Magnification so what! Case report 2. Precision Tooth Preparation. Dr. Shamir B. Mehta BDS BSC MClinDent(Prosth) GDP Harrow, Senior clinical teacher & Deputy Director MSc Aesthetic Dentistry, King s College London. Dr.Subir Banerji BDS MClinDent(Prosth) Private practice Ealing London, Director MSc Aesthetic Dentistry and Senior Clinical Teacher King s College London Dental Institute. As conscientious restorative dental practitioners we face the daily quandary of balancing the aesthetic needs and demands of our patients with that of providing restorations that are functionally stable and respectful of the biological health of the dental hard and soft tissues respectively. It is well accepted that the preparation of teeth to receive full coverage crown restoration is associated with a significant risk of irreversible pulp tissue damage (1). Furthermore, it has been shown that approximately 60% to 70% of the coronal structure of a tooth may be lost when preparing a tooth to receive full coverage porcelain fused to metal and all ceramic crowns respectively (2). Consequently, dental manufacturers are continually evolving their products so as to attain superior aesthetics with the least compromise to healthy tooth tissue. As a result of the rapid rate at which new ceramic and resin based materials are being introduced into the marketplace, the dental practitioner is forced to continually update their knowledge of these novel products (all too often just after having developed the confidence and competency with a given system). The latter can place considerable demands on the contemporary dental practitioner. At King s College London, postgraduate students enrolled on the MSc in Aesthetic Dentistry programme receive copious hands-on training on the art and science of precision tooth preparation. The latter crudely refers to the ability to prepare teeth to receive any form of indirect restorations to comply with the exact requirements for the material and cementation as per the manufacturer s instructions. There is no doubt that failure to prepare teeth to the required dimensions, clearance, taper, resistance and retention form according to a given prescription will culminate in an indirect restoration which is either mechanically (and functionally) sub-optimal, displays poor aesthetic value, or may not be conducive to supporting and maintaining the biological health of the dental hard and soft tissues respectively. In essence, the latter will result in a restoration which will most probably be doomed to premature failure. Whilst there are a number of basic tools available to assist with the concept of precision of tooth preparation such as well fabricated pre-operative indices, callipers, depth reduction burs etc., as shown by Figure 1, there is absolutely no doubt that the value of optical magnification and superior illumination cannot be over emphasised as part of the process.
Figure 1. Selection of burs which may be used to accurately plan levels of tooth reduction. The use of optical magnification in other medical disciplines is by no means a novel concept. Whilst the use of optical magnification in the field of endodontic practice has been popular for the past 15 years or so (3), it is not uncommon to find an operating microscope in the restorative dentist s outfit, as shown by Figure 2, where there is a quest to attain superior standards of clinical care, and the ultimate goal of well formed, bacteria free sealed margins. Indeed, it has shown that been the resolution power of an unaided eye is 0.2mm (3), which is ten times less than the film thickness of conventional crown and bridge cements (0.025mm) rendering it almost impossible to ascertain excellent marginal adaptation of indirect restorations to the underlying tooth preparation without any form of magnification. The use of a common optical microscope can raise the resolving limit from 0.2mm to 0.006mm. Figure 2. Use of Carl Zeiss optical microscope for restorative dental practice. It has been suggested that the use of head/loupe mounted high quality LED light sources can increase the level of illumination attained by a standard dental chair side operating light by a factor of four (3). Optical magnification and enhanced illumination are particularly helpful when planning and preparing restorative margins, avoiding iatrogenic damage to the adjacent hard tissues (and restorations), supporting soft tissues, determining the exact levels of tooth reduction, taper, retention and resistance form, ascertaining the accuracy of any dental impressions, the fit and adaptation of both the provisional and definitive restorations respectively. This has been illustrated by two cases below. Case One, is that of a male patient presenting with pain and swelling from a 4 unit double cantilevered bridge at LL3,4,5 and 6 (with LL3,LL4 as bridge retainers) Figure 3. Following the removal of the bridge (by sectioning) it was noted that the LL3 had fractured off to gingival level. Electro-surgery (Sensimatic, Electrosurge, Parkell) was performed to increase the height of the clinical crown (so as to attain a ferrule of the desired dimension), and root canal therapy performed at this tooth, Figure 4. Provisional custom indirect (laboratory fabricated), single unit crowns (Ceramage, Shofu) were placed at LL3 (post retained; temporary stainless steel Parapost XP, Coltene, Whaledent) and LL4 Figure 5.
Following a period of 6 weeks, so as to permit soft tissue resolution and an opportunity to assess the efficacy of the endodontic therapy, LL3 was prepared to receive a cast gold alloy post and core, and porcelain fused to metal crowns placed to definitively restore LL3 and LL4 as individual units. Impressions were taken using President Putty and wash (Coltene, Whaledent). The cast gold post and core was cemented in initially (using RelyX automix, 3M, ESPE) Figure 6, and an impression taken thereafter for the fabrication of the definitive crowns. Working casts were mounted on a semi-adjustable articulator, so as to copy the occlusal prescription established with the provisional restorations, with the aid of a customised anterior guidance table, as depicted by Figure 7. Hence, occlusal stops were provided in the intercuspal position at both LL3 and LL4, and the patient was restored with left lateral excursive guidance provided by the LL4 and no non-working side contacts on either the LL3 or LL4. The crowns were cemented using a glass ionomer cement, Aquacem (Dentsply), Figures 8. The entire case was performed using Carl Zeiss, EyeMag ProF 4.5 to 5.0 optical loupes and EyeMag LED Light, (Figure9). The use of the latter proved particularly useful in attaining the desired preparation form, avoidance of damage to the hard and soft tissues, assessing the fit and form the provisional and definitive restorations and the working impression. The crowns have been designed with rest seats, guide planes and necessary undercuts (in porcelain at the patient wishes) for a subsequent tooth and mucosally supported removable partial denture. Figure 3. Pre-operative view of failed 4 unit double cantilevered fixed bridge. Figure 4 Right. Abutment fractured to gum level. Figure 5. Provisional crowns cemented and occlusion checked.
Figure 6. Cast post and core in situ; 6 weeks post-gingivectomy. The LR3 has been prepared with adequate ferrule. Figure 7 Anterior guidance table used to copy the occlusal prescription as established with the use of the custom indirect provisional restorations. Figure 8. Completed restorations in situ; individual porcelain fused to metal crowns. Figure 9. Zeiss EyeMag Pro (F) and LED light used in clinical practice. TBI The use of onlays (both of the conventional and adhesive variety; metallic and non-metallic variety) in restorative dentistry has been well documented recently. The popularity of the adhesive onlay restoration has been enhanced in recent times with concurrent advances in ceramic and resin technology, together with an increased desire by dentists and patients alike to reduce the need for tooth preparation. It has been estimated that approximately 40% of coronal tooth structure may be compromised when preparing a tooth to receive an onlay restoration; whilst this statistic appears rather gloomy, it contrasts favourably to that of 60-70% for full coverage tooth coloured crown restorations (2). The use of adhesive resin lutes to provide retention form (as opposed to the need to rely on mechanical tooth preparation form, which frequently often requires the removal of healthy, sound dental hard tissues), is most likely to culminate in an even more conservative preparation, where adhesive onlay restorations are concerned. In summary, the applications of onlay restorations include: The treatment of worn posterior occlusal surfaces (4)
The management of cracked tooth syndrome (5) Restoration of posterior teeth where hard tissue may have been lost as a consequence of trauma Where there is a need to provide cuspal coverage where two or more walls remain intact (6) Regardless of the material used to fabricate the restoration, it remains doubtless that success with such restorations is based on the operator having an excellent appreciation of the principles of adhesive dentistry such as effective moisture control and tooth conditioning, occlusal dynamics and the presence of a copious quantity and quality of tooth enamel. However a poor tooth preparation, resulting in a less that optimal fit will culminate in a resin lute film thickness of resin which is mechanically sub-optimal, as well as a poor contact angle being formed between the resin lute, tooth preparation and restoration respectively will culminate in premature failure. The use of optical magnification and illumination can prove to be very helpful in planning, making, placing and evaluating the adhesive onlay restoration. Case Two, Figure 10 is that of a gold adhesive onlay restoration replacing a failed amalgam in a molar tooth using the concepts of minimal precision preparation, with the aid of a Carl Zeiss operating microscope. The restoration was cemented using Panavia F (Kuraray, Japan). Figure 10. Cast gold adhesive onlay restoration. In conclusion, the use of optical magnification and enhanced illumination are key components in enabling the dental operator to undertake high quality, precision tooth preparation. References: 1. Edelhoff D, Sorenson, J. Tooth structure removal associated with various preparation designs of posterior teeth, Int J Perio Restor Dent. 2002; 22: 241-249. 2. Saunders WP, Saunders EM. Prevalence of periradicular periodontitis associated with crowned teeth in an adult Scottish subpopulation. Br Dent J. 1998; 185: 137-140. 3. Carr GB, Murgel CAF. The use of the operating microscope in endodontics. Dent Clin N Am 2010; 54: 191-214. 4. King PA. Adhesive techniques. Tooth Surface Loss chpt 5. BDJ Books 2000. 5. Banerji S, Mehta SB, Millar BJ. Cracked tooth syndrome part 2: Restorative options for the management of cracked tooth syndrome. Br Dent Jour 2010; 208: 503-514. 6. Yap A. Cuspal coverage with resin bonded metal onlays. Dental Update 1995; 403-406. For more information please call Nuview on 01453 872266,
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