小照射野歯科用CT撮影における画像測定精度の検討

Objectives: Limited cone-beam CT (CBCT) has recently become popular in dentistry, but there has been no collective opinion on the accuracy of length measurement using CBCT. Some studies suggest that CBCT is superior to conventional CT, but others disagree and believe that CBCT is associated with considerable measurement errors. The purpose of this study was to assess the measurement accuracy of image examination for dental implant treatment.<br/> Material and Methods: A mandibular phantom was designed with precise geometrical morphology and 6 implants (Replace^® Select Tapered, Nobel Biocare). The phantom was made with a Teflon sheet surface imitating the cortical bone and internal acrylic resin imitating the cancellous bone. Six implants were placed in the following respective positions: right first molar, right first premolar, right lateral incisor, left second molar, left second premolar, and left canine. The phantom was set in a cylinder-type water tank made of acrylic resin and scanned with an output of 80 kV and 3 mA, and an interval thickness of 1.0 mm using a CBCT apparatus (3DX multi-image micro CT FPD, Morita: 3DX-FPD). The diameter and length of the implants, the distance between the implants, the buccolingual bone width, the lingual cortical bone width, and the vertical length from the apex of the dental implant to the upper rim of the mandibular canal were measured on the CT images and compared to the actual length of the implant.<br/> Results: The measured length of the implants was statistically longer than their actual lengths although the diameter in measurements was very accurate. The measured distance between the outside/center of the two implants was also statistically longer although the internal measurement was very accurate. Noise such as poor granularity and image distortion of the lingual cortical margin shifting buccally were observed in the images of a case with no implant in the image region and implants in the opposite site. Therefore, the patterns of implant placement affected both the measurement of bone width in the horizontal plane and also the measurement of vertical length from the implant to the mandibular canal to a lesser extent.<br/> Conclusion: It was concluded that the vertical length and the distance between implants measured by a CBCT apparatus (3DX-FPD) were longer than their actual length and distance, the existence of a dental implant affected the measurement of bone width in the horizontal plane, and the degree of influence depended on the number of implants and the sites at which they were placed.