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Available online at www.sciencedirect.com

ScienceDirect journal homepage: www.intl.elsevierhealth.com/journals/jden 1 2 3

Strain measurements and fracture resistance of endodontically treated premolars restored with all-ceramic restorations

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Liang Lin Seow a,*, Chooi Gait Toh a, Nairn H.F. Wilson b a b

School of Dentistry, International Medical University, 126, Jalan Jalil Perkasa 19, 57000 Kuala Lumpur, Malaysia King’s College London Dental Institute, Guy’s Hospital, London SE1 9RT, England, United Kingdom

article info

abstract

Article history:

Objectives: The aim of this study was to investigate the recovery of cuspal stiffness and

Received 27 June 2014

fracture resistance in endodontically treated maxillary premolars restored with bonded

Received in revised form

ceramic inlays and onlays of various designs.

29 September 2014

Methods: Seventy intact premolars were selected for this study; six cavity designs were

Accepted 1 October 2014

investigated: (i) mesio-occlusal-distal (MOD) inlay (I), (ii) MOD inlay with palatal cusp

Available online xxx

coverage (IPC), (iii) MOD onlay (O), (iv) MOD inlay with pulp chamber extension (IPE), (v) MOD inlay with palatal cusp coverage and pulp chamber extension (IPCPE), and (vi) MOD

Keywords:

onlay with pulp chamber extension (OPE). Intact teeth acted as control. Strain gauges were

Strains

attached to the buccal and palatal surfaces of the teeth to measure cuspal stiffness under

Fracture strengths

static loading. All specimens were eventually subjected to compressive load to failure.

Endodontically treated teeth

Cuspal stiffness and fracture resistance data were analyzed using ANOVA and Tukey test.

Inlay

Results: The I and IPE restorations restored cuspal stiffness to 75% of the sound tooth value.

Onlay

The O and OPE restored teeth had stiffness values greater than that of a sound tooth. The I,

Pulp chamber extension

IPC, O, IPE, IPCPE and OPE restored teeth demonstrated fracture strength values of 938 N  113 N (s.d.), 1073 N  176 N and 1317 N  219 N, 893 N  129 N, 1062 N  153 N and 1347 N  191 N respectively. Conclusions: Within the limitations of this study, it was concluded that the all-ceramic onlay or inlay with palatal cusp coverage provided best biomechanical advantage in restoring an endodontically treated maxillary premolar tooth. Clinical significance: The onlay approach which is more conservative compared to full coverage restoration is considered an appropriate approach to the restoration of endodontically treated maxillary premolars. The addition of a pulpal extension to the all-ceramic restorations, apart from being technically challenging, was not found to offer any biomechanical advantage to the restored teeth. # 2014 Published by Elsevier Ltd.

11 9 10 12 13 * Corresponding author. Tel.: +60 3 86567228; fax: +60 3 86567229. E-mail address: [email protected] (L.L. Seow). http://dx.doi.org/10.1016/j.jdent.2014.10.001 0300-5712/# 2014 Published by Elsevier Ltd.

Please cite this article in press as: Seow LL, et al. Strain measurements and fracture resistance of endodontically treated premolars restored with all-ceramic restorations. Journal of Dentistry (2014), http://dx.doi.org/10.1016/j.jdent.2014.10.001

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1.

Introduction

Traditionally, endodontically treated posterior teeth tended to be restored with the aid of a radicular metallic post in the unsubstantiated belief that a rigid post would reinforce the tooth.1–3 The use of a rigid, metallic post in the restoration of an endodontically treated tooth is now known to weaken rather than strengthen remaining tooth tissues with post canal preparation4,5 and post placement being associated with the risk of root perforation and radicular fracture in clinical service. One of the important criteria to ensure clinical success of endodontically treated teeth is the provision of cuspal coverage. This has been shown in a retrospective study done by Sorensen and Martinoff2 – a study which include 1273 endodontically treated teeth restored with/without coronal coverage restorations. The study found that coronal coverage did not significantly improve the success of endodontically treated anterior teeth. However, the study found a significant improvement in the clinical success of maxillary and mandibular premolars and molars when coronal coverage restorations were present. Currently, adhesive techniques are widely used in the restoration of endodontically treated teeth to increase the stiffness of the restored tooth unit, and protect the restored tooth against fracture in clinical service.6–8 This has been considered to be of particular importance in the restoration of endodontically treated posterior teeth.9 The strengthening effect of adhesive restorative techniques in such situations has been shown to restore cuspal stiffness, often to a level comparable to that found in intact teeth.10–13 Clinical studies have provided supportive evidence, demonstrating the effectiveness of, in particular, various all-ceramic systems.14,15 To avoid post canal preparation and take advantage of the versatility of CAD-CAM technology in the restoration of endodontically treated posterior teeth, it has been suggested that the pulp chamber be used to create and retain an ‘‘endotype’’ crown– a ceramic crown extending into the pulp chamber for additional retention and resistance.16 The extension of restorations into the pulp chamber, with or without conservation of coronal tooth tissues8,17 may, however, adversely affect the biomechanical behaviour of the restored tooth unit.18 In this laboratory-based study, endodontically treated maxillary premolar teeth were restored with bonded ceramic inlays and onlays, including designs having pulp chamber extensions to investigate the recovery of cuspal stiffness and fracture resistance of the restored tooth units. The aim of the study was to identify the design of all-ceramic restoration best able to restore the biomechanical properties of a maxillary premolar tooth subsequent to endodontic treatment. The specific hypothesis was that the addition of a pulpal extension to certain designs of all-ceramic restorations would offer no advantage in terms of the stiffness and fracture resistance of prepared and restored endodontically treated maxillary premolar teeth.

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Seventy sound, single-rooted maxillary premolar teeth, extracted for either orthodontic or periodontal reasons, and

Materials and methods

which satisfied a series of selection criteria aimed at limiting the effects of tooth related factors, were collected and stored in 0.5% chloramine solution. The teeth were individually mounted perpendicular in cold-cure epoxy resin (Mirapox 950230, Miracon, Malaysia) to a level 2.0 mm apical to the cementoenamel junction (CEJ). The mounted teeth were divided at random into seven test groups of 10 teeth, each of which was assigned to receive one of the selected preparation designs: (a) mesio-occlusal-distal (MOD) inlay (I), (b) MOD inlay with palatal cusp coverage (IPC), (c) MOD onlay (O), (d) MOD inlay with pulp chamber extension (IPE), (e) MOD inlay with palatal cusp coverage and pulp chamber extension (IPCPE) and (f) MOD onlay with pulp chamber extension (OPE) (Fig. 1). The seventh group of teeth acted as a control group. The preparations for groups (d)–(f) extended into the pulp chamber to the level of the CEJ and have a butt joint with the composite restoration in the pulp chamber. Root canal treatment (RCT) was performed in all the intact teeth assigned for preparation. The laterally condensed guttapercha was removed to 2.0 mm below the CEJ. The access cavity was then fully restored by means of a bonded resin composite restoration (Prime & Bond1 NTTM, SPECTRUM1 TPH13, Dentsply, Germany). The preparations had an isthmus width of one-third of the intercuspal distance. The occlusal section was 2.0 mm deep, with the pulpal floor prepared at right angles to the long axis of the mounted tooth. The buccolingual width of each proximal box was half the width of the proximal surface of the tooth. The 1.5 mm wide gingival floors were placed 1.0 mm above the CEJ. The palatal cusp was reduced by 2.0 mm and the buccal cusp by 1.5 mm in the cuspal coverage preparations. The preparations with pulp chamber extensions were prepared apically to the level of the CEJ. The all-ceramic restorations were constructed from ProCAD1 blocks (Ivoclar Vivadent AG, FL-9494 Schaan, Liechtenstein) using a CEREC machine (Sirona Dental Systems GmbH, Bensheim, Germany). All the restorations were adhesively cemented using Prime & Bond NT and Calibra resin luting cement (Dentsply Caulk, Milford, USA). Strain gauges with an internal resistance of 120 V and gauge length of 1.0 mm (TML FLA-1-17-3L, Tokyo Sokki Kenkyujo Co., Tokyo, Japan) were bonded to the buccal and palatal surfaces of the teeth. During investigation the strain gauges were connected to a strain data logger (NMB Multipoint Data Processor DPU-103, MINEBEA Co. Ltd., Japan) to complete a Wheatstone bridge circuit, with the voltage output from the bridge being directly proportional to the deformation of the specimens. The output signal was recorded as microstrain (me). Strain readings of the teeth under axial static loading (150 N) were obtained at three stages: (1) prior to preparation of the tooth, (2) following endodontic treatment and cavity preparation, and (3) after restoration with the assigned bonded all-ceramic restoration. The strain data were then computed to obtain relative stiffness (RS) values19: Relative stiffness ðRSÞ maximum strain in the unaltered tooth ¼ maximum strain under the test condition These calculations provided unitless values. The sound tooth was given the value of one. Relative stiffness values of less than one reflected a decrease in the stiffness of the tooth,

Please cite this article in press as: Seow LL, et al. Strain measurements and fracture resistance of endodontically treated premolars restored with all-ceramic restorations. Journal of Dentistry (2014), http://dx.doi.org/10.1016/j.jdent.2014.10.001

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Fig. 1 – Preparation designs (a)–(f).

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with an increase in flexure. Conversely, a relative stiffness value greater than 1.0 reflected an increase in the stiffness of the tooth and a reduction in flexure. Following the successful completion of the non-destructive testing, the restored tooth units and control teeth were subjected to compressive loading, using an Instron Universal Testing Machine (Intron 4302, Instron Ltd., High Wycombe, England) operating at a cross head speed of 1.0 mm/min to obtain fracture resistance values in Newtons (N). The findings were analyzed using SPSS package version 14.0 to apply one-way analysis of variance (ANOVA) at p < 0.05 and Tukey (multiple comparison) tests.

3.

Results

3.1.

Relative stiffness

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The mean relative cuspal stiffness values for the buccal and palatal cusps of the prepared and restored tooth units are illustrated in Figs. 2 and 3 respectively. The standard inlay and inlay with pulp chamber extension approaches resulted in the greatest loss of stiffness (80%), compared to the other cavity designs. Although restoration of the standard inlay and inlay with pulp chamber extension cavities resulted in an increase in the stiffness of the buccal and palatal cusps to approximately 75% of the values obtained for a sound tooth, it was significantly less (p < 0.05) than that achieved with the other restorative approaches. Buccal cusps lost 50% of their stiffness when teeth were prepared for the inlay with palatal cusp coverage, standard onlay and onlay with pulp chamber extension. Including a

pulp chamber extension preparation resulted in a 65% reduction in the stiffness of the buccal cusps in the teeth prepared for the inlay with palatal cusp coverage. When the inlay with palatal cusp coverage and inlay with palatal cusp coverage and pulp chamber extension cavity preparations were restored, the buccal cusps were found to have statistically similar (p < 0.05) stiffness values, equivalent to 92% of a sound tooth (Fig. 2). The buccal cusps in the restored standard onlay and onlay with pulp chamber extension groups were found to be stiffer than the buccal cusps in sound tooth with relative stiffness values of 1.24 (s.d. 0.18) and 1.12 (s.d. 0.12) respectively. Preparation for the inlay with palatal cusp coverage, standard onlay, inlay with palatal cusp coverage and pulp chamber extension and onlay with pulp chamber extension, resulted in approximately a one-third decrease in palatal cusp stiffness (Fig. 3). The bonded all-ceramic restorations restored the relative stiffness of the teeth prepared for the inlay with palatal cusp coverage and the inlay with palatal cusp coverage and pulp chamber extension to a level comparable to that of a sound tooth (Figs. 2 and 3). Restoring teeth prepared for onlay and onlay with pulp chamber extension designs increased the cuspal stiffness to a level above that of a sound tooth.

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3.2.

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Fracture resistance

The mean compressive fracture strengths for each test group are shown in Fig. 4. The values obtained for the standard inlay (I), inlay with palatal cusp coverage (IPC), inlay with pulp chamber extension (IPE) and inlay with palatal cusp coverage and pulp chamber extension (IPCPE) were not significantly

Please cite this article in press as: Seow LL, et al. Strain measurements and fracture resistance of endodontically treated premolars restored with all-ceramic restorations. Journal of Dentistry (2014), http://dx.doi.org/10.1016/j.jdent.2014.10.001

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Fig. 2 – Relative stiffness values for the buccal cusps, according to the various preparation designs. Key: A, prepared standard MOD inlay (I); B, restored standard MOD inlay (I); C, prepared standard MOD inlay with palatal cusp coverage (IPC); D, restored standard MOD inlay with palatal cusp coverage (IPC); E, prepared standard onlay (O); F, restored standard onlay (O); G, prepared MOD inlay with pulp chamber extension (IPE); H, restored MOD inlay with pulp chamber extension (IPE); I, prepared MOD inlay with palatal cusp coverage and pulp chamber extension (IPCPE); J, restored MOD inlay with palatal cusp coverage and pulp chamber extension (IPCPE); K, prepared onlay with pulp chamber extension (OPE); L, restored onlay with pulp chamber extension (OPE).

Fracture resistance (N)

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Restored tooth

Fig. 3 – Relative stiffness values for the palatal cusps, according to the various preparation designs. Key: A, prepared standard MOD inlay (I); B, restored standard MOD inlay (I); C, prepared standard MOD inlay with palatal cusp coverage (IPC); D, restored standard MOD inlay with palatal cusp coverage (IPC); E, prepared standard onlay (O); F, restored standard onlay (O); G, prepared MOD inlay with pulp chamber extension (IPE); H, restored MOD inlay with pulp chamber extension (IPE); I, prepared MOD inlay with palatal cusp coverage and pulp chamber extension (IPCPE); J, restored MOD inlay with palatal cusp coverage and pulp chamber extension (IPCPE); K, prepared onlay with pulp chamber extension (OPE); L, restored onlay with pulp chamber extension (OPE).

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Cavity preparation designs Fig. 4 – Mean fracture resistance values (N) for the restored tooth. Key: I, standard MOD inlay; IPC, standard MOD inlay with palatal cusp coverage; O, standard onlay; IPE, MOD inlay with pulp chamber extension; IPCPE, MOD inlay with palatal cusp coverage and pulp chamber extension; OPE, onlay with pulp chamber extension; C, control. Please cite this article in press as: Seow LL, et al. Strain measurements and fracture resistance of endodontically treated premolars restored with all-ceramic restorations. Journal of Dentistry (2014), http://dx.doi.org/10.1016/j.jdent.2014.10.001

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different from each other (I cf. IPC p = 0.057; I cf. IPE p = 0.412; I cf. IPCPE p = 0.054), but significantly different from the values obtained for the standard onlay and onlay with pulp chamber extension (I cf. O p = 0.000; I cf. OPE p = 0.000). The values for the standard inlay and inlay with pulp chamber extension were significantly different from the values obtained for the control group (C) (I cf. C p = 0.000; IPE cf. C p = 0.000), but the values for the inlay with palatal cusp coverage, and inlay with palatal cusp coverage and pulp chamber extension were not significantly different from the control group (IPC cf. C p = 0.052; IPCPE cf. C p = 0.052).

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Clinical experience has long been recognized to indicate that restored teeth, in particular restored endodontically treated posterior teeth are prone to cuspal fracture with cuspal separation rarely occurring in intact teeth ,9 given the buttressing effect of the roof of pulp chamber and marginal ridges .20 Reeh et al.19 were amongst the first to report that the preparation of endodontic access alone had little effect on cuspal stiffness, compared to the effect of the preparation of a MOD cavity, which makes the palatal cusps of maxillary premolars especially prone to subsequent cuspal fracture .9 The use of dentine adhesive systems, together with resin luting cements have the ability to strengthen restored tooth units .7,21–24 The bonding of intracoronal restorations offers advantages in terms of cuspal splinting, decreasing cuspal flexure and strengthening of the remaining tooth structure.25,26 The loss of cuspal stiffness following preparation has been shown to be substantially, but may be restored with the placement of a bonded restoration .8,10,13,27 As a consequence, it was considered appropriate in the present investigation to restore the endodontic access cavities with adhesively bonded composite resin and to use dual-cured resin to lute the all-ceramic restorations, prior to the provision and investigation of the selected forms of allceramic restoration. Mounting strain gauges on tooth surfaces has been suggested to be a reliable means of measuring relative stress/strain generated by non-destructive occlusal loading .13,19,27 Bonding strain gauges to the buccal and palatal surfaces of the test teeth in the present study enabled nondestructive evaluations of the effects of different cavity preparations and approaches to restoration on the deformation of endodontically treated maxillary premolars under static, axial occlusal loading. Fracture resistance testing, using increasing compressive loading generated by a universal testing machine, has been criticized because the load required to fracture specimens tends to be non-physiological.28 Loads generated intraorally vary in magnitude, speed and direction, while loads applied to teeth in laboratory fracture resistance testing are generally constant in their speed of application and direction, and increase continuously until fracture occurs. Notwithstanding these considerations, both non-destructive and destructive testing methods were employed in the present study to best understand the effects of restoring endodotically treated

Discussion

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maxillary premolar teeth, using bonded restorations of different design. The MOD inlay (I) and MOD inlay with pulp chamber extension cavity (IPE) preparations were found to reduce the stiffness of the endodontically treated maxillary premolars by almost 80%. Previous studies found that maxillary premolars lost on average 53% of stiffness when prepared with a MOD cavity with an isthmus width of one-third of the intercuspal distance,13 and when an isthmus width of half of the intercuspal distance was selected, prepared premolar teeth displayed only 33% of the stiffness of the original tooth – a 67% loss in stiffness.27 When the I and IPE preparations were restored, the level of stiffness was found to be significantly lower (p < 0.05)than that of a sound tooth, fracture testing indicating that the inlay (I) and inlay with pulp chamber extension (IPE) restored only 75% of the strength of a sound tooth. The inlay only approach, including the IPE was therefore considered to give an unfavourable biomechanical outcome to the restoration of endodontically treated maxillary premolar teeth. The loss of stiffness of cusps following preparation for an inlay, without no provision for cuspal coverage may be explained by the effect of deflection of a cantilever beam.29 The depth of the preparation is the most critical factor in predisposing the prepared tooth to fracture.20,30 As the buccal wall of the cavity will be longer than the palatal wall, the buccal cusp will flex to a greater extent than the palatal cusp under axial occlusal loading. Whatever advantage may be gained by extending an all-ceramic inlay preparation into the pulp chamber, this will be offset by an increased risk of cuspal fracture. Cuspal coverage restorations are superior to intracoronal inlay restorations in protecting remaining tooth tissue from the effects of repeated occlusal loading and improve clinical success rate for posterior teeth.2 In the present study, restoring endodontically treated maxillary premolars with the various designs of resin bonded all-ceramic onlay restorations restored tooth stiffness to different levels. Restoration with the inlay with palatal cusp coverage (IPC) and the inlay with palatal cusp coverage and pulp chamber extension (IPCPE) resulted in substantial recovery of tooth stiffness, with the restored tooth units having a level of stiffness similar to that of a sound tooth. Also, these restored tooth units demonstrated fracture resistance not significantly different from that recorded for intact teeth. These findings suggested that the inlay with palatal coverage approach may be considered to be an appropriate biomechanical approach to the restoration of endodontically treated teeth, with the added advantage of preserving buccal tooth structure. The addition of the pulp chamber extension to the IPC was not found to offer any advantage. The resin bonded all-ceramic MOD onlay (O) and onlay with pulp chamber extension (OPE) restorations restored the stiffness of the endodontically treated maxillary premolar teeth to a level above that recorded for the intact tooth. This splinting effect has been demonstrated by other researchers .25,31 The MOD onlay (O) and onlay with pulp chamber extension (OPE) restored tooth units also exhibited fracture resistance values greater than that of the intact tooth, the difference was not, however, statistically significant ( p > 0.05).

Please cite this article in press as: Seow LL, et al. Strain measurements and fracture resistance of endodontically treated premolars restored with all-ceramic restorations. Journal of Dentistry (2014), http://dx.doi.org/10.1016/j.jdent.2014.10.001

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These findings demonstrated the effectiveness of the onlay approach in splinting and protecting the buccal and palatal cusps of the endodontically treated tooth. Again, no biomechanical advantage was found to be associated with the addition of a pulp chamber extension to the restoration. Retrospective clinical studies have demonstrated a favourable long-term prognosis for endodontically treated posterior teeth restored with a cuspal coverage restoration .2 The findings of the present study lend support to the possible reasons behind the favourable performance of onlay restorations in clinical service. The onlay approach, while requiring occlusal reduction sufficient to accommodate a viable thickness of ceramic, may therefore be considered to be an appropriate approach to the restoration of endodontically treated maxillary premolar teeth, with the advantage of being more conservative than a full coverage restoration .17 The selection between a full or partial (palatal) coverage onlay and full coverage restoration may be largely determined by aesthetic considerations, including the status and appearance of the cervical portion of the tooth. Where a favourable aesthetic outcome may be achieved using an onlay rather than a full coverage restoration, albeit possibly more technically demanding, preference should be given to the onlay. The rationale for the ‘‘endo-type’’ crown was based on the versatility of the machining capability of the CEREC system and opportunity to avoid the then norm of placing a post in one or more of the root canals of an endodontically treated tooth .16,32 It has been advocated that this endo-crown fits the concept of biointegration and is a restorative options for posterior endodontically treated and badly damaged molars .32 There has been suggestion to extend the concept to maxillary premolars .33 Apart from the technical difficulties, as experienced in the present study, of the CAD-CAM manufacture of well-fitting restorations with a pulp chamber extension, the findings of the present study indicate that the addition of a pulp chamber extension to an all-ceramic for the restoration of an endodontically treated maxillary premolar tooth offers no biomechanical advantage to the restore tooth unit, As a consequence the specific study hypothesis was confirmed.

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Within the limitations of the present study, it was concluded that, in restoring an endodontically treated maxillary premolar tooth by means of an all-ceramic restoration, best biomechanical advantage may be gained by providing an onlay or inlay with palatal cusp coverage. The addition of a pulpal extension to the all-ceramic restorations investigated, notwithstanding technical difficulties, was not found to offer any biomechanical advantage to the restored tooth units.

Conclusions

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Acknowledgment Q2 This study was supported by research grant from High Energy

Physics Group, University of Mississippi (UM03/213), Kuala Lumpur, Malaysia.

references

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Please cite this article in press as: Seow LL, et al. Strain measurements and fracture resistance of endodontically treated premolars restored with all-ceramic restorations. Journal of Dentistry (2014), http://dx.doi.org/10.1016/j.jdent.2014.10.001

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Please cite this article in press as: Seow LL, et al. Strain measurements and fracture resistance of endodontically treated premolars restored with all-ceramic restorations. Journal of Dentistry (2014), http://dx.doi.org/10.1016/j.jdent.2014.10.001

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Strain measurements and fracture resistance of endodontically treated premolars restored with all-ceramic restorations.

The aim of this study was to investigate the recovery of cuspal stiffness and fracture resistance in endodontically treated maxillary premolars restor...
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