Posterior Fat Pad Sign as an Indicator of Occult Elbow Fracture

abstract & commentary

Source: Skaggs DL, Mirzayan R. The posterior fat pad sign in association with occult fracture of the elbow in children. J Bone Joint Surg 1999;81-A:1429-1433.

Consecutive pediatric subjects were prospectively enrolled in this study if they had an elevated posterior fat pad noted on initial radiographic evaluation of the post-traumatic elbow, together with the absence of radiographic bony evidence of fracture. All subjects initially received anteroposterior, lateral, and two oblique views; if the attending pediatric radiologist, attending pediatric emergency medicine physician, or orthopedic resident noted a fracture, the subject was excluded. All patients were casted above the elbow. The main outcome measure was the incidence of fracture healing (periosteal elevation, callus formation, or both) seen at follow-up, which was designed to be at three weeks after the initial visit.

Forty-five children were included in the study, with a mean age of 4.5 years. Thirty-four (76%) had evidence of fracture healing, and were thus judged to have had an occult fracture on initial evaluation. Anatomic breakdown was as follows: supracondylar fracture in 18 patients (53%); proximal ulna in 9 patients (26%); lateral condyle in 4 patients (12%); and radial neck in 3 patients (9%). Skaggs and associates conclude that presence of a posterior fat pad sign on initial radiographic evaluation of the post-traumatic pediatric elbow occurring in the absence of detectable bony fracture should lead the practitioner to treat such patients as though they have a non-displaced fracture of the elbow. Although this is not a new recommendation, the researchers’ results were inconsistent with earlier studies that found lower rates of fracture at follow-up. The authors speculate that those studies suffered from design flaws (all were retrospective studies), including follow-up radiography that may have been too early (14 days or earlier). The average time to follow-up imaging in the present study was 20 days (range, 8-37 days); more importantly, the average follow-up x-ray that was negative for fracture was 21.5 days (range, 18-35 days).

Comment by Richard A. Harrigan, MD, faaem, facep

Skaggs et al are to be congratulated on performing a prospective study. The high incidence of radiographic evidence of fracture healing seen in follow-up of these elbows that initially had only posterior fat pads as evidence of fracture should remind us to be vigilant for this radiographic sign. There were a few study design flaws worth discussing, however. Whereas both the pediatric radiology and pediatric emergency medicine attending were blinded to the study, the orthopedic resident was not; a positive finding by any of the three led to exclusion from the study population. Thus, all initial evaluators of the data were not on equal footing. It would have been more consistent to make all three aware of the study. The main outcome measure was evidence of new bone formation (signifying fracture healing) seen on follow-up x-ray. No mention is made of who was reading these films, although it is alluded that the attending orthopedists followed these patients, during a discussion of why they were not involved in the initial radiograph evaluation (fear of introducing bias into the study). It is unclear how inconsistency between the attending orthopedist and the attending pediatric radiologist in the interpretation of follow-up films would be reconciled in determining the main outcome measure. Inter-rater reliability data would have been important to report for both initial and follow-up films. Moreover, were those who read the follow-up films blinded to the study? Significant bias would exist if those charged with reading follow-up films were aware of the hypothesis and design of the study.

In the pediatric patient with elbow trauma:

a. the posterior fat pad sign is always indicative of fracture.

b. occult fracture may be implied by the presence of the anterior fat pad.

c. evidence of occult fracture has been found at follow-up in 76% of patients with only a posterior fat pad on initial radiographs.

d. occult fat pad signs have been found at follow-up in 76% of patients with bony evidence of fracture on initial plain films.