A broken elbow can look dramatically obvious on an x-ray, with a clear crack or displaced bone fragment, or it can be nearly invisible, detectable only through subtle clues like displaced fat pads or shifted alignment lines. What you see depends on which bone is broken, how severely, and whether the patient is a child or an adult.
The elbow joint involves three bones: the bottom end of the upper arm bone (humerus), the larger forearm bone (ulna) with its bony point at the back of the elbow, and the smaller forearm bone (radius) with its disc-shaped head. A fracture can occur in any of these, and each produces different patterns on imaging.
What Doctors Look For First
Radiologists and emergency physicians typically evaluate elbow x-rays using two standard views: one from the front (anteroposterior) and one from the side (lateral). The lateral view is the single most important image for identifying fractures, because it reveals displacement, the number of bone fragments, and how much the joint surface is disrupted. Many fractures that are invisible on the front view become clear from the side.
Before even looking at the bones themselves, doctors check for indirect signs of a fracture. The most reliable of these is the posterior fat pad sign.
The Fat Pad Sign: A Hidden Fracture Clue
Inside your elbow joint, small pads of fat sit tucked against the bone, normally hidden from view on a lateral x-ray. When a bone breaks inside the joint, bleeding fills the joint capsule and pushes these fat pads outward, making them visible as dark triangular shadows lifted away from the bone. The posterior fat pad, which sits behind the elbow on the lateral view, is especially telling. It should never be visible on a normal x-ray, so when it appears after an injury, it strongly suggests a fracture is hiding somewhere in the joint.
A study of 45 children who had a visible posterior fat pad but no obvious fracture on initial x-rays found that 76% turned out to have an occult fracture. In a larger study comparing x-rays to CT scans, about 86% of patients with a fat pad sign on x-ray were confirmed to have a fracture. The fat pad sign is most commonly associated with radial head fractures in adults and supracondylar fractures in children.
If you’re looking at an elbow x-ray and notice a dark, sail-shaped shadow lifted off the front or back of the joint, that’s a fat pad being pushed outward by fluid, likely blood from a fracture you can’t directly see on the image.
Radial Head Fractures in Adults
Radial head fractures account for roughly one-third of all adult elbow fractures. They happen at the top of the smaller forearm bone, right where it meets the upper arm bone. On an x-ray, a displaced radial head fracture shows up as a visible crack or step-off in the smooth, rounded disc of bone at the top of the radius. You might see a fragment of bone shifted out of its normal position, or the head may appear tilted or angulated.
The tricky part is that nondisplaced radial head fractures often cannot be seen directly on plain x-rays. The bone is cracked but hasn’t shifted, so the fracture line blends into the surrounding bone. In these cases, the only visible clue may be an enlarged posterior fat pad on the lateral view. Doctors will sometimes order an additional angled view called a radiocapitellar view to try to catch a subtle crack, but it’s also common to simply diagnose a nondisplaced fracture based on the fat pad sign combined with tenderness over the radial head.
Olecranon Fractures
The olecranon is the bony point at the tip of your elbow, the part of the ulna you lean on. Fractures here tend to be more visible on x-ray because the powerful triceps muscle pulls the broken piece upward, creating an obvious gap. On a lateral x-ray, you can see a clear break across the curved bone at the back of the elbow, with the fragment displaced upward and a visible gap at the joint surface. In more severe injuries, the bone may be shattered into multiple pieces (comminuted), which appears as several fragments scattered around the joint with irregular edges and widened spaces between them.
A true lateral view is critical for olecranon fractures. Doctors use it to assess how far the fragments have separated and how much of the joint surface is disrupted, both of which determine whether surgery is needed.
Alignment Lines That Reveal Fractures
Even when a fracture line isn’t directly visible, shifts in normal bone alignment can give it away. Two key lines help doctors evaluate elbow x-rays:
- The anterior humeral line: On a lateral x-ray, a line drawn along the front surface of the upper arm bone should pass through the middle third of the rounded knob (capitellum) at the bottom of the humerus. If the line passes in front of the capitellum or misses it entirely, the bone has shifted backward, indicating a supracondylar fracture with extension. This is one of the most reliable signs in pediatric elbow injuries.
- The radiocapitellar line: A line drawn through the center of the radius bone should point directly at the capitellum in every view. If it doesn’t, the radial head may be dislocated, which can accompany a fracture of the ulna.
Why Children’s Elbow X-Rays Are Harder to Read
Children’s elbows look strikingly different from adults’ on x-ray because much of a child’s elbow is still made of cartilage, which doesn’t show up on x-rays at all. As children grow, six separate ossification centers (small islands of bone) gradually appear and eventually fuse together. These centers appear in a predictable order remembered by the mnemonic CRITOE: capitellum, radial head, internal (medial) epicondyle, trochlea, olecranon, and external (lateral) epicondyle. They show up at roughly ages 1, 3, 5, 7, 9, and 11, though there’s significant individual variation.
The problem is that these normal bone islands can look like fracture fragments to an untrained eye, and actual fracture fragments can be mistaken for normal ossification centers. One key rule: the centers always appear in the CRITOE sequence. If you see what looks like a trochlea ossification center but the medial epicondyle hasn’t appeared yet, that “center” is likely a fracture fragment that’s been pulled out of position.
Supracondylar Fractures in Children
Supracondylar fractures are the most common elbow fracture in children, making up 50% to 70% of pediatric elbow breaks. They peak between ages 5 and 7, and occur more often in boys. The fracture crosses through the lower humerus just above the joint.
On x-ray, a displaced supracondylar fracture is usually easy to spot: the bottom portion of the humerus is tilted or shifted backward, and a visible fracture line crosses the bone above the condyles. In severe cases, the fragments may overlap or angulate dramatically. Less obvious fractures may show only a subtle buckling of the bone cortex on one side, or no visible fracture line at all. In those cases, the anterior humeral line becomes the key diagnostic tool. If the line passes in front of the capitellum rather than through its middle third, there’s likely a fracture with posterior displacement.
Research on post-treatment alignment shows that children whose anterior humeral line passed through the middle or posterior third of the capitellum had better elbow motion (average total range of about 136 degrees) compared to those whose line fell anterior to the capitellum (about 128 degrees). When the line missed the capitellum entirely, outcomes were significantly worse.
What a Normal Elbow X-Ray Looks Like
Understanding what’s abnormal is easier when you know what normal looks like. On a true lateral x-ray of an adult elbow, the three circles of the trochlea, capitellum, and inner trochlea should overlap concentrically, like a target. The bones should have smooth, unbroken outer edges (cortex) with no steps, gaps, or angulation. No posterior fat pad should be visible. The anterior humeral line should pass through the middle third of the capitellum, and the radiocapitellar line should point directly at it.
Any break in the smooth cortical outline, any visible fat pad that shouldn’t be there, or any deviation in the alignment lines raises suspicion for a fracture, even if no obvious crack is visible. When plain x-rays are inconclusive but clinical suspicion is high, a CT scan can catch fractures that x-rays miss.