October 1, 2011
by David Chvartszaid, DDS, MSc (Prostho), MSc (Perio), FRCD(C)
Excellent theoretical knowledge and clinical skills provide a firm foundation for success in implant dentistry. However, inability to see “inside” the tissues and the prosthetic replacements may hamper the ability to deliver optimum treatment outcomes. Radiographic prescription is often needed to provide a more complete picture of the current clinical situation and to guide further clinical steps. Radiographs can be utilized at various steps during implant treatment planning, the treatment itself, and the follow-up. The objectives of radiographic analysis and the types of radiographic imaging modalities vary at each phase of therapy. This article highlights some of the common principles in the use and interpretation of radiographic imaging within the scope of implant treatment planning and delivery. This overview is intended to provide a practical guide for the dental practitioner.
Pre-operative assessment focuses on the proposed surgical sites as well as the condition of adjacent teeth and osseous structures to ensure that their status is consistent with the proposed treatment plan. Ultimately, the dentist needs to ensure that the proposed surgical site can host an implant (possibly with some modification, such as osseous grafting) and that the proposed surgical site is consistent with prosthodontic demands.
The treatment team must be able to identify or visualize radiographically certain key structures as well as to estimate the distance to them. This will enable the proposed surgical intervention to be carried out safely without undue risk of engagement of these key structures.
Many features (and proximity to them) are of interest in planning an implant surgical intervention including:
• osseous anatomy (e.g., sinus floor, sinus septa, nasal floor) and dimensions (height, width, volume) [Fig. 1],
• vital structures (inferior alveolar canal, mental foramen, anterior loop),
• mesio-distal width between adjacent roots and unusual root anatomy (e.g., root dilaceration),
• pathologic or disease entities as well as other clinical entities of interest (periapical pathology, bone loss due to periodontal disease, soft tissue lesions associated with Schneiderian membrane of the sinus [Fig. 2], periapical cemental dysphasia, dense bone islands, retained root tips, impacted teeth etc.).
The types of imaging modalities ordered during treatment planning depend on multiple factors such as location of proposed surgical site, size of edentulous span, type of planned restoration, and clinical suspicion of osseous width or height deficit. Local anatomy is typically assessed first on plain films. Advanced imaging modalities (such as cone beam CT) are utilized once other simpler imaging modalities failed to fully visualize the structures of interest. In this manner, basic images obtained early in the process (such as periapical and panoramic films) in combination with the clinical exam and treatment needs help guide the selection of additional imaging modalities (such as occlusal films, cephalometric films, tomography, or cone beam CT) if needed. Advanced imaging modalities can also help in reconciling local anatomy with prosthodontic needs, typically through the use of a well-designed radiographic guide. [Figs. 3, 6]
A radiographic guide enables two key processes. First, it allows an accurate assessment of the presence of adequate bony dimensions at precise prosthodontically-planned locations and angulations. Second, through conversion to a surgical guide, it allows the radiographic planning information to be related to the surgical field.
Occasionally, slight variations in technique can be used to help visualize structures of interest. For example, sometimes it may be difficult to visualize mental foramen and inferior alveolar canal on a panoramic image. Changing the angle of the head by 5° may help in demonstrating these important landmarks.
Pre-operative imaging and intra-operative surgical steps are linked. Greater anatomic knowledge or certainty derived from a thorough radiographic investigation may permit the clinician to operate closer to vital structures than would otherwise be possible without this detailed prior knowledge. In fact, in specific situations, CT-guided surgery utilizing extremely precise radiographic and surgical guides may allow a surgical intervention to be carried out with limited flap reflection. In order for this particular treatment approach to be successful great care must be utilized in ensuring the accuracy of the guide fit both during the radiographic image acquisition and during the surgical intervention.
SURGICAL IMPLANT PLACEMENT
During the surgical implant placement, radiography helps in visualizing implant angulation and proximity to adjacent entities such as teeth, vital structures, or other implants. Difficulties in surgical access, presence of blood, malocclusion, and differences in the slope of osseous crest relative to occlusal plane may make accurate clinical determination of drill angulation and depth difficult. The use of a surgical guide is always advised during surgical implant placement, and its use is likely to lead to more optimal implant placement. If a radiographic guide was utilized during the radiographic treatment planning, this guide can be converted directly into a surgical guide (for conventional implant placement) or the information gathered during the treatment planning can be used to make an entirely new surgical guide (typically, for CT-guided surgery).
Radiographic film for verification of implant osteotomy depth and angulation should be taken as early in the surgical installation as possible. This usually means taking the radiograph right after the use of the first drill – typically, a 2 mm twist drill. This allows for an early recognition and correction of depth or mesio-distal angulation issues. By contrast, if a radiograph is taken after the final drill has prepared the osteotomy to the final depth, irreversible damage may have been done already. In the case of a very poor implant location or angulation, the prepared osteotomy site may have to be abandoned.
Drills themselves are quite long, and it is advisable to take the radiographs with the direction indicators corresponding to the drill diameter rather than with the drill itself [Fig. 9]. If the implant company does not manufacture a direction indicator for a particular drill diameter, one can be fabricated easily by shortening an old drill. It is imperative to secure a direction indicator with a piece of floss to avoid its disappearance in the patient’s mouth. [Fig. 4]
If limited bone height is a concern, a film should be taken before the osteotomy is prepared to full depth. Bone height may be a concern in the posterior mandible where limited height of bone may be present superior to the mental foramen or the inferior alveolar nerve. Another scenario would be the presence of a deep impacted tooth, where an effort is being made to place the implant superior to the impaction. In addition to measuring the depth of osteotomy preparation directly in the mouth, the depth of the preparation can also be read off the radiograph (for some implant systems where direction indicators have radiographically visible measurement markers). The amount of residual bone apical to the preliminary osteotomy can then be inferred.
Image interpretation is often hampered by an over-prepared osteotomy site. If the direction indicator is loose within the osteotomy site, its position on the radiographic image may not be representative of the intended osteotomy angulation.
Very few drills are needed to place narrow diameter implants. In most implant systems, only two drills are used – a 2 mm twist drill and a final drill. This short drill sequence leaves very little room to correct for errors in angulation. Hence, for narrow diameter implants, it is beneficial
to use an additional drill – a precision drill – and to take a radiographic film checking for angulation at the precision drill stage (rather than at a 2mm twist drill stage). [Fig. 5] This will give the operator two more drill diameters to make angulation and depth corrections. Furthermore, it is usually easier to make modifications in osteotomy angulation when transitioning from a precision drill to a 2 mm twist drill, than when transitioning from one twist drill diameter to another. Once again, the earlier the image is taken in the surgical implant installation process, the greater the opportunity for correction and the lesser the chance that significant damage has already been done inadvertently.
IMMEDICATELY AFTER THE SURGICAL INTERVENTION
Right after the implant has been placed, radiographs are taken as a final confirmation of implant position and to ensure complete seating of a cover screw or healing abutment. [Fig. 11] The image should be taken perpendicular to the implant so that the implant-abutment interface is clearly visible. A simple visual check to ensure that the radiograph was taken at the correct angulation is to examine the appearance of the threads on the sides of a threaded implant. Sharp clear threads on both sides of the implant indicate optimal image angulation. Blurriness of the threads on one side of the implant indicates that the image was not taken perpendicular to the implant. Poor definition of the threads on both sides of the implant indicates significant misangulation of the film relative to the implant angulation. If the image is taken correctly, but the implant appears to have blurry threads on both sides, this may indicate a substantial misangulation of the implant bucco-lingually and should be investigated further. [Fig. 7]
It is important to keep in mind that the film taken at this point is only a confirmation film – the dentist should already have a good idea that the cover screw or healing abutment is fully seated based on tactile feedback or direct visual observation at its placement. Since observation of an open interface between the implant and the cover screw or healing abutment would indicate a need for additional procedures, it is advantageous to take this film before suturing the flaps (rather than after).
The final post-surgical film should show the implant in full. If radiographic proximity to adjacent structures is apparent, a decision has to be made as to whether this is a true clinical proximity or merely a result of overlap of 3D objects on a 2D projection. This overlap is commonly observed radiographically between the maxillary implant in the first premolar position and the adjacent canine. Tactile sensation during osteotomy preparation should alert the dentist if true damage to adjacent teeth has been made. Modification in horizontal beam angulation often allows the true relationship between the implant and the adjacent tooth (or other entities) to be clarified. [Figs. 12, 13]
A periapical image is preferred as a post-operative film due to its clarity and ability to precisely visualize the area of interest. However, for multiple implants placed into very large edentulous spans periapical images may be difficult to take and may be poorly angulated. Furthermore, it may be difficult to relate the periapical images to each other or to the surgical field. Hence, for large edentulous spans (including, fully edentulous arches) a panoramic image is frequently the image of choice.
Post-operative radiographic assessment is procedure-specific. For example, immediately following a direct sinus lift grafting procedure, an image (typically, a panoramic image) should be taken to visualize the body of the graft as well as its most apical extent. It is important to visualize the most apical periphery of the graft to ensure that no gross perforation or overfill had occurred. At the same time, the image of the body of the graft can reveal if any large voids are present within the graft. Their identification at this stage may avoid them being mistaken for sites of infection at a later time.
Radiographic assessment prior to Stage II surgery focuses on signs of successful osseointegration, bone levels around the implant, and evidence that the cover screw is fully seated. [Fig. 8] Periapical images are typically used for this purpose, but a panoramic image may be used for large partially edentulous spans and, especially, full arches.
Although a non-seating cover screw should have been noted at the time of implant placement, this may have been missed due to variation in angulation of image projection. Identification of a gap between the implant platform and the cover screw should alert the dentist to the presence of a possible obstruction between them. This same obstruction may prevent full seating of the healing abutment unless measures are taken to address this. Observation of an incompletely seating cover screw should guide the dentist towards conventional Stage II procedure and away from a “punch” uncovery. A “punch” uncovery may not allow the dentist to visualize the implant platform clearly and will complicate ensuring full seating of the healing abutment.
The dentist should also note the depth of implant platform relative to adjacent bone levels. Healing abutments come in different heights and different diameters – typically, straight and flared. If the use of a flared healing abutment is anticipated for prosthetic reasons, the dentist should anticipate that osseous re-contouring may be necessary to enable a flared healing abutment to seat fully. Once again, a “punch” uncovery would be inappropriate in this situation.
Pre-prosthetic check is done at the conclusion of the time-dependent healing phase procedure and just before the dentist embarks on the restorative phase. Radiographic assessment prior to prosthetic treatment focuses on signs of successful osseointegration, bone levels around the implant, and evidence that the healing abutment is fully seated. Some self-limited bone remodeling is typically observed around dental implants, but the precise amount is variable and partly implant design-dependent.
Although a non-seating healing abutment should have been noted at the time of Stage-II surgery (or at the time of Single-stage surgical implant placement), this may have been missed due to a variation in angulation of image projection. Another possibility is that the healing abutment may have loosened during the healing period due to inadequate tightening or excessive dislodging forces. It is important to determine the cause of a non-seating healing abutment and to correct this to enable uneventful full seating of all subsequent prosthodontic components such as impression copings and prostheses themselves.
If the implant placement was done by someone other than the restoring dentist, it is a good idea for the restoring dentist to be familiar with the general appearance of different implant brands and healing abutments so as to be able to confirm visually that the information provided by a surgical colleague is, in fact, correct. Most dentists order parts for implant restorations based on the “sticker” information provided by the surgical team member. Unpleasant surprises can occur if incorrect “sticker” information is provided inadvertently to the restoring dentist.
During prosthodontic treatment, several distinct stages may require radiographic evidence of proper fit (complete seating and passive fit) of components to the implant or to the intermediary abutment: impression, verification jig, try-in, and insertion. [Fig. 15] Component fit of screw-retained components is usually apparent with tactile sensation and radiographic image is taken largely as a confirmation. [Fig. 10]
The dentist must know what correct component fitting is supposed to look like radiographically. Significant difference exists in the radiographic appearance of “fit” among different
implant systems and especially between internal connection and external connection systems. Inexperience may lead to an erroneous judgement of poor fit where, in fact, optimal fit is present. The opposite situation may also occur.
If cement is used as part of implant treatment, it should be radiopaque. This will help in radiographic cement identification should it inadvertently escape into the adjacent tissues. Of course, only cement present on the mesial or distal of the implant would be visualized on a plain film image.
As was the case earlier in ensuring that the cover screws and healing abutments were fully seated on the implants, image geometry is important in confirming full seating of prosthodontic components. Unless the image is optimally taken, the implant platform will not be visualized precisely and misfit of components may be missed. Assessing the clarity of the threads on both sides of the implant is a good guide to proper image angulation. Vertical bitewings can be useful at this stage instead of regular periapical images. (Horizontal bitewings may not show the implant platform if vertical bone loss has taken place and the implant is positioned “deep” within the tissues.)
FOLLOW-UP MAINTENANCE COMPLICATIONS
Patients with implant-supported prostheses need to return for regular assessment and maintenance just like all other dental patients. No clear guidelines exist as to how often clinical and radiographic exam should be conducted on these patients. Studies are clear that most implant failures occur within 2 years of implant placement. Hence, a reasonable approach would call for a closer follow-up for patients soon after restoration placement, and a more spaced out follow-up frequency as the time goes on. This approach is particularly appropriate for single implant-supported restorations.
The images typically utilized at follow-up appointments are periapical images. Panoramic images are occasionally used to survey full-arch fixed prostheses. However, limitations of this approach must be readily acknowledged. [Fig. 14]
Items assessed during the radiographic follow-up exam (as part of an overall comprehensive exam) include signs of osseointegration, bone levels, component fit, component and implant integrity (i.e., signs of fracture), as well as prosthetic screw integrity and proper location (i.e., signs of screw fracture or loosening). It is important to note that several key features may preclude patients from noting a problem with their implant-supported restorations or with the implants themselves until severe irreversible damage has already occurred. The implant will remain immobile as long as even a small fraction of the implant surface remains osseointegrated. Hence, the patient may not be aware of substantial degrees of bone loss around the implant(s) until complete loss of osseointegration has taken place. Furthermore, a prosthesis that is supported by multiple implants will remain immobile as long as the retention mechanism (screw or cement) continues holding it in place to even a single implant (even though all the other implants may have fractured or failed). For obvious reasons, it is beneficial to make the appropriate diagnosis early before complete loss of osseointegration or fracture has occurred of all the implants. [Fig. 14]
Occasionally, patients will present with signs and symptoms that manifest secondary to implant treatment complications. These may include prosthesis mobility or soreness in the area of implant treatment. Clinical findings and patient history will guide radiographic assessment in these cases. For example, prosthesis mobility or dislodgement may be due to prosthesis fracture, decementation, screw loosening/fracture, or implant failure/fracture. [Fig. 16] Signs of tissue inflammation can be due to plaque accumulation, soft tissue irritation, cement retention, implant failure/fracture, or loosening of the prosthesis. Where appropriate, periapical radiographic images would help assess the integrity of retaining screws and implants as well as the hard tissues around them.
Radiographic imaging modalities guide the practitioner’s surgical and prosthodontic decision making at multiple steps during implant treatment planning, execution, and follow-up. Radiographic imaging provides information that is not readily apparent to a clinician. Implant therapy is partly a “blind” procedure, since much of the surgical procedure occurs inside a dense object (bone) and the implant platform level is often hidden deep within the soft tissues during the prosthodontic phase and at follow-up. The knowledge of regional anatomy and treatment requirements will help the clinician undertake successful implant interventions and minimize occurrence of complications.
THINGS TO REMEMBER:
• radiographic assessment complements clinical exam and patient history
• provisional diagnosis (or differential diagnosis) guides the ordering of a radiographic film
• proper image geometry is crucial in enabling proper image interpretation
• radiographic prescription is always objective-driven and is limited by judicious application of the ALARA principle
• advanced imaging modalities are prescribed when more basic imaging modalities have failed to reveal the desired information. OH
Acknowledgement: The author would like to thank Drs. Peter Birek and Milan Madhavji for their support and expert editorial input during the preparation of this article.
Dr. David Chvartszaid is a specialist in both Prosthodontics and Periodontics. He is an assistant professor at the University of Toronto and maintains a private practice in Toronto. He can be reached at: firstname.lastname@example.org .
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