Specimen Preparation

Thirty-two sound mandibular first premolars were extracted for orthodontic reasons. The teeth were obtained with the consent of healthy Chinese adults 20 to 30 years of age, living in the same locality, without water fluoridation. After cleaning, the teeth were examined stereoscopically at 10× magnification to verify the absence of cracks and before being stored in 0.9% saline solution at 4°C for no longer than two weeks. The natural crowns were sectioned perpendicular to their long axes 1.0 mm coronal to the buccal cementoenamel junction using water-cooled diamond disks.

Standardized root canal preparations were completed using size 4 drills (Gates-Glidden, Dentsply Maillefer SA, Ballaigues, Switzerland) before placing laterally-condensed gutta percha points (Dentsply International Inc, York, PA, USA) to obturate the canals. Post-hole preparation channels, 10 mm deep, for tapered carbon fiber-reinforced dowels or posts (#2 C-POST, BISCO, Inc, Schaumburg, IL, USA) were made using the matching drills provided in a slow-speed contra-angle handpiece. The 32 prepared roots were randomly assigned to four equal groups (A1, A2, B1, B2) according to a table of random numbers. The root lengths and the mesiodistal and buccolingual diameters of the roots at the root face and the cross-sectional width of the canal walls at the mesial, buccal, distal and lingual root face sites were measured to 0.02 mm with a vernier caliper (Vernier Caliper Model 93218-0654, Harbin Measuring & Cutting Tool Group Co Ltd, Harbin, PR China). There were no statistically significant differences in these dimensions among the four groups, as shown in Table 1.

Table 1 Mean Dimensions (mm) of Randomly Assigned Mandibular First Premolar Roots in Each Group

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For Groups A1 and B1, the root face was left unprepared, with a 5.0 mm high tapered core being fabricated, leaving an 0.8 mm wide encircling shoulder of dentin (Figure 1a). For Groups A2 and B2, a 2.0 mm long ferrule with an 0.8 mm wide shoulder was prepared to extend apical from the root face to encircle the root, and a 5.0 mm high tapered core was fabricated (Figure 1b). All cores were prepared with a 12° convergence angle using a milling machine (F3/Ergo, Dusseldorf, Germany).

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Group A1: After taking a vinyl polysiloxane impression (Aquasil, Dentsply International) of the prepared post-hole, a tapered dowel-core was fabricated in a cast nickel-chromium (Ni-Cr) alloy (Chang Ping Shi Ye Inc, Shanghai, PR China) and sandblasted. The post-hole walls were conditioned with 10% polyacrylic acid (Dentsply International) for 15 seconds, then rinsed thoroughly and dried lightly with paper points. Before milling, the Ni-Cr alloy dowel-core was placed using glass-ionomer cement (GIC) (Glasionomer, Shofu Inc, Kyoto, Japan) according to the manufacturer's instructions.

Group A2: The root was prepared with an apical extended 2.0 mm long ferrule. A Ni-Cr alloy dowel-core was fabricated and cemented, as in Group A1.

Group B1: The prepared post-hole walls were etched with 32% phosphoric acid gel (UNI-ETCH, BISCO, Inc) for 15 seconds, then rinsed thoroughly and dried lightly with paper points. A resin-based adhesive (ONESTEP PLUS, BISCO, Inc) was applied twice as a thin layer over the walls of the post-hole and once over the surfaces of a prefabricated carbon fiber-reinforced dowel (#2 C-POST, BISCO, Inc). After thinning lightly with dry, oil-free air, the adhesive was light-cured (Variable Intensity Polymerizer Junior, BISCO, Inc) from a coronal direction for 10 seconds at 600 mW/cm². The post hole was completely filled by injecting resin luting cement (POST CEMENT HI-X, BISCO, Inc) before inserting the carbon fiber-reinforced dowel. A resin composite core (LIGHT-CORE, BISCO, Inc) was built-up around the dowel and light cured for 40 seconds before milling.

Group B2: The root was prepared with an apical extended 2.0 mm long ferrule. A carbon fiber-reinforced dowel and a resin composite core were placed, as in Group B1.

After 24 hours in the saline storage medium, a standardized Ni-Cr alloy crown was fabricated in the dental laboratory for each of the prepared teeth and cemented with GIC (Shofu Inc). The specimens were kept in the storage medium at all times except during the experimental testing.

Fracture Strength Testing

Each root was coated with a 0.1~0.2 mm thin layer of vinyl polysiloxane silicone (modulus of elasticity 0.3 MPa) (Aquasil, Dentsply International) to simulate the periodontal ligament before being embedded from 2.0 mm apical to the crown preparation margins in a block of self-cured acrylic resin (modulus of elasticity 2~3 GPa) (Shanghai Dental Materials Manufacturing Co, Shanghai, PR China). A unidirectional static load was then applied to the buccal cusp of the Ni-Cr alloy crown at an angle 150° from the long axis of the root using a cylindrical Ni-Cr alloy rod (3.0 cm long × 0.8 cm diameter) in a universal load-testing machine (Model CSS-2202, Changchun Tester Institute, Changchun, PR China) at a crosshead speed of 1.0 mm/minute (Figure 2). The 150° angle simulated the application of an oblique occlusal force. The force (kN) for initial root fracture was recorded and the failure site patterns noted.

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Statistical Analysis

All analyses were conducted using a commercial software package (SPSS v11.0, SPSS Inc, Chicago, IL, USA). One-way and two-way ANOVA with Tukey HSD tests, Student's t-test and Fisher's exact test were used to detect any significant differences between the groups. The probability level for statistical significance was set at α=0.05.

Specimen Preparation and Fracture Strength Testing

Mandibular first premolars were selected for this study, because these teeth are vulnerable to vertical root fracture after endodontic treatment.19 The teeth were obtained from young adults who lived in the same locality and had very similar root forms and dimensions (Table 1). Gross destruction of the coronal tissues was simulated using standardized flat root face preparations. Grossly broken down endodontically-treated teeth may also require surgical crown lengthening,20 which facilitates placement of a long apical extended ferrule to potentially enhance the resistance of the remaining root to fracture.21–22

The roots of the restored teeth were coated with silicone rubber and embedded in acylic resin. The moduli of elasticity of these materials approximated those of the viscoelastic periodontal ligament and the alveolar bone, respectively.23–24 To minimize the experimental variables, a unidirectional static loading force was used in this and many other studies of root fractures. However, further in vitro testing should more closely simulate the complex dynamic forces present during mastication and para-function and the effects of single severe impacts. An oblique force applied at 150° from the long axis of the mandibular premolar was employed to simulate functional working-side buccal cusp loading. Similar in vitro studies of maxillary incisor fracture resistance have employed angles from 110°–150°.2125–27

Fracture Resistance of Restored Premolars

Endodontically-treated teeth requiring placement of deep subgingival preparation margins may need surgical crown lengthening or orthodontic extrusion to gain the retention and resistance forms necessary for successful coronal restorations. Crown lengthening surgery is considered an effective method for restoring endodontically-treated teeth with subgingival defects.20 The alveolar bone level can be lowered surgically for apical extension of the preparation margins and to re-establish biological width, which is necessary for periodontal health.28 Crown lengthening would facilitate the apical placement of a ferrule and, presumably, also increase fracture resistance of the root.

However, the static loading test results showed that the mean fracture strength for Groups A1 and B1 with no ferrules was 35.5% and 10.8%, higher than for Groups A2 and B2, which were prepared with 2.0 mm ferrules (Table 2). Other in vitro studies employing metal alloy dowels have found either no significant difference in fracture resistance17 or a significant reduction in fracture resistance16 for the crown lengthening and ferrule placement when compared to no crown lengthening and no ferrules. One in vitro study involving a quartz fiber-reinforced dowel-resin composite found that a 2.0 mm apical extended ferrule was significantly more effective in increasing fracture resistance than shorter ferrules.11 A recent finite element analysis study of maxillary incisors found that, when subjected to a palatal load, the placement of either conventional coronal or apical extended ferrules produced high tensile stresses in dentin at the palatal cervical margins of the preparations.18 These stresses approached the tensile strength of dentin and may possibly result in crack propagation.

The effective clinical crown length (C_e) in the apical extended ferrule groups was 11.0 mm, which is 2.0 mm more than that in the no ferrule groups, and the embedded root length (R_b) was 10.0 mm, which is 2.0 mm less than what is in the no-ferrule groups. Therefore, the clinical crown/root ratio (C_e/R_b) was 1.10 for the ferrule groups and 0.75 for the no-ferrule groups (Figure 3). In addition, the diameter of the root decreases towards the root apex, because of root taper. The placement of a 2.0 mm long ferrule, which increased the clinical crown/root ratio, reduced the fracture strength of the endodontically-treated teeth (Table 2). Thus, caution is needed when preparing a long, heavy ferrule in the clinical situation, as the reduced volume of the root dentin that remains may then compromise the fracture resistance of the root to heavy occlusal stresses.

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Ideally, dowels should have a similar modulus of elasticity to dentin in order to better distribute occlusal loads and prevent high stress concentrations in the remaining sound root substance.1429 Moduli of elasticity are approximately 21 GPa for carbon fiber-reinforced dowels (BISCO, Inc), 150–200 GPa for cast Ni-Cr alloys and 15 GPa for human dentin.23 In a finite element study, irrespective of dowel length and diameter, when compared to more rigid cast gold and prefabricated stainless steel dowels, fiber-reinforced dowels reduced stress on root dentin around the apical dowel tips by 5–14%.30 Moduli of elasticity are approximately 16–25 GPa for universal and posterior resin composites,23 which may be used as core materials with fiber-reinforced dowels. When subjected to fatigue loading, the latter combination provided significantly stronger artificial crown retention than cast gold alloy dowel-cores and titanium alloy dowel-resin composite cores.13 The use of a resin-based luting cement with a low modulus of elasticity would also reduce the transfer of mechanical stresses from a rigid metal dowel to the remaining root dentin,2931 as would the use of a resin composite to reinforce thin-walled root canals.27

The root fracture sites were significantly different for Ni-Cr alloy dowels and carbon fiber-reinforced dowels (Table 2). Most failures for the Ni-Cr alloy dowel-core system were catastrophic fractures, such as vertical root fractures and oblique root fractures located below the cervical third of the roots; whereas, most failures for the fiber-reinforced dowel-core system were less severe oblique fractures located at or above the cervical third of the roots and were potentially repairable.