What does a fat pad sign mean (in children)?

27 Nov

Results

In the study of Al-Aubaidi 23% (6 / 25) of patients with positive fat pad sign and no visible fractures had a fracture on MRI examination some days later.3 Two of these were supracondylar humeral fractures, one proximal radial fracture, one proximal ulnar fracture, one lateral humeral condyl avulsion and one coronoid process avulsion.3

Donnelly included 54 children with an average age of 7 who had a joint effusion but no identifiable fracture on initial radiographs. The presence of periosteal reaction or bony slerosis on follow-up radiographs was considered to be evidence of occult fracture. Only 9 out of 54 patients showed evidence of a healing occult fracture upon follow-up radiographs. The location of the fractures was supracondylar 5, radial neck 2, olecranon 1 and coronoid 1. Skaggs in his study looked specifically at the posterior fat pad sign in children and found that 76% (34/45) had evidence of a fracture.2

Blumberg’s purpose was to look for the positive predictive value of an anterior fat pad sign following trauma in children. The positive predictive value of a normal fat pad sign was 96.4% The negative predictive value of a normal fat pad was 98,2%.5

Rabiner et al researched the use of ultrasonography in the use of diagnosing cortical disruptions.6 They compared this to a standard elbow x-rays. Ultrasonography is more sensitive than x-ray in detecting posterior fat pad elevation. The emergency physicians in this pediatric hospital all received a 30 minute hands-on practical session with live models. A positive elbow ultrasonographic result was defined as the enrolling pediatric emergency physician’s determination of elevation of the posterior fat pad. Elevation of the posterior fat pad was defined as rise of the fat pad above the extension of the distal humeral line on longitudinal view or above a line connecting both lips of the olecranon fossa on transverse view. 130 patients were enrolled with a mean age of 7,5 years. The majority of elbow fractures were located in the distal humerus 86% with 23% supracondylar fractures. 14% of fractures were the proximal ulna and radius. 44% of patients had an elevated fat pad 63% of these had a fracture at the initial visit 19% had a fracture identified on follow-up. Two patients had a visible fracture without a posterior fat pad. The sensitivity of elbow ultrasonography was 97%. Only one sonologist who enrolled twelve patients had experience with ultrasonography before the study. There was only one patient with a fracture and no positive fat pad or lipohemarthosis on ultrasonography. This was also missed on x-ray. One patient had a positive fat pad sign with ultrasonography with a negative x-ray who on follow-up had a lateral condyle fracture. Limitations were that only when a trained physician was available patients were enrolled. Not all patients received follow-up x-rays as this was deemed unethical. Patients were contacted by phone if they had no complaints they were diagnosed as not having a fracture. There is literature evidence that ultrasound is more sensitive in diagnosing joint effusions than x-ray.8 O’Dwyer researched adults with a positive fat pad sign and no visible fracture and performed an MRI 75% of these patients had an occult fracture. Most of these fractures were in the radial head.7

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tabel fat pad

Discussion

Finding a fracture does not alter the treatment as it should be treated as an undisplaced supracondylar fracture in children and as such should be treated with a cast. Independent of whether there’s a visible fracture or only a positive fat pad sign.

MRI is the golden standard in fracture diagnosis as this can detect bony injury as well as small fractures. In only two studies an MRI exam was included of these only Al-Aubaidi examined children.3  MRI is often not available in an acute setting and costs a lot more than an x-ray. O’Dwyer et al. only included adults in their study. Since  adults have different fracture sites (mainly radial head) compared to children (mainly condylar) the fat pad sign may have a different sensitivity and specificity in the adult population.Rabiner looked at posterior fat pad sign elevation, not at cortical disruption. It can be difficult to directly visualize fracture at the curved end of long bones and areas adjacent to joints. Ultrasonography may be of use in reducing the need for x-rays. Although this is a very interesting diagnostic modality the study will have to be repeated before we can really switch to ultrasound. It would be interesting to combine to try and directly visualize the fracture and visualize the ventral and posterior fat pad.

References

  1. Norell HG, Roentgenologic visualization of the extracapsular fat. Act Radiol 1954; 42; 205-210
  2. Skaggs DL et al., The posterior fat pad sign in association with occult fracture of the elbow in children. J Bone Joint Surge Am 1999: 81: 1429-1433.
  3. Al-Aubaidi, The role of fat pad sign in diagnosing occult elbow fractures in the pediatric patient a prospective magnetic resonance imaging study, Journal of Pediatric orthopedics 2012 vol 21 6 pa 514-519
  4. Donnelly traumatic elbow effusions in pediatric patients: are occult fractures the rule? AJR 1998 243-245.
  5. Blumberg et al., The predictive value of a normal radiographic anterior fat pad sign following elbow trauma in children
  6. Rabiner et al., Accuracy of ultrasonographic diagnosis of pediatric elbow fractures, Annals of emergency medicine: Jan 2013 pages 9-17
  7. O’Dwyer et al., The fat pad sign following elbow trauma in adults J Comput Assist Tomogr 28;4: 562-565
  8. De Maesseneer et al., Elbow effusions: distribution of joint fluid with flexion and extension and imaging implications. Invest Radiol 1998; 33: 117-125

Carolina Spruyt

Emergency Medicine Resident – Albert Schweitzer hospital

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