A line drawn along the radial shaft misses the capitellum in 16% of radiographs of normal elbows.

BACKGROUND: The radiocapitellar line (RCL) is a routinely referenced radiographic measurement for evaluating injury of the pediatric elbow, such as a Monteggia fracture. It is most commonly described as a line drawn along the radius that should intercept the center of the capitellum in a normal elbow. However, the RCL has not been rigorously defined or validated in young children.

METHODS: The RCL was drawn out on the anteroposterior and lateral radiographs of 116 normal pediatric elbows by 3 examiners. On each radiograph, lines were drawn along the longitudinal center of the radial neck as well as the radial shaft. The distance from each of these lines to the center of the capitellum was defined as the line-capitellar distance (LCD). The LCD was standardized to the patient size and correlated with patient variables.

RESULTS: Of the 2052 RCLs drawn, 50% (1029/2052) fell outside the middle third of the capitellum and 8.6% (176/2052) missed the capitellum completely. Lines drawn along the radial neck in the anteroposterior and lateral views missed the capitellum less frequently (1.2% vs. 12.7%, P<0.0001) and passed through its central third less (77.8% vs. 44.7%, P<0.0001) when compared with lines drawn using the radial shaft. Lines along the radial neck intersected the capitellum significantly more often on the lateral view than on the anteroposterior view (98.8% vs. 96.8%, P=0.06) and were more likely to pass through the center third of the capitellum (77.8% vs. 55.5%, P<0.0001). With both techniques of drawing the RCL, the percent of measurements falling within the middle third of the capitellum was higher in patients above 5 years of age (P<0.001). The RCL is more reliable in older patients and shows considerable variation in infants.

CONCLUSIONS: The RCL does not invariably bisect or fall within the middle third of the capitellum in normal pediatric elbows. Therefore, an abnormal RCL is suggestive but not pathognomonic for injury such as Monteggia fracture. An RCL drawn along the radial shaft will miss the capitellum in 15.6% of elbows. The most consistent RCL is drawn along the radial neck on the lateral view. Patient variables such as age and sex should be considered in the ability of the RCL to identify elbow injury.

LEVEL OF EVIDENCE: Diagnostic Level 3.