Klein line on the anteroposterior radiograph is not a sensitive diagnostic radiologic test for slipped capital femoral epiphysis

Objective: To correlate the amount of slipped capital femoral epiphysis (SCFE) that results in an abnormality of the Klein line. In mild slips, the Klein line on the anteroposterior (AP) radiograph may remain normal, potentially leading to a delay in diagnosis and emphasizing the importance of obtaining a frog lateral radiograph in patients with a suspected SCFE. Study design: A retrospective review of 55 patients with SCFE was performed from January 2004-March 2009. Of the 55 patients, 32 were missing radiographs and were excluded, leaving 23. The 23 sets of radiographs were reviewed. Results: Of the 23 hips studied with SCFE, only 9 (39%) were able to be diagnosed on the AP radiograph using the classic definition of the Klein line. Twenty cases (87%) of SCFE were identified on the AP radiograph using the modified Klein line. All 23 cases (100%) of SCFE were identified on frog lateral radiographs. Conclusions: Relying on the Klein line will fail to identify many mild or moderate slips. An AP and a frog lateral pelvic radiograph should be obtained in any child when an SCFE is suspected to accurately identify the disorder and to prevent delays in diagnosis.