The changes in bone-mineral content in the distal aspect of a cadaveric femur that had been prepared for insertion of the femoral component of a total condylar knee prosthesis were evaluated with visual inspection and computer-processing of roentgenograms and with dual-energy x-ray absorptiometry. Seventeen small slices of bone, each three millimeters wide, were removed so that, finally, 89 per cent of the bone was lost from the distal femoral metaphysis. Standardized lateral roentgenograms of the specimen were made with use of a reference step-wedge of hydroxyapatite, and dual-energy x-ray absorptiometry studies were performed with the x-ray beam tangential to the interface. The roentgenograms were digitized and the bone mineral was measured with use of computer analysis. Dual-energy x-ray absorptiometry was performed with and without the femoral prosthesis in place, in order to determine the effect of the metallic prosthesis on the accuracy of the measurement.
A bone loss of 25 per cent or more was identified visually by all five of the readers 100 per cent of the time; losses of 20 to 24 per cent, 15 to 19 per cent, 10 to 14 per cent, and 3 to 9 per cent were correctly identified 92, 75, 66, and 59 per cent of the time, respectively.The measurements of bone-mineral content that were obtained from the digitized roentgenograms were linearly correlated with the actual bone-mineral content (the ash content) (r = 0.97, p < 0.001) and were three times more accurate than the visual readings. The determinations of bone-mineral content with dualenergy x-ray absorptiometry correlated highly with the ash content (r = 1.00, p < 0.001) and were seven times more accurate than the visual readings. There was only a 4 per cent difference between the measurements with dual-energy x-ray absorptiometry made with the prosthesis in place and those made without it in place (p < 0.01).
Dual-energy x-ray absorptiometry was the most accurate of the three methods and could detect the smallest experimentally created loss; computer-processing and visual-processing of roentgenograms detected losses of 8 per cent or more and 25 per cent or more, respectively. Dual-energy x-ray absorptiometry and computer-processing of the roentgenograms quantified the bone loss, while visual-processing could detect only the presence or absence of bone loss. These studies indicate that dual-energy x-ray absorptiometry may be used to quantify changes in bone mineral in metaphyseal regions accurately after a joint replacement.