The evaluation of the bone mineral content in renal osteodystrophy by the CT-numbers of the mid-shaft and lateral condyle of the femur

To estimate the bone mineral content in patients with chronic renal failure, the CT-scans of the femurs together with calibration phantoms were taken. Two levels (at 20 cm and 2 cm proximal to the distal end of the femur) were scanned, and the CT-numbers of cortical bone of the mid-shaft (CT20) and of spongy bone of the lateral condyle (CT02) were calculated. The materials of phantoms used to calibrate the CT-numbers were aluminium and teflon for the mid-shaft, and the solution of 40 g K2HPO4 in 100 ml of H2O and acrylic acid resin for the lateral condyle. To estimate the reproducibility, the volunteers (group-III) were examined twice within three months. The means of CT20 and CT02 of them were 1677 and 424 (HU), and the mean differences of the two examinations were 29 and 15 (HU), respectively. The CT-numbers of group-II correlated significantly to the duration of hemodialysis (r = -0.63, p = 0.0004 for CT20, and r = -0.63, p = 0.0007 for CT02). The CT-numbers of group-II may become significantly lower than those of group-III after 45 months of hemodialysis for CT20, and 21 months of hemodialysis for CT02. The change of the mineral content equal to the ambiguity based on the reproducibility may occur in 16 months for CT20, and 10 months for CT02. The correlation coefficients between the CT-numbers and the age in group-I were -0.11 (p = 0.49) for CT20, and -0.52 (p = 0.0002) for CT02. The sex difference in group-I was not significant for CT20, but CT02 of females was significantly lower than that of males. These results indicate that the CT-numbers of the femurs can be used to quantitatively estimate the bone mineral content, and it is recommended to examine the CT-numbers, especially CT02, once or twice a year, for evaluating the progression of renal osteodystrophy. (J.P.N.)