Written by George F. Warga, DDS Tuesday, 13 September 2011 09:25
For complex multiunit cases, traditional dental tools and procedures do not always help us accomplish our goals as efficiently and accurately as we would like. The process of capturing impressions of multiple preparations and then creating the necessary models is time and labor intensive for both the dentist and laboratory technician. Furthermore, in cases with complex bites, the situation can become even more difficult. New technology, however, is changing this and offers the potential to dramatically streamline the way we treat complex cases. One important technology in this trend is the introduction of digital impression devices such as the Lava Chairside Oral Scanner (C.O.S.) [3M ESPE]) or iTero (Cadent).
We purchased the Lava C.O.S. and have been very impressed with its capabilities and the accurate restorations that it helps the dental laboratory team produce for our patients. This article will focus on this system and our personal experience and feedback after using it since 2008 in our practice.
A Brief Review of the Technology
The Lava C.O.S. uses an intraoral wand to capture continuous video images in the mouth, made up of 2,400 data sets per arch. Therefore, a 2-minute, full-arch scan produces 24 million data points. The images captured are displayed on a chairside monitor, allowing the dentist to simultaneously review the images and to ensure that the digital scans are complete. The dentist can prescribe any type of laboratory or CAD/ CAM restoration in the same manner as with a traditional impression technique. Software is then used to cut the dies and mark the margins of the scans, after which the information is transmitted digitally to a model production facility. The models are produced using stereolithography (SLA) equipment using an automated process to create the model from a polymer resin. The models are created and cured in an additive process, and the final result is a model that is highly accurate and durable. A study performed on the SLA models has shown that they are as accurate as stone models made from a vinyl polysiloxane impression.1
|Figure 1. The patient had significant plaque and an unusual pattern of caries.||Figure 2. A face-bow record was used to relate the maxillary cast relative to the condyles and the arc of closure.|
|Figure 3. A centric bite record was taken to relate the maxillary and mandibular study casts in the most predictably repeatable joint position.||Figure 4. A protrusive bite record was taken to set the angle of the articulator eminentia relative to the face-bow record.|
|Figure 5. The Class II Division I bite, as demonstrated on the articulated study casts.||Figure 6. The teeth needed only light powdering before the preparations were |
digitally scanned (Lava C.O.S. [3M ESPE]).
The SLA models are then shipped to our dental laboratory team, who then uses them to create a wide range of restorations from a variety of materials. This versatility appealed to me when I was researching digital impression systems, as I did not want to be locked into a limited range of materials. While there is some appeal to being able to use an in-office system to place restorations in one appointment, I believe that for most cases it is more appropriate to allow more time between the preparation of teeth and cementation of final crowns. I often find that it is necessary to evaluate provisional restorations over time to gain important feedback such as periodontal and pulpal health, aesthetics, and function. Additionally, for our practice needs, the in-office CAD/CAM systems are very limited in use of dental materials, aesthetic control, and occlusal function.
I have used the Lava C.O.S. to create restorations from pressed porcelain, lithium disilicate, full-gold (cast) restorations, and porcelain layered over gold and zirconia. In addition to single-unit crowns, the device can also be used for inlays and onlays, bridgework (up to 4 units), and lab-processed provisionals and veneers. This degree of flexibility enables the system to be used for many different types of cases, and the device saves a significant amount of time. For example, if a zirconia coping is desired, additional time can be saved because it can be made directly from the digital scanning data while the SLA models are simultaneously being constructed. In this way, both the zirconia coping and the SLA models can arrive at your dental laboratory within a short period of time.
With traditional impression materials, it is common to retake impressions due to issues such as delamination, facial-lingual pulls, margin tearing, or tray-tooth contact. In my practice, I estimate that we performed retakes for up to 20% of our traditional impressions, and retaking impressions was more common for multiunit cases. Additionally, I estimate that we asked our dental laboratory team for a remake of a restoration approximately once a month prior to our acquisition of the scanner. However, with the Lava C.O.S. technology, the accuracy of the digital scans and the resulting predictability of the restorations significantly reduced chair time. Studies done by 3M ESPE have shown that seating times of single unit crowns made with the Lava C.O.S. are an average of 41% less than those made with traditional impressions.2 Laboratory teams have also seen added efficiencies with the Lava C.O.S., for which the rate of remakes due to marginal fit is up to 80% below the industry average.2
In addition to these benefits, the device is also much more patient friendly than a traditional impression and can boost a practice's high-tech image, as well as help it create a reputation for gentle dentistry.
The following case demonstrates the use of digital impression technology on a disabled patient, for whom the comfort of the oral scan was a major benefit.
Diagnosis and Treatment Planning
A 58-year-old female patient had significant areas of decay involving teeth in her anterior sextants and maxillary left and mandibular right quadrants. She presented with a severe Class II Division 1 bite. The patient was somewhat compromised in physical dexterity and thus had difficulty with her oral hygiene (Figure 1). She also had a very active tongue thrust and a significant gag reflex. We discussed the treatment options and cautioned the patient that unless she was able to improve her oral hygiene, any dental treatment would be fruitless. She returned for her next appointment 6 weeks later and had in fact improved her hygiene enough that we felt it would be acceptable to move forward with dental treatment.
It was determined that we would place PFM (high noble) crowns on teeth Nos. 12 to 14, 28 to 30, and a gold crown on tooth No. 31. Full gold was selected for the second molar to preserve tooth structure and to accommodate future arthritic changes in the joint. The choice of gold would allow the occlusion to "self-adjust" more readily.
|Figure 7. The scan |
captured a 3-dimensional image of the teeth.
|Figure 8. Software was then used to insert die spacers on the digital model.|
|Figure 9. Digital models are extremely useful for visualizing the centric occlusion.||Figure 10. A single, highly durable and accurate model was created. (Note the almost invisible die cuts that contributed to the accuracy of the interproximal contacts.)|
|Figure 11. The cemented crowns on the mandibular right.||Figure 12. Right lateral excursive movement demonstrated the appropriate anterior group-guidance function.|
A face-bow (Figure 2) and protrusive (Figure 3) and centric bite records (Figure 4) were taken to create diagnostic study casts articulated on a PROTAR evo 5 (KaVo) instrument (Figure 5). The articulated casts were equilibrated to create bilaterally balanced centric occlusion stops and a smooth transition in all excursive movements from posterior tooth guidance to anterior tooth guidance. For Class II Division 1 bites, it is often necessary to initiate group function on the posterior teeth. For the patient, this can be done successfully by careful evaluation of the masticatory muscles and the patient's clinical response. The patient should be able to move with ease in all excursive movements without creating fremitus or mobility in any teeth. Typically, the group function guidance will start on the molars and/or bicuspids and transition anteriorly to the canines and then finishing on the incisors.
After the trial equilibration of the study casts, the morphology of the teeth was refined with wax, and putty stents (Express Registration Material [3M ESPE]) were made for provisional fabrication. The preparations and digital impressions (Lava C.O.S.) were performed in separate appointments for each quadrant due to the patient's time restrictions. Work on the maxillary left quadrant was performed first. The teeth preparations were begun with a tungsten carbide H245.314.008 (long pear) bur (KOMET USA), used to break the interproximal contacts and remove old dental materials. An edge wheel round (donut diamond) 6909-040 was used to cut 1.5 mm reduction grooves and then to start gross planing of the axial and occlusal walls. A round end taper diamond (fat chamber) 5856-025 was then used to continue the gross planing, and a modified shoulder parallel diamond (parallel final prep) 837KR-012 was used to finalize the margins and smooth the axial walls and occlusal table. This bur was also used to place a small retentive proximal box in each preparation at a depth approximating 1.5 mm to resist buccal-lingual tipping forces.
Prior to scanning the preparations, non-impregnated retraction cord (GingiBraid, size 1n [DUX Dental]) soaked in Hemodent (Premier Dental Products) was placed in the sulcus circumferentially. A larger size (2n) cord was similarly placed on top of the first cord where the sulcus depth allowed. The teeth were lightly powdered (Figure 6), the 2n cord was removed, and the digital impression wand was then used to capture a scan of the prepped teeth and the opposing arch (Figure 7). The digital scans are displayed in real time on the chairside monitor, enabling the doctor to review them for accuracy and completeness before moving on. The clinician's ability to evaluate the tooth preparation contours and marginal precision with the digital scanner surpasses any direct vision technique traditionally used for physical impressions. The clinician is viewing a positive 3-dimensional highly-magnified view instead of a physical impression which represents a negative image of the teeth and preparations.
Putty stents (these can be fabricated from the unprepared teeth, or in this case from prepared study casts) were then placed and temporaries were created using a filled bis-acrylic material (Protemp Plus Temporization Material [3M ESPE]). The temporaries were seated with a suitable temporary cement that was selected for its long-term strength (HY-Bond Polycarboxylate Temporary Cement [Shofu]), due to the fact that we expected the patient to wear the provisionals for several months. Since the provisionals were replicated from the idealized study casts, the teeth opposing the provisionals were appropriately adjusted. Sequencing treatment in this way allows the clinical equilibration to be done more efficiently and to also closely mimic the trial equilibration done on the mounted study casts.
Using the Lava C.O.S. software, the digital dies were cut on the impression and the margins were marked (Figure 8). (I perform this step personally utilizing a separate software package from 3M ESPE, but many dentists have their dental laboratory team perform this task.) The data was then submitted to 3M ESPE for the automated manufacturing of the model, and was also concurrently submitted to my laboratory for creation of the restorations (Figure 9). Once the model was complete, it was also shipped to the laboratory for completion of the crowns (Figure 10).
The patient returned one month later for work on the lower right quadrant, and Nos. 28 to 31 were prepped, scanned, and temporized in much the same fashion as for the maxillary left quadrant. Once all of the final crowns were completed by the dental laboratory team, the patient returned to the office and they were tried in the mouth. No interproximal adjustments were necessary, and only minimal occlusal adjustments were performed. The crowns were then seated with a self-etching, self-adhesive resin cement (RelyX Unicem [3M ESPE]) (Figure 11).
The patient was thrilled with the final result and also appreciated the comfort of the digital impression taken in the clinical process of fabricating her restorations. She is now on an every 3 month recare schedule, and ongoing monitoring has shown improved hygiene and healthier tissues (Figures 12 to 14).
|Figure 13. Left lateral excursive movement; appropriate anterior group-guidance function.||Figure 14. The completed case. Gingival inflammation was still present, but the patient's hygiene had improved and |
continued to do so over time.
To treat a case such as this one using traditional impression techniques would have been extremely difficult. Multiple physical impressions and models would have been necessary, as well as far more aggressive tissue retraction procedures. The patient's tongue thrust and gag reflex would have made traditional impression procedures extremely uncomfortable for her, keeping in mind that her disability also prevented her from being able to spend much time in the chair. Furthermore, although the patient did improve her hygiene before the impressions were made, areas of bleeding were still present, which could have also compromised the final impressions had they been made with traditional material. The digital impression technology used here allowed isolation of each preparation and seamlessly married the multiple images into one model. This saved a tremendous amount of chair time and stress on the patient and doctor.
Significant advantages are realized by using virtual digital imaging during die and model construction. Many errors are inherently incorporated during the traditional impression process and model construction, such as impression distortion, stone expansion and porosity distortion, as well as chipping and wearing of stone dies. The dies of an SLA model are significantly more stable than a stone model because the computer generated die cuts are extremely refined and virtually seamless. The durability of the completed SLA model also adds to the accuracy of the final product, as it does not abrade during use. The articulator system used here, while not as anatomically sophisticated as an arcon articulator (defined as an articulator with the equivalent condylar guides fixed to the upper member and the hinge axis to the lower member), is flexible, allowing for careful evaluation of the wear facets of the teeth. (Note: Manufacturers are currently working to create adaptors for standard practice articulators.)
The computer images of the model allowed us to design the restorations to fit together in maximum intercuspation. Additionally, the ability to use the computer to magnify the scans allows for much more accurate marking of the margins than could be possible with a stone model; the user can easily see the difference between tissue and tooth. And while making a mistake on a stone model renders it useless, an error on the computer design can be quickly undone with the click of a mouse.
Overall, the case presented in this article is an ideal illustration of the capabilities and advantages found when using digital impression systems. The use of this technology removed the stress (for both the doctor and the patient) usually found with the use of traditional physical impression materials. The final restorations were seated in a fraction of the time that it would have taken had they been created with traditional impressions, and the patient was very pleased with the final result. The ability to help patients be more comfortable in the office, while at the same time efficiently delivering dentistry that is far more accurate, is a much appreciated feature of these modern digital technologies.
- Ogledzki M, Wenzel K, Doherty E, et al. Accuracy of 3M-Brontes stereolithography models compared to plaster models. J Dent Res. 2010;89(special issue A). Abstract 1060. iadr.confex.com/iadr/2010dc/ preliminaryprogram/abstract_129542.htm. Accessed on May 17, 2011.
- Lava Chairside Oral Scanner from 3M ESPE now available nationwide with powerful software upgrade [press release]. solutions.3m.com/wps/portal/3M/ en_US/LavaCOS/3MESPE-LavaCOS/PressRoom/ PressReleases/Nationwide. Accessed on May 17, 2011.
Disclosure: Dr. Warga received financial compensation from 3M ESPE as a member of its speakers bureau for public speaking engagements. He has not received any financial support for authoring this article.
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