Table 4 Relative risks (95% CI) for smokers with a life long consumption of 20 cigarettes/day <15 years (light smokers) and >15 years (smokers) respectively, users of oral contraceptives and oestrogen to develop knee osteoarthrosis, and freqency of exposed cases and referents. The smokers are compared with those who have never smoked, and oestrogen and oral contraceptive users with those who have never been users or were users for less than one year. The relative risks are controlled for potential confounding from age, body weight, physical load, and sports