Some of the methods to evaluate the bone quality that influences the primary implant stability have already been applied in a clinical practice. Lekholm and Zarb classified bone density into four types in terms of radiography, with the thickness of the cortical bone and the density of the spongy bone as the indexes [14]. This classification method is accepted most commonly at present but is problematic with accuracy and reproducibility because it is a subjective evaluation. On the other hand, Misch classified CT values (Hounsfield unit) into five steps (D1: >1,250 HU; D2: 850 to 1,250 HU; D3: 350 to 850 HU; D4: 150 to 350 HU; D5: <150 HU) to evaluate the bone quality [15]. The CT value is the value obtained by multi-detector CT (MDCT) and is defined as the relative value of the X-ray attenuation coefficient of the object for water, with the X-ray attenuation by water defined as zero. Today, this classification has been used for the evaluation of the bone quality because it is an objective method compared with that of Lekholm and Zarb.

Turkyilmaz et al. placed 24 implants in human dry bones and calculated CT values using MDCT images before surgery to examine correlations with the ITVs and the ISQ values [16]. They reported that there were significant correlations of CT values with both ITVs and ISQ values and that bone density (CT value) was one of the factors that had an influence on the primary implant stability.

In recent years, cone beam CT (CBCT) has been used for preoperative diagnosis in implant treatment. CBCT is superior for its high definition, reduction of the exposure dose, low cost, and usability compared with MDCT [17-21]. However, CBCT does not have a linear relationship compared with the CT values obtained by MDCT, and therefore, it is considered difficult to evaluate bone density quantitatively [22-24]. The major causes arise from the lack of calibration of X-rays, the localized imaging area that allows various external anatomical structures, and too many scattered radiations.