Fat Pad Sign Elbow: Anterior vs Posterior on X-ray

Answer: An intra-articular elbow effusion. Blood or inflammatory exudate distends the joint capsule and pushes the fat pads out of the humeral fossae. A thin anterior fat pad is a normal finding. The sign is positive when the anterior pad is lifted 16 degrees or more off the anterior humeral line, or when any posterior fat pad becomes visible.

The usual cause is a supracondylar fracture in a child and a radial head or neck fracture in an adult. Non-traumatic causes include septic arthritis, juvenile idiopathic arthritis and osteomyelitis.

Why is it called so?

Norell described the displaced extracapsular fat of the injured elbow in 1954. The name records what is displaced, not what is broken. The elevated anterior pad stands off the humerus as a sharp triangle resembling a filled sail, giving the sail sign; the posterior pad is named simply for appearing at all.

Pathophysiology

The fat pads are extrasynovial but intracapsular, so rising intracapsular volume lifts the capsule and carries them outward. The anterior pad swings away from the humeral shaft and increases its angle; the posterior pad is levered out of the olecranon fossa and appears for the first time. The pads therefore report capsular distension, not cortical disruption, which is what makes them positive with no visible fracture line and negative once the capsule has torn.

Alternative names: Sail sign (for the elevated anterior fat pad), elevated or displaced fat pad sign, positive fat pad sign

Other associated named signs: Spinnaker sail sign and thymic sail sign, unrelated thoracic signs sharing the metaphor; lipohaemarthrosis; the anterior humeral line

Anterior versus posterior fat pad: which one is abnormal?

The two pads are not equivalent. A visible anterior pad is normal on most lateral films; the posterior pad has no normal counterpart.

How often is there an occult fracture?

Less often than classic teaching claims. Pooled across 10 studies and 250 children, the rate behind a positive sign with normal radiographs is 44.6% (95% CI 30.4 to 59.7).

What the sign does not tell you

• It is sensitive but not specific. For radial head and neck fractures, sensitivity was 85.4% and specificity only 50%, so its absence is more informative.
• There is no agreed pathway. Among 133 surgeons, the definition, the imaging and the treatment all varied widely, which is why the objective 16-degree definition was proposed.
• Report it, then look again. Say which pad is displaced, then re-interrogate the supracondylar region, the radial head and neck, and the anterior humeral line.

Frequently asked questions

Is the anterior fat pad sign normal?

Yes. A thin anterior fat pad is visible on 57% of normal lateral elbow radiographs. It becomes the abnormal sail sign only once lifted 16 degrees or more off the anterior humeral line. A normal anterior pad carries a negative predictive value of 98.2% for fracture in children.

Does a negative fat pad sign rule out an elbow fracture?

No. It was absent in 38% of displaced extension-type supracondylar fractures, because a torn capsule lets the haemarthrosis escape instead of displacing the pads. A negative sign lowers the probability of an intra-articular fracture but never excludes one, and says least about extra-articular injuries.

How often is there an occult fracture if the X-ray looks normal?

About 44.6% of children, pooled across 10 studies and 250 children (95% CI 30.4 to 59.7). Series range from 17% to 76% depending on whether fractures were confirmed on follow-up radiographs or on MRI. In adults, 15 of 20 had one on MRI.

Does a positive fat pad sign change management?

Usually not beyond immobilisation and clinical review. In a prospective MRI study, identifying the occult fracture did not alter final treatment, and 2 to 3 weeks of immobilisation sufficed. With no consensus pathway, advanced imaging is better reserved for persistent or worsening symptoms.

References

1. Norell HG. Roentgenologic visualization of the extracapsular fat; its importance in the diagnosis of traumatic injuries to the elbow. Acta Radiol. 1954;42(3):205-10.
2. Kappelhof B, Roorda BL, Poppelaars MA, The B, Eygendaal D, Mulder PGH, et al. Occult fractures in children with a radiographic fat pad sign of the elbow: a meta-analysis of 10 published studies. JBJS Rev. 2022;10(10).
3. Skaggs DL, Mirzayan R. The posterior fat pad sign in association with occult fracture of the elbow in children. J Bone Joint Surg Am. 1999;81(10):1429-33.
4. Donnelly LF, Klostermeier TT, Klosterman LA. Traumatic elbow effusions in pediatric patients: are occult fractures the rule? AJR Am J Roentgenol. 1998;171(1):243-5.
5. Blumberg SM, Kunkov S, Crain EF, Goldman HS. The predictive value of a normal radiographic anterior fat pad sign following elbow trauma in children. Pediatr Emerg Care. 2011;27(7):596-600.
6. Poppelaars MA, Eygendaal D, The B, van Oost I, van Bergen CJA. Diagnosis and treatment of children with a radiological fat pad sign without visible elbow fracture vary widely: an international online survey and development of an objective definition. Children (Basel). 2022;9(7):950.
7. Al-Aubaidi Z, Torfing T. The role of fat pad sign in diagnosing occult elbow fractures in the pediatric patient: a prospective magnetic resonance imaging study. J Pediatr Orthop B. 2012;21(6):514-9.
8. Samelis PV, Papagrigorakis E, Ellinas S. Role of the posterior fat pad sign in treating displaced extension type supracondylar fractures of the pediatric elbow using the Blount method. Cureus. 2019;11(10):e6024.
9. O’Dwyer H, O’Sullivan P, Fitzgerald D, Lee MJ, McGrath F, Logan PM. The fat pad sign following elbow trauma in adults: its usefulness and reliability in suspecting occult fracture. J Comput Assist Tomogr. 2004;28(4):562-5.
10. Irshad F, Shaw NJ, Gregory RJ. Reliability of fat-pad sign in radial head/neck fractures of the elbow. Injury. 1997;28(7):433-5.