forces of various
post and core
K. W. Hemmings, BDS, MSc, FDS, RCS,a P. A. King, BDS, MSc, FDS, RCS,b and D. J. Setchjell, BDS, MS(Mich.), FDS, RCSC Institute
Surgery, Eastman Dental Hospital,
This in vitro, study investigated the resistance of various post and core designs to torsion.al forces. Cast gold, parallel-sided, serrated post and cores were cemented in extracted teeth. The test groups were made up of the following designs: controls without an antirotational feature, keyway form, coronal flare form, auxiliary pin form, a cervical collar form, and included a tapered post group. The specimens were mounted on an Instron Universal testing machine and a torsional force was applied to the core using a lever arm until failure. All the antirotational features tested elevated resistance to torque. Failure occurred through fracture of the tooth, the cement, or an optional auxiliary pin. The cervical collar was the most favorable design, embracing resistance and reducing tooth fractures. (J PROSTHET DENT 1991;66:325-9.1
ost in vitro studies have been performed to evaluate post and core designs subjected to tensile,lT6 compressive/shear fo:rces,7-11 or trauma?14 However, posts are also subjected clinically to torsional or rotational forces produced by functional tooth cant acts,15so various antirotational locks ‘have been advocated. These locks included a shallow parallel-sided slot across the root face,12an auxiliary pin,” coronal flares, l7 keyways or notches prepared in the coronal surface of the post hole,ls and an extracoronal collar or diaphragm.1g~20Preservation of the remaining tooth tissue is the chief concern in preparing an effective antirotational feature. If failure does occur, it would be preferable to involve cementation failure rather than catastrophic root. fracture. There are diverse opinions as to whether antirotational features are necessary, considering the natural eccentricity of root canal morphology. If further resistance to rotational forces is indicated, there have been no specific recommendations for a preferred design. This in vitro study tested the resistance of various post and core designs to torsional forces.
Sixty freshly extracted, sound human canine teeth of comparable size were collected for experimentation. The coronal surfaces of the crowns were removed 2 mm above the facial cementoenamel junction. The root was then
Fig. Condensed from a research report presented as a requirement for the degree of M.Sc. in Conservative Dentistry, University of London. KRegistrar, Department of Conservative Dentistry. b~Consultant, Department of Conservative Dentistry. CConsultant and Head, Department of Conservative Dentistry.
Six test groups.
mounted vertically in individual brass electrical fittings (Logic MK Ltd., London, UK), using heavily filled acrylic resin (Formatray, Kerr/Sybron, Romulus, Mich.). The root face was then ground perpendicular to the long axis of the tooth. 325
Fig. 2. Castings of post and cores. A, From left, control, auxiliary pin, cervical collar. B, From left, tapered, keyway, coronal flare.
A post hole 10 mm deep and 1.5 mm in diameter was prepared with files (Kerr/Sybron), Gates-Glidden burs (Produits Dentaires, SA, Vevey, Suisse), and twist drills (Whaledent International, New York, N.Y.). Endodontics was simulated by packing zinc polycarboxylate cement (A. D. International Ltd., Potters Bar, England) into the apical surface of the root canal. The specimens were stored in distilled water at room temperature and were randomly assigned to one of six groups (Fig. 1). Group A (control group)-No further tooth preparation Group B (keyway)--Keyway was prepared in the greatest bulk of dentin to a depth of 3 mm using a tungsten carbide 170 bur (Claudius Ash Sons & Co. Ltd., Potters Bar, England). Group C (coronal flare)-The coronal 3 mm of the post hole was flared to leave a minimum of 1.5 mm of circumferential dentin in the mesiodistal plane and 2 mm in the faciolingual plane. Group D (auxiliary pin)-A pin hole 3 mm in depth was
prepared in the greatest bulk of dentin using the Parapost (Whaledent International) pin jig and twist drill. Group E (cervical collar)-A 45-degree bevel was prepared at the periphery of the root face using a tungsten carbide chamfer bur (RCB 11-15 Wright Dental Ltd., London, England). Group F (tapered)-The preparation was flared from the base of the post hole coronally to leave the same surface post hole dimensions as in group C and an ovalshaped cross section. The sharp edges in the preparations were smoothed using a diamond finishing bur (Hi-D1 Diamond Precisions Tools Ltd., London, England). Direct wax patterns were shaped around a serrated laboratory burnout post, and in group D Metforte (Whaledent International) platinum iridium pins 0.7 mm in diameter were used to cast type III gold (Englehard, Surrey, England) (Fig. 2). Two parallel walls on the core facilitated mounting on the testing apparatus. The dowels were sandblasted with 50
:I$+ 3. Instron testing machine and specimen assembly. rl, Mounting ‘lever arm:. D, tension spring; and E, load head.
jig: R. specimen: (‘.
.ioad (newton meters) Group No. B
2 ‘3 I $5
Mear SC SD, Standard
pm aluminum oxide abrasive ;and were then cemented to the teeth with zinc phosphate cement (powder:liquid ratio of 0.25 gm:O.l ml; A. D. International Ltd.) using finger pressure. The specimens were stored in distilled water for a minimum of 24 hours prior .~otesting. Specimens were inserted into a mounting block so that a torque lever arm could be aligned parallel to the base of the block (Fig. 3). A tension spring exerting a pull of 200 gmf was att.ached to the lever arm. A controlled load was applied to t:le lever arm 30 m:n from the long axis of t,he
post by an Instron Universal testing machine (Instron Corp., Canton, Mass.), at a crosshead speed of 5 cm/min. Multiple comparisons of the mean values at failure were computed using the Kruskal-Wallis test and the MannWhitney rank sum tests.
RESULTS Failure was established as the point at which the specimen could not withstand an increase in load (Table I). The mean load at failure and the standard deviation are also il-
KING, AND SETCHELL
Fig. 4. Mean torque with standard deviations at failure. A, Control; B, keyway; C, coronal flare; I1, auxiliary pin; E, cervical collar; and F, tapered.
Mode of failure for test groups Mode
C D E
Auxiliary pin fracture
lustrated in Fig. 4. These differences between the groups were statistically significant at a level of probability of p < 0.05 except for group F (tapered form), that could not be differentiated from either group B (keyway form) or group C (coronal flare form). Failure occurred through cementation failure, tooth fracture, or fracture of the auxiliary pin (Table II).
DISCUSSION The antirotational features in this study substantially increased the resistance to torsional forces. When compared with group A (control), the relative increase in resistance is evident: auxiliary pin X3; keyway x6; tapered X8; coronal flare x9; and cervical collar ~13. These results conflict with those of Tjan and Miller,15 who concluded that antirotational features did not improve
retention. However, their method of testing involved the application of intermittent rotational forces prior to tensile testing. All specimens were made from the same materials, so thermal cycling or aging tests would not influence the aims of the study. The standard deviations were comparable with similar studies that used human teeth. The results of group A (control) were similar to those of Ruemping et a1.21 The keyway form in this study exceeded the anticipated performance, with a reduced number of tooth fractures in 6 of 10 specimens. This may have resulted from a preliminary smoothing of the sharp preparation angles and redistributing stress concentration or from the beneficial effect of preserving dentin with respect to the other designs. The antirotational feature should provide resistance to torsional forces but should not elevate the fracture failure rate. The desirable attributes of antirotational features were best demonstrated by the cervical collar, but clinically the appearance with this design is unacceptable to many patients. Occasionally there is insufficient clinical crown height for supragingival margins or to avoid undue encroachment into the gingival sulcus and violation of the biologic width.
Six different forms of cast gold post and cores (round, parallel-sided; keyway; coronal flare; auxiliary pin; cervical collar; and tapered) were subjected to torsional forces. All the antirotational features in this experiment improved resistance to torsional forces. The cervical collar was the
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