Limb Positioning
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Direct answer
Two projections at right angles — typically AP and lateral — with the joint above and below any long-bone injury included on the film, is the non-negotiable rule of limb radiography. Beyond it live the named views: the scaphoid series with ulnar deviation, the elbow lateral in exactly 90 degrees of flexion, the ankle mortise view with 15-20 degrees internal rotation, the knee tunnel view for the intercondylar notch, the Merchant skyline for the patella, the scapular Y and axillary views for the shoulder, and the internally-rotated hip AP. A displaced fracture may hide completely on one projection, and the elbow and scaphoid are the two spots where normal-looking films most famously lie.
What you must remember
- Two views at 90 degrees always; for long-bone injuries include the joint proximal and distal to the fracture site — a tibial fracture film without the ankle and knee is incomplete.
- Hand: PA, oblique and fan lateral; wrist: PA, lateral and oblique.
- Scaphoid series: PA with the hand in ulnar deviation (plus an angled PA) elongates the scaphoid and opens the waist; clinical snuffbox tenderness with normal films still means cast and repeat in 10-14 days.
- Elbow: AP in full extension and a true lateral in exactly 90 degrees flexion; the fat pad sign (anterior sail) on a normal film hints at an occult intra-articular fracture — supracondylar fractures in children hide here.
- Shoulder: AP in external and internal rotation; a dislocation needs the scapular Y (60-degree anterior oblique) or axillary view to confirm direction — AP alone can miss posterior dislocation.
- Pelvis and hip AP: feet internally rotated about 15 degrees to profile the femoral necks; trauma hips take a cross-table lateral instead of frog-leg.
- Knee: AP, lateral, and the intercondylar tunnel view with 40-50 degrees flexion; skyline patella views (Merchant: knee flexed about 45 degrees with a 30-degree caudal beam) are intolerable in acute injury.
- Ankle: AP, lateral and the mortise view — 15-20 degrees internal rotation so the ankle mortise joint space opens evenly; foot AP, oblique and the 40-45 degree axial calcaneal view complete the hindfoot.
- Comparison views of the opposite side (children's elbows especially) are taken selectively, not routinely.
A fall on the outstretched hand
Follow the commonest limb presentation and the views assemble themselves. A 24-year-old falls off a bike onto an outstretched hand: pain at the wrist snuffbox. The clinical story demands the scaphoid series: PA in ulnar deviation elongates the scaphoid so a waist fracture no longer overlaps itself, and an angled projection profiles the bone lengthwise. Scaphoid fractures are notorious for being invisible on day one; snuffbox tenderness plus normal films equals immobilise and re-image at 10-14 days, a clinical rule to know, not argue with.
Transmitted force also points to the elbow. The elbow AP needs the arm fully extended and supinated; the lateral needs exactly 90 degrees of flexion with true sideways positioning, because obliquity either way distorts the radial head. Watch for the fat pad sign: the anterior fat pad is normally a slim lucency, but a bulbous "sail" rising from the coronoid fossa means haemarthrosis and, in a child, a supracondylar fracture until proven otherwise. The lesson generalises: know what each view is for, include the joints the mechanism implicates, and flag films that look normal but should not be.
Where students slip
The elbow lateral is taken at 40 degrees "because the patient could not straighten" — a lateral in any flexion other than 90 degrees cannot be assessed, and if the arm cannot extend, the AP is taken in two positions rather than one dishonest one. Second, the mortise view: 15-20 degrees of internal rotation is the number; a "mortise" rotated 10 degrees mimics or hides a Tillaux or malleolar alignment error. Third, the shoulder: relying on AP alone for dislocation — posterior dislocations are notoriously subtle on AP, and the Y or axillary view is the answer. Fourth, the scaphoid: forgetting ulnar deviation, or believing the first normal film excludes fracture. Finally, paediatric elbows: compare the other side, and know the six ossification centres by age (capitellum, radius, internal epicondyle, trochlea, olecranon, lateral epicondyle).
Frequently asked questions
Why are two projections at right angles mandatory?
A fracture displaced in the plane of one projection may be perfectly aligned on it; the orthogonal view reveals displacement, angulation and joint involvement that a single film cannot.
How is the scaphoid specifically demonstrated?
By a scaphoid series: PA wrist with the hand in ulnar deviation plus supplementary angled projections, elongating the scaphoid to open the waist, where most fractures occur.
What is the mortise view of the ankle?
An AP ankle with 15-20 degrees of internal rotation, projecting the ankle mortise joint spaces evenly — essential for judging malleolar fractures and syndesmotic widening.
How is a posterior shoulder dislocation confirmed?
By the scapular Y view (60-degree anterior oblique) or the axillary projection, since AP films can look deceptively normal with the humeral head posteriorly displaced.