Key points

Introduction

The advent of cone beam CT (CBCT) as a relatively low-dose 3-D dental imaging technology has improved both diagnostic accuracy and implant planning. The 3-D visualization of the alveolar ridge enables clinicians to easily identify anatomic structures and anomalies. CBCT is essential to assess alveolar ridge deficiencies and bone augmentation outcomes. A 3-D evaluation is key in particular techniques, such as zygomatic implants, pterygoid implants, and osteogenic distraction.

CBCT images enable clinicians to discuss the patient-specific clinical scenario while presenting an individualized treatment plan. In this review, the authors discuss the added value of CBCT in implant dentistry and investigate the question of whether CBCT should be routinely obtained for preoperative implant planning.

Cone beam CT radiation doses

The effective radiation dose associated with CBCT imaging in implant dentistry is varied, ranging from 10 μSv to 271 μSv, depending on the model of CBCT scanner and field of view (FOV) selected. CBCT provides an equivalent patient radiation dose of 3 days to 48 days of background radiation or 5 times to 74 times that of a conventional panoramic radiograph ( Table 1 ). The use of additional personal protection (thyroid collar) and patient positioning modifications (tilting the chin) can substantially reduce the dose by up to 40%. Other methods of reducing patient exposure include decreasing scan time or scan arc and increasing voxel size, which decreases the image resolution but does not affect measurement accuracy. Based on a systematic review, it has been suggested that a voxel size of 0.3 to 0.4 mm was adequate to provide CBCT images of acceptable diagnostic quality for implant treatment planning. Many CBCT systems offer ultra–low-dose imaging that has radiation doses comparable to conventional panoramic images, without significant reduction in image quality.

Clinicians should apply the As Low As Diagnostically Acceptable (ALADA) principle in reducing patient exposure during the acquisition of CBCT images. This includes appropriate justification of CBCT use, optimizing exposure parameters, using the smallest FOV necessary for the diagnostic task, and using appropriate personal protective shielding.

Cone beam CT versus multidetector CT

In a CBCT machine, the x-ray beam emerges as a divergent cone-shaped or pyramidal beam. Data are acquired as a series of sequential planar projection images (basis images) made with angular differences as the beam rotates around a patient’s head. Projection data are reconstructed through software algorithms, producing a volumetric data set used to generate primary reconstruction images in 3 orthogonal planes (axial, sagittal, and coronal).

In contrast, the x-ray beam in a multidetector CT (MDCT) scanner emerges as a flat fan beam. Data are acquired as a series of consecutive axial slices of a patient’s head obtained from multiple rotations of the x-ray source around the patient. In MDCT, the average density of each voxel is measured in a Hounsfield unit (also known as a CT number); this unit represents the average attenuation coefficient in that voxel. The Hounsfield units that are derived from various CBCT systems, however, are not consistently representative of the actual density values, sometimes even within the same system.

The radiation dose of the CBCT is less than that for an MDCT examination (the dose reduction is between 76.2% and 98.5%). Most CBCT systems use x-ray tubes similar to those in panoramic x-ray machines, operating in the range of 2 mA to 14 mA, whereas for MDCT, the tube current may be in excess of 250 mA. CBCT is more reliable for linear measurements than MDCT and less affected by metal artifacts.

Patient selection criteria

In addition to a thorough clinical examination, a radiographic assessment is essential to evaluate the potential implant sites. Various conventional and tomographic (cross-sectional) imaging techniques are available for evaluation of the implant site and related anatomical structures. Intraoral periapical images are a valuable tool to estimate the mesiodistal and vertical dimensions of an implant site. Panoramic radiographs are less suited because of inherent distortion. Unlike cross-sectional techniques, however, neither panoramic nor periapical radiographs provide information on the buccolingual width or angulation of the alveolar ridge.

The selection of an appropriate radiograph should be based on the needed diagnostic information while balancing the potential benefits and risks. Evidence-based guidelines for patient selection criteria were published in an American Academy of Oral and Maxillofacial Radiology (AAOMR) position paper. In their guidelines, the initial assessment of a potential dental implant site should be completed using intraoral periapical and/or panoramic radiographs. CBCT is recommended as the imaging modality of choice for presurgical implant planning ( Table 2 ).

The use of CBCT for presurgical implant planning provides a morphometric and skeletal analysis of the alveolar ridge, highlighting the proximity to vital anatomic structures. When the clinical examination and conventional radiograph reveals the need for bone augmentation or site development procedure, CBCT should be considered for preoperative and postoperative evaluation ( Figs. 1–3 ). In addition, CBCT can be used for an accurate, prosthetically driven implant planning. Intraoral periapical and panoramic radiographs are indicated for postoperative assessment. CBCT imaging, however, may be indicated postoperatively if the implant placement is complicated by local infection or nerve injury ( Fig. 4 ).

Comparative sets of axial, reformatted panoramic, and sequential 1-mm–thick cross-sectional CBCT images show consolidation of the graft material to the right mandibular posterior edentulous region. This graft increases the alveolar bone width necessary for implant placement.

Parasagittal CBCT image of a partially edentulous maxilla showing homogenous hyperdense sinus-lift graft material with minimal mucosal thickening.

( A ) Comparative sets of cross-sectional CBCT images in the anterior maxillary edentulous region show labial cortical concavity. ( B ) Postoperative images show the width gain and bone graft consolidation.

Comparative sets of axial, reformatted panoramic, and consecutive 1-mm–thick cross-sectional CBCT images of the right mandibular posterior edentulous region demonstrate embarrassment and compression of the mandibular canal by a dental implant. The patient reported postoperative paresthesia of the right lip.

Anatomic considerations