Atypical and typical (idiopathic) slipped capital femoral epiphysis. Reconfirmation of the age-weight test and description of the height and age-height tests.

BACKGROUND: The age-weight test was described to aid the clinician in defining demographic predictors of an atypical slipped capital femoral epiphysis. We wished to retest the accuracy and applicability of the age-weight test and height differences in children with atypical and typical slipped capital femoral epiphyses.
METHODS: A retrospective review of the records for all children with slipped capital femoral epiphysis from 1998 through 2003 was performed. Gender, race, chronological age, weight, height, the duration of symptoms, and the laterality of the slip were recorded. The slip angle was classified as mild (< 30 degrees), moderate (30 to 50 degrees), or severe (> 50 degrees). Statistical analyses were performed.
RESULTS: The study included 105 children (thirty-eight girls and sixty-seven boys) with 141 slipped capital femoral epiphyses; ten children had fifteen atypical slipped capital femoral epiphyses, and ninety-five children had 126 typical slipped capital femoral epiphyses. Sixty-nine children had unilateral involvement, and thirty-six had bilateral involvement. The average age at the time of presentation for the first slipped capital femoral epiphysis was 12.1 +/- 2.0 years. The average duration of symptoms was 3.7 +/- 5.5 months. In the group of 128 slipped capital femoral epiphyses for which the slip angle was known, there were ninety-three mild, twenty-seven moderate, and eight severe slips. The average slip angle was 24 degrees +/- 18 degrees. The age-weight test demonstrated a sensitivity of 50%, a specificity of 89%, a positive predictive value of 33%, and a negative predictive value of 94%. The age-height test, involving the same definition as the age-weight test except that the percentiles apply to height and not weight, demonstrated a sensitivity of 88%, a specificity of 73%, a positive predictive value of 30%, and a negative predictive value of 98%. The height test, which was defined as positive if the child's height was at or below the tenth percentile for age and as negative if it was above the tenth percentile, demonstrated a sensitivity of 75%, a specificity of 97%, a positive predictive value of 75%, and a negative predictive value of 97%.
CONCLUSIONS: The present study reaffirmed the accuracy and applicability of the age-weight test for differentiating between typical and atypical slipped capital femoral epiphyses, and it further defined the age-height and height tests. If the height of a child can be obtained, the height test is likely to be most useful for differentiating between typical and atypical slipped capital femoral epiphysis. When height is not known, the age-weight test will result in a similar negative predictive value but with a lower sensitivity, specificity, and positive predictive value. LEVEL OF EVIDENCE: Diagnostic Level I. See Instructions to Authors for a complete description of levels of evidence.