Mechanism-equivalent age quantifies how many years of ageing equal a road traffic injury after limb fracture in 547,024 patients

Abstract Purpose. We expressed the in-hospital mortality of a road traffic injury, against a fall, in years of ageing (mechanism-equivalent age, MEA) and asked whether it depends on age and bone. Methods. Retrospective cohort of adults with hip, femoral shaft, tibial or forearm shaft fracture after a fall or road traffic injury in Brazilian, Mexican and US hospital databases. MEA at age x was the number of years a fall related patient had to be older to match the standardized mortality of a traffic patient aged x. Added value of mechanism over age and fracture site was validated in later years and held-out hospitals. Results. Among 547,024 patients and 10,253 deaths, a traffic injury at 40 in Brazil was worth 17.6 years of ageing after femoral shaft fracture (95% CI 10.9 to 21.9) and 17.2 after tibial fracture (12.6 to 20.8), but 3.1 (minus 7.7 to 11.4) after hip fracture. By 80 it was 2.9 years (minus 2.0 to 6.7) for femoral shaft, 8.5 (5.3 to 15.0) for tibia and nil for hip. Ten years of ageing outweighed mechanism from age 31 for hip and the early sixties for tibia and femoral shaft. Mechanism raised the area under the curve by 0.001 with unchanged calibration; Mexico agreed. Conclusion. A road traffic injury equals about 17 years of ageing in middle age after femoral shaft and tibial fracture, a few years in old age, and nothing after hip fracture. Mechanism loses weight as host vulnerability rises; a fall does not mean low risk.