Hip fracture seasonality in a mild climate: the joint association of daylight duration and temperature.

PURPOSE: To evaluate the seasonality of hip fractures and determine whether daylight duration and temperature are associated with monthly fracture volume; and likelihood of a hip fracture occurring during high-volume months in Southern California, a region with mild seasonal climate variation. METHODS: We conducted a serial cross-sectional study of adults aged ≥ 50 years who underwent operative treatment for hip fracture at a single academic health system between 2017 and 2022. Patients were identified using ICD-10 codes and operative CPT codes. Monthly hip fracture counts were analyzed for seasonality using year-adjusted cosinor analysis and categorized into volume tertiles: high (≥ 9 fractures/month; top tertile) and lower (< 9 fractures/month; lower two tertiles: low < 6, middle 6-8). We used multivariable logistic regression to evaluate the interaction of daylight duration (short: ≤ 10.7 h, medium: 11-13 h, and long: > 13 h) and temperature (cooler < 65°F, warmer ≥ 65°F) on the odds of a hip fracture occurring during a high-volume month, adjusting for age, sex, and American Society of Anesthesiologists (ASA) Classification. RESULTS: Among 499 patients, average age was 77.2 ± 11.8 years and 62.7% were women. Monthly hip fracture counts demonstrated significant seasonality (p = 0.02), peaking in January with a trough in July. There was a significant interaction between daylight and temperature on the odds of a hip fracture occurring during a high-volume month, independent of age, sex, and ASA classification (pinteraction = 0.02). At cooler temperatures, shorter daylight duration was associated with higher odds of a hip fracture occurring during a high-volume month (OR = 2.6, 95% CI: 1.48-4.58). At warmer temperatures, both shorter (OR = 0.17, 95% CI: 0.06-0.50) and longer (OR = 0.52, 95% CI: 0.29-0.92) daylight durations were associated with lower odds compared to medium daylight duration. CONCLUSION: Hip fractures followed a seasonal pattern, despite Southern California's mild climate, with peak hip fracture volume during winter months. The association between daylight duration and hip fracture occurrence varied by temperature, suggesting seasonal influences beyond weather-related fall hazards.