Seasonal variation of hip fracture at three latitudes.

We studied the seasonal variation of hip fracture admissions at three different latitudes: Scotland (56 degrees North; 54,399 admissions); Shatin, Hong Kong (22 degrees North; 4180 admissions); and Auckland, New Zealand (36 degrees South; 2257 admissions). We calculated the extent of seasonal variation (amplitude) and the time of year of the peak value (acrophase) by fitting a sine curve to monthly data using cosinor analysis. A significant seasonal variation was found in all three countries, at a high level in Scotland (p < 0.01) and Hong Kong (p < 0.001), but just significant in New Zealand (p < 0.05). The extent of the seasonal change was very similar in Scotland and New Zealand, but, as expected, the peak in New Zealand (early September) was approximately six months ahead of Scotland (mid February). In Hong Kong, the amplitude was three times greater than in Scotland and the peak occurred a month earlier. There is neither snow nor ice in Hong Kong, and this provides powerful evidence against a major influence of conditions underfoot causing extra falls in winter. In Scotland there was a significant increase in the proportion of deaths in winter as compared to summer. The Scotland/Hong Kong amplitude difference is striking, but it is unknown whether this has a genetic or environmental explanation. The cause of seasonal death difference to a given injury is also unknown. Possible mechanisms are discussed, but the purpose is to report two new epidemiological features, without wild speculative hypotheses. The findings should be viewed as leads to further epidemiological, clinical and more basic research.