Conversion of bone marrow in the extremities from red to yellow is a normal maturation process. Marrow reconversion is present when expected yellow marrow is replaced with active red marrow, and it tends to occur at times of physiologic stress. The significance of this finding on MR imaging is not always clear. Accordingly, we studied the prevalence of bone marrow reconversion in adults undergoing MR imaging of the knee for reasons other than marrow disorders. Possible relationships between marrow reconversion and age, sex, weight, and smoking were sought.

Fifty-nine outpatients over 24 years of age who had no evidence of hematopoietic disorders and for whom MR images of the knee were available for review were selected for study. Patients younger than 25 years old were eliminated to ensure that all patients in the study were old enough to have achieved normal adult marrow conversion. The 59 subjects were classified according to their smoking histories. Twenty-eight were nonsmokers, 10 were intermediate smokers, and 21 were heavy smokers. The MR images were classified as showing either red marrow (reconverted from yellow to red) or yellow marrow (converted) by two independent observers who had no knowledge of the patients' smoking histories.

MR images of six heavy smokers, two intermediate smokers, and one nonsmoker showed evidence of reconversion. A statistically significant association between marrow reconversion and heavy smoking (p = .02) was found. Reconversion also was found to be more prevalent in patients less than 39 years old and in obese women who smoked. No association was found between weight, sex, or obese male smokers and reconversion.

Our results show marrow reconversion at the knee is most prevalent in heavy smokers, younger patients, and especially obese women who smoke heavily. In these patients, marrow reconversion can be a normal finding on MR imaging. In other patients, other causes should be considered.