Gianluca Gambarini
University of Rome "La Sapienza"

New instruments for the preparation of the dowel space.

Abstract
The ability of the instruments to stay centered is essential to provide a correct enlargement, without excessive weakening of the root structure, during preparation of the dowel space. Several studies have shown that nickel-titanium endodontic instruments remain significantly more centered and demonstrated less canal transportation than stainless steel files .Two different instrumentation techniques (1. New NiTi rotary instruments (Profile Orifice Shapers) and 2. Traditional instrumentation with SS Peezo burs) have been used to prepare dowel space in extracted teeth. Evaluation of the channel has been made by comparing cross-section photographs of before (Fig.1) and after instrumentation (Fig.2),using a modified Bramante technique. Dentin removal and centering ratio were evaluated and compared.The results of the present study showed statistically significant differences between the two techniques:ProFile Orifice Shapers produced a more symmetric,rounder enlargement of the root canal, and well planed canal walls.They can be successfully used for dowel preparation, provided that dimensions and tapers are compatible with those needed for the dowel.

Fig.1: Cross-section photograph

Fig.2: Dowel preparation with Orifice Shapers

Introduction
The placement of a dowel into the prepared root canal is often part of the final restoration of an endodontically restored tooth. The preparation of the dowel space involves removal of a part of the root canal filling materials and usually enlargement and shaping of the root canal itself. (1). However, instrumentation for dowel space is a quite difficult task,that requires considerable skill and an accurate understanding of individual root canal systems. It's important that excessive dentin not to be removed in the preparation of the dowel space, to mantain good resistance of the root to fracture. Studies have shown that the dowel which provides adequate reinforcement for the core without endangering the root has a diameter varying from 0.7 mm ( on mandibular incisors) to 1.7 mm (on maxillary incisors) , and has more than 1 mm of tooth structure remaining around the post(2).

Peezo burs are often used to form a dowel space within the root ( custom cast dowels). The number 3 and 4 reamers ( with a diameter of 1.1 through 1.3 mm.) have dimensions which can be paired with the majority of castable plastic patterns ( with a diameter of 0.8 mm. through 1.0 mm) , and can estabilish a clean, parallel- sided dowel channel without overinstrumentation ( 3). However, the stiffness of stainless steel alloy and the cutting tips of Peezo reamers can be sometimes dangerous, especially in curved or tapered canals. In those cases a tapered form is a better alternative to the excessive instrumentation that would need for a propoerly adapted parallel post.

ProFile Orifice Shapers (Maillefer-Dentsply, Baillagues, Switzerland) are nickel titanium rotary intruments which have been recently proposed for early coronal enlargment of the root canal space, using a Crown-Down preparation technique( 4). The mechanical properties of the alloy and the specific design of the instruments ( radial lands and non-cutting tips) prevent them from cutting into the canal walls in an uncontrolled fshion and causing unwanted transportation which may weaken the tooth structure. Several studies have shown that nickel-titanium rotary instrumentation produce rounder canals and remain more centered in the original canal than traditional stainless-steel instrumentation techniques.( 5,6 ) Moreover, these new instruments have diameters and tapers which can be paired with castable plastic patterns (Table 1).

On these basis, Profile Orifice Shapers have been used to prepare dowel space in extracted teeth and evaluation of the channel has been made by comparing cross-section photographs of before and after instrumentation,using a modified Bramante technique. Dentin removal and centering ratio were evaluated and compared with traditional stainless steel instrumentation.

Fig.3: ProFile Orifice Shapers

TABLE 1: ProFile Orifice Shapers. Diameters and Taper.

Materials and methods
20 palatal roots of extracted human maxillary molars were selected for this study Root canals were prepared using ProFile .04 and .06 Tapers (Maillefer,Baillagues,Switzerland) nickel-titanium rotary instrumentation with a .06-25 as a master apical file. The twenty root canals were obturated using vertical compaction of warmed guttapercha and then randomply divided into two groups. To accomplish comparison(Fig.4) of the obturated canal shape with postinstrumentation ( after dowel preparation) shape a modified Bramante technique was used (7). Root were embedded in a casting resin and each block was cut perpendicular to the root surface , 6.0 mm from the apical foramen (Fig.1), using an Isomet low-speed saw refrigerated with water. The section were photographed using a digital camera (Sony SC-100E) and transferred to a computer (McPerson EOS, with Windows '98 system software) using a DV still image capture board ( DVBK-2000E, Sony). Digital images were analyzed and measurements were accomplished using an imaging software package (Corel Draw).

The resin blocks were reassembled for instrumentation by repositioning the apical and coronal sections in the stone locking jigs. Dowel space was prepared using the following techniques:

GROUP A: ProFile Orifice Shapers ( Fig.3) no.4 through 6 were sequentially inserted into the canal, reaching a working length of 9-10 mm ( approximately 4-5 mm from the apical foramen.). Rotation speed was selected at approximately 600 rpm.

GROUP B: Peezo burs no.2 through 4 were sequentially used at the same working length of 9-10 mm.( approximately 4-5 mm from the apical foramen).

The blocks were disassembled and imaged (Figg.2,5) The amount of dentin removed and the mean centering ratio ( the ability of the istruments to remain centered in the canal) were quantitated and statistically analized using a chi-square test and the Fisher's test. The level of significance was set at P<0.05.

Results

TABLE 2: Amount of Dentin Removed and Ability of the Instruments to Stay Centered After Dowel Preparation ( Data are expressed as the mean +/- SD)

Results are shown by Table 2.The area of dentin removed was calculated by subtracting the area of the canal before and after dowel preparation.Overall, ProFile Orifice Shapers tended to produce rounder canals and remove more dentin.However,there was no significant difference in the total amount of dentin removed between the two groups.

As suggested by Calhoun and Montgomery (8),the mean centering ratio was calculated using the following formula: X1-X2/Y. X1 represents the maximun extension of canal movement in one direction, X2 is the movement in the opposite direction and Y is the largest diameter of canal preparation. Canals instrumented with Profile Orifice Shapers remained significantly more centered (Fig.4) than those instrumented with Peezo burs.

No instrument separation occured in any of the two groups.

Fig.4: Superimposed image

Fig.5: Dowel Prep.with Peezo burs

Discussion
Several studies have shown that nickel-titanium instruments remain significantly more centered and demonstrated less canal transportation than stainless steel files (4-6).Such favourable properties are extremely useful for dowel prepation. Canals must be enlarged to allow the dowel to withstand the forces of mastication. On the other hand,dowel diameters must be gauged carefully to avoid unnecessary removal of dentin which may lead to root fracture (2). The ability of the instruments to stay centered is therefore essential to provide a correct enlargement and without excessive weakening of the root structure.

Peezo burs are rigid,aggressive instruments with a tendency to straighten the root canals. They should be used carefully in order to avoid excessive or poorly centered preparation (Fig 5). A certain degree of transportation was observed in many canals, with a tendency to transport toward the inner side on the root. However,this trend was not entirely consistent as some canals displayed either no transportation or tranportation in the opposite direction.

On the contrary, ProFile Orifice Shapers rotary intrumentats are more flexible and incorporates two factors ( specific file design and nickel-titanium alloy) which independently are believed to reduce canal transportation. The results of the present study confirm the potential of this new generation of instruments to shape canals safely and in the appropriate manner.Overall, .ProFile Orifice Shapers produced a more symmetric,rounder enlargement of the root canal, and well planed canal walls. A minimal tranportation was often observed, but its magnitude was so small as to be unimportant with the result that the original shape of the obturated canals was largely mantained. Rotation speed at 600 rpm allowed easy removal of guttapercha-and-sealer root canal obturation.

The present study suggested that nickel-titanium rotary instruments can be successfully used for dowel preparation, provided that dimensions and tapers are compatible with those needed for the dowel. They remained more centered in the canals,allowing rounder preparations and a safer enlargement of the endodontic space.

Bibliografy

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