If a plastic dual arch impression tray can negatively affect the fit of a restoration clinically1,2,3,4,5,8 you better believe that the SMALL stuff can have a tremendous impact on the quality of your dentistry and the overall success of your business/practice.
In 2007, 3M/ESPE reported the cost in lost revenue to the dental practitioner for a crown remake is approximately $713 USD.6
After numerous adjustments you come to the painful realization that all your chair time and materials have gone to waste and the once beautiful yet ill fitting crown now has to be remade. Now think about the additional material costs such as impression materials, tips, disinfection, the added appointment, wasted chair time and inconvenience to the patient to come in again to seat the new restoration. Take all this in consideration and you too will conclude that… The SMALL stuff does add up!
When a clinician or technician evaluates the quality of a PVS impression there are a number of SMALL things that can be easily identified that contribute to a compromised final restoration.
A physical gap or ledge between light and heavy body material (Fig. 1) can and in most cases will result in heavy occlusal contacts of the final restoration.7 There are number of reasons this can occur, primarily caused by unmatched fluid viscosity of light and heavy body materials hence one of the reasons for the decline of condensation silicone or putty impression materials. In addition, the working times listed for impression materials are at room temperature and not at the higher temperature where light body is applied, directly in the mouth. Often when impressing multiple units the impression material expressed around the first preparation will start setting before the last is completed. This is the reason why a seam or ledge can be seen on the first preparation in the series. Clinicians must be aware that when impressing multiple units they must work faster than what is stated as “working time” from the manufacturer or switch to a material designed specifically for large cases such as Multi Prep from Clinicians Choice. Multi Prep was specifically designed to give you a longer working time to avoid this common occurrence.
Ideally clinicians should use impression materials that have a similar fluid viscosity regardless of durometer / setting hardness with the same working and set times. This can be seen in some advanced second generation PVS materials but is one of the key benefits in third generation materials such as Affinity from Clinicians Choice. Affinity has an added benefit for the both the clinician and patient due to a fast and independent intra oral set time.
Pulls or drags, can be caused either by the clinician overextending a materials working time or a patient that swallows, yawns or moves while the impression is setting. These pulls can be harmless as long as they do not extend up onto a preparation or to the middle third of the mandibular dentition. If so, the altered shape of these teeth on the model can interfere with articulation causing a SMALL unnoticeable gap between maxillary and mandibular articulated models leading to heavy occlusal contacts.
Although many laboratories are still on the fence in regards to the use of a dual arch impression tray they have been proven to be accurate, time saving and offer a significant cost advantage to the clinician provided the tray and impression material work together. Dual arch trays require a high durometer, inflexible heavy body material such as Inflex from Clinicians Choice to provide the support and resistance to distortion required for dual arch trays. They have also been proven to produce mounted casts with significantly more accurate maximal intercuspal relationships than with mounted full arch impressions. 9, 11
It has been proven both in research as well as by a significant number of lecturing and practicing clinicians that the flex and high sidewalls of plastic dual arch trays are problematic due to their elastic properties.10 When a patient bites into an impression material within a plastic tray the sidewalls are often deflected laterally (Fig. 3). Plastic trays have even been known to distort if the patient swallows while the impression material is in its gel phase pulling the lingual sidewall.7 If this was dead soft aluminum such as with the Quad Tray Xtreme from Clinicians Choice which has no side walls it would not be a problem. However, when a plastic tray is removed from the mouth it will spring back a SMALL unnoticeable amount which will squeeze the impression bucco-lingually elongating the impression mesio-distally thus resulting in a space on the model larger than present in the mouth. The result is tight interproximal contacts of the final restoration.
There is one impression material on the market today designed specifically for the dual arch impression technique, Affinity Inflex from Clinicians Choice. Due to its unique third generation chemistry this viscosity has similar flow characteristics to their light body materials but has a setting hardness higher than that of a heavy body or putty material. This will stabilize the impression and prevent distortion when poured in stone. Traditional heavy body materials were designed to flex in order to release from the mouth when delivered in rigid stock impression tray. These materials are not suited for the dual arch technique due to the excess flex and the propensity for distortion.
It is common practice, especially with a full arch crown & bridge impression for the clinician to take an opposing impression using alginate. Where the problem lies is in the SMALL but crucial occlusal detail an alginate lacks to reproduce. If you have ever tried to fit a PVS bite registration material onto an alginate generated opposing model you have probably noticed a SMALL gap between the models and bite registration (Fig. 4). PVS bite registration materials are high durometer impression materials that capture more detail than alginate, therefore not allowing proper seating onto the model due its lack of detail in fossa and secondary anatomy. The clinical implication of this SMALL inaccuracy is a heavy occlusal contact of the final restoration.
Manufacturers of impression material have come to realize the shortcomings of this use of alginate and have since developed alginate alternatives/substitute materials such as Alginot -Kerr, Status Blue — Zenith / DMG, Penta — 3M/Espe. Silgimix –Sultan Chemists and the newest technology is an improved 2nd generation multipurpose intra oral duplicating materialcalled Counter fit from — Clinicians Choice. Counter fit has ideal flow characteristics, excellent replication of detail and does not tend flow down the back of the throat. These materials are a lower cost PVS impression material that is much more accurate than alginate while maintaining a few important benefits. Dimensional stability means the impression can be taken on Friday and poured Monday without shrinkage or distortion and because it is silicone allows multiple pours without the concern of dehydration or the ripping and tearing commonly associated with alginate. When choosing an alginate substitute remember they are PVS materials which prefer a dry field for an accurate impression therefore stay away from flavoured or scented materials that can cause the patient to salivate.
Taking a bite registration to ensure models are articulated in centric occlusion is viewed by many clinicians as a relatively simple procedure, and it is. However if done incorrectly or if the wrong materials are used for this application the clinician is wasting money and laboratory time.
Bite registration materials today are available in a variety forms including different waxes and polyvinyl materials, the question is, which indications should they be used for?
Wax bite registration material is cost effective but should only be used for study models, diagnostic wax ups, and orthodontic records. The reason, most orthodontic impressions are taken with an alginate impression material and occlusal detail is not a priority. The use of a wax bite for mounting crown and bridge models should not be done! Wax does not capture occlusal detail and depending on its temperature can prevent the patient from closing properly into centric. In addition it is soft, pliable and can distort during transport and will deform if the clinician or technician is required to carve it in order to fit both working and opposing models.
PVS bite registration materials are the material of choice for C&B bite registration due to the high durometer. A high durometer bite registration is required to stabilize models and prevent the rocking which is problematic especially when articulating quadrant models. High durometer bite registration materials include Blu Mousse — Pentron, Memoreg –Heraeus Kulzer and Quick Bite –Clinicians Choice. Although all have a high durometer there is only one resin filled product, Quick Bite from Clinicians Choice. The benefit of resin filler is that it will more readily flow into finer occlusal detail, also when cured allows the clinician or technician to carve without it crumbling or cracking which is known to occur with other silica filled materials
Many laboratories have made it common practice to utilize plastic disposable articulators for one or two unit cases that have been delivered in a quadrant tray impression. This only becomes an issue when trying to fabricate a terminal end preparation. Unless a distal and mesial stop have been established either by natural dentition or a stop that has been integrated into the articulator. This SMALL detail can affect the fit of the final restoration by creating open occlusal contacts when seated intra orally.
How? As with plastic impression trays, plastic articulators will flex. If a restoration is being fabricated on a terminal end tooth without a mesial and distal stop the models will rotate on the last unprepped tooth creating a fulcrum which leads to open occlusal contacts. The recommendation is to mount these cases on a metal hinged articulator unless mesial and distal stops are integrated into the design of the disposable plastic.
Sound familiar? “Hello Dr… this is the lab, unfortunately we cannot fabricate the restoration you prescribed for Mr. Smith, and we just do not have enough clearance. Would it be possible to get the patient back in to take another. 5 mm from the distal lingual cusp and retake the impression?”
1 – The laboratory has poured, based and articulated models that cannot be used and will have to start over when the new impression arrives.
2 – The clinician now has to re-prepare, re-impress and reship the new impression to the laboratory.
3 – The patient now has to come in for an additional appointment for the preparation adjustments and impression and now has to wait longer for the final restoration
Not only does this waste time for all, but incurs the additional expense of materials.
Tooth Reduction Guides are used for the same reason we use caries detecting mediums, to ensure the job is done right.
In the age of minimally invasive dentistry a tooth reductions guide is an excellent tool that if utilized can eliminate the loss excess tooth structure, as well as save the time and money associated with an underprepared tooth surface. Tooth reduction guides are available in an assortment of shapes and designs all off which give you the option of a 1, 1.5 or 2 mm reduction. Unfortunately a few of the designs do not help indicate the exact location of the underprepared area and lead to over preparation. The Occlusal Reduction Rings available from Clinicians Choice have a unique design that fits over the preparation, when the patient closes the ring will rotate over the exact location (Fig. 5) where additional preparation is required ensuring that enough reduction has been achieved but more importantly without over preparation.
It can be a challenge when fabricating a large span anterior bridge, partial denture or a full denture either traditional or implant retained to determine the facial midline. Specifically when anatomical landmarks are not present on the model or are not in alignment the facial midline.
Up until now many clinicians have been using a technique that utilizes Popsicle sticks or tongue depressors and glue to fashion a device to send to the laboratory.
Symmetry from Clinicians Choice is the first device designed specifically for accurately determining and communicating the facial midline (Fig. 6) to the laboratory and was used and supported by LVI.
No longer will you have to send a denture set up back to the laboratory to adjust the midline nor will you need to remake a fixed retainer due to poor positioning of the pontics.
Symmetry is low cost, but indispensible tool to eliminate the loss of time associated with multiple appointments for set up adjustments or worse the remake of long span anterior bridge.
In today’s marketplace there is a number of SMALL but indispensible tools and techniques that help enhance case communication between clinician and technician and if taken advantage of can offer a significant amount of cost and time saving benefits.
The SMALL stuff does make a difference! oh
Gregg Tousignant, CDT graduated Dental Technology from George Brown College in 1992, Certified Dental Technician since 1994 from the National Board for Certification in the USA. He has worked for 12 years as a dental technician, was certified as an instructor for Heraeus Kulzer where he provided a number of hands-on programmes for indirect composites and denture injection systems. Gregg currently provides lectures and hands-on courses for both the General / Cosmetic dentist as well as dental and hygiene schools across Canada. Course topics include Tooth Whitening, Impressioning, Temporization, Direct Veneer Colour Change including the invisible Class IV restoration and the Sensitivity Free Direct Posterior Composite. Gregg is currently serving as Technical Support Manager for Clinical Research Dental where he provides technical support and hands-on courses consistent with the Company’s philosophy, Teaching Better Dentistry.
Oral Health welcomes this original article.
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nd Bridge- And Investigation and Review, British Dental Journal, 1998
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8. Gary J. Kaplowitz, DDS, MA, M.ED, Trouble Shooting Dual Arch Impressions, JADA, Feb 1996, Volume127, page 234-240
9. Merle H. Parker, MS, DDS, Steven M. Cameron, DDS, James C. Hughbanks, DMD, David E. Reid, DDS, Comparison of Occlusal Contacts in Maximum Intercuspation for two impression techniques, US Army Activity, Fort Gordon, Nov 2005
10. Igor J. Pesun, DMD, MS, FACD, Three way trays, easy to use and abuse, Clinical Practice
11. Michael DiTolla, DDS, Achieving Accurate Bite Registrations, Dental Economics, June 2006.
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