Procedures · Upper Extremity

Elbow Positioning Series

Compare the AP, flexion alternates, obliques, and lateral projections of the routine elbow series.

elbow · Extended

Right

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The routine elbow series

The projections this region is examined with. Set them up yourself in Practice positioning.

AP elbow

Anteroposterior projection · elbow fully extended, hand supinated

Demonstrates: The distal humerus, the elbow joint space, and the proximal radius and ulna together. The olecranon process sits down in the olecranon fossa, the joint space opens with the arm fully extended and the beam centred correctly, and the radial head, neck and tubercles stand clear of the ulna or are only slightly covered by it.
Part position: Seated at the end of the table with the elbow fully extended, the hand supinated, and the arm and forearm aligned with the long axis of the receptor. The elbow joint is centred to the receptor, and the patient leans laterally as far as it takes to bring the joint truly frontal. The hand is supported to stop it drifting.
Central ray: 90° to the image receptor, centered to the mid-elbow joint — about ¾ inch (2 cm) distal to the midpoint of a line between the epicondyles.
Why it is positioned this way: Everything in this position serves one check, and it is a check you make with your fingers rather than your eyes: palpate both epicondyles and confirm they are the same distance from the receptor. That is the interepicondylar plane lying parallel to the receptor, and when it does, the beam meets both epicondyles edge-on and throws them into profile — which is the whole test for rotation on this projection. Leaning the patient rather than turning the arm is how you get there without pronating the hand, and that matters because pronation crosses the radius over the ulna and hides the radial head behind it. Full extension is what seats the olecranon in its fossa and opens the joint space; anything less and both close down.

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Where "mid-elbow joint" actually isThe joint is not level with the epicondyles. It sits about ¾ inch — 2 cm — distal to the midpoint of the line between them, and that is where the central ray goes. Centring on the epicondyles themselves puts the beam through the distal humerus instead of through the joint, and the space closes.

AP elbow — partial flexion

Alternate AP · when the elbow cannot be fully extended

Demonstrates: The same anatomy as the AP, but split across two exposures rather than one, because a bent elbow cannot present both bones flat to the receptor at once. The projection shown here is the one taken with the forearm parallel to the receptor, which is where the proximal radius and ulna are seen best. Structures in the joint region are partly obscured and slightly distorted either way, and how much depends on how far the elbow will open.
Part position: Seated at the end of the table with the elbow partially flexed — as far as it will comfortably go and no further. Two AP projections are taken from that one position: one with the forearm parallel to the receptor, and one with the humerus parallel to it, with a support under the wrist and forearm for the second so nothing moves.
Central ray: 90° to the image receptor, centered to the mid-elbow joint — about ¾ inch (2 cm) distal to the midpoint of a line between the epicondyles.
Why it is positioned this way: A partly flexed elbow puts the humerus and the forearm on two different planes, and one beam set at 90° to the receptor can only meet one of them squarely. So the answer is not to compromise between them, it is to take both: whichever segment lies parallel to the receptor is the one that comes out undistorted, so the humerus-parallel exposure is read for the distal humerus and the forearm-parallel one for the proximal radius and ulna. Notice how little the picture changes here from the routine AP — the forearm has not moved at all, only the humerus has swung, which is exactly the difference a real partial-flexion elbow presents.

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The second exposure — humerus parallelThe companion projection puts the humerus flat on the receptor instead, with the forearm angled up and supported. It is the one that shows the distal humerus and its epicondyles cleanly; the forearm-parallel exposure shown here is the one that shows the proximal radius and ulna. Neither replaces the other, which is why the routine asks for two.

How far is "partially flexed"?There is no fixed number for it — partial flexion is whatever that patient's elbow will do, and no evaluation criterion depends on the figure. What does depend on it is which of the two exposures you take first.

When the elbow will not open past 90°If the elbow stays near 90°, take the same two projections but angle the central ray 10° to 15° into the joint. If it will not open even that far — more than 90° of flexion — stop and use the acute flexion projections instead.

AP elbow — acute flexion

Acute flexion · when the elbow cannot be extended at all

Demonstrates: The distal humerus, seen through the forearm folded across it. Both bones are superimposed, so the exposure has to carry through two thicknesses of bone: the medial and lateral epicondyles and parts of the trochlea, capitulum and olecranon process come out in profile, and soft tissue detail is not readable on this projection at all.
Part position: Seated at the end of the table with the acutely flexed arm resting on the receptor, fingertips on the shoulder. The humerus is aligned and centred to the long axis of the receptor and the receptor is adjusted to the elbow joint region. Palpate the epicondyles and make sure the interepicondylar plane is parallel to the receptor, the same rotation check as the routine AP.
Central ray: 90° to the image receptor and to the humerus, centered to a point midway between the epicondyles.
Why it is positioned this way: With the elbow shut, the humerus and the forearm lie on top of one another and no single beam can be square to both — so, as with partial flexion, the answer is two exposures rather than a compromise. This one is square to the humerus and reads the distal humerus; its companion is angled square to the forearm and reads the proximal radius and ulna through the overlying humerus. The central ray is midway between the epicondyles here rather than ¾ inch distal to them, because with the joint closed the target is the distal humerus itself, not a joint space that no longer has anywhere to open.

Also covered by this projection

How far is "acute flexion"?Acute flexion is described by where the hand ends up — fingertips resting on the shoulder — rather than by a number of degrees. Read it as "as far as this elbow closes", not as a prescribed angle.

The second exposure — proximal forearmAngle the central ray until it meets the forearm at 90° and centre about 2 inches (5 cm) proximal to the olecranon process. That projection shows the proximal ulna and radius, including the outline of the radial head and neck, through the superimposed distal humerus.

AP oblique elbow — lateral (external) rotation

AP oblique · whole arm rotated laterally 45°

Demonstrates: An oblique of the distal humerus and proximal forearm in which the radial head, neck and tuberosity are clear of the ulna, and the lateral epicondyle and the capitulum are elongated and thrown into profile. It is the projection for the radial head and neck, which is the pair of structures a fall on an outstretched hand most often breaks.
Part position: Seated at the end of the table with the arm fully extended and the shoulder and elbow on the same horizontal plane, dropping the shoulder as needed. The arm and forearm are aligned with the long axis of the receptor and the elbow joint is centred to it. The hand is supinated and the whole arm is then rotated laterally until the distal humerus and the front of the elbow are about 45° to the receptor, which usually means leaning the patient laterally as well.
Central ray: 90° to the image receptor, centered to the mid-elbow joint — about ¾ inch (2 cm) distal to the midpoint of a line between the epicondyles as seen from the tube.
Why it is positioned this way: The instruction is to rotate the whole arm, not to turn the hand, and the difference is the point. Turning the hand alone pronates the forearm and rolls the radius across the ulna while leaving the humerus where it was, so the elbow never obliques at all. Rotating from the shoulder carries the humerus with it, which is what puts the interepicondylar plane at 45° and swings the capitulum and lateral epicondyle around into the edge of the beam where they elongate. The hand stays supinated throughout so that the radius and ulna keep the side-by-side relationship the AP established, and the radial head simply rotates into the open.

Also covered by this projection

What "45°" is measured onNot the hand, and not the forearm. The angle is the interepicondylar plane against the receptor — the same plane that has to be parallel for the AP and edge-on at 90° for the lateral. Every position in this series is stated as an angle of that one plane, which is why palpating the epicondyles is the check for all of them.

AP oblique elbow — medial (internal) rotation

AP oblique · hand pronated, arm rotated medially 45°

Demonstrates: An oblique in which the coronoid process of the ulna is thrown into profile and the radial head and neck are projected over the proximal ulna rather than beside it. The medial epicondyle and the trochlea elongate and come into partial profile, the olecranon stays seated in its fossa, and the trochlear notch opens partly with the arm fully extended.
Part position: Seated at the end of the table with the arm fully extended and the shoulder and elbow on the same horizontal plane. The arm and forearm are aligned with the long axis of the receptor and the elbow is centred to it. The hand is pronated into a natural palm-down position and the arm is rotated as far as needed to bring the distal humerus and the front of the elbow to 45° — again, the interepicondylar plane at 45° to the receptor.
Central ray: 90° to the image receptor, centered to the mid-elbow joint — about ¾ inch (2 cm) distal to the midpoint of a line between the epicondyles as seen from the tube.
Why it is positioned this way: This is the mirror of the lateral oblique and it is the one taken for the coronoid process, which is a small anterior beak of the proximal ulna and is invisible on an AP because the ulna is projected end-on over it. Rotating medially swings that beak around until the beam runs tangent to it, and a structure the beam runs tangent to is a structure seen in profile. The radial head goes the other way in the same movement, sliding across to sit centred over the ulna instead of beside it — which is why "radial head superimposed and centred over the proximal ulna" is the check that the rotation went far enough, and in the right direction.

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Why the radial head superimposing is a PASS hereOn every other view in this routine, the radius covering the ulna is a fault. On this one it is the criterion. The two obliques are graded on opposite outcomes at the same landmark — free of the ulna on the lateral oblique, superimposed and centred over it on the medial — which makes that single relationship the fastest way to tell at a glance which oblique you are looking at.

Lateromedial elbow

Lateromedial projection · elbow flexed 90°, thumb side up

Demonstrates: A lateral of the distal humerus and proximal forearm with the humeral epicondyles stacked one on the other and the olecranon process in profile. About half the radial head is covered by the coronoid process. A true lateral announces itself by three concentric arcs — the trochlear sulcus, the double ridges of the capitulum and trochlea, and the trochlear notch of the ulna. It is also the view the fat pads are read on.
Part position: Seated at the end of the table with the elbow flexed 90°, the long axis of the forearm aligned with the receptor and the joint centred to it. The shoulder is dropped so that the humerus and forearm lie on the same horizontal plane, and the hand and wrist are then turned into a true lateral, thumb side up, which stands the interepicondylar plane at 90° to the receptor. A support under the hand and wrist keeps a heavy forearm parallel to the receptor.
Central ray: 90° to the image receptor, centered to the mid-elbow joint — a point about 1½ inches (4 cm) medial to the easily palpated posterior surface of the olecranon process.
Why it is positioned this way: Two separate movements build this projection and neither can do the other's job. Flexing to 90° is what stacks the epicondyles; turning the hand thumb-up is what brings the interepicondylar plane to 90° with the receptor. The 90° is not a rough figure: the posterior fat pad only lifts into view at 90° of flexion, and that raised fat pad is often the only sign of an elbow fracture on the whole image, so a lateral taken at 60° can be diagnostically empty while looking perfectly acceptable. Note also where the central ray goes — not on the olecranon, which is what your fingers find first, but 1½ inches medial to it, because the joint lies medial to that ridge.

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When less than 90° is allowedSome soft tissue questions call for 30° to 35° of flexion instead. Those are taken only when specifically indicated — at that angle the posterior fat pad has not lifted, so the projection cannot answer the question the routine lateral exists to answer.

A heavy or muscular forearmPut a support under the hand and wrist to raise the distal end until the forearm lies parallel to the receptor. Without it the forearm slopes away, the epicondyles come apart, and the three concentric arcs never resolve.

Procedures lessonForearm & elbow