|Year : 2019 | Volume
| Issue : 4 | Page : 363-369
Why CBCT is imperative for implant placement
Aveek Mukherji1, Mohit Pal Singh1, Prashant Nahar1, Saurabh Goel1, Hemant Mathur1, Zibran Khan2
1 Department of OMDR, Pacific Dental College, Udaipur, Rajasthan, India
2 Oral Surgery, Pacific Dental College, Udaipur, Rajasthan, India
|Date of Submission||06-Oct-2019|
|Date of Acceptance||19-Nov-2019|
|Date of Web Publication||03-Mar-2020|
Dr. Aveek Mukherji
33-D, Prasanna Naskar Lane, Budir Bagan Complex, Picnic Gardens, Kolkata - 700 039, West Bengal
Source of Support: None, Conflict of Interest: None
| Abstract|| |
Dental implants are widely used for the rehabilitation of edentulous spaces. Diagnostic images of the surgical site is essential for preoperative planning, intra-operative assessment, and postoperative evaluation. At present, Cone-Beam Computed Tomography(CBCT) is the best imaging modality for qualitative and quantitative analysis of the surgical site for implant placement. It allows pragmatic planning and subsequent post operative evaluation with submillimetric accuracy. CBCT software enables simulation of implant placement to choose the ideal dimension, position and orientation. This article briefly describes why CBCT is crucial for dental implants and explains how to utilise its advantages. Radiological pictures of a few cases have been discussed to highlight the merits and demerits of this imaging modality for pre-surgical and post surgical evaluation of implant placement.
Keywords: Anatomical landmarks, CBCT, Implant Safe zone, Implant dimension, Implant assessment
|How to cite this article:|
Mukherji A, Singh MP, Nahar P, Goel S, Mathur H, Khan Z. Why CBCT is imperative for implant placement. J Indian Acad Oral Med Radiol 2019;31:363-9
|How to cite this URL:|
Mukherji A, Singh MP, Nahar P, Goel S, Mathur H, Khan Z. Why CBCT is imperative for implant placement. J Indian Acad Oral Med Radiol [serial online] 2019 [cited 2020 Mar 30];31:363-9. Available from: http://www.jiaomr.in/text.asp?2019/31/4/363/279858
| Introduction|| |
Dental implants are widely being implemented for the rehabilitation of edentulous spaces. Their success to restore adequate function and aesthetics depend on unambiguous treatment planning and its execution. Assessment of the site of implant placement in all dimensions, relative to the hard and soft tissues and addressing the inadequacies can significantly reduce the chances of complication.
Diagnostic imaging is essential for the preoperative planning of dental implant placement. It also helps in intraoperative assessment and postoperative evaluation. Cone-beam computed tomography (CBCT) has become a very popular radiographic diagnostic tool for the assessment of dental implant therapy to ensure predictable results.
Expectations from an exemplary imaging technique for implants
An ideal radiograph should enlighten us about the quality and quantity of bone, spatial relationship of the important anatomical structures, number and dimension of placed implants, occlusal pattern, and prosthetic design, apart from ruling out the presence of relevant pathologies in the area of interest. It must also be inexpensive, easily accessible with minimal radiation risk and capable of providing infallible measurements. Simulation of implant to choose its ideal dimension, position, and orientation will be advantageous. International Team for Implantology, in 2013, recommended cross-sectional imaging to detect the topography and relationship of adjacent anatomical structures with the implant.
Demerits of two-dimensional images
Two-dimensional (2D) radiographs utilised in implantology include intraoral periapical radiography (IOPAR), orthopantomography (OPG), and occlusal radiography, while the three-dimensional (3D) imaging modalities include computed tomography (CT), and CBCT.
Although easily available, IOPARs have anatomical limitations and any imperfection in its technique of acquisition leads to either foreshortening or elongation of image. OPGs exhibit asymmetrical distortion, leading to erroneous interpretation and measurements. The occlusal radiographs, used to determine bucco-lingual dimensions (not provided by OPG or IOPAR) of the mandibular alveolar ridge, display the widest portion (normally positioned inferior to the alveolar ridge) of the mandible, giving a false impression of excess availability of bone. Occlusal radiographs fail to provide such measurements for maxilla because of its anatomic limitations.
This precariousness of conventional images projects the necessity of a superior diagnostic procedure for implants.
Advantages of CBCT
CBCT gained prominence as a radiographic diagnostic tool immediately after its introduction in 1982 for providing undistorted images, free from superimposition by neighbouring structures. This improved the visibility of anatomical structures that were unclearly observed by intraoral and panoramic radiographs. CBCT provides more precise 2D diagnostic information (from reformatted images like axial, coronal, sagittal, cross-sectional, panoramic images) in all three dimensions compared to CT. Moreover, it has lesser radiation dose (15 times lower) and scanning time, compared to a multislice CT. Simulated implant placement using CBCT software allows evaluation of multiple variations of individual implant position (different depth and mesiodistal ororofacial angulations) and encourages interdisciplinary communication between the radiologist and referring dentist, to attain the optimal treatment plan. Pre- and postoperative CBCT images can also be utilised to compare alveolar ridge resorption/preservation following grafting procedures, before implant placement. Thus, CBCT is presently the most proficient imaging modality for dental implants.
Optimal dimension and safe zone of implants
Selecting implants with the most favourable size ensures positive outcome. Diagnostic and planning software of CBCT allow virtual mock surgery of implant (of different diameters and lengths) placement, so that the optimal dimension can be selected. Moreover, provision by most software for 360° rotating visualisation [Figure 1] of the anatomical structures around such simulated implants enable detailed scrutiny of the region.
|Figure 1: Visualisation around a simulated implant. Degrees of rotation are encircled in red|
Click here to view
Increase in the diameter of an implant improves its stability and strength by increasing the bone to implant contact area. Inadequate initial stability allows micromovement of the implant, promoting the formation of fibrous tissue into the implant-bone interface, hindering osseointegration. Thus, the optimal diameter is the largest implant diameter, within morphologic limits. Since insufficient surrounding bone thickness can compromise implant success, at least 1 mm of bone should surround the orofacial (bucco/labio-lingual/palatal) sides of the whole implant. Even a gap of 2mm can jeopardise the unification of the implant with the bone. Although 1.5 mm of orofacial bone thickness around the point of emergence of the implant shoulder is considered adequate, 2 mm of bone on the facial side is recommended in the aesthetic zone to avert crestal bone loss and soft tissue recession. The implant is to be placed 2-3 mm below the cementoenamel junction (CEJ) of the adjacent natural teeth, to create an ideal emergence profile and its diameter should not exceed the diameter of natural tooth root at this level. A distance of 1-1.5mm must be maintained between an implant and a natural tooth root. No less than 3mm of bone should be retained between two adjacent implants at the implant-abutment level.
A shorter implant will have lesser contact with bone, leading to lower initial implant stability. During immediate placement, the apical portion of the implant should be engaged at least 3-5 mm within the host bone, to ensure primary stability. The available bone height from the alveolar crest region of a potential implant site is limited apically by anatomical structures like mandibular canal, floor of the maxillary sinus, and nasal floor. A safety margin of 1.5 mm is generally recommended while calculating the available bone height. Ideally, the long axis of the implant should coincide with those of the prosthetic teeth and occlusal table of the final prosthesis.
These obligatory mensurations highlight the importance of unequivocal measurements provided by CBCT. Since all implantologists have their specific preferences of safety margin and implant dimension, the customised measurements should be communicated before selecting the ideal implant for mock surgery. CBCT software usually have inbuilt database of implants with different size and shape (in accordance with the manufacturing companies) to choose from, so that an accurate analysis is attainable.
Essential anatomical landmarks for implant placement
Appropriate knowledge of anatomical structures and meticulous preoperative radiological evaluation limits operative and post-surgical complications. Since variations exist in every patient, CBCT helps in determining the limits of safe area for surgery. The noteworthy anatomical structures and their significance are discussed below:
Incisive (Nasopalatine) canal: It is commonly a single canal of cylindrical shape, having neurovascular content. The incisive canal occupies up to 58%(mean36%) of the alveolar ridge width of central incisor region and connects the nasal floor with the oral cavity. Some accessory canals (mostly on the palatal side of left central incisors) with a mean diameter of 1.31mm have been reported. Thus, tracing this canal is essential while placing implants near the midline of anterior maxilla.
Nasal floor: Precise assessment of the alveolar bone dimension in the nasal area (or maxillary sinus area) can avoid implant penetration into the nasal cavity (or maxillary sinus) and subsequently prevent complications like rhinosinusitis.
Maxillary sinus: The roots of maxillary posterior teeth are in close proximity with the maxillary sinus. Ruling out sinusitis and other potential pathological conditions is necessary before sinus floor elevation. With a prevalence of 9.5%-55.2%, sinus septa is a major contributing factor for sinus membrane perforation. During sinus augmentation, the posterior superior alveolar artery (which most commonly runs within the bone) may cause bleeding, especially in the lateral wall of the maxillary sinus, where it joins the infra-orbital artery.
Mandibular canal: It runs near the roots of mandibular molars and second premolars, occasionally with its bifid and trifid variations. Mandibular canals, carrying the inferior alveolar nerves, arteries, and veins, extend bilaterally from the mandibular foramen to the mental foramen, changing its course from lingual position posteriorly (near second molars) to a buccal position anteriorly (second premolar region). Thus, assessment of its buccolingual position, ideally with cross-sectional images, is of considerable importance.
Anterior loop: Sometimes, instead of dividing into two branches (mental and incisive) near the mental foramen, the undivided terminal portion of the inferior alveolar nerve passes below and forward to the mental foramen and gives off the incisive nerve branch. The main branch curves back and emerges out of the mental foramen as the mental nerve. This mental neurovascular bundle that crosses anterior to the mental foramen, then doubles back to exit through the mental foramen, is called anterior loop. It has a high cadaveric prevalence (61.5%-96%) with symmetric occurrence in 76.2% cases. Anterior loops, larger than 2 mm are more likely to cause sensory disturbances or hemorrhagic complications when dental implants are installed in the most distal area of the interforaminal region.
Mandibular incisive canal: This is the intrabony continuation of the mandibular canal and has about 100% cadaveric presence. It gradually becomes smaller anteriorly and is surmised to form an untraceable neurovascular plexus near the midline. The mandibular incisive canal is said to prevent osteointegration by migrating soft tissues around the implant.
Midline lingual canal: Most patients have at least one midline lingual foramen in the midline of the mandible, superior to, or at the level of the mental spines or genial tubercles, leading into the midline lingual canal, which contains blood vessels. Bleeding caused by accidental perforation of these arteries, close to the lingual border of the mandible, may spread to the floor of the mouth, with chances of obstruction of the airway by pushing the tongue against the palate (especially if the size of the artery exceeds 1 mm). A safety margin of 2mm is advisable during the placement of implant.
Lingual undercut: These concavities in the lingual side of the posterior mandible, are well-visualised in cross-sectional images. Their accidental perforation during implant placement (especially in the first and second molar region) can lead to severe surgical complication. Studies reveal that 38.83% of lingual undercuts are present above the mandibular canal and greatly influence the implant placement.
Soft tissue: The contour and dimension of the peri-implant mucosa can be assessed with CBCT by applying radio-opaque contrast materials on the surface of the mucosa and by displacing the lips and the cheeks from the alveolar process using lip retractors or cotton rolls.
Many vital structures are ill documented due to their dubious representation in conventional radiographs. CBCT software enables tracing of canals (containing blood vessels and nerves) and allows precise evaluation of size and distance of anatomical structures. Their relative position with a simulated/actual implant is also revealed accurately in different planes. These measurements and tracings are often done in disparate colours to avoid confusion.
Evaluation of bone quality
In conventional 2D images, the suitability of an implant site was assessed by measuring the bone density using Hounsfield Units (HU). These grey values are unreliable in CBCT due to its variability with machine model, patients, and different sites of the same patient. However, the bone structure parameters revealed in CBCT can provide better estimation of the implant success because the bone quality is not only a matter of mineral content but also of the structure. The most commonly used classification (amongst other classifications given by Seibert, Allen et al., Misch and Judy, Studer et al., Chen et al., etc.) for presurgical assessment of the bone is the Lekholm and Zarb index[Table 1], which roughly predicts the time required for osseointegration, based on the radiographic proportion and structure of compact and trabecular bone.
Mandible usually has more cortical density compared to maxilla, and their cortical thickness generally increases anteriorly. Prediction of primary implant stability is more accurate by evaluating the bone structure, compared to assessing the bone density. A ridge with a cortical thickness >0.75 mm and a normal appearance of the inferior mandibular cortex gives high peak implant torque. Thus, CBCT can prognosticate the time needed for osseointegration of implants and can predict the suitable interval between implant insertion and prosthetic loading.
Acrylic stents and surgical templates
Acrylic stents with radiopaque markers [Figure 2] positioned in the patient's mouth during radiological scanning can demarcate the intended implant sites. These may subsequently be utilised as surgical guides to orient the insertion angle of the guide bur and hence the angle of the implant. Computer-aided surgical templates may be fabricated to direct the surgical positioning of the drills and place implants along guide sleeves. This ensures accuracy of implant position, avoids harming important anatomical structures, and takes account of restorations and biomechanics. These are especially helpful in cases where precision of angulations and depth is of utmost importance (as in all-on-four implant placements).
|Figure 2: (a) Radiograph of an acrylic stent with radiopaque markers. (b) Implant planning in the intended sites|
Click here to view
The induced osseointegration process, characterised by an intimate interfacial contact between the bone and implant surface, determines the clinical success. Resorptive changes are disclosed radiologically by the presence of apical migration of alveolar bone or by a distinct radiolucent osseous margin. As per a proposed criteria,”an implant can be considered successful if there is no clinically observable movement, no peri-implant radiolucency, vertical bone loss <2 mm in the first year and <0.2 mm in subsequent years, and no persistent signs or symptoms, such as pain, infection, neuropathy, paresthesia, and injury to the mandibular canal”.
Early failure, due to lack of osseointigration is linked to impaired healing ability of the bone, disruption of a weak bone-to-implant interface and infection. Late failure after the successful osseointegration is associated with occlusal overload or peri-implantitis(irreversible and progressive marginal bone loss after initial bone remodelling). [Figure 3]a shows an implant with a radiolucent border (signifying failed osseointegration) palatally and no bony covering buccally. The apical end of the implant lies within the soft tissue lining of the nasal floor. [Figure 3]b shows an implant with successful osseointegration.
|Figure 3: Postsurgical evaluation of implants in (a) maxilla and (b) mandible|
Click here to view
CBCT images can pinpoint peri-implant bone defects in all three planes, true to scale, and without distortion– as revealed by histological correlation. While suspecting neuropathic pain triggered by implant placement, CBCT scans can be utilised to perceive the relative position of the implant and the nerve, so that decompression of the nerve can be planned when required. In case of accidental displacement (due to poor primary stability and an uncorrected planning program) of implants and migration into the craniofacial structures like maxillary sinus, the ethmoid sinus, sphenoid sinus, orbit, cranial fossae, and submandibular fossa, CBCT can locate the exact position of such implants and assist in subsequent treatment.
Shortcomings of CBCT for implants
Lack of availability, higher cost compared to two-dimensional images and beam-hardening artefacts[Figure 3] around titanium implants are the major drawbacks of CBCT. Due to this artefact, less than 6 mm of bone adjoining an implant can be either underestimated or be imperceptible.
Assessment of a probable implant placement site with CBCT
Explanatory CBCT evaluation[Figure 4] having pictorial demonstrations with marked measurements are obligatory for a quintessential planning of implant placement. Preferably, the quantification of available alveolar bone is to be illustrated in accordance with the position of a simulated implant of optimal dimension and location. Alternatively, disclosing the bone width at different heights along the apparent path of insertion of the implant can be equally useful. However, an arbitrary measurement of the bone dimension will generate fallacious perception [Figure 5]. The implantologists are usually consulted to identify their expedient preference of safety margins and positions. Defects, inadequacies, and recommended modifications are also conveyed. Pictures of virtual crown placement[Figure 6] are often provided to exemplify the desired outcome and further motivate and convince the patient. Specifying the locus of the central point of the simulated implant in relation to a clinically visible anatomical landmark (usually the proximal surface of an adjacent tooth) can help in localising the desired spot for insertion of the pilot drill. Soft and hard copies of the scan are also provided, to enable reassessment by the referring dentist.
| Conclusion|| |
CBCT has revolutionised planning and evaluation of implants by providing unmatched quality and precision of measurements. Its unique interactive software help in accurate diagnosis and treatment planning, so that implant surgeries can proceed uneventfully, fulfilling functional and aesthetic demands. It is also the best non-invasive tool for the successive re-evaluation of those dental implants.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6]