Avaliou-se "in vitro" a influência da utilização de diferentes alargadores cervicais na determinação do diâmetro anatômico. Foram selecionados cinquenta pré-molares superiores. Após o acesso endodôntico e a remoção do tecido pulpar, os dentes foram divididos aleatoriamente em cinco grupos distintos, de acordo com o tipo de alargamento realizado: Grupo I- sem alargamento cervical; Grupo II- brocas Gates-Glidden (#2, #3); Grupo III- Orifice Opener (25.08, 25.10); Grupo IV- Protaper (S1, SX); Grupo V- LA Axxess (20.06, 35.06). A determinação do primeiro instrumento que travou em cada canal radicular, no comprimento real de trabalho, foi determinado por meio de uma lima tipo K, e o diâmetro das limas foram registrados. Depois de fixados os instrumentos em posição, os ápices foram seccionados, e levados ao microscópio eletrônico de varredura. Obtidas as fotomicrografias, estas foram digitalizadas e procedeu-se à mensuração do menor diâmetro do canal e do diâmetro do instrumento, com o auxílio de um "software". A diferença entre esses diâmetros, de acordo com cada grupo, foi submetida à análise estatística. A análise de variância indicou diferença estatisticamente significante entre os grupos (p<0,001). O teste complementar de Tukey evidenciou que o grupo cujo alargamento cervical foi efetuado com brocas LA Axxess apresentou a menor discrepância (x̄ = 0,0008mm ± 0,0007) e resultados estatisticamente diferentes dos demais, seguido pelo grupo cujo alargamento foi realizado com instrumentos Protaper (x̄ = 0,0355mm ± 0,0151). Os grupos em que se utilizaram as brocas Gates-Glidden (x̄ = 0,0832mm ± 0,0308) e instrumentos Orifice Opener (x̄ = 0,0736mm ± 0,0249) apresentaram-se em posição intermediária e diferente dos demais. As maiores discrepâncias (x̄ = 0,1589mm ± 0,0294) foram representadas pelas amostras que não receberam alargamento. Pode-se concluir que o alargamento dos terços cervical e médio permitiu melhor determinação do instrumento apical inicial. No grupo em que foram utilizados instrumentos LA Axxess, a determinação do diâmetro anatômico apical do canal radicular foi mais fiel.

To investigate the influence of cervical preflaring with different instruments (Gates-Glidden drills, Orifice Opener, ProTaper instruments and LA Axxess burs) on the first file that binds at working length (WL) in buccal roots of maxillary premolars. Fifty human maxillary premolars with two roots, separated from the cervical level, were used. After standard access cavities, a size 06 K-file was inserted into the buccal canals until the apical foramen was reached and working length was set 1 mm short of it. Group 1 received the initial apical instrument without previous preflaring of the cervical and middle thirds of the root canal. Group 2 had the cervical and middle portion of the root canals enlarged with Gates-Glidden drills sizes 90 and 110. Group 3 had the cervical and middle thirds of the root canals enlarged with Orifice Opener instruments. Group 4 had the cervical and middle portions enlarged with ProTaper instruments. Titanium-nitrite treated, stainless steel LA Axxess burs were used for preflaring the cervical and middle portions of root canals from Group 5. Each canal was sized using manual K-files, starting with size 08 files with passive movements until the WL was reached. File sizes were increased until a binding sensation was felt at the WL, and the instrument size was recorded for each tooth. The apical region at WL was then observed under a scanning electron microscope, images were recorded digitally and the difference between root canal and maximum file diameters were evaluated for each sample. Significant differences were found between experimental groups regarding anatomical diameter at the working length and the first file to bind in the canal (P < 0.0001, 95% confidence interval). The major discrepancy was found when no preflaring was performed (0.1589mm average). The LA Axxess burs produced the smallest differences between anatomical diameter and first file to bind (0.0008 mm average). Gates-Glidden drills and Orifice Opener instruments showed no statistically significant differences between them (0.0832mm and 0.0736 mm average, respectively). ProTaper instruments presented an average of 0.0355mm for the discrepancy values. The instrument binding technique for determining anatomical diameter at working length is not precise. Preflaring of the cervical and middle thirds of the root canal improved anatomical diameter determination; the instrument used for preflaring played a major role in determining the anatomical diameter at the working length. Canals preflared with LA Axxess burs created a more accurate relationship between file size and anatomical diameter.