CT determination of bone mineral density and structural investigations on the axial skeleton for estimating the osteoporosis-related fracture risk bymeans of a risk score

In addition to the pure measurement of bone mineral density (BMD) in osteod ensitometry, the investigation of bone structure is becoming increasingly i mportant for estimating fracture risk. In a clinical study, a risk score wa s proposed which separately assesses BMD and structural parameters for spon gious and cortical bone and aggregates them into a single diagnostic para m eter. In 120 lumbar vertebrae from 40 patients, BMD was determined separate ly for spongious and cortical bone by means of quantitative CT using a sing le energy procedure (SE-QCT/ 85 kV). In addition, structural parameters bas ed on high resolution CT were calculated for the spongiosa and cortical bon e. For all patients the number of osteoporosis-related fractures was determ ined on the entire skeletal system. According to WHO criteria, the patients were subdivided into four groups: 1, normal; 2, osteopenic; 3, osteoporoti c without fractures; 4, severely osteoporotic. Weighting factors were deter mined by means of multivariate least-squares analysis and used to calculate a risk score of all parameters. The ability of the individual parameters a nd of the sum to discriminate between the individual groups was tested. If one considers the individual parameters (BMD and the fractal structural val ues for spongious and cortical bone), they allow a statistically significan t separation of the four groups, although there is overlapping in the value ranges. In patients with fractures, there was a significant reduction in t he cortical mineral density, accompanied by a deterioration in structural p roperties. The following individual values were obtained (minimum-mean-maxi mum): spongiosa BMD (mg ml(-1)), unfractured: 62-112-163, fractured: 9-48-7 7; cortical BMD (mg ml(-1)), unfractured: 190-287-405, fractured: 133-191-2 69; spongiosa structural parameter, unfractured: 0.35-0.73-1.01, fractured: 0.95-1.24-1.58; cortical structural parameter, unfractured: 18-31-65, frac tured: 21-44-66. Above 77 mg ml(-1) CaHA in the spongiosa and 270 mg ml(-1) CaHA in cortical bone, no fractures were observed. By appropriately select ing the weighting factors, the score is free of overlapping between the gro ups with and without fractures (values: unfractured 1-9-15, fractured 16-21 -29). With higher score values, the fracture risk is increasing.