Abstract
Objectives
Test the hypotheses that there are equivalent wear rates for enamel-versus-enamel and ceramic-versus-enamel, analyzing the in vivo wear of crown ceramics, their natural enamel antagonists, and the corresponding two contralateral teeth; and, that bite force does not correlate with the wear.
Methods
A controlled, clinical trial was conducted involving patients needing full coverage crowns opposing enamel antagonists. Bite forces were measured using a bilateral gnathodynamometer. Single-unit restorations of metal/ceramic (Argedent 62, Argen Corp/IPS d.SIGN veneer); or, core-ceramic/veneer from either, Empress2/Eris, or e.max Press core/e.max Ceram glaze (ceramics: Ivoclar Vivadent, USA) were randomly assigned, fabricated and cemented. Impressions were made of the ceramic crowns, as well as each maxillary and mandibular quadrant at one week (baseline) and one, two and three years. Resulting models were scanned (3D laser scanner). Maximum wear was calculated by superimposing baseline with annual images.
Results
There were a total of thirty-six crowns required for thirty-one patients. Each restoration had three associated enamel teeth: crown, (1) antagonist, (2) contralateral and (3) contralateral–antagonist. SAS PROC MIXED ( α = 0.05) indicated no statistical significance for mean maximum wear among crown ceramics, enamel antagonists and contralaterals. However, enamel wear was statistically significant in relation to intraoral location ( p = 0.04) and among years ( p < 0.02). Analyzed alone, the enamel contralateral–antagonist exhibited significantly greater wear ( p < 0.001). Considering all wear sites, there was no correlation with bite force ( p = 0.15).
Significance
The ceramics and their antagonists exhibited in vivo wear rates within the range of normal enamel. Future studies should examine the wear implications of the contralateral–antagonist enamel.
1
Introduction
Recent demand for esthetic improvement has led to the introduction of ceramic products for crowns and fixed dental prostheses (FDPs) before the clinical performances and limitations of these products have been fully explored and reported. One limitation of restorative ceramics is the abrasiveness to natural tooth structure that may accelerate the wear of natural enamel . Unfortunately, very little is understood of the mechanisms, occurrence of wear and patterns among individuals , especially since more clinical in vivo studies analyze survival rates and fracture rates, not wear . Wear occurs based on complex masticatory movements, as the jaw moves in different directions, and the patterns vary depending upon joint pathology, occlusion and muscle tone.
Given the complexity of our masticatory system, bite force, long considered a contributing factor to prostheses wear and survival, has been a point of interest since Borelli’s work “ De Motu Animalium ” in biomechanics in 1680 . Few measurement techniques represent the multiple directional forces and muscles used while elevating or depressing the mandible and maxilla during mastication . However, some gnathodynamometers do test molar mastication and maximum bite force bilaterally , such as in this study. Until recently, the vast majority of wear studies concentrated on composite wear conducted in vitro without involving bite force. Testing mechanical properties in vitro may be a preliminary indicator for events such as subcritical crack formation and failure over time . However, while this yields important material property information, there is essentially no scientific correlation for in vitro findings with clinical occurrences.
An in vitro method for measuring one of the seven types of wear as defined in the International Organization for Standardization (ISO) is a pin-on-disk system in which an experimental stylus (pin) of restorative material is placed in contact with a flat, rotating, enamel disk . The resulting decrease in stylus length is then quantified based upon specified loads and frictional forces. However, teeth have asperities and do not rotate on a 360-degree plane. Furthermore, the effects of solid and liquid dietary components cannot be simulated effectively. The in vivo wear process is multifactorial.
Given these considerations, three-dimensional laser scanners are used to measure intraoral wear using non-contact optical sensors on attentively fabricated replicate dental models . The scanners project a laserbeam through an optical system onto the target surface, scanning without physical contact within an estimated precision from 5 to 8 μm . Surface differences are quantified using reference-free digital subtraction analysis . While few in vivo studies have used this technology to date, two were relatively comprehensive . However, they have provided no correlation with the ceramic characteristics that may affect the materials’ abrasiveness.
This in vivo study was conducted to quantify the wear of natural enamel opposing three ceramic materials as influenced by the ceramic, observation time and bite force testing the following hypotheses: (1) wear for enamel-versus-enamel and ceramic-versus-enamel is commensurate; (2) wear loss for enamel antagonists opposing each veneer and core ceramics is similar; and (3) biting force does not correlate with the amount of wear.
2
Materials and methods
2.1
Study design
To analyze in vivo wear of both enamel and ceramic, a randomized, controlled, clinical trial was conducted. Prior to the single-blind pilot study initiation, three different commercially available ceramic systems were selected for fabricating full coverage restorative crowns as needed on a second premolar, first molar or second molar that opposed natural antagonist teeth in any arch. A random number table was formulated by the study’s statistician for assignment of the clinical restorative materials. Restorable teeth required a crown to root ratio of at least 1:1, with a full complement of opposing non-restored or minimally restored natural teeth. “Minimally restored” indicated that nothing more extensive than a Class II amalgam restoration was present in the opposing arch, and that there was a natural contralateral tooth.
Prior to the study, two investigators (dentists) were identified to prepare the restoration sites and place the crowns that would be fabricated by one professional commercial lab. Two different investigators (dentists), not involved in the crown preparations, were selected to evaluate the crowns post cementation.
2.2
Study population criteria
The clinical protocol for treating patients was approved by the Institutional Review Board of University of Texas Health Science Center at San Antonio (UTHSCSA). Flyers and e-mail advertisements were used to recruit patients meeting the following criteria: minimum age of 18 years; good overall general health with no contraindications to dental treatment; good overall dental health ( i.e ., no caries, periodontal disease or pocket depths in excess of 4 mm); no evidence of temporomandibular disorders ( e.g . clicking, popping, or pain on opening); no parafunctional habits ( e.g ., bruxism or clenching); good oral hygiene ( e.g ., compliant with instructions as determined by plaque present); normal saliva flow ( e.g ., no medical pathologies or chronic medication intake that limited salivary volume or flow); capable of paying the crown laboratory cost of two hundred dollars (USD); and, agreeable to three consecutive yearly appointments. The informed consent for each participant included the following baseline data: general medical history with physical examination, primary dental models, maximum bite force measurement, periodontal pocket depths and, periapical radiographs of abutment teeth.
2.3
Crown materials
The three restorative material combinations included: one metal/ceramic – (1) Argedent 62 alloy, Argen Corp., USA/IPS d.SIGN glass-veneer; and two all-ceramics – (2) IPS Empress 2 core ceramic/IPS Eris veneer; or (3) IPS e.max Press core ceramic/e.max Ceram glaze (ceramics, Ivoclar Vivadent AG, Schaan, Liechtenstein).
The gold-based, high noble metal alloy (Argedent 62) has a reported composition of 61.7% Au, 24.2% Pd, 8.75% Ag. The glass–ceramic veneer used in conjunction with it was sintered at 890 °C and is composed of both needle-like apatite crystals and leucite crystals . One core ceramic, lithium disilicate and lithium orthophosphate ceramic (Empress 2) , was pressed at 920 °C. A glass–matrix ceramic containing fluorapatite crystals sintered at 755 °C, was designed specifically for application on Empress 2 ceramic core . The lithium-disilicate-based core (e.max Press) was pressed at 920 °C, with the glaze paste and stain (e.max Ceram) applied directly to the core and fired at 770 °C. The ceramics used in the study met the criteria for ISO 6872 .
2.4
Impressions and models
Impressions for crown fabrication and the wear measurement were made using both light and medium body vinylpolysiloxane (Affinis, Coltene/Whaledent, OH, USA) to capture all possible detail. After cleaning, rinsing and drying the patient’s teeth, a first impression was made to remove plaque and salivary residue and discarded. A second impression was made immediately, maximizing the marginal quality. After 24 h at ambient temperature, the impressions were cleaned with alcohol and misted with a silicone relaxation liquid (Smoothex, Whip Mix Corp, KY, USA). The impressions were poured with white Type IV gypsum material (GC Fuji Super Hardrock, Leuven, Belgium), vibrated, set under pressure (2 bar) for 30 min, then stored in ambient conditions for a minimum of 24 h. The baseline impressions were made after crown cementation.
2.5
Study intervention
Prior to restorative preparation procedures, bite force measurements were recorded with each patient. The bilateral gnathodynamometer bite fork was positioned along the posterior teeth, extending back with the second molar as the forward contact point. Each patient was instructed to “bite down” exerting pressure until there was an initial sense of pain. The resulting maximum bilateral bite force was recorded.
Teeth identified for restoration were prepared for full crowns and provisional restorations were fabricated using a bis-acryl resin (Integrity, Dentsply, USA). Impressions were made and poured. The master models were mounted in centric relation, representing the most posterior relation of the lower to the upper jaw from which lateral movements can be made . Once a final crown was fabricated and tried-in place, occlusal adjustments were made using a high-speed dental handpiece (Kavo Dental GmbH, Germany) and a fine diamond bur (Brasseler, USA). Prior to cementation, all adjusted surfaces were polished (Shofu porcelain polishing kit, Shofu Dental Corporation, JP) or glazed depending on the adjustment areas. Areas involving more than two cusps were glazed, while smaller areas were polished to the same smoothness as the non-adjusted surfaces. Final crowns were cemented with a dual-cure resin luting agent (Variolink II, Ivoclar Vivadent). All crown fabrication was conducted according to the manufacturer’s recommendations at one professional commercial lab (Creative Smiles Inc., Dental Laboratory, San Antonio, TX, USA).
2.6
Wear determination methods
Models were produced from all of the post cementation impressions. A three-dimensional laser scanner (es 1 Scanner, etkon, Germany) was used to record the anatomical surfaces of the white model replicates (Scan 3D software). White stone maximizes the diffuse reflection areas using the high resolution charge-coupled device (CCD) with 10 mobile axes under a light angle of 45° establishing 28,500 measuring points per second.
The baseline scan images were superimposed over each of the successive annual images. Wear amounts (μm) were calculated (Match 3D software) as the maximum loss in height of the occlusal surface. Reported wear data were determined from baseline for each of years one, two and three. Statistical analyses were performed using SAS PROC MIXED.
2.7
Micrographs
In order to examine surface aspects of in vivo wear, representative models of crowns and enamel antagonists with measured wear values were fixed on aluminum mounts using conductive carbon paint (SPI-Chem, PA, USA) and sputter coated with 250 Å gold–palladium (AuPd; Hummer II, Technics, CA, USA). Micrographs, secondary electron images (SEI) and backscattered images (BSI), were recorded with a scanning electron microscope (SEM, JSM 6400, Joel, Ltd., Tokyo, JP).
2
Materials and methods
2.1
Study design
To analyze in vivo wear of both enamel and ceramic, a randomized, controlled, clinical trial was conducted. Prior to the single-blind pilot study initiation, three different commercially available ceramic systems were selected for fabricating full coverage restorative crowns as needed on a second premolar, first molar or second molar that opposed natural antagonist teeth in any arch. A random number table was formulated by the study’s statistician for assignment of the clinical restorative materials. Restorable teeth required a crown to root ratio of at least 1:1, with a full complement of opposing non-restored or minimally restored natural teeth. “Minimally restored” indicated that nothing more extensive than a Class II amalgam restoration was present in the opposing arch, and that there was a natural contralateral tooth.
Prior to the study, two investigators (dentists) were identified to prepare the restoration sites and place the crowns that would be fabricated by one professional commercial lab. Two different investigators (dentists), not involved in the crown preparations, were selected to evaluate the crowns post cementation.
2.2
Study population criteria
The clinical protocol for treating patients was approved by the Institutional Review Board of University of Texas Health Science Center at San Antonio (UTHSCSA). Flyers and e-mail advertisements were used to recruit patients meeting the following criteria: minimum age of 18 years; good overall general health with no contraindications to dental treatment; good overall dental health ( i.e ., no caries, periodontal disease or pocket depths in excess of 4 mm); no evidence of temporomandibular disorders ( e.g . clicking, popping, or pain on opening); no parafunctional habits ( e.g ., bruxism or clenching); good oral hygiene ( e.g ., compliant with instructions as determined by plaque present); normal saliva flow ( e.g ., no medical pathologies or chronic medication intake that limited salivary volume or flow); capable of paying the crown laboratory cost of two hundred dollars (USD); and, agreeable to three consecutive yearly appointments. The informed consent for each participant included the following baseline data: general medical history with physical examination, primary dental models, maximum bite force measurement, periodontal pocket depths and, periapical radiographs of abutment teeth.
2.3
Crown materials
The three restorative material combinations included: one metal/ceramic – (1) Argedent 62 alloy, Argen Corp., USA/IPS d.SIGN glass-veneer; and two all-ceramics – (2) IPS Empress 2 core ceramic/IPS Eris veneer; or (3) IPS e.max Press core ceramic/e.max Ceram glaze (ceramics, Ivoclar Vivadent AG, Schaan, Liechtenstein).
The gold-based, high noble metal alloy (Argedent 62) has a reported composition of 61.7% Au, 24.2% Pd, 8.75% Ag. The glass–ceramic veneer used in conjunction with it was sintered at 890 °C and is composed of both needle-like apatite crystals and leucite crystals . One core ceramic, lithium disilicate and lithium orthophosphate ceramic (Empress 2) , was pressed at 920 °C. A glass–matrix ceramic containing fluorapatite crystals sintered at 755 °C, was designed specifically for application on Empress 2 ceramic core . The lithium-disilicate-based core (e.max Press) was pressed at 920 °C, with the glaze paste and stain (e.max Ceram) applied directly to the core and fired at 770 °C. The ceramics used in the study met the criteria for ISO 6872 .
2.4
Impressions and models
Impressions for crown fabrication and the wear measurement were made using both light and medium body vinylpolysiloxane (Affinis, Coltene/Whaledent, OH, USA) to capture all possible detail. After cleaning, rinsing and drying the patient’s teeth, a first impression was made to remove plaque and salivary residue and discarded. A second impression was made immediately, maximizing the marginal quality. After 24 h at ambient temperature, the impressions were cleaned with alcohol and misted with a silicone relaxation liquid (Smoothex, Whip Mix Corp, KY, USA). The impressions were poured with white Type IV gypsum material (GC Fuji Super Hardrock, Leuven, Belgium), vibrated, set under pressure (2 bar) for 30 min, then stored in ambient conditions for a minimum of 24 h. The baseline impressions were made after crown cementation.
2.5
Study intervention
Prior to restorative preparation procedures, bite force measurements were recorded with each patient. The bilateral gnathodynamometer bite fork was positioned along the posterior teeth, extending back with the second molar as the forward contact point. Each patient was instructed to “bite down” exerting pressure until there was an initial sense of pain. The resulting maximum bilateral bite force was recorded.
Teeth identified for restoration were prepared for full crowns and provisional restorations were fabricated using a bis-acryl resin (Integrity, Dentsply, USA). Impressions were made and poured. The master models were mounted in centric relation, representing the most posterior relation of the lower to the upper jaw from which lateral movements can be made . Once a final crown was fabricated and tried-in place, occlusal adjustments were made using a high-speed dental handpiece (Kavo Dental GmbH, Germany) and a fine diamond bur (Brasseler, USA). Prior to cementation, all adjusted surfaces were polished (Shofu porcelain polishing kit, Shofu Dental Corporation, JP) or glazed depending on the adjustment areas. Areas involving more than two cusps were glazed, while smaller areas were polished to the same smoothness as the non-adjusted surfaces. Final crowns were cemented with a dual-cure resin luting agent (Variolink II, Ivoclar Vivadent). All crown fabrication was conducted according to the manufacturer’s recommendations at one professional commercial lab (Creative Smiles Inc., Dental Laboratory, San Antonio, TX, USA).
2.6
Wear determination methods
Models were produced from all of the post cementation impressions. A three-dimensional laser scanner (es 1 Scanner, etkon, Germany) was used to record the anatomical surfaces of the white model replicates (Scan 3D software). White stone maximizes the diffuse reflection areas using the high resolution charge-coupled device (CCD) with 10 mobile axes under a light angle of 45° establishing 28,500 measuring points per second.
The baseline scan images were superimposed over each of the successive annual images. Wear amounts (μm) were calculated (Match 3D software) as the maximum loss in height of the occlusal surface. Reported wear data were determined from baseline for each of years one, two and three. Statistical analyses were performed using SAS PROC MIXED.
2.7
Micrographs
In order to examine surface aspects of in vivo wear, representative models of crowns and enamel antagonists with measured wear values were fixed on aluminum mounts using conductive carbon paint (SPI-Chem, PA, USA) and sputter coated with 250 Å gold–palladium (AuPd; Hummer II, Technics, CA, USA). Micrographs, secondary electron images (SEI) and backscattered images (BSI), were recorded with a scanning electron microscope (SEM, JSM 6400, Joel, Ltd., Tokyo, JP).
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