International Endodontic Journat (1990) 23,304-312

An in vitro study to compare impact fracture resistance of intact root-treated teeth A. V. M C D O N A L D ,

P. A. K I N G & D . J. S E T C H E L L Department Dentistry, Institute of Dental Surgery, Gray's Inn Road, London, UK Summarj. The aim of this in vitro study was to determine the effect on impact fracture resistance of three methods for restoration of root-treated lower incisor teeth with otherwise intact natural crowns. In a control group (Group I) lateraiiy condensed gutta-percha fiiied the entire root canal, whereas pan of the root filiing was removed to enabie insertion of a i mm diameter post in the other two groups. Stainless steel posts were placed in Group ii and experimenui carbon fibre reinforced carbon (CFRC) in Group III. A composite resin iuting agent was used to iute the posts, and standardized composite resin restorations were piaced in each access cavit), Fifteen specimens of each group were tested to faiiure with a single impact force applied at 90 degrees to the mid-point inciso-cervicaily on the labial surface. The peak force, peak energj, and first peak total energy' required to fracture each specimen were recorded. The results showed no significant difference between the three groups, nor was a difference in the mode or site of fracture observed. The results suggested that there is no advantage from the point of view of fracture mechanics in 'restoring' intact root-treated teeth with either stainless steel or carbon fibre reinforced carbon rods. Ititroduction Endodontically treated teeth are considered by many clinicians to have an increased susceptibility to fracture. The reasons postulated relate mainly to the loss of internal dentine during root canal therapy (Rosen 1961), and a change in the mechanical properties of dentine due to desiccation (Heifer f^ a/. 1972). It is common to find the suggestion that a post can serve not only to retain an artificial crown but also to 'reinforce' the remaining tooth structure (Frank 1959, Silverstein, 1964, Correspondence: Miss A. V. McDonald, Department of Restorative Dentistry, Dublin Dental Hospital, Lincoln Place, Dublin 2, Republic of Ireland. 304

of Conservative

Lau 1976, Michnick & Raskin 1978). For this reason, posts are sometimes inserted in roottreated teeth even when the crown remains relatively intact. Until recently this practice has been based on anecdotal evidence. Laboratory investigations have produced conflicting results; Trabert et al. (1978) found no difference in impact fracture resistance between root-treated teeth and untreated teeth. Insertion of a large post could weaken a root, whereas posts of smaller diameter might provide some reinforcing effect. Conversely, Guzy and Nicholls (1979) reported no difference in fracture behaviour under oblique compression between root filled teeth with or without a 'reinforcing' post. Lovdahl and Nicholls (1977) showed that 'unreinforced' teeth with restored access cavities required a greater force to fracture under similar conditions than those with two types of core. Tjan and Whang (1985) found that a large post produced a weaker root than a small post. Although inconsistent, all of these results suggest that preservation of internal dentine, rather than provision of a post, may be a critical factor in determining the strength of a tooth. Clinical studies provide little direct evidence of the value of the 'reinforcing' effect of a post, but Sorensen and Martinoff (1984) observed no greater success rate where posts had been placed. Ross (1980) found no evidence of tooth fracture, irrespective of whether or not 'internal support' in the form of a post was used. Recent technology has produced a rigid non-metallic composite in the form of C F R C which can be formed in various configurations in order to make maximum use of its properties. When fibres are aligned at an angle to the principal axis of the composite, high transverse strengths and moduli of elasticity

Impact fracture resistance of root-treated teeth

similar to those of bone are achieved (Hobkirk 1982). CFRC in the form of a prefabricated post has been developed'. Its mechanical and biocompatible properties render it theoretically acceptable for consideration in an endodontic post system (Lovell 1983). Initial experiments in vitro have shown that post-retained crowns using a prefabricated CRFC post exhibited properties comparable with, and in some cases, better than those of either wrought precious alloy or stainless steel prefabricated posts. The mode of failure of specimens restored with a CFRC post was less damaging to the remaining tooth tissue than that of specimens restored with a metal post (King 1984). There have been no experiments to determine the effect on the strength of intact root-treated teeth of 'reinforcement' with a prefabricated CFRC post. The aim of this study was to test the impact fracture resistance of root-treated mandibular incisors restored with either stainless steel or CFRC prefabricated posts. Materials and methods Forty-five freshly extracted lower incisor teeth were selected for study on the basis of similar dimensions and absence of caries, restorations, cracks or fractures in either root or crown. The bucco-lingual and mesio-distal dimensions of all specimens were measured at the level of the buccal amelo-cemental junction, using a thickness gauge accurate to within 0.! mm. In order to meet the criteria for inclusion in the study, the bucco-lingual dimension was required to be 6 + 1 mm, the mesio-distal 4 + 1 mm, and the length from incisal edge to apex 22 + 0.5 mm. The teeth were stored in 5 per cent formol saline solution until required for preparation. All teeth were prepared in a standardized clinical manner. The access cavity was prepared incisally at half the mesio-distal distance. Initial access was made lingual to the incisal edge to allow straight-line access to the root canal whilst preserving the labial enamel. A 12-fluted tungsten carbide bur was used to standardize the access cavity size and shape. The teeth were held in tissues soaked in ' Dunlop Medical Products, Coventry, UK.

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water to ensure maximum hydration during preparation. Following canal location, the canal was prepared by hand with K-flex files using a standardized stepback technique (ISO sizes 08-40). The apicai preparation was enlarged to size 25, and Gates Glidden burs (sizes 2-5) were used to enlarge the canal orifice. During instrumentation the canals were irrigated with 1.6 per cent (w/v) sodium hypochlorite. When the preparation was complete, the root canal was obturated. A size 25 master gutta-percha point was lightly coated with Tubliseal" and inserted. Lateral condensation with a finger spreader and accessory points was used to complete the obturation. The specimens were randomly allocated to one of three groups for completion of the restoration. Group I Gutta-percha was removed to a level 1 mm apical to the lingual margin of the access cavity. The cavity walls were cleaned and Scotchbond"' dental adhesive was applied in a thin layer and light cured using a Luxor light*. The cavity was filled with P30' composite resin which was light cured. The composite resin was finished on a subsequent day using a white stone under waterspray. Groups II and III To facilitate placement of either a stainless steel' or CFRC rod', gutta-percha was removed in the remaining 30 teeth to within 5 mm of the apex of each specimen using Gates Glidden burs (sizes 2-5) and finally a Parapost drilP of diameter 1 mm (yellow). These 30 teeth were divided into two groups of 15 teeth each. Group II were fitted with stainless steel cemented posts and Group III with carbon fibre reinforced carbon. The posts were 12 mm long and ended 5 mm from the apices of the teeth. Since the " Kerr, Bretton, Peterborough, UK. ' 3M Dental Products, St Paul, MN, USA. *iCI Pharmaceuticais, Macclesfieid, Cheshire, UK. 'Parapost, Whaiedeiw Imernationai, New York, USA.

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A. V. McDonald, P. .4. King (S D. J. Setchell

5 rnm

12 mm

2 mm

11 mm

J9mm 5 mm

2 0 mm 22 mm

Fig. 1. Diagrammatic representation of specimen. teeth were all of similar length, approxitnately 8 mtn of the post was within the root and 4 mm was within the anatomicai crown. The coronal end of the post was 5 mm below the incisal edge. Conclude' composite resin luting agent was mixed according to the manufacturer's instructions and introduced into the canal using a spiral root filler^. The entire rod was coated with the resin and seated using finger

pressure. The access cavity was filled as descrihed for Group I. To allow for testing, the teeth were mounted vertically in 20 mm diameter brass bushes^ to a level 1 mtn below the ameiocemental junction using a highly filled acrylic resin'* (Fig. 1). During the exothermic setting reaction of the acrylic resin, the specimens were placed in water to dissipate heat. All specimens were numbered and stored in distilled water at room temperature.

' 3M Dental Products, St Paul, MN, USA. ^Zipperer, Giover Dentai Supplies, Shrewsbury, UK.

'Logic M.IC. Ltd, RS Components Part No, 608222, Corby, Northants, UK. •" Formatray, Kerr Uk, Peterborough, UK.

Impact fracture resistance ofroot-lreuted teeth

307

transient recorder and sampled at a preselected rate. The resulting data enabled measurement of the loss of kinetic energy caused by the impact between the striker and the specimen. Theinformation was processed by a con version and measuring module before storage in an .4pple II micro-computer" which generated force/time and energy/time graphs. The peak force was derived from the highest peak on the force/time curve and thus represented the largest force developed as a result of impact. The. peak energy was read from the energy/time curve at the time corresponding to development of the peak force and represented the energv absorbed by the specimen prior to failure. However, the energy/time curves indicated that the energy le-sei continued to rise after the peak force had de\eioped, as the disruption of the specimen progressed. Therefore the first peak total energy was also

Fig. 2. Ceast penduluro machine. Experimental method

A Ceast pendulum machine' {Fig. 2) was used to apply an impact load to the specimens using an Izod technique, whereby the specimens were supported as a cantilever. The impact striker contained a tup which delivered the impact force. This had a radiused shape with a diameter of 6.34 mm and a mass of 2.3 kg. The velocity of the striker was calculated to be 1 m second"'. A metal mounting block was designed and machined to accept the threaded specimen holder. It contained two locking devices which allowed hoth rotational movement ofthe specimen and linear movement in a horizontal plane. These adjustments were essential to align each specimen correctly so that the tup impacted each specimen midway between the incisal edge and the labial amelo-cemental junction, at 90 degrees to the surface (Fig. 3). The tup contained a semiconductor strain gauge having electrical properties which varied with force. The output voltage was fed into a ' Ceast, Turin, Italy.

recorded as the highest point reached on the energy axis of these plots. The mode of fracture was noted and a diagram of the fracture pathways was recorded. All specimens were viewed under a Wild Photomicroscope^, and photomicrographs were taken to record the type and position of the fracture. Comparisons of the group means for peak energy, and first peak total energy were made using a one-way analysis of variance. The Kruskal-Wallis one-way analysis of variance by Ranks was applied to the results for peak force for all three groups. Results The results are represented graphically in Figs 5, 6 and 7. All the specimens demonstrated complete fracture. Twenty-eight samples showed a single oblique fracture occurring near the embedment site of the specimens in the acrylic resin. The remaining 17 samples demonstrated a double fracture. These specimens tended to fracture near the embedment site and also at the site of impact. Both fracture types were equally distributed between the three groups. The typical mode of fracture occurring in each group was as follows. Group III demonstrated a 'greenstick' fracture of the CFRC Apple, Cupertino, California, USA. •' Wild, Heebrugg, Germany.

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A. I'. .McDonald. P. A. King IS D.J. Setchell

Fig. 3. Alignment of specimen with tup.

Fig. 4. Typical mode of fracture in each group; sfainles.s steel (left), gutta-percha (centre), CFRC (right).

post. Group I fractured with the coronal portion retaining 6-8 mm of the condensed gutta-percha. In Group II the stainless steel rod was usually retained In the crown and bent to some degree (Fig. 4). One-way analysis of variance demonstrated no statistically significant difference between group means for peak energy and first peak total energy (Table I). The variances of the peak force values for stainless steel and those for CFRC were significantly different (P

In vitro study to compare impact fracture resistance of intact root-treated teeth.

The aim of this in vitro study was to determine the effect on impact fracture resistance of three methods for restoration of root-treated lower inciso...
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