Plain Radiography Principles

Plain radiography remains the first-line imaging modality in orthopaedic practice. Despite the proliferation of advanced cross-sectional imaging, it is the starting point for the vast majority of orthopaedic consultations and the gold standard for fracture diagnosis, alignment assessment, arthritis grading and implant surveillance.

Why it stays first. Acquisition takes seconds, the cost is low globally, and it is universally available in emergency departments and clinics. It gives excellent cortical bone detail with high spatial resolution, and weight-bearing views provide functional information.

What it cannot do.

• It superimposes three-dimensional anatomy onto a two-dimensional projection
• Its soft-tissue contrast is limited, so ligaments, tendons and cartilage cannot be reliably evaluated
• It is projection-dependent, which is why every study needs two views (see the two-view minimum below)
• Its sensitivity for occult fractures is low, with 10-20% of scaphoid fractures initially radiograph-negative
• It uses ionising radiation, though the dose is low for extremity imaging

Three linked factors. The clinical value of a radiograph depends on the physics of image generation (what creates the image), the technical parameters chosen (diagnostic quality with minimum dose) and the reader's interpretive framework (systematic review to avoid missed pathology). All three are examined at fellowship level.

What is examined. Plain radiography physics is commonly examined in both written and viva formats: kVp versus mAs, the two mechanisms of X-ray production, the photoelectric effect versus Compton scatter, 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.

X-rays are electromagnetic radiation with wavelengths between 0.01 and 10 nanometres, which places them between ultraviolet light and gamma rays on the electromagnetic spectrum. In medical imaging they are produced artificially in an X-ray tube, where high-energy electrons interact with a metal target.

X-Ray Tube Components

The tube. A vacuum-sealed glass or metal envelope holds two electrodes. The cathode is a coiled tungsten filament that emits electrons when heated to high temperatures, by thermionic emission. The anode is a large rotating tungsten disc that serves as the electron target, chosen for tungsten's high atomic number of 74 and extremely high melting point of 3422 degrees Celsius.

Acceleration. A high potential difference, measured in kilovolts peak (kVp), is applied across the tube and accelerates the cathode electrons toward the anode at tremendous speed. The X-ray photons they generate exit through a window in the tube housing and form the imaging beam.

Inefficiency. When the electrons strike the anode, their kinetic energy becomes heat (over 99%) and X-ray photons (less than 1%). That extreme inefficiency is why anode rotation and heat dissipation are critical engineering challenges.

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Two Mechanisms of X-Ray Production

Bremsstrahlung is German for "braking radiation", and in a viva it is worth explaining in full. When a fast-moving electron passes close to a tungsten nucleus, the strong electrostatic attraction decelerates it, and the kinetic energy lost 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.

Filtration, Half-Value Layer and Beam Hardening

Soft photons. The raw Bremsstrahlung beam contains a large number of low-energy ("soft") photons that are too weak to pass through the patient and reach the detector. They contribute nothing to the image and are entirely absorbed in superficial tissue, adding only skin dose. Filtration removes them, and it is a high-yield but frequently overlooked exam topic.

The viva question. "How do you reduce patient dose without changing the image?" One correct answer is adequate beam filtration: aluminium filters remove the low-energy photons that would only be absorbed in the skin, cutting skin dose while barely affecting the image-forming photons. Beam quality, quantified by the HVL, is increased by both higher kVp and greater filtration. Do not confuse HVL, a measure of penetration, with mAs, a measure of quantity.

The two most important operator-controlled parameters are kilovoltage peak (kVp) and milliampere-seconds (mAs). Their effects on the X-ray beam are distinct, and understanding the difference is fundamental.

kVp: Penetration

kVp controls beam quality, the penetrating ability of the beam. Higher kVp accelerates electrons to greater speed before they strike the anode, producing higher-energy, shorter-wavelength photons that penetrate tissue more easily. The cost is contrast: more photons pass through both bone and soft tissue without being absorbed, and the image becomes more uniformly grey.

The 15% rule. 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.

mAs: Quantity

mAs controls beam quantity, the number of X-ray photons produced. It is the product of tube current (mA) and exposure time in seconds. A higher mA means a hotter cathode filament releasing more electrons per second, and a longer exposure lets electrons flow for longer; both increase the total number of photons and produce a darker, more exposed image. By the reciprocity law, 100 mA for 0.1 s (10 mAs) gives an identical exposure to 200 mA for 0.05 s (10 mAs).

Dose and noise. Doubling mAs doubles patient dose, a directly proportional relationship, but mAs does not change beam energy or penetration, only the number of photons. It primarily affects image noise: insufficient mAs produces a grainy image (quantum mottle).

Automatic exposure control. In digital radiography the AEC system adjusts mAs in real time from the detector signal, which makes manual mAs selection less common. Understanding the principle remains essential, and it is foundational to radiation dose discussions.

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: the discovery of one obvious abnormality causes the reader to stop looking for others. Satisfaction of search is the most dangerous error in radiograph interpretation.

The ABCS Approach

The Two-View Minimum

Two views at 90 degrees. Every long bone and joint requires a minimum of two views at 90 degrees to each other, because a fracture that is invisible on an AP view may be obvious on a lateral. 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 an incomplete study. It should prompt a request for the orthogonal view before clinical decisions are made.

View Selection by Region

Knowing which views to request, and why, is a significant advantage in the viva.

Differential of a Linear Lucency: Fracture vs Mimics

Fracture or mimic. Over-calling or under-calling a lucent line is a common interpretive trap. 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.

Certain radiographic signs are pathognomonic or highly suggestive of specific diagnoses, and they are commonly tested at fellowship level.

Fat Pad Signs

Fat pads are radiolucent structures that become visible or displaced when the adjacent joint develops an effusion, so recognising them gives indirect evidence of occult pathology. These signs are frequently used as viva image stations.

The elbow. The posterior fat pad 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 is normally a thin lucent stripe, and when it is elevated and sail-shaped (the sail sign) it indicates an intra-articular effusion.

Elbow trauma with no visible fracture. A positive fat pad sign indicates an occult fracture, most commonly a radial head fracture in adults. It warrants further evaluation, or immobilisation and follow-up imaging.

Knee lipohaemarthrosis. A horizontal-beam lateral radiograph of the knee may show a fat-fluid level within the suprapatellar pouch. It occurs when an intra-articular fracture releases marrow fat into the joint, where the fat layers above the blood because of its lower density. The sign indicates an occult intra-articular fracture, most commonly of the tibial plateau, even when no fracture line is visible on standard views.

Key Alignment Lines

These lines are essential viva knowledge.

• Shenton line. A smooth continuous arc runs from the inferior border of the superior pubic ramus along the medial border of the femoral neck. Disruption of the arc indicates hip pathology: dislocation, fracture or developmental dysplasia.
• Anterior humeral line. Drawn along the anterior cortex of the humerus on the lateral elbow view, it should pass through the middle third of the capitellum. If it passes through the anterior third, consider a posteriorly displaced supracondylar fracture.
• Radiocapitellar line. Drawn along the long axis of the proximal radius, it should pass through the centre of the capitellum on all views. Failure indicates radial head dislocation (Monteggia equivalent).
• Böhler angle. The angle formed between the posterior facet and anterior process of the calcaneus on the lateral view is normally 20-40 degrees. Flattening to less than 20 degrees suggests a calcaneal compression fracture.

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

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Protection Principles

The three principles of protection are time, distance and shielding.

• Time. Minimise exposure duration. In fluoroscopy, use pulsed rather than continuous mode, use last-image-hold to review without ongoing radiation, and plan the procedure to minimise screening time; in plain radiography, avoid unnecessary repeat exposures by ensuring correct positioning before exposure.
• Distance. By the inverse square law, doubling the distance from the radiation source reduces dose to one-quarter, so 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.
• Shielding. A lead apron (minimum 0.25 mm Pb equivalent, preferably 0.5 mm for wrap-around) is mandatory during fluoroscopy. A thyroid shield is strongly recommended, and lead glasses (0.75 mm Pb equivalent) should be worn by high-volume fluoroscopy users.

Gonadal Shielding

The guidance has reversed, and this is now a viva discriminator. Both the American Association of Physicists in Medicine (2019) and the British Institute of Radiology (2020) issued position statements recommending that routine gonadal contact shielding be discontinued. Three reasons converged.

• Mispositioning. Shields are frequently mispositioned: a meta-analysis of 2,187 paediatric pelvic radiographs found them significantly more often placed inaccurately in girls than boys. A misplaced shield obscures the very anatomy being imaged and buys a repeat exposure.
• Automatic exposure control. A shield entering the field makes the AEC increase output, raising rather than lowering dose.
• No demonstrated hereditary risk. The hereditary risk that motivated the practice in the early 1900s has never been demonstrated in humans, and the gonadal tissue weighting factor has been revised sharply downwards as doses fell.

What to say instead. The evidence is strongest for stopping in girls; some centres retain shielding selectively in boys or for reassurance, and that is defensible only with training and audit. Say that dose is controlled by collimation, exposure factors and avoiding the repeat: shielding was never the primary protective measure.

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

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.

Paediatric Radiography

Children are more radiosensitive than adults because of their rapidly dividing cells and the longer remaining lifespan over which stochastic effects (cancer) can develop. The ALARA principle is therefore even more important in paediatric imaging.

• Dose reduction. Use lower kVp and mAs settings. Modern protocols reduce paediatric extremity doses by 30-50% compared with adult settings.
• Gonadal shielding. Routine contact shielding is no longer recommended, as set out under radiation safety above.
• 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 when the normal appearance of ossification centres makes it difficult to distinguish a fracture from a developing physis. Routine comparison views should not be performed unless clinically indicated, because each additional radiograph adds dose.
• Growth plate awareness. Open physes can mimic fracture lines, and knowledge of the age-dependent ossification centres (CRITOE for the elbow) is essential to avoid misdiagnosis.

Imaging in Pregnancy

Risk depends on dose and gestational age. The highest-risk period for teratogenesis is organogenesis (2-8 weeks); beyond 15 weeks, the threshold for major malformation is well above any diagnostic imaging dose. The stochastic risk of future cancer is estimated at approximately 1 in 10,000 per mGy of fetal dose, and fetal dose should be kept below 1 mGy throughout pregnancy.

Extremity radiographs are safe. The fetal dose from a hand, wrist, ankle or knee radiograph is effectively zero, and the fetal risk negligible, because the beam is directed far from the uterus.

Pelvic and abdominal imaging. It requires careful justification and optimisation. A single AP pelvis radiograph delivers approximately 1 mGy to the uterus, well below the 100 mGy threshold for deterministic effects, but repeated exposures and CT scanning can accumulate meaningful doses.

Alternatives. MRI is preferred for non-urgent indications: it does not use ionising radiation and is considered safe in the second and third trimesters (avoid gadolinium if possible). Ultrasound is first-line for many soft-tissue and periarticular questions.

Never withhold clinically essential imaging in pregnancy, but always document the risk-benefit discussion.

Recognising radiographic artefacts prevents misdiagnosis and unnecessary further investigation, and it matters in quality assurance discussions.

Radiographic physics is settled. What to do with it is not, and three of these have changed within the working lifetime of current consultants.

Patient contact shielding. Routine gonadal shielding was near-universal for a century before the reversal described under radiation safety. The residual debate is whether to retain it selectively in boys, or for patient reassurance, and how to explain the change to families who expect a lead apron. It stays unresolved because the harm of a misplaced shield is measurable while the benefit was always theoretical, and abandoning a visible safety ritual is a communication problem as much as a physics one.

Dose creep in digital radiography. Film was self-policing: overexpose it and the image went black. On a digital detector an overexposed image looks excellent, the extra dose simply producing a cleaner picture, and the result is a documented tendency for departmental doses to drift upward over time. The exposure index exists to detect it, but it is vendor-specific, inconsistently monitored and invisible to the clinician requesting the film, so the person who could notice has no feedback loop.

The linear no-threshold model at radiographic doses. Every dose figure quoted here is interpreted through the LNT assumption, that risk is proportional to dose with no safe threshold. It is a deliberately conservative protection convention extrapolated from high-dose data, not a measured effect at the microsievert level of an extremity radiograph. At these doses the predicted excess is far below the statistical noise around a lifetime cancer incidence of roughly 40%, so no study can settle it. Quote risks with the model named, and neither claim "there is no risk" nor imply a measured one.

How many views, and when. The two-view minimum is a sound rule that is nonetheless applied unevenly: additional obliques and stress views are ordered by habit as often as by question, and each carries dose and cost for a variable yield. The value of an extra view depends on the specific fracture being excluded and on who is reading it, so blanket protocols either over-image the simple case or under-image the subtle one. The defensible position is to name the question each view answers before requesting it.