OBJECTIVETo analyze the relationship of genetic and mechanical factors with age of onset of knee osteoarthritis (OA).

METHODSCross-sectional and longitudinal studies were conducted in 863 patients with knee OA and 999 age-matched controls from Chinese Han population with complete clinical data. Single nucleotide polymorphisms of + 104T/C in rs143383 of the growth differentiation factor 5 (GDF5) gene and the aspartic acid repeat polymorphism of the asporin gene ASPN were genotyped by taqman probe and polyacrylamide gel electrophoresis respectively.

RESULTSBody mass index (BMI) in subjects with knee OA was significantly higher than that in controls (P< 0.01). Obesity (BMI≥ 25) was a high risk factor for knee OA (P< 0.01). Regression analysis showed that there was a certain correlation between the BMI and age of onset of the knee OA (Pearson= 0.15, P< 0.01). To analyze the BMI distribution, the two groups were subgrouped by age with 5-year interval. The BMI in early-onset patients was lower than that in the late-onset patients (F= 2.497, P= 0.011). However, there was no significant difference in the control subgroups. Frequencies of the GDF5 and ASPN alleles distributed differently between the early- and late-onset patients (P< 0.05).

CONCLUSIONGenetic factors play an important role in knee osteoarthritis of early-onset patients, whereas mechanical factor play an important part in knee osteoarthritis of late-onset patients.