Optimal Timing for Sail Sign and Fat Pad Sign in Occult Olecranon Fracture
The sail sign (anterior fat pad) and posterior fat pad sign are visible immediately after injury on initial radiographs and do not require delayed imaging to appear, as joint effusion develops acutely following elbow trauma.
Immediate Visibility on Initial Radiographs
Both fat pad signs appear on initial post-trauma radiographs taken at presentation, as the joint effusion that displaces these fat pads develops immediately after injury 1, 2.
The posterior fat pad sign has been demonstrated to be predictive of occult fracture in 76% of children when present on initial radiographs, without requiring follow-up imaging to become visible 3.
In adults, 75% of patients with an elevated fat pad sign on initial radiographs (without visible fracture) were found to have radiographically occult fractures when evaluated by MRI performed 0-12 days post-injury (mean 4 days), indicating the fat pad elevation was present from the time of initial imaging 2.
Measurement Characteristics
A normal anterior fat pad has a mean apical angle of 14.7 degrees (SD 3.3), while an abnormal anterior fat pad measures 27.0 degrees (SD 6.8) 4.
The anterior fat pad displacement in patients with occult fractures ranges from 5-15 mm (mean 9.25 mm), and posterior fat pad elevation ranges from 1-6 mm (mean 3.2 mm) 2.
Point-of-care ultrasound can identify the elevated posterior fat pad and associated anechoic joint fluid with hyperechogenic spicules in the olecranon fossa at the time of initial evaluation 1.
Clinical Implications for Timing
There is no need to wait for delayed radiographs to see fat pad signs—if they are going to be present, they will be visible on the initial lateral radiograph taken at presentation 4, 3.
Follow-up radiographs at 10-14 days are more sensitive for detecting the fracture line itself (sensitivity increases from 15-35% initially to 30-70% on follow-up) due to overt bone reaction and callus formation, but this is distinct from fat pad visibility 5.
A normal anterior fat pad on initial radiographs has a 96.4% sensitivity and 98.2% negative predictive value for excluding fracture, making it a reliable immediate indicator 4.
Important Caveats
The posterior fat pad sign can be falsely negative if subtle, and falsely positive if the elbow is not positioned at 90 degrees of flexion on the lateral view 1.
The size of fat pad elevation (anterior or posterior) does not correlate with the likelihood of underlying fracture—even minimal elevation should be considered significant 2.
In the specific context of olecranon fractures, 9% of occult fractures in children with isolated posterior fat pad signs were located in the proximal ulna (olecranon region) 3.