Fracture resistance of molar teeth with mesial-occlusal-distal (MOD) restorations

Abstract

Objective

Filled MOD restorations show near-complete recovery of tooth strength relative to the newly prepared, unfilled state. The present study examines the underlying mechanics of this recovery by more closely quantifying the mode of splitting fracture from the cavity base. By understanding the role of specific cavity dimensions on fracture resistance, useful clinical guidelines concerning MOD morphologies are formulated.

Methods

A systematic in vitro study is made of the load-bearing capacity of filled and unfilled MOD cavities by axially loading extracted molar teeth with a hard metal ball. Filled and unfilled cavities are considered as bounding cases. Focus is placed on drillings with rectangular or rounded tips, covering a range of cavity widths and depths. The failure process is monitored during loading by a video camera, enabling the entire damage evolution from first contact to ultimate failure to be recorded.

Significance

While respecting the widely accepted clinical practice of drilling cavities with internal widths less than one third that of the entire tooth, a stronger correlation is obtained between critical splitting load P C and ratio of cavity wall thickness h (distance between cavity wall and outer tooth surface) to cavity depth D . Imposing a conservative upper limit on P C for tooth survival, the study recommends that MOD cavities be prepared such that the ratio remains in the region h > D , regardless of the tooth size.

Introduction

Molar teeth with significant damage or carious infection are routinely subjected to mesial-occlusal-distal (MOD) cavity filling . Such cavities are generally drilled with a diamond or carbide bur, most often with a rectangular or base-rounded profile, sometimes with undercuts. Current procedure involves sequentially filling the drilled cavities with layers of dental composite, with intermittent curing. Modern-day adhesive treatments have led to improved bonding between composite and tooth structure, and are thus less likely to open up fissures at the filler margins. While producing successful immediate outcomes, MOD cavity filling involves a substantial amount of tooth removal and constitutes a potential weakness over the long term in the event of any degradation of the composite-tooth interface.

Several empirical studies have been reported on the load-bearing capacity of extracted molar or premolar teeth with MOD fillings . In all such studies tests are usually done by pressing a large hard ball or cylinder onto the occlusal fossae so that cuspal contact wedges open the MOD cavity walls. Failure is marked by a precipitous drop in applied load. Critical failure loads in extracted molars and premolars have been measured for different cavity designs, with and without fillings . The data indicate a considerable loss in structural integrity for cavities without fillings as compared to intact (undrilled) specimens, but strong recovery after filling. However, while the mode of failure is broadly reported as fracture from the base region of the cavity, little attempt has been made to observe the evolution of such fractures during loading. More importantly, nor has there been any systematic quantitative study of the role of characteristic cavity dimensions in determining failure resistance. While some stress analyses of MOD cavities have been reported , none has hitherto addressed the critical issue of crack evolution. Accordingly, there appears to be little scientific basis for optimizing MOD morphologies for optimum fracture resistance.

To answer these questions, this paper examines the fracture behavior of simple MOD-drilled cavities geometries such as rectangular or rounded bases. Filled and unfilled cavities are considered, with cavity depth and width systematically varied. The evolution of damage is observed in real time during the loading, and a basic strength of materials approach used to analyze failure load data.

Materials and methods

A quantitative study of MOD cavity-drilled extracted molar teeth in axial loading with a hard ball is made. Filled and unfilled cavities are considered as bounding cases, the former representative of a near-intact occlusal surface and the latter of a compromised filling completely detached from the tooth walls. Extracted mandibular molar teeth for drilling MOD cavities were obtained from the American Dental Association laboratories at the US National Institute of Standards and Technology, with patient consent and IRB approval. Any teeth with visible pre-existing cracks or carious damage were eliminated from the supply. The remaining teeth ( n = 28) were embedded with their roots in self-curing polymethlmethacrylate resin 1–2 mm below the cemento-enamel junction for support during ensuing preparation and testing. These selected teeth were kept in distilled water until just prior to preparation and testing. Rotary drills of various sizes with either a flat-end or round-end diamond-coated burs (A&M Instruments, Inc., Alpharetta, GA) were used to produce cavities of depth D and width 2 w as indicated in Fig. 1 . The drills were mounted on a high-speed dental drill attached to an x, y, z stage. Specified cavity dimensions were obtained by repeat passes of the drill. The resulting wall thickness h = R w was measured close to the cavity base. To help understand general trends the cavity dimensions covered broad ranges, w = 0.7–2.3 mm and D = 2.1–6.8 mm. The depth D was such that the cavity always penetrated some way into the dentin but not as far as the pulp.

Fig. 1
Schematic of MOD cavity in molar tooth mounted into epoxy support and axially loaded at force P with a tungsten carbide ball of radius r , for cavity of half-width w and depth D , wall-thickness h = R w , where R is the tooth half-width in the near-base region of the cavity.

Drilled specimens were cleaned and examined by optical microscopy to ensure freedom from preparation defects. Three teeth groups were studied: filled rectangular base cavities ( n = 9); as-drilled unfilled rectangular base cavities ( n = 12); as-drilled unfilled rounded base cavities ( n = 7). The teeth in the first group were restored with a dental composite according to manufacturer’s specifications. This latter entailed first etching the cavity walls with 37% acid gel (Phosphoric Acid Etching Gel, Pentron Clinical Technologies, Orange, CA), followed by application of a primer (Clearfil SE, Kurary America, New York, NY). After drying, a bonding agent (Clearfil SE Bond, Kurary America, New York, NY) was applied to the primed walls and light-cured for 20 s (Max 100, Dentsply Caulk, Milford, DE). Resin composite (Clearfil Majesty Posterior, Kurary America, New York, NY) was then filled into the cavity layer by layer , with intermittent light curing for 20 s at each step (Max 100, Dentsply Caulk, Milford, DE). The fillings were ground with a carbide bur and polished to fit the natural occlusal contour. All such filled specimens were left to age in water for 24 h at room temperature before failure testing.

The failure testing was conducted by mounting the epoxy-supported tooth specimens onto the platen of a screw-driven testing machine (Instron 4501, Instron Corp, Canton, MA) and by placing a tungsten carbide ball of radius r = 3.17 mm laid freely in the central fossa. This ball size was chosen to ensure contact with the inner surfaces of the enamel cusps ( r > w , Fig. 1 ). A vertical force was applied monotonically to the ball at a fixed displacement rate 0.1 mm/min up to failure at a critical load P C , as marked by a precipitous drop in the load-displacement record. A video camera was placed side-on to observe the damage evolution during the testing. In this way the entire fracture sequence could be monitored, both at the side walls and in the occlusal regions immediately adjacent to the contact. Dimensions R , h, w and D were determined from video images with the help of a calibrated digital ruler using a Photoshop package. The tooth radius of the selected specimens showed relatively small variation, R = 5.3 ± 0.4 mm (mean and standard deviation).

The statistical significance of the ensuing data sets was analyzed by one-way ANOVA. The significance level was set at p ≤ 0.05 for all analyses.

Materials and methods

A quantitative study of MOD cavity-drilled extracted molar teeth in axial loading with a hard ball is made. Filled and unfilled cavities are considered as bounding cases, the former representative of a near-intact occlusal surface and the latter of a compromised filling completely detached from the tooth walls. Extracted mandibular molar teeth for drilling MOD cavities were obtained from the American Dental Association laboratories at the US National Institute of Standards and Technology, with patient consent and IRB approval. Any teeth with visible pre-existing cracks or carious damage were eliminated from the supply. The remaining teeth ( n = 28) were embedded with their roots in self-curing polymethlmethacrylate resin 1–2 mm below the cemento-enamel junction for support during ensuing preparation and testing. These selected teeth were kept in distilled water until just prior to preparation and testing. Rotary drills of various sizes with either a flat-end or round-end diamond-coated burs (A&M Instruments, Inc., Alpharetta, GA) were used to produce cavities of depth D and width 2 w as indicated in Fig. 1 . The drills were mounted on a high-speed dental drill attached to an x, y, z stage. Specified cavity dimensions were obtained by repeat passes of the drill. The resulting wall thickness h = R w was measured close to the cavity base. To help understand general trends the cavity dimensions covered broad ranges, w = 0.7–2.3 mm and D = 2.1–6.8 mm. The depth D was such that the cavity always penetrated some way into the dentin but not as far as the pulp.

Fig. 1
Schematic of MOD cavity in molar tooth mounted into epoxy support and axially loaded at force P with a tungsten carbide ball of radius r , for cavity of half-width w and depth D , wall-thickness h = R w , where R is the tooth half-width in the near-base region of the cavity.

Drilled specimens were cleaned and examined by optical microscopy to ensure freedom from preparation defects. Three teeth groups were studied: filled rectangular base cavities ( n = 9); as-drilled unfilled rectangular base cavities ( n = 12); as-drilled unfilled rounded base cavities ( n = 7). The teeth in the first group were restored with a dental composite according to manufacturer’s specifications. This latter entailed first etching the cavity walls with 37% acid gel (Phosphoric Acid Etching Gel, Pentron Clinical Technologies, Orange, CA), followed by application of a primer (Clearfil SE, Kurary America, New York, NY). After drying, a bonding agent (Clearfil SE Bond, Kurary America, New York, NY) was applied to the primed walls and light-cured for 20 s (Max 100, Dentsply Caulk, Milford, DE). Resin composite (Clearfil Majesty Posterior, Kurary America, New York, NY) was then filled into the cavity layer by layer , with intermittent light curing for 20 s at each step (Max 100, Dentsply Caulk, Milford, DE). The fillings were ground with a carbide bur and polished to fit the natural occlusal contour. All such filled specimens were left to age in water for 24 h at room temperature before failure testing.

The failure testing was conducted by mounting the epoxy-supported tooth specimens onto the platen of a screw-driven testing machine (Instron 4501, Instron Corp, Canton, MA) and by placing a tungsten carbide ball of radius r = 3.17 mm laid freely in the central fossa. This ball size was chosen to ensure contact with the inner surfaces of the enamel cusps ( r > w , Fig. 1 ). A vertical force was applied monotonically to the ball at a fixed displacement rate 0.1 mm/min up to failure at a critical load P C , as marked by a precipitous drop in the load-displacement record. A video camera was placed side-on to observe the damage evolution during the testing. In this way the entire fracture sequence could be monitored, both at the side walls and in the occlusal regions immediately adjacent to the contact. Dimensions R , h, w and D were determined from video images with the help of a calibrated digital ruler using a Photoshop package. The tooth radius of the selected specimens showed relatively small variation, R = 5.3 ± 0.4 mm (mean and standard deviation).

The statistical significance of the ensuing data sets was analyzed by one-way ANOVA. The significance level was set at p ≤ 0.05 for all analyses.

Results

A series of control tests were conducted on filled teeth with rectangular MOD cavities ( n = 9). A representative example of in situ side-wall observations of fracture in a filled tooth is shown in Fig. 2 a, photographed during loading (ball in place) and immediately after failure (ball removed). In a majority of cases, failure occurred by debonding at the filler interface and flaking of the entire tooth side wall, a singular event marked by an abrupt load drop. Critical loads P C in all such instances exceeded 1 kN, regardless of cavity dimensions. These loads lie well in excess of measured normal bite forces of several hundreds of Newtons exerted on human molars during mastication . While filler-debonding/side-wall-flaking was confirmed as the principal failure mode, some specimens, notably those with narrower filled cavities (i.e. larger h / D , Fig. 1 ), instead disintegrated at even higher loads by severe crushing and chipping at the cusps adjacent to the contact site, or even by splitting through the entire composites filler. Also apparent during the testing were precursor ‘longitudinal’ cracks which grew steadily along the enamel side walls as load increased. These latter cracks, which remain fully contained within the enamel shell , became visible at loads well below those required to produce ultimate splitting, but tended to close up somewhat on unloading.

Fig. 2
Examples of failure at side walls of MOD molar tooth specimens observed during testing (ball in place) and after failure (ball removed), for axial loading with tungsten carbide ball of radius r = 3.17 mm: (a) filled cavity with rectangular base; (b) unfilled cavity with rectangular base; (c) unfilled cavity with rounded base. Failure in the filled specimen in (a) occurs by debonding at the composite–tooth interface, with immediate flaking of the tooth side wall. Failure in the unfilled specimens occurs by fracture at the cavity base, with location of crack initiation shifting from a rectangular corner toward the cylindrical tip in (b) and (c). Note precursor cracks in enamel (arrowed) prior to failure, more visible during loading owing to wedge-opening action of ball indenter.

Comparative tests were run on unfilled teeth with rectangular ( n = 12) or rounded ( n = 7) MOD cavities. In contrast to their filled counterparts, these specimens fractured at markedly lower loads, depending on the cavity dimensions. Representative examples of in situ side-wall observations of fracture are shown in Fig. 2 b for specimens with rectangular cavity profiles, and in Fig. 2 c for specimens with rounded profiles. As with filled teeth, some specimens with narrower cavity walls disintegrated by chipping and crushing at loads above 1 kN. But final failure in the remaining specimens occurred near a base corner of rectangular cavities and near the deepest points of rounded cavities. Again, stably extending precursor longitudinal cracks were evident during the loading sequence.

Individual data showing the variation of critical splitting load on characteristic MOD cavity dimensions are listed in Table 1 . Within each data set, P C values are ranked in descending order. The degree of dependence of P C on individual dimensions, or on ratios of these dimensions, is quantified at the bottom of each data set by Pearson correlation coefficients (CC). Note that the strongest correlation averaged over all three data sets, especially for unfilled cavities, is not the traditional one between P C and w / D but rather between P C and h / D .

Table 1
Failure loads P C for molar teeth with MOD cavities of specified dimensions D , R , w and h as defined in Fig. 1 . Failure modes TS, IC and OD corresponds to tooth splitting, interfacial cracking following by tooth splitting and occlusal damage. Rows CC indicates correlation coefficient between load and each dimension (Section 2 ).
P c (N) Mode D (mm) R (mm) w (mm) h (mm) w / R w / D h / R h / D
Filled rectangular cavities
4009 OD 3.9 5.8 2.1 3.8 0.36 0.54 0.66 0.97
3739 OD 4.0 5.9 1.5 3.9 0.25 0.38 0.66 0.98
3268 OD 3.5 5.1 1.6 3.5 0.31 0.46 0.69 1.00
2832 OD 3.0 5.7 0.9 3.8 0.16 0.30 0.67 1.27
2067 IC 4.0 5.3 1.6 2.9 0.30 0.40 0.55 0.73
1733 IC 4.5 5.5 2.0 2.6 0.36 0.44 0.47 0.58
1657 IC 4.7 4.6 2.1 2.3 0.46 0.45 0.50 0.49
1623 IC 5.8 5.3 2.0 2.4 0.38 0.34 0.45 0.41
1482 IC 5.0 4.9 1.9 2.4 0.39 0.38 0.49 0.48
CC −0.67 0.68 −0.32 0.94 −0.49 0.33 0.91 0.81
Unfilled rectangular cavities
2606 OD 2.1 5.5 0.9 4.3 0.17 0.43 0.78 2.00
996 TS 3.4 5.3 1.4 2.8 0.26 0.41 0.53 0.82
988 TS 3.1 5.2 1.3 3.8 0.25 0.42 0.73 1.2
798 TS 4.7 5.3 0.7 4.0 0.13 0.15 0.75 0.85
727 TS 6.8 4.9 0.8 2.9 0.16 0.12 0.59 0.43
698 TS 5.8 4.8 1.4 3.4 0.29 0.24 0.71 0.59
655 TS 5.7 4.7 1.9 2.7 0.40 0.33 0.57 0.47
606 TS 4.3 5.6 1.5 3.5 0.27 0.35 0.63 0.81
543 TS 3.7 4.8 1.7 2.6 0.35 0.46 0.54 0.70
308 TS 5.5 4.9 2.0 2.7 0.41 0.36 0.55 0.49
237 TS 5.5 5.0 2.0 1.8 0.40 0.36 0.36 0.33
192 TS 6.0 5.7 2.3 1.6 0.40 0.38 0.28 0.27
CC −0.71 0.28 −0.62 0.72 −0.63 0.17 0.65 0.94
Unfilled rounded cavities
2377 OD 2.2 5.3 0.9 4.3 0.17 0.41 0.81 1.95
1686 TS 2.2 5.3 0.9 4.0 0.17 0.41 0.75 1.82
1117 TS 3.4 5.5 1.5 3.7 0.27 0.44 0.67 1.09
730 TS 4.6 5.7 1.5 3.0 0.26 0.33 0.53 0.65
598 TS 4.1 5.3 1.4 3.4 0.26 0.34 0.64 0.83
410 TS 3.6 5.1 2.0 2.7 0.39 0.56 0.53 0.75
349 TS 4.9 5.5 1.4 2.5 0.25 0.29 0.45 0.51
CC −0.87 −0.14 −0.81 0.93 −0.77 0.13 0.92 0.96
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Nov 22, 2017 | Posted by in Dental Materials | Comments Off on Fracture resistance of molar teeth with mesial-occlusal-distal (MOD) restorations
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