Distribution of bone mineral density in the proximal tibia in mid-teens

The distribution of bone mineral density (BMD) in the proximal tibia was studied in 94 knees from 47 mid-teens. The subjects were healthy high school freshmen (19 male; 28 female) who engaged in daily physical exercise. We measured BMD at six regions of interest (ROI) at the bilateral proximal tibia (three immediately above and three immediately be-low the physeal line) using dual-energy X-ray absorptiometry (QDR-2000; Hologic, Waltham, MA, USA). Subjects were supine during the measurements, with the knee extended and the foot fixed in a special holding device, ensuring correct rotational alignment of the knee. Measurements were performed using the forearm array scan mode. The precision of this method (coefficient of variation) was 1.1%-4.9% (mean 2.5%) for six ROIs. The BMD of the medial side was higher than that of the lateral side, and the BMD at the center of the metaphysis was higher than that of the epiphysis. The subjects in the present study did not show any misalignment of the knee joints. These findings indicate that the medial side of the proximal tibia already has higher BMD than the lateral side in teenagers with normal alignment of the knee joint, and that the increase in epiphyseal BMD may be slower than that of metaphyseal BMD. It is suggested that this phenomenon should be life-long and may play a role in the progression of osteoarthritis of the knee.