Abstract
Objective
To evaluate in vitro the pre-cementation resistance of CAD/CAM onlays subjected to functional occlusal tapping.
Methods
An extracted tooth model (molar and premolar) with simulated bone and periodontal ligament was used to make a mesio-occlusal onlay preparation (two mesial cusps covered). Immediate dentin sealing was applied to the prepared tooth. The corresponding onlays were fabricated with Cerec either using composite resin (Paradigm MZ100) or ceramic (e.max CAD and Mark II) ( n = 14). An elevated marginal ridge was designed with the intention of generating hyper-occlusion. Pre-cementation occlusal tapping was simulated using closed-loop servo-hydraulics at 2 Hz, starting with a load of 40 N, followed by 80, 120, 160, 200, 240 and 280 N (10 cycles each). All samples were loaded until fracture or to a maximum of 70 cycles. Groups were compared using the life table survival analysis ( p = .016, Bonferroni method).
Results
Survival probability was MZ100 > e.max CAD > Mark II. The restorations made from e.max CAD and Mark II failed at an average load of 157 N and 123 N, respectively with no specimen withstanding all 70 load cycles (survival 0%); with MZ100 the survival rate was 36%.
Significance
Material selection has a significant effect on the risk of CAD/CAM onlay fracture during pre-cementation functional occlusal tapping with composite resin onlays showing the minimum risk compared to ceramic ones.
1
Introduction
Bonded restorations are undoubtedly the most conservative and biomimetic option allowing the minimum loss of sound structure when compared to conventional procedures, namely amalgam and crowns . Standard fixed prosthodontics procedures include testing the occlusion of the restoration before cementation. Such a step is usually not recommended for partial indirect bonded restorations (inlays, onlays and veneers) . The main reason is the vulnerability of these brittle restorations prior to cementation. They require meticulous tooth preparation (due to the more complex geometry with multiple inner angles and walls) and delivery sessions, requiring special attention to interproximal contacts, marginal fit, and occlusal contacts . There are significant drawbacks for not being able to carry out pre-cementation occlusal adjustments. Even in the presence of optimal clinical conditions and dental laboratory support, it is very likely that minor adjustments of the restoration will be needed. While minor changes can easily be carried out after cementation, other more significant adjustments such as major hyperocclusion call for substantial work, which cannot be ideally performed intraorally. Onlays present additional challenges compared to inlays because of their increased occlusal participation. Both the patient’s and clinician’s discomfort at the end of the cementation appointment can be amplified when major occlusion discrepancies are detected. This will require prolonged corrective procedures, possibly resulting in flawed anatomy, surface roughness and color discrepancy.
Maximum bite force varies considerably and is in the range of 234–597 N for women and 306–847 N for men . However it has been demonstrated that the controlled bite force during tapping is much lower, approximately 22 N somewhat similar to the maximum cementation force, approximately 25 N . Given the development of stronger materials in combination with CAD/CAM techniques, it calls into question whether those new CAD/CAM restorations actually require special handling during try-in and cementation. It can be speculated that most modern materials used to fabricate bonded restorations have flexural strength and toughness that will sustain tapping/cementation forces. This issue, however, has not been addressed in the literature.
The aim of the present study was to evaluate in vitro the pre-cementation resistance of CAD/CAM onlays subjected to functional occlusal tapping. The influence of different machinable materials was assessed: high-strength ceramic, composite resin, and feldspathic porcelain. The null hypothesis stated that (1) there is no influence of material selection on the try-in resistance of the onlays and (2) restoration design (onlays versus inlays) has no influence of the try-in resistance of the indirect partial restorations. Part 2 of the null-hypothesis was formulated by including previous data concerning the pre-cementation resistance of CAD/CAM inlays subjected to occlusal tapping by the same research group in strictly identical conditions .
2
Materials and methods
2.1
Specimen preparation
Two freshly extracted maxillary teeth – one molar and one premolar – stored in solution saturated with 0.1% thymol, were used upon approval from the University of Southern California Institutional Review Board ( Fig. 1 A ). Two layers of water-based liquid latex (Rubber-Sep; Kerr Corporation, Orange, CA) were applied on the roots in order to simulate the periodontal ligament ( Fig. 1 B) . Teeth were positioned in proximal contact and the roots were embedded in acrylic resin (Palapress; Haereus Kulzer, Armonk, NY) up to 3.0 mm below the cement–enamel junction (CEJ).
The molar received a mesio-occlusal onlay preparation derived from a preexisting inlay preparation used in a previous study by reducing the two mesial cusps 1.5-mm with a round-ended tapered diamond bur (6856-027; Brasseler, Savannah, GA) (detailed dimensions in Fig. 2 ). The dentin was sealed with 3-step etch-and-rinse dentin bonding agent (Optibond FL; Kerr, Orange, CA) immediately following tooth preparation. After the standard light curing time of 20 s, an air-blocking barrier (K-Y Jelly; Personal Products Company, Skillman, NJ) was then applied and followed by 10 s of additional light exposure (Allegro; Den-Mat, Santa Maria, CA) to optimize the polymerization of the oxygen-inhibition layer. The use of “immediate dentin sealing” technique (IDS) is supported by the prerogative of using limited anesthesia to allow the patient to better control tapping forces and improve their ability to detect minor occlusal discrepancies. Studies have reported an increase in bite force development induced by anesthetization . By utilizing IDS technique , in which all the exposed dentin is etched, primed and resin-coated immediately after tooth preparation, before impression making, the patient does not require anesthesia during delivery of the restorations.
2.2
Restoration design and manufacturing
Standardized onlays were generated with the Cerec 3 CAD/CAM system (Cerec software v. 3.03., Sirona Dental Systems GmbH, Bensheim, Germany) ( Fig. 3 A ). All restorations were identical in size and anatomy (by means of the correlation mode the original shape of the intact molar was scanned before preparation and replicated on the restorations) because they were produced by the multiple milling of the same design. The latter included a marginal ridge slightly higher than the neighboring premolar marginal ridge. This anatomical flaw was included to simulate hyperocclusion. Fourteen inlays were milled for each restorative material: e.max CAD (Ivoclar; Schaan, Liechtenstein), Paradigm MZ100 (3 M/ESPE; Saint Paul, MN), and Vita MarK II Blocks (Vident; Brea, CA). Detailed description of the materials is presented in Table 1 . All the restorations were milled in Endo mode (optimized fit for smooth preparations) with the sprue at the distal edge. The restorations milled with lithium disilicate blocks were cerammed in a ceramic furnace (Austromat D4, DEKEMA Dental-Keramiköfen GmbH, Freilassing, Germany) following the manufacturer’s instructions (Ivoclar Vivadent AG). The surface polishing of the ceramic onlays was performed mechanically using diamond ceramic polishers (Dialite, Brasseler), while the composite resin onlays were finished with brushes (Jiffy Composite Polishing Brushes, Ultradent, South Jordan, UT). A stone replica of the preparation was used for holding the onlays during finishing procedures.
| Material | Content | Particle size |
|---|---|---|
| e.max (lithium disilicate glassceramic) | ≈58% in volume of needle-like lithium disilicate homogeneously dispersed in the glassy matrix a | Lengths and thicknesses of elongated crystals: ∼10 and ∼1 μm, respectively a |
| MZ100 (composite resin) | 85 wt% zirconia–silica particles bisGMA/TEGDMA polymer matrix c | 0.6 μm c |
| MK II (feldspathic porcelain) | ≈30% in volume of feldspar uniformly embedded in the glassy matrix c | 1–7 μm b |
2.3
Occlusal tapping test
The onlay was placed inside the wet prepared tooth. An artificial mouth using closed-loop servohydraulics (Mini Bionix II; MTS Systems, Eden Prairie, MN) was used to simulate occlusal tapping forces. The try-in cycle was simulated by an isometric contraction (load control) applied through a 7-mm-diameter composite resin sphere (Filtek Z100, 3M/ESPE). Due to their identical occlusal anatomy, all specimens could be positioned in the same reproducible location with the sphere contacting the inner slope of the mesial marginal ridge ( Fig. 3 B). Cyclic occlusal tapping was applied at a frequency of 2 Hz, starting with a load of 40 N for 10 cycles followed by stages of 80, 120, 160, 200, 240, and 280 N at a maximum of 10 cycles each. The specimens were loaded until fracture or to a maximum of 70 cycles. The failure load was recorded.
2.4
Statistical analysis
The fracture resistance of the three groups was compared using the life table survival analysis (MedCalc, v. 11.0.1; MedCalc Software, Mariakerke, Belgium). At each time interval (defined by each load step), the number of onlays starting the interval intact and the number of onlays that fractured during the interval were counted, allowing the calculation of survival probability at each interval. The influence of the restorative material on the fracture resistance was determined by comparing the survival curves using the log rank test at a significance level of 0.05. Differences were identified using pairwise post hoc comparisons with the same test at a significance level of 0.016 (Bonferroni correction for 3 comparisons). Additional computations were made by including three experimental groups of a previous study about the pre-cementation resistance of CAD/CAM inlays subjected to occlusal tapping . The study was carried out by the same authors and produced in rigorously identical conditions (operators, same teeth, and experimental setup). The life table survival analysis was used to compare the fatigue resistance of the six groups (differences in fracture strength detected by the log-rank test at a significant level of .05). Pairwise post hoc comparisons were used to locate the differences at a significant level of 0.003 (Bonferroni correction for 15 comparisons).
2
Materials and methods
2.1
Specimen preparation
Two freshly extracted maxillary teeth – one molar and one premolar – stored in solution saturated with 0.1% thymol, were used upon approval from the University of Southern California Institutional Review Board ( Fig. 1 A ). Two layers of water-based liquid latex (Rubber-Sep; Kerr Corporation, Orange, CA) were applied on the roots in order to simulate the periodontal ligament ( Fig. 1 B) . Teeth were positioned in proximal contact and the roots were embedded in acrylic resin (Palapress; Haereus Kulzer, Armonk, NY) up to 3.0 mm below the cement–enamel junction (CEJ).
