Objective The alveolar bone density quantitative measuring system was established on the basis of grey level measuring on standardized-exposed direct digital periapical radiograph by referring to an aluminum step wedge with the aid of computer technique. We tested it′s accuracy and sensibility by measuring a series of specimens and made it possible to estimate the area density of hydroxyl-phosphorite（HP） in the alveolar bone with the system. Methods The average grey level intensity of 19 specimens containing different amount of HP within certain area was measured with the system. The equivalent aluminum thickness（EAT） and corrected grey level intensity（CGL） of each specimen were calculated according to known grey level intensity of the aluminum step wedge and it′ s corresponding thickness. The liner regression equations between EAT, CGL and HP density（mg/mm2） were established. We put the values of EAT and CGL into the equations and calculated the corresponding values of estimated HP density. The bias ratios of estimation were then calculated. We measured two randomly chosen specimens 10 times respectively to calculate the system′s measurement error range, then the sensibility of the system for measuring HP density was calculated according to the regression equations. Results EAT and CGL were significantly correlated with HP density. When EAT was larger than 0.67 mm or CGL larger than 41, the system′s bias ratio of estimated HP density was lower than 5%. When we estimated the HP density with the use of EAT, the system could effectively check out a HP density change larger than 0.17 mg/mm2, with the use of CGL, a HP density change larger than 0.18 mg/mm2 could be effectively checked out. Conclusion The accuracy and sensibility of the quantitative alveolar bone density measuring system are acceptable, it can be widely used to compare alveolar bone density longitudinally and cross-sectionally.
