Elbow Imaging: Systematic Interpretation

A visible posterior fat pad on a lateral elbow radiograph is ALWAYS abnormal. Normally, the posterior fat pad is hidden within the olecranon fossa and only becomes visible when displaced by joint effusion - typically hemarthrosis from an intra-articular fracture. Even though no fracture is visible on the initial X-ray, I must presume this patient has an occult fracture until proven otherwise. The most common occult fracture in this scenario is a radial head fracture, which can be difficult to see especially if non-displaced. I would obtain additional views - a radial head view (45° external rotation) may reveal the fracture. If X-rays remain negative, I have two options: treat as presumed fracture with immobilization and repeat X-rays in 7-10 days when bone resorption may make the fracture visible, or obtain CT/MRI for immediate diagnosis. Given the high likelihood of fracture with a positive posterior fat pad sign, I would treat this patient in a sling or splint, advise elevation and ice, and arrange follow-up. I would also document the fat pad finding clearly.

In a 6-year-old with fall on outstretched hand and displacement of the anterior humeral line, I suspect a supracondylar humerus fracture, which is the most common elbow fracture in children. The anterior humeral line should normally pass through the middle third of the capitellum on a true lateral view. If it passes anterior to this (or the capitellum is completely posterior to the line), this indicates posterior displacement of the distal fragment, consistent with an extension-type supracondylar fracture, which accounts for 98% of these injuries. I would assess the Gartland classification: Type I is non-displaced with intact posterior cortex, Type II has posterior cortex intact but angulation, and Type III has complete displacement. I would also check for the posterior fat pad sign confirming intra-articular effusion. In assessing this injury, I must evaluate neurovascular status - the anterior interosseous nerve (FPL to thumb, FDP to index) and brachial artery are at risk. Clinically, I would check for a pink, pulseless hand indicating vascular compromise. I would also use CRITOE to check the ossification centers - at age 6, the child should have Capitellum, Radial head, and Internal (medial) epicondyle visible.

I use three main lines to assess elbow alignment. First, the radiocapitellar line - I draw a line through the center of the radial shaft, and this should pass through the center of the capitellum on ALL views (AP, lateral, and any obliques). If this line doesn't pass through the capitellum, the radial head is dislocated. This is critical for detecting Monteggia fracture-dislocations where an ulna fracture is accompanied by radial head dislocation. Second, the anterior humeral line - on a true lateral view, I draw a line along the anterior cortex of the distal humerus and extend it distally. Normally, this line passes through the middle third of the capitellum. In extension-type supracondylar fractures, posterior displacement of the distal fragment causes this line to pass anterior to the middle third or even anterior to the entire capitellum. Third, the carrying angle - on the AP view with elbow extended and forearm supinated, I measure the angle between the long axis of the humerus and ulna. Normal is 5-15 degrees of valgus (slightly greater in females). Increased valgus (cubitus valgus) can result from lateral condyle fracture malunion; decreased angle (cubitus varus or gunstock deformity) results from supracondylar fracture malunion.