Head and Neck Imaging

Dental implant

In dental implant surgery, fully or partly edentulous patients can be provided with a fixed prothesis. This technique provides a better result than a removable prosthesis. It consists of endossous supports (the implants) that are placed into predrilled holes into the bone, and a superstructure (the fixed prosthesis). The implants and prosthesis are connected by so-called abutments, that perforate the gums.

The implants must be placed in bone of sufficient density and height/width to obtain a good osseointegration. In edentulous patients, atrophy of the alveolar process may interfere with adequate implant placement; a surgical reconstruction of the alveolar process may be needed in such patients. To avoid complications, the exact relationship of the alveolar process to the maxillary sinus and mandibular canal must be predetermined. Pantomography does not allow estimation of the width of the alveolar bone; these images also suffer from distortion. The bony dimensions can be estimated using conventional tomography, but this method is only suitable for studying a limited region because of high radiation exposure.

CT is commonly used for implant treatment planning, especially in patients requiring a large number of implants. Multiple scans are made in the axial plane. After the scanning procedure, these images undergo computer manipulation with a special software package. This software generates orthogonal cross-sectional images through the entire jaw (each image is perpendicular to the alveolar process) and panoramic images along the alveolar arch. Such images allow precise determination of the bone dimensions at the site of planned implant surgery (Fig.1). Dental splints with opaque markers are sometimes used during the scanning procedure, allowing to be correlated to the position and inclination of a proposed fixture placement.

Sometimes the position of the mandibular canal is difficult to visualize on these images; in a number of cases, it can be determined by comparison with the opposite side, by using triangulation (relating similar anatomical points on the different images produced by the software, using the reference scales) or by searching for the cortical niche sign.

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Fig.1

a. Axial reference image showing the position of the secondary orthogonal reconstructions through the mandibular alveolar process. b. Sample of orthogonal reconstructions through the mandibular bone. Bone dimensions can be accurately determined on such images; the cross-section of the mandibular canal (arrow) is clearly visible on successive images.
Dental implant, Fig.1 (a)
Dental implant, Fig.1 (b)