14.3 Elbow & Humerus

The Elbow: Routine Projections

The elbow joint is formed by the distal humerus and the proximal radius and ulna. Positioning revolves around the relationship of the humeral epicondyles to the IR. For the AP elbow, the arm is fully extended and the hand supinated (palm up) so the epicondyles are parallel to the IR and the joint opens with no radioulnar overlap. The CR is perpendicular to the mid-elbow. When the patient cannot fully extend, trauma AP views are needed (see below).

For the lateral elbow, the elbow is flexed 90 degrees and the hand is turned into a true lateral (thumb up) position so the epicondyles are stacked (perpendicular to the IR). This 90-degree flexion is critical: it superimposes the epicondyles, shows the olecranon in profile, and displays the anterior and posterior fat pads. A visible or displaced posterior fat pad sign indicates joint effusion and a possible occult radial head fracture - the exam loves this clinical link.

Oblique Elbow Projections

Remember the pairing: pronate for the coronoid (medial oblique) and externally rotate for the radial head (lateral oblique). A memory aid is that the radial head - on the lateral (thumb) side - is freed when you rotate the elbow laterally.

Trauma Elbow: Partial Flexion and the Coyle Method

When an injured elbow cannot be fully extended, a single AP is not valid because either the proximal or distal segment is foreshortened. The rule is to take two AP projections: one with the humerus parallel to the IR (demonstrates the distal humerus, with the forearm foreshortened) and one with the forearm parallel to the IR (demonstrates the proximal radius and ulna, with the humerus foreshortened). The CR remains perpendicular to the IR.

The Coyle (axial trauma) method obtains a well-projected radial head or coronoid process without full extension:

• Radial head: elbow flexed 90 degrees, hand pronated, CR angled 45 degrees toward the shoulder (proximally).
• Coronoid process: elbow flexed about 80 degrees, CR angled 45 degrees away from the shoulder (distally).

The radial head can also be studied with a four-position lateral series, rotating the hand from supination to pronation to roll the radial head into different aspects.

The Humerus

The humerus is imaged in AP and lateral projections, each including the shoulder and elbow joints when possible. For the AP humerus, the arm is extended and the hand supinated so the epicondyles are parallel to the IR; the greater tubercle is seen laterally and the humeral head is in an anatomic AP. For the lateral humerus, the epicondyles are placed perpendicular to the IR - a lateromedial or mediolateral projection with the elbow partially flexed and the hand internally rotated.

Trauma Humerus

When a proximal humeral fracture or dislocation prevents moving the arm, do not rotate it. Two options preserve a 90-degree relationship between projections:

1. Transthoracic lateral (Lawrence method): the affected arm hangs at the side, the CR passes horizontally through the thorax to project the humerus between the thoracic vertebrae and the sternum; a breathing technique blurs the ribs.
2. Scapular Y (PA oblique): an anterior oblique that also demonstrates the proximal humerus and any dislocation.

Always obtain two projections 90 degrees apart for any suspected fracture, and keep the anatomic side marker in the field at the time of exposure. Confusing the oblique rules (pronate vs. supinate) or omitting the second trauma projection are the most common tested mistakes for the elbow and humerus.

The Fat Pad Signs

The elbow has three fat pads that are radiographic clues to occult injury. The anterior fat pad is normally a thin lucency just anterior to the distal humerus and may be visible on a normal 90-degree lateral. The posterior fat pad sits in the olecranon fossa and is normally invisible; when a joint effusion (blood from a fracture) pushes it out of the fossa, a positive posterior fat pad sign appears. In adults this most often means a radial head fracture; in children it suggests a supracondylar fracture. Because the sign depends on a properly flexed, non-rotated 90-degree lateral, sloppy positioning can create a false negative - a reason the exam stresses exact lateral technique.

Evaluation Criteria and Scenario

A correct AP elbow shows the joint open with the radial head, neck, and tuberosity slightly superimposing the proximal ulna, and the epicondyles in the same plane. A correct lateral shows the humeral epicondyles superimposed, the trochlea and capitulum as concentric arcs, and the olecranon in profile. Scenario: an adult who fell on an outstretched hand cannot straighten the elbow and reports lateral elbow pain. Rather than force extension for a standard AP, the technologist performs the two-part trauma AP (humerus-parallel and forearm-parallel projections) plus a Coyle radial-head view (90-degree flexion, CR 45 degrees toward the shoulder). The follow-up image confirms a radial head fracture that a single foreshortened AP would have obscured.

Common Elbow and Humerus Mistakes

• Pronating for the radial head - the radial head needs the lateral (external) oblique with the hand supinated; pronation is for the coronoid.
• Under-flexing the lateral elbow - anything other than a true 90-degree lateral distorts the fat pads and can hide an effusion.
• A single AP on a partially flexed elbow - always split into humerus-parallel and forearm-parallel projections.
• Rotating a trauma humerus - use the transthoracic lateral or scapular-Y instead of a forced lateral.
• Excluding a joint on the humerus - include the shoulder and elbow; use a longer IR diagonally if needed.