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. 2014 Feb;8(1):11-22.
doi: 10.1007/s11832-014-0558-3. Epub 2014 Feb 6.

Seasonal variation in children with developmental dysplasia of the hip

Randall T Loder  1 , Cody Shafer
Affiliations

Seasonal variation in children with developmental dysplasia of the hip

Randall T Loder et al. J Child Orthop. 2014 Feb.

Abstract

Background: It has been postulated that developmental dysplasia of the hip (DDH) is more frequent in infants born in the winter months. It was the purpose of this study to ascertain if there was any seasonal variation in DDH at the author's institution and compare/contrast our results with those in the literature using rigorous mathematical fitting.

Methods: All children with DDH treated at the author's institution from 1993 to 2012 were identified. The month of birth was recorded and temporal variation was analyzed using cosinor analysis. Similar data from the literature was analyzed.

Results: There were 424 children (363 girls, 61 boys). An additional 22,936 children were added from the literature for a total of 23,360. Pearson's Chi-square test demonstrated a non-uniform distribution in the month of birth for both our 424 children as well as the combined literature series in both the Northern and Southern hemispheres. Cosinor analysis of the 424 children demonstrated double peaks in mid-March and mid-October. For the entire 23,360 children, no seasonal variation was observed in 2,205 (9.4 %), a single winter peak in 16,425 (70.3 %), a single summer peak in 1,280 (5.5 %), and double peaks in the spring and autumn in 3,450 (14.8 %).

Conclusions: This study partly supports the hypothesis of tight clothing/cold temperature as one factor in the etiology of DDH with the tighter clothing/swaddling increasing the risk of DDH. However ~20 % of the DDH births demonstrated a non-winter peak. The single summer and double spring/autumn peaks, as well as in those series where no seasonal variation was noted, refutes the cold winter clothing hypothesis. Perhaps these different patterns in seasonal variation represent the heterogeneity of the genetic factors in DDH interacting with external factors (temperature and clothing) and internal factors (metabolic). Further study will be required to understand these different patterns in DDH seasonal variation.

Level of evidence: IV-case series.

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Figures

Fig. 1
Fig. 1
Month of birth for 424 children with developmental dysplasia of the hip. Cosinor analysis demonstrated an excellent fit using a 7-month periodicity with the equation: number of DDH births = 33.61 + 11.45(cos(51.4t − 25.7) − 122), where t = 1 is January, 2 is February, 3 is March, etc. This was statistically significant (r2 = 0.70, p = 0.005). The peaks are March 13 and October 12 (solid arrows). The data points are the black triangles and the best fit represented by the bold black line. The numbers of births in the state of Indiana are represented by the open circles and the 12-month cosinor fit by the hatched line represented by the equation: number of live births = 58,234 + 2,586(cos(30t − 15) − 208), where t = 1 is January, 2 = February, 11 = November, 12 = December. This was statistically significant (r2 = 0.60, p = 0.017). The peak was July 30 (hatched arrow)
Fig. 2
Fig. 2
The three different patterns of DDH birth month. The month of birth data is represented by the open square; the cosinor fit by the solid black line; the average monthly temperature (°F) and precipitation (in.) are shown by the dotted lines. a A double peak in Sweden [9]. This double fit with a period of seven months was statistically significant and represented by the equation: proportion DDH births = 0.0824 + 0.00786(cos(51.40t − 15) − 122). This was statistically significant (r2 = 0.53, p = 0.034). The two peaks were February 20 and September 21. The seasonal variation was small, with a mesor/amplitude ratio of 0.10. b A single winter peak seen in Kōchi, Japan [2]. This was statistically significant (r2 = 0.843, p = 0.002) and represented by the equation: proportion DDH births = 0.0853 + 0.00842(cos(30t − 15) − 25); the peak was January 26. Note the large seasonal variation, with a mesor/amplitude ratio of 0.99. c A single winter peak seen in Leipzig, Germany [13]. This was statistically significant (r2 = 0.58, p = 0.021) and represented by the equation: proportion DDH births = 0.0833 + 0.0133(cos(30t − 15) − 350); the peak was December 21. Note the small seasonal variation, with a mesor/amplitude ratio of 0.16. d A single summer peak seen in Córdoba, Spain [34]. This was statistically significant (r2 = 0.84, p < 0.001) and represented by the equation: proportion DDH births = 0.0849 + 0.0276(cos(30t − 15) − 146); the peak was May 28. Note the moderate seasonal variation, with a mesor/amplitude ratio of 0.33
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