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Single Tooth Prosthetic Restoration Through Surgical Crown Lengthening, Conservative Therapies and CAD-CAM Milled Restoration in Lithium-Disilicate. A Case Report

Objective The restorative treatment of a decayed, non-vital upper premolar often requires an interdisciplinary approach. Esthetics and the entity of the masticatory loads are usually affecting the choice of procedures and materials. A conservative treatment in a case with esthetic needs and functional requirements is investigated in this case report. Materials and methods A case of a 45- year- old woman with a severely decayed upper premolar is presented. The tooth has been previously treated in an emergency department with a provisional endodontic medication, but a further treatment has been then performed. The tooth was painful at the moment of the visit and the carious lesion appeared to invade the subgingival level. A surgical-conservative approach has been selected involving crown lengthening, endodontic treatment and a prosthetic crown with a monolithic Lithium-Silicate. Results The rehabilitative process required 3 months, including maturation times after surgery and the placement of the provisional crown. The patient claimed to be both esthetically and functionally satisfied with the restoration. Conclusion A conservative treatment of a severely involved tooth requires a critical evaluation of the remaining structures and a precise selection of the restorative materials. A monolithic crown could represent an ideal solution for restoring an upper premolar requiring esthetics and solid function, but the choice of a high translucent, easily polishable and possibly even chair-side prosthetic material might be an appreciable added value for clinicians.


Introduction
The restoration of serious tooth decay often requires an interdisciplinary approach. For example, the dentist can be required to restore esthetics and function of an upper premolar, where the esthetics and the entity of the masticatory loads are often affecting the choice of procedures and materials and even general prognosis of the tooth.
In fact, there are lots of influences to consider in this case: position and size of the cavity; requirement of endodontic treatment; size of the future restoration; requirement of endodontic post; requirement of prosthetic rehabilitation; position of the bone crest; length of the root; status of teeth adjacent to a single-tooth; oral hygiene and the patient's compliance; cost.
In many cases, when the rehabilitation plan of a premolar includes different and complex treatments such as endodontic treatments, posts or extensive restorations, the dentist tends to choose an implant-supported prosthetic rehabilitation, because the costs of the above mentioned conservative treatments could be similar to those of the implant-supported ones, but the overall prognosis of a tooth-supported prosthesis may be, in the mind of professionals and in the literature, lower than that of a crown over implant (1,2). The prognosis of both rehabilitations is strongly affected by different factors such as the surgeon, his skills, used materials, the patient's habits, and a definite conclusion concerning the best solution to the problem; either endodontic treatment or implant-supported therapy, still cannot be reached (3).
However, the rehabilitative treatment of a natural tooth is usually faster and the conservative treatment represents, biologically and when possible, always the best choice. In this case, the right selection of prosthetic materials could be decisive.

Case report
A 45-year-old woman patient, B.S., visited the dental clinic of the IRCCS Istituto Ortopedico Galeazzi (Milan, Italy) reporting the pain in her upper left dental arch. When her medical history was taken, she stated that she was not a smoker, was not affected by any chronic disease and was in good health.
During the clinical oral examination, she did not exhibit any mucosal lesions, she presented a good level of oral hygiene but there were numerous incongruous restorations, especially a temporary-like restoration on tooth #25.
This tooth was not stimulated with the cold test and the radiographic image revealed an incomplete endodontic treatment (Figures 1-2). The tooth seemed to present a suitable root length for a conservative restoration, but the apical position of the carious lesion and the proximity of the interdental bone ridge did not allow a correct rehabilitation with prosthetic crown, respecting the biological width (4).

Operational phases
The first step consisted of the removal of the decayed tissue of the distal face of the tooth 25 in its coronal portion and in the first third of the root.
The cervical limit of the decay has been exposed, then the wall has been reconstructed with a glass ionomer material (EQUIA Forte™, GC Corp, Tokyo, Japan) coated with the light curable EQUIA Forte Coat™ to obtain a more resistant material even in case of occlusal loads (12)(13)(14). Glass ionomer material has been chosen because the apical edge of the tooth cavity was under the gingival margin; therefore, it was impossible to obtain a correct isolation for a composite restoration. It has been confirmed that the glass ionomer materials can tolerate acid and humid environments more than composites (12)(13)(14).
The EQUIA Forte was selected among different glass ionomer cements since it has been reported in the literature that it has better long-term results (12)(13)(14)(15)(16).
After that, the endodontic treatment of the tooth has been performed. The tooth had only one root canal, processed with a hand file READY STEEL K-File™ (Dentsply Sirona, Italy) and, for the shaping and refinement, with mechanical file PROTAPER GOLD™ (Dentsply Sirona, Italy) at a working length of 20 mm.
The sealing of the canal has been performed with a Ther-mafil™ cone (Dentsply Sirona, Italy) with an apical diameter of 0.30 mm. (Figure 3) The third step of the treatment consisted of the crown lengthening, necessary to expose an adequate part of the root, to obtain, after healing, the correct adhesion of the composite for the pre-prosthetic restoration and the successive prosthetic rehabilitation.
After the surgical flap elevation of the tissue and the bone remodeling, the flap was repositioned apically and sutured with vertical mattress suture anchored in the periosteum. The suture was removed after 7 days.
During the fourth phase, after waiting for a postsurgical healing time of 4 weeks necessary for the correct maturation of the tissues, the glass ionomer and coronal part of the endodontic material has been removed with the Gates Glidden™ cutters (Dentsply Sirona, Italy) with 01-02-03 size, it has been inserted a glass fiber post with a medium size truncated cone form, Anatomical Post (DENTALICA, Italy), fixed by a self-adhesive dual-cure cement (GCem LinkAce™ translucent, GC Corp, Tokyo, Japan). The permanent composite restoration was completed with the Gaenial Posterior™ shade A3 composite (GC Corp, Tokyo, Japan) bonded with its respective self-etch adhesive (Gaenial Bond (™)).
After the restoration, the tooth was prepared with the B.O.P.T technique and a provisional crown in PMMA, obtained with an optical scan done before the preparation of the element with a AADVA IOS100 scanner (GC Corp, Tokyo, Japan), was placed. During this phase the old restorations of the teeth 24 and 26 were replaced (Figures 4-5), (10,11).
After another 4 weeks, the refinement of the prosthetic abutment was performed and the definitive dental impression with Polyvinylsiloxan-Ether (PVS-E, Exa'Lence™, GC Corp., Tokyo, Japan) ( figure 6) was taken. The dental im-
pression was sent to the laboratory, where it has been optically scanned and a CAD-CAM path was set.
As a consequence of the new surgically modified prosthetic margin, the appropriate length of the abutment for an adhesive cementation has been obtained, and the highly esthetic requirement and the contemporary need of covering costs with a monolithic crown, led to the clinical choice of lithium-disilicate crown with high translucency, which is suitable for the CAD-CAM technology.
The chosen material was a lithium disilicate block (GC Corp, Tokyo, Japan), with an ultra-thin structure and two important advantages: first of all, the selected block was made for chairside use and it was easy to be milled, and secondly this kind of materials do not require other steps in the oven to be sintered or glazed. In fact, the selected block is in the group of those completely crystalized, hence the time for its crystallization could be saved and the software for the finishing and glazing did not have to compensate any material contraction due to the temperature in the crystallizing oven. 17 In this way, the margins remain extremely thin and clear and it was particularly useful in our case. Also, it was possible to reduce costs compared to the use of a high esthetic and functional material.
Additionally, the ultra-thin structure of selected lithium disilicate blocks permits an easily polishing of the restoration even after the occlusal adjustments, leaving the area extremely uniform and smooth. This reduces the finishing times, the brightness lasts longer and the occlusal contacts produce less abrasion on the restoration and on the antagonists (Figure 7).
Regarding the luting phase, the tooth abutment was etched with 37% orthophosphoric acid for 40 seconds, rinsed and then dried with compressed air. The definitive crown was etched with 9% hydrofluoric acid for 40 seconds, rinsed and dried with compressed air. Since the acid etching with the hydrofluoric acid may result in the formation of crystals of lithium salts on the inner surface of the crown 18 , it is important to put the crown in hot water for 1 minute after the acid removal, to eliminate the crystals and prevent any interference with the adhesive cementation, and then it must be carefully dried. Before the placement of the adhesive cement, a specific coupling agent was placed in the internal face of the crown in order to obtain a stronger adherence between the ceramic and the luting resin. For this reason the G-Multi Primer (GC Corp, Tokyo, Japan) was selected and applied. The luting phase was finally performed with the G-CEM Link-Force™ (GC Corp, Tokyo, Japan), after the placement of the dedicated adhesive system (G-Premio Bond™) on the adhesive surfaces of tooth and crown, blowing them with compressed air for 20 seconds, without curing them before the luting phase in order to allow the correct fitting of the crown. 40 seconds of curing time for each crown surface have been executed (120" in total in order to correctly cure both adhesive and resin cement even through the ceramic) and the excess of the luting material have been finally removed.

Konačni rezultat
Na kraju postupaka restauracija je točno sjedila na razini gingivnih rubova i činilo se da je morfološki integrirana phologically integrated in the dental arch, with correct contact points and with a good chromatic match with the adjacent elements (Figures 8-9). If we look at the x-ray image, it is possible to see that the sub-gingival margins are also integrated, without any step or any plaque-retaining area that could be a problem for the home dental hygiene (Figure 10). The patient did not report any pain and was completely satisfied with the prosthetic rehabilitation. She stated that the restoration was perfectly integrated at both functional and chromatic levels. She also believed that the color of the prosthesis was better than the color of her natural teeth. Also, she has requested information about tooth whitening.

Discussion
This case presents a very frequent case that dentists face during their practice. Due to their position and function, premolars are often affected by tooth decay; hence there is a need for endodontic treatments, posts and even extensive restorations. In this case report, the dentist had to primarily solve pain, consequent to an incomplete endodontic treatment, but then to guarantee enough solidity to whatever is the planned, final rehabilitation. CAD-CAM technology and chairside procedures, including the milling process that takes place completely inside the dental clinic, are actually a valuable reality and these kinds of digital machines have been easily available for many years (12,17,19).
In this specific case, the choice has been directed towards a lithium-disilicate ceramic block for the manufacturing of both an esthetic and durable crown. The lithium-disilicate is in fact an ideal material for restorations in molars and premolars, having a very high flexural strength and compressive resistance, but preserving highly esthetic properties, such as translucency, proper light reflection, natural opacity and fluorescence. Even the choice of a monolithic material such as presented in our case is a clear example of how these categories of new millable materials can fit even if there is a demand for esthetic results.
The required time for the complete crown fabrication inside the dental clinic, from designing, milling to finishing, may vary depending on the material: the required fabrication time ranges normally from 1 hour (for the simplest materials) to more than 4 hours (for the materials that after milling require more tests and passages in finishing furnaces. For this reason, and due to the diffusion of CAD-CAM procedures in laboratories, chairside procedures are actually less appreciated by many dentists, who see them as a possible waste of time, diverting them from the real clinical activity 12 . However, the reliability of modern CAD-CAM systems and new materials allow dentists and laboratories to make new choices, even in the name of a smoother workflow and cost control, when possible. A millable material that has considerable esthetic properties is in any case indispensable, whether a clinician decides for a full chairside procedure, or for sending the impression in the laboratory. Also the "monolithic" materials cannot actually afford to be opaque, not very natural and not very translucent, because very few professionals and patients are inclined to accept esthetic compromises in modern times u zubni luk, s ispravnim kontaktnim točkama i dobrom podudarnošću boje sa susjednim zubima (slike 8. i 9.). Na rendgenskoj snimci može se vidjeti da su i rubovi ispod gingive integrirani, bez stepenice ili prostora za zadržavanje plaka koji bi mogli biti problem tijekom održavanja higijene kod kuće (slika 10.). Pacijentica se više ne žali na bolove i zadovoljna je protetičkom rehabilitacijom za koju smatra da se savršeno integrirala i s funkcionalnog i s estetskog stajališta. Također vjeruje da je boja krunice ljepša od prirodnih zuba te se raspitivala o izbjeljivanju.
The patient in this case report represents a stereotype of a highly demanding patient. This normally does not mean that she necessarily needs perfect esthetics, but the resolution of the initial problems (spontaneous pain, also stimulated on chewing, trapping of food in proximal areas, marginal inflammation) is mandatory to achieve the success. Endodontic treatment is the main part of the rehabilitation, because only an appropriate endodontic therapy can solve the pain and permit a solid restoration. However, the position of the restoration, very close to the marginal crest, and the need of performing adhesion for the luting of both composite and ceramic, suggest to clinician to carefully evaluate all the possible complications during the prosthetic procedures, since the wrong positioning of the crown margins could lead to chronical inflammation, bone resorption, pain and definitely to tooth loss. The choice of performing a surgical procedure such as the crown lengthening is a simple, strategical step that normally allows creating in a single appointment all the ideal conditions that a complex tooth such as an upper premolar is not offering after extensive tooth decay. In this case, the dentist must respect all ideal healing times, in order to allow the periodontal ligament to recreate a physiologic architecture that normally occurs after at least 21 days of healing process. For this reason, after the surgery but also after the initial preparation for the provisional crown, the authors of this case report decided to wait for 4 weeks (28 days).
The choice of the BOPT technique has been identified as the ideal one for this specific case. First, the mesial carious lesion is close to the bone crest, but also involving the root (Figure 2). In that area, a shoulder preparation requires a huge waste of tooth structure in respect to a vertical preparation. Even in the case the crown lengthening, this sacrifice of sound tissue cannot be limited. A vertical preparation on the other side can be limited to the surface areas, with minimal sacrifice of root tissues, and a good control during luting procedures. Even a margin relocation using a glassionomer can be considered in this case, but the depth of the lesion and good control after crown lengthening suggested to put directly the crown margin on the sound tissue of the root. The CAD design in case of a vertical preparation of teeth normally requires higher skills from the technician, since good fitting and lack of steps, notches and/or marginal chippings are fundamental for the biological integration of the crown under the gingival margin.

Zaključci
Monolitna CAD-CAM rješenja čine se kao pouzdano rješenje za protetičku rehabilitaciju, čak i u područjima u kojima bi estetika, translucencija i refleksija prirodne svjetlosti mogle biti primarno važne za pacijentovo zadovoljstvo. No izbor materijala idealnih svojstava za određeni slučaj i dalje ostaje najvažniji. Idealan monolitni materijal mora ima-terial must have the following characteristics, which a dentist must know in order to make an appropriate choice for esthetic and functional rehabilitations: Availability of different shades; Adequate translucency; Ease of processing and milling (little wear on milling burs and machinery); High mechanical strength; Durable in physiologic and pathological oral environment (16,19). Possibility of simple luting with most common adhesive systems or cements; Quick intraoral polishing phase; Availability for the most common chairside and laboratory milling machines (compatibility).
Only a good mix of these characteristics can determine a clinical success of a prosthetic rehabilitation procedure with monolithic materials and, ultimately, full patient satisfaction.