Seasonal variation of hip fracture at three latitudes

We studied the seasonal variation of hip fracture admissions at three diffe rent latitudes: Scotland (56 degrees North; 54,399 admissions); Shatin, Hon g Kong (22 degrees North; 4180 admissions); and Auckland, New Zealand (36 d egrees South, 2257 admissions). We calculated the extent of seasonal variat ion (amplitude) and the time of year of the peak value (acrophase) by fitti ng a sine curve to monthly data using cosinor analysis. A significant seaso nal 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 Ko ng, the amplitude was three times greater than in Scotland and the peak occ urred a month earlier. There is neither snow nor ice in Hong Kong, and this provides powerful evidence against a major influence of conditions underfo ot causing extra falls in winter. In Scotland there was a significant incre ase in the proportion of deaths in winter as compared to summer. The Scotla nd/Hong Kong amplitude difference is striking, but it is unknown whether th is has a genetic or environmental explanation. The cause of seasonal death difference to a given injury is also unknown. Possible mechanisms are discu ssed, but the purpose is to report two new epidemiological features, withou t wild speculative hypotheses. The findings should be viewed as leads to fu rther epidemiological, clinical and more basic research. (C) 2000 Elsevier Science Ltd. All rights reserved.