Ideally, crown preparations strike a careful balance between maximizing the preservation of natural tooth structure and creating adequate space for a strong and aesthetic restoration. Inaccurate preps can sacrifice this balance, jeopardizing the long-term viability of the restoration and the patient’s oral health.
On one hand, prepping too aggressively risks serious pulpal damage, whether from heat generation or exposure to bacteria due to the increased permeability of deeper dentin layers.1 On the other hand, prepping too conservatively can lead to an embarrassing second impression at best or the fabrication of a thin and far more fragile crown at worst.2 Ironically, the conservative option, though well-intentioned and at least superficially consistent with the minimally invasive principles that are rightly championed wherever possible, often leads to less-than-desirable outcomes. The cracks, punctures, and stress fractures that can result from even minor shortfalls in material thickness end up providing virtually unimpeded access to the underlying tooth structure, facilitating further dental decay.
With the above being common knowledge, and the difference between a successful restoration and a failure often being only a fraction of a millimeter, this raises the question: Why do so many dental professionals still rely on just their naked eyes to confirm accurate preparations? It is, to be frank, unrealistic and unnecessary to expect dentists to consistently and accurately discern measurements down to a tenth of a millimeter unaided and within the visually restrictive confines of the mouth. Even amongst experienced clinicians, relying only on visual confirmation of adequate reduction on an abutment can lead to under-preparation3 or over-preparation,4 largely dependent on the surrounding visual context. This is especially true when accounting for the wide variety of different techniques for crown preparation: Free-handing, using specialized depth-gauging burs, and using a wax or silicon index can all frequently provide different results, even under identical circumstances.4
Knowing the fundamental importance of accurate, ideally balanced preparation and the understandable difficulty of consistently ensuring that high level of accuracy across the countless preps most clinicians will make in a career, it is only prudent to find a simple and reliable way to confirm the presence of adequate clearance. Most products currently on the market tend to be flat and tab-shaped: designed to be placed between a prepared abutment and the opposing tooth with the jaw closed. While they can be helpful for checking occlusal clearance, they often lack the versatility to easily confirm other aspects of the prep (marginal, mesial, distal, etc) and, in addition, can be difficult to use in the anterior region.
To get around these limitations, the PrepSure (ContacEZ) instrument was developed. Instead of the standard tab-shaped design, PrepSure utilizes long, round tips with diameters matching common reduction indications for various restorative materials: 1.0, 1.5, and 2.0 mm (Figure 1). By gliding these tips over the cusps of a prepared abutment while the jaw is closed, the presence of adequate occlusal clearance can still be confirmed by the absence of resistance from the opposing tooth (Figures 2 and 3). In addition, however, these tips can also be placed against prepared margins or inside initial depth cuts on other aspects of the prep to more completely confirm the accuracy of the entire preparation. Moreover, this added utility makes it easy to periodically check for necessary clearance throughout the procedure rather than simply at the end of the preparation procedure, when any over-reduction would have already occurred.
Diagnosis and Treatment Planning
The case described here centers on a patient who presented with an aging porcelain-fused-to-metal (PFM) crown on tooth No. 14.
Upon initial examination, the PFM crown displayed a significant loss of its layering porcelain, causing diminished aesthetics and limited occlusion with the opposing tooth. Upon further examination, recurrent decay was noted under the crown, which further necessitated the replacement of the existing restoration.
Treatment options were discussed with the patient. Because the restoration would be outside the aesthetic zone, and with additional consideration being given to an option that would be long-lasting and avoid the same sort of fracturing, a monolithic zirconia crown replacement was recommended. This treatment plan was accepted by the patient.
|Figure 1. The 3 sizes of PrepSure (ContacEZ) are shown here and listed with the common materials they work with. Shown above in classic colors, but also available in modern colors gradient.||Figure 2. The PrepSure tip glides over the cuspal curves to detect areas of insufficient occlusal reduction.|
|Figure 3. Each PrepSure has designated mesial and distal ends to make access as simple and accurate as possible.||Figure 4. The distal tip was used to check occlusal clearance on the distal side of the abutment.|
|Figure 5. Similarly, the mesial tip was then used to check the mesial side.||Figure 6. A radiograph, taken prior to cementation, confirmed the successful contact adjustment and complete marginal seating.|
The old, damaged PFM crown was cut and removed, and the occlusal clearance on the existing abutment was checked using the 1.0-mm PrepSure instrument. Using the angled mesial and distal sides of the instrument, the tips were passed over the top of the abutment while the patient’s jaw was closed. By gliding the tip over the cusps in this manner from buccal to lingual and back, the operator can immediately detect any spot on the abutment that has been inadequately prepared via resistance against the tip’s passage. Surprisingly, resistance was immediately detected between the abutment and the adjacent tooth, demonstrating that less than 1.0 mm of occlusal clearance had been prepared. While 1.0 mm is a viable thickness for high-strength zirconia, the less than 1.0 mm of occlusal reduction that had been created for the previously placed PFM crown all but guaranteed its limited survival time.
Additional reduction of the tooth structure was performed to meet the 1.0-mm minimum occlusal requirement for the new zirconia crown. While doing the occlusal prep reduction, the 1.0-mm PrepSure was frequently used to check for the point at which adequate space had been created (Figures 4 and 5). This method of actively monitoring the creation of sufficient clearance also helps prevent over-reduction, thus preserving as much of the patient’s own tooth structure as possible. Once the necessary space was made, the tip was able to glide across the cuspal waves of the prepared abutment without resistance.
After the crown was successfully fabricated and returned by the dental laboratory team, it was tried in the patient’s mouth. Using a single-handed diamond strip (Black Diamond Strip [ContacEZ]), the proximal contacts of the new zirconia crown (NexxZr+ [Sagemax]) were adjusted to ensure interproximal relief and complete marginal seating5 (Figure 6). This step is especially noteworthy, as ensuring the crown is fully and securely seated not only prevents iatrogenic bacterial buildup on exposed dentin from an open margin but also helps minimize the need for occlusal refinement. Improperly seated crowns will be raised by a small amount off the abutment, necessitating the removal of some amount of the coronal surface to restore proper occlusion. Achieving complete marginal seating prevents this situation, maximizing the structural integrity of the restoration.
Once accurate fit was verified, the crown was then cemented (GC Fuji PLUS [GC America]) into place. The occlusion was checked and adjusted as needed, and the patient was dismissed.
With the provision of crowns being one of the most common procedures and revenue generators in many dental practices,6 the ability to minimize failures and provide long-lasting, aesthetic restorations is vital. This is especially true during the difficult economic circumstances dentists currently find themselves in as the COVID-19 pandemic continues to disrupt normal operations. Avoiding the waste of expensive materials and valuable chair time can help to boost the profit margin of a practice during times like these. Furthermore, this is without mentioning the long-term financial benefits of making sure patients stay satisfied over time with the new crowns that have been delivered to them.
However, beyond the financial element, ensuring the accuracy of every crown preparation also speaks to the heart of why many of us are dentists. As dedicated healthcare providers, we pride ourselves on giving our patients the highest quality of care possible and take a great deal of personal and professional satisfaction in knowing that our work can restore beauty and proper function for years to come. To rely on just the naked eye to confirm the accuracy of our crown preps is to knowingly risk that quality of care by potentially providing thin, fragile, and ultimately short-lived crowns.
With all of this taken into consideration, there simply is no reason not to use some reliable means of confirming the accuracy of the preparation. All of the currently available products on the market today are inexpensive enough that, if they save you even a single second impression or one crown redo, they will likely have paid for themselves for the next several years. Not only that, but any method of confirming adequate clearance on your preps is likely so quick and easy that there would be virtually no negative impact on the efficiency of your day-to-day procedures. Regardless of what technical solution you may choose, you owe it to your practice, your patients, and yourself to take this one small step to ensure every crown is the best it can be.
- Whitworth JM, Walls AW, Wassell RW. Crowns and extra-coronal restorations: endodontic considerations: the pulp, the root-treated tooth and the crown. Br Dent J. 2002;192:315-327.
- Zimmermann M, Egli G, Zaruba M, et al. Influence of material thickness on fractural strength of CAD/CAM fabricated ceramic crowns. Dent Mater J. 2017;36:778-783.
- Ram HK, Shah RJ, Agrawal HS. Evaluation of three different tooth preparation techniques for metal ceramic crowns by comparing preparation depths: an in vitro study. J Indian Prosthodont Soc. 2015;15:162-167.
- Aminian A, Brunton PA. A comparison of the depths produced using three different tooth preparation techniques. J Prosthet Dent. 2003;89:19-22.
- Kim DS, Suh KW. A proximal contact adjustment and interproximal relief method. J Prosthet Dent. 2007;97:244-245.
- State of the Cosmetic Dentistry Industry: 2019 Survey Report. American Academy of Cosmetic Dentistry; 2019:16-18. https://aacd.realmagnet.land/SOI-survey-2019.
Dr. Kim is in private practice in Vancouver, Wash. He lectures at dental scientific sessions, seminars, and study clubs. He is a former part-time faculty member at Yonsei University College of Dentistry and a visiting professor at Korea University College of Medicine, both in Seoul, South Korea. He can be reached via email at firstname.lastname@example.org.
Disclosure: Dr. Kim is the inventor and founder of ContacEZ.
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Instruments Monitor and Confirm Adequate Clearance in Crown Preparation