle) | Random statistical fluctuation creating a grainy appearance | Adequate mAs (sufficient photon count), slower detector readout | Low mAs (insufficient photons), fast detector readout, high receptor sensitivity settings |
| Distortion | Magnification or shape change of the imaged structure | Long source-to-image distance (SID, standard 100cm), short OID, perpendicular beam alignment | Short SID, long OID, angled beam (unless intentionally used for specific views) |
Digital vs Film Radiography
Modern digital radiography systems (computed radiography using phosphor plates, or direct digital radiography using flat-panel detectors) have largely replaced film-screen systems. Digital systems offer several advantages: wider dynamic range (more forgiving of exposure errors), post-processing capability (window/level adjustment, edge enhancement), immediate image availability, and electronic storage and retrieval. However, the wider dynamic range can mask overexposure, potentially leading to 'dose creep' — a gradual increase in exposure settings because the image remains diagnostic even at unnecessarily high doses. Quality assurance programmes must monitor exposure indices to prevent this.
Understanding the difference is important for modern practice.
Special Considerations
Paediatric Radiography Principles
Children are more radiosensitive than adults due to their rapidly dividing cells and longer remaining lifespan over which stochastic effects (cancer) can develop. The ALARA principle is therefore even more important in paediatric imaging.
Key modifications for paediatric radiography:
- Dose reduction: Use lower kVp and mAs settings. Modern protocols reduce paediatric extremity doses by 30-50% compared to adult settings.
- Gonadal shielding: Particularly important in children when the gonads are near the primary beam (pelvis, hip imaging).
- Immobilisation without restraint: Use positioning aids rather than manual holding by staff or parents where possible. If a parent must assist, they must wear full protective equipment and the exposed parent should ideally not be pregnant.
- Comparison views: A contralateral comparison radiograph may be justified in children when the normal appearance of ossification centres makes it difficult to distinguish a fracture from a developing physis. However, routine comparison views should not be performed unless clinically indicated — each additional radiograph adds dose.
- Growth plate awareness: Open physes can mimic fracture lines. Knowledge of age-dependent ossification centres (CRITOE for elbow) is essential to avoid misdiagnosis.
Paediatric imaging is always a balance between diagnostic need and radiation risk.
Common Artefacts
Recognising radiographic artefacts prevents misdiagnosis and unnecessary further investigation.
| Artefact | Cause | Appearance | Solution |
|---|---|---|---|
| Motion blur | Patient or tube movement during exposure | Blurred cortical margins, loss of fine detail | Shorter exposure time, immobilisation, repeat if non-diagnostic |
| Grid cutoff | Misaligned anti-scatter grid | Peripheral darkening or banding pattern | Correct grid alignment, ensure grid is centred to beam |
| Scatter fog | Compton scatter photons reaching detector | Overall greying with reduced contrast | Collimation, grid use, lower kVp if appropriate |
| Double exposure | Two exposures on the same detector | Superimposed anatomy creating confusing appearance | Digital systems prevent this; check for processing errors |
| Metal artefact | Jewellery, clothing snaps, surgical implants | Bright white opacity obscuring underlying anatomy | Remove external objects; for implants, use adjusted technique and additional views |
| Quantum mottle | Insufficient mAs (too few photons) | Grainy, speckled appearance with poor signal-to-noise ratio | Increase mAs (with corresponding dose increase) |
Understanding artefacts is important for quality assurance discussions.
Evidence Base
ICRP Publication 135: Diagnostic Reference Levels in Medical Imaging
- Established international diagnostic reference levels (DRLs) for common radiographic examinations across all anatomical regions.
- DRLs are not dose limits but investigation levels — facilities consistently exceeding them should review and optimise their protocols.
- Plain radiography delivers the lowest effective doses among ionising imaging modalities, supporting its role as the first-line investigation.
ARPANSA Diagnostic Reference Levels for Medical Imaging
- Regional Australia-New Zealand diagnostic reference levels cover common radiographic procedures including chest, pelvis, lumbar spine, and extremity examinations.
- These DRLs are broadly consistent with international benchmarks while reflecting regional equipment profiles and practice patterns.
- Recommends regular dose audits with comparison to DRLs as part of quality assurance programmes.
European Guidelines on Diagnostic Reference Levels
- European DRLs for paediatric and adult radiographic examinations with specific recommendations for dose-sensitive populations.
- Emphasises the importance of DRLs as a quality improvement tool, not a regulatory limit.
- Highlights the significant dose reductions achievable through modern digital radiography compared to film-screen systems.
Guidelines provide the regulatory framework for safe radiographic practice.
Guidelines, Registries & Global Practice
Plain radiography is the highest-volume diagnostic imaging examination worldwide, and medical exposure is now the largest man-made source of population radiation dose. The governing principles — justification, optimisation and dose limitation — are internationally harmonised through the International Commission on Radiological Protection (ICRP), with diagnostic reference levels (DRLs) operationalised regionally. The framework is the same on every continent; only the numerical DRLs and the regulator differ.
Global Regulatory and Guideline Landscape
| Body / Region | Role | Key Position | Basis |
|---|---|---|---|
| ICRP (international) | Source framework for all national rules | Justification, optimisation (ALARA), dose limitation; publishes DRLs concept | Consensus, expert |
| IAEA Basic Safety Standards (global) | Adopted by member states into national law | Mandates DRLs, QA programmes and patient dose recording | Consensus, expert |
| ACR Appropriateness Criteria (USA) | Evidence-based ordering guidance | Radiograph first-line for most acute musculoskeletal trauma; advanced imaging for occult/complex injury | Systematic review |
| RCR iRefer / NICE (UK) | Referral guidelines and dose governance | Two orthogonal views standard; clinical decision rules to limit unnecessary radiographs | Guideline |
| EuroSafe Imaging / EC RP185 (Europe) | DRLs and dose-optimisation campaign | Paediatric and adult DRLs; promotes digital optimisation over film | Consensus, expert |
| ARPANSA / RANZCR (Australia-New Zealand) | Regional DRLs and requesting standards | Regional DRLs broadly aligned with international values; standardised justified requesting | Survey, guideline |
Clinical Decision Rules — the Global Standard for Justification
The single most exam-relevant point is that radiographs should be justified, not reflexive. Validated decision rules reduce unnecessary radiography by 25-40% while maintaining near-100% sensitivity for clinically important fractures, and are recommended across the ACR, NICE/RCR and European frameworks.
- Ottawa Ankle and Foot Rules — image only if there is bony tenderness at the posterior malleolar edges, the navicular or base of the fifth metatarsal, or inability to weight-bear; validated at 100% sensitivity for ankle and midfoot fracture.
- Ottawa Knee Rule — image only with age 55 years or older, isolated patellar tenderness, fibular head tenderness, inability to flex to 90 degrees, or inability to weight-bear.
- Canadian C-spine Rule / NEXUS — identify which alert, stable trauma patients can be cleared clinically without cervical radiography.
Occupational Dose Governance (Globally Harmonised)
Orthopaedic and trauma surgeons receive among the highest occupational radiation exposures of any non-radiology specialty because of intra-operative fluoroscopy. The ICRP occupational effective-dose limit — adopted near-identically by national regulators worldwide — is 20 mSv per year averaged over five years, with no single year exceeding 50 mSv, and a 20 mSv equivalent-dose limit to the lens of the eye. Personal dosimetry (thermoluminescent or optically stimulated luminescence badges, worn at collar level outside the apron) is mandatory for regularly exposed staff in every jurisdiction.
Practice Variation
In high-resource settings the debate centres on optimisation: pulsed and low-dose fluoroscopy, dose-creep surveillance on digital systems, and reducing repeat exposures. In limited-resource settings the constraints differ — film-screen systems may persist, automatic exposure control may be unavailable, and access to cross-sectional imaging for occult fractures is restricted, increasing reliance on serial radiographs and clinical follow-up. The principle of justified, optimised, two-view radiography is universal; the technology available to deliver it is not.
Clinical Decision Scenarios
Practise clinical reasoning and management decisions out loud
“You are shown a radiograph of a patient's elbow after a fall. The AP view appears normal but the lateral view shows an elevated anterior fat pad (sail sign) and a visible posterior fat pad.”
“A 28-year-old woman requires a pelvic radiograph following a road traffic collision. She tells you she might be pregnant.”
“An examiner asks you to explain the physics of X-ray production and describe why bone appears white on a radiograph.”
X-Ray Physics
- kVp = beam quality (penetration); mAs = beam quantity (number of photons)
- Bremsstrahlung (80-90%) = continuous spectrum; Characteristic (10-20%) = discrete peaks
- Inverse Square Law: Intensity proportional to 1/d squared — double distance = quarter dose
- Photoelectric effect proportional to Z cubed — creates bone vs soft tissue contrast
- Over 99% of electron kinetic energy becomes heat; less than 1% produces X-rays
ABCS Systematic Interpretation
- A = Alignment (joint congruity, angular deformity, subluxation)
- B = Bone (cortex, trabeculae, density, lesions, periosteal reaction)
- C = Cartilage and Joints (joint space, subchondral sclerosis, osteophytes, erosions)
- S = Soft tissues (swelling, effusion, calcification, foreign bodies, gas)
Radiation Safety (ALARA)
- Three principles: Time, Distance, Shielding
- Extremity radiograph 0.001-0.01 mSv; Pelvis 0.3-0.7 mSv; CT 6-20 mSv
- Pregnancy: fetal deterministic threshold approximately 100 mGy
- Lead apron attenuates approximately 95% of scatter at diagnostic energies
- 15% increase in kVp has the same density effect as doubling mAs
Key Radiographic Signs
- Posterior fat pad sign (elbow) = occult fracture until proven otherwise
- Lipohemarthrosis (knee) = intra-articular fracture releasing marrow fat
- Shenton line disruption = hip pathology (fracture, dislocation, DDH)
- Anterior humeral line should pass through middle third of capitellum
- Radiocapitellar line must pass through capitellum on ALL views
Standard Views
- Minimum 2 views at 90 degrees for every long bone and joint
- Elbow: AP in extension, lateral at 90 degrees flexion
- Shoulder: AP (IR/ER) plus axillary lateral
- Knee: Weight-bearing AP, true lateral, Rosenberg for posterior condyles
- Ankle: AP, Mortise (15-20 degrees IR), Lateral