Effect of core materials on stress distribution of posts

Effect

of core materials

on stress distribution

Peter Yaman, DDS, MSya and Thorsteinn University

of Michigan,

School of Dentistry,

S. Thorsteinsson,

of posts

DDS, MSb

Ann Arbor, Mich.

Severely damaged endodontically treated teeth require the use of a post and core for the retention of the restoration. The choice of material for a post and core and their stress-producing characteristics must be considered by the clinician. Twodimensional photoelastic analysis of birefringent models was used to investigate the influence of different core materials on the stress distribution of nonthreaded cylindrical posts. Cast posts and cores and prefabricated posts with amalgam and composite resin cores were compared with posts without cores at installation and under two loading conditions. The findings indicate a significant difference between posts with and without cores. It appears that the stiffer core materials can shift the load from the apex to the coronal region, (J PROSTHET DENT 1992;68:416-20.)

e&oration of the pulpless tooth is critical for successful endodontic therapy. If the tooth is severely damaged, a post and core is indicated for retention of the restoration and reinforcement of the tooth. A post and core restoration has many aspects for the dentist to consider. Among these are the choices of materials and the stressproducing characteristics in relation to the size and shape of the selected post. Gold alloys have been the traditional material for cast posts and cores. Casting alloys have superior physical properties (Table I), but are costly and their use requires a two-step clinical procedure. Composite resin is economical, time-efficient, and has been recommended for the core in conjunction with a cemented reinforcing rod.lm4However, the physical properties of composite resins do not compare favorably with amalgam and cast metal. They are also dimensionally unstable because of water absorption that can affect the seating of crowns.5’ 6 Amalgam that has better physical properties than composite resins can form the core, but the major clinical disadvantage is the slow setting rate requiring longer chair time.7 Stress transfer from the restoration to the remaining root structure should ideally be uniform and of low magnitude. Smooth or serrated tapered. and cylindrical posts are benign on installation. 8-11However, upon loading, the tapered posts show high wedging forces and the cylindrical shaped posts create high apical stresses.8a11-14Threaded cylindrical posts show stress concentration where they engage the supporting structure and a relative increase in stress upon loading.ls, I6 Threaded conical posts create high wedging forces at installation and upon loading.g This study used the photoelastic stress analysis method to investigate the influence of different core materials on the stress distribution of serrated cylindrical shaped posts

aAssistant Professor, Department tistry. bPrivate practice, Iceland. 10/1/35666

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of Cariology

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Cervical l/3 Middle l/3 Apical 113

Fig. 1. Experimental model.

at installation and upon loading. Concepts, application, basic principles of photoelasticity, and photoelasticity as a predictor have been described in the literature.17 MATERIAL

AND

METHODS

Forty blocks, 13/4 by 1% square and % in thick were prepared from PSM-1 photoelastic material (Measurement Group, Inc., Raleigh, N.C.). Manufacturer’s directions were followed in the handling, preparation, and annealing during mechanical fabrication of the blocks. The Parapost Plus post system (Whaledent International, New York, N.Y.) was used to prepare post spaces in the blocks. After the individual blocks were prepared, they were divided randomly into four equal groups. A titanium post, 1.6 mm in diameter, was used for three of the groups. This post was shortened to 14 mm in length, 5 mm for the head length, and 9 mm for the embedment depth. The groups were as follows: group l-a Parapost Plus post with no core serving as the control; group 2-a Parapost Plus post with composite resin core; group 3-a Parapost Plus post with amalgam core; and group 4-a cast gold post and core.

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AREAS I

v-

Cervical l/3

Middle l/3

Apical l/3

Parapost Plus without core Parapost Plus+composite resin axe Parapost Plus+amalgam core Cast Parapost and core

Fig. 2. Fringe order in principal surfaces for all groups at installation. Vertical lines denote SD. and bars joined by a horizontal line are not significantly different at the 95 % confidence level @ > 0.05).

All the posts, centered on one side, were installed with zinc phosphate cement (L. D. Caulk Co., Milford, Del.). In an attempt to simulate a common clinical situation, the cores were made 1.5minutes after the insertion of the posts. This timing was to correlate the setting time difference of the cement at mouth and at room temperature. Group 2 received a Concise (3M Dental Products, St. Paul, Minn.) crown build-up composite resin core, and group 3 received a Tytin (Kerr Mfg., Co., Romulus, Mich.) amalgam core. To facilitate exact placement of the cores on the models, a jig was made. This matrix analogue, with a hole 3/16 inch in diameter and 1/44inch high, was accurately machined from brass to be assembled and seated on the blocks so that the hole was centered over the post head (Fig. 1). When the core material was set, the jig was disassembled and moved to the next block, leaving a precisely formed and placed core. The jig was used with a Parapost laboratory burnout post (Whaledent International) to wax the pattern for the cast posts and cores. The patterns were invested with Beauty Cast Material (Whip Mix Corp., Louisville, Ky.) and were cast with Forticast gold (J. F. Jelenko & Co., New Rochelle, N.Y.). The castings were cleaned and any surface irregularities that would prevent complete seating were removed. After insertion and fabrication of the cores, each group was randomly divided into two subgroups, five samples to be tested at 30 lb vertical load and five samples to be tested at 30 lb at 26 degrees inclined load. The models were subjected to stress analysis in a circular polariscope (Photoelastic, Inc., Malvern, Pa.). The resulting colorful pattern of fringes, whose number and closeness will indicate relative magnitude and concentration of stress, were recorded with an OM-4 camera (Olympus Corp., Wood-

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bury, N.Y.) at insertion and at the determined loading condition. The color prints, 3.5 X 5 in, were evaluated in three locations-cervical, middle, and apical one third (Fig. 1). The maximum fringe order to the nearest one half order was recorded for each location. The data were analyzed with a one-way analysis of variance and t,he Student t test to compare the groups. Further, the t tests were recalculated using the method of Bonferroni to minimize the possibility of type 1 error.

RESULTS The statistical data were recorded and are presented in Figs. 2 through 4. As can be seen in Fig. 2, the tested posts were very benign on installation. The overall picture shows the cast post relatively free of installation stresses and the Parapost Plus post demonstrating slight stress for all the groups in the middle and the apical regions, especially the group with the composite resin core. No significant difference between the groups was found in the cervical and the middle regions, while the Parapost Plus post group with the composite resin core was significantly different from all the other groups in the apical region. A considerable increase in stress was observed in all regions when the posts were loaded vertically (Fig. 3). In the cervical region the posts without the cores registered the lowest stresses, followed by the composite resin core and the amalgam core groups. The cast post and core demonstrated almost twice the stress of the control group. The stress was concentrated in close proximity to the cores in the experimental groups but was more evenly distributed for the control group. There was a statistically significant

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AREAS l

,

Cervical 113

Middle 1I3

Apical 113

q

Parapst Plus without core •C Parapst Plustmmpsite resin core q Parapost Phamalgam core n Cast Parapst and core

Fig. 3. Fringe order in principal surfaces for all groups at vertical loading. Vertical lines denote S.D. and bars joined by a horizontal line are not significantly different at the 95 % confidence level (p > 0.05).

difference between the groups at the 95 % confidence level (p < 0.05). In the middle region the groups demonstrated evenly distributed stresses of similar magnitude and concentration without a significant difference. The stress picture of the apical region was almost the opposite of that of the cervical region. Here the control group registered the highest stress, followed by the composite resin and the amalgam groups, with the cast post and core registering the lowest stress. The stress was always concentrated at the apex of the posts. A statistical difference was not found between the core groups, while a highly significant difference was found between the cast and amalgam core groups with respect to the control group. The data for the inclined loading are presented in Fig. 4. Characteristic of this loading condition was the accumulation of stress on the compressive side, resulting in an asymmetric distribution of stress. In the cervical region the stress was concentrated beneath the cores with significantly more for the cast core group than for the other experimental groups. However, like the control group, the cast post concentrated the stress closer to the post itself. The middle region was stressed to a similar magnitude and concentration for all groups. Although of different magnitude, the apical stresses at inclined loading appear similar to the apical stresses at vertical loading. Again, the stress was concentrated at the very apex, more for the control group than in the experimental groups.

DISCUSSION It has been postulated that some design features of dental restorations possess stress induction of a magnitude that is harmful to teeth. If the magnitude and concentra-

428

tion exceed the strength of the supporting structure, failure might result. Stress from installation is important, since it does not dissipate and must ultimately combine with functional stress. The apical stresses encountered at installation have been described before as a result of finger pressure or hydrostatic back pressure.8f l1 Apical stress should not be generated if the post apex does not interact with the channel apex and also if the post is adequately vented. However, hydrostatic pressure in the post hole can reach the level of 1.7 x lo4 kN/m2 for a 1.3 mm. diameter post.i8 This pressure must be a function of cement thickness, length of the post, and the way the post is vented. The observed difference between the cast post and the Parapost Plus post might be explained by the difference in their design. The cast post has one straight venting groove while the Parapost Plus post has nine shallow spiralling flutes, possibly leading to decreased venting and increased hydrostatic pressure for the latter. During setting, composite resin materials contract, creating setting forces at the restorationtooth interface as high as 130 kgJcm2.ig If the composite resin used for the cores in this study binds to the photoelastic material, it could possibly push the post apically. The result would be interaction with the channel apex and increased apical stresses (Fig. 2). The ability of a post to distribute stress can be affected by the core, the core material, and eventually by the final restoration. The core should be made from materials that have adequate modulus and yield strength to complement the mechanical properties of the underlying post and tooth structure.17 For illustrative purposes, Table I contains representative values to reflect the physical properties of

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I Cervical 113 7.2 7.0

Middle II3

Apical II3 a

Parapost Plus wihout core ampost Plustmmpos~te resin mre

n

Cast Parapost and rn@

Fig. 4. Fringe order in principal surfaces for all groups at inclined loading. Vertical liraes denote S.D. and bars joined by a horizontal line are not significantly different at the 95% confidence level (p > 0.05).

the core and post materials used in this study. The Concise core build-up material is filled with large particles and exhibits favorable physical properties within the group of composite resins. Tytin is a high copper spherical amalgam that was selected because of its fast setting rate and popularity. The Forticast gold fulfills the physical properties for a suitable material for cast posts and cores. The literature is limited with respect to the effect of a core material on the stress distribution of a post. However, there is a report showing a reduction in stress eoncentration around loaded serrated posts with composite resin cores when compared with posts without cores.ll Fig. 3 indicates that cervical stresses increased in concentration and magnitude as the physical properties of the core materials improved, while the middle surface was unaffected by the difference. Cores appear to inhibit the intrusion of the loaded posts, resulting in diminished apical stress concentration: allowing the load to be shifted to the cervical surface. It is likely that increased stiffness (a combination of modulus of elasticity and cross-sectional geometry) is responsible for this phenomenon, more than any other function of the physical property of the core materials. Teeth with diminished bone support suffer from peripheral root stress that moves apically with increased bone 10ss.l~ At some level, a critical situation could develop if peripheral stress coincided with high internal stresses. This potential danger facilitates the shift of the stress from the apex to the cervical surface by using the stiffer core materials. The bulk of the root structure in the cervical surface is also more suitable to withstand loading. In addition, dentists could protect the tooth by incorporating a collar around the coronal margin. This collar, known as the fer-

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Table I. Properties

of core and post materials*

Yield strength (MPa)

Composite resin 150 Amalgam Gold type IV 350-615 Titanium 850 (Ti 6AI 4V)

Tensile strength W’4 40-70 50-60 470-520 970

Modulus

of elasticity @Pa) ‘7-16 35-62 85-105 100-110

Compressive strength W’a) 210-340 443-516 -

Properties for specific products may vary from the values presented. *Compiled from Craig RG. Restorative dental materials. 8th ed. St. Louis: CV Mosby, 1989, and Caputo AA, Standlee JP, Biomechanics in Dentistry. Chicago: Quintessence Publ, 198’7.

rule design, has been recommended as an aid in holding the tooth together and in preventing fracture.20 Recent studies support the reinforcing role of the collar.21 The fringe patterns found in the cervical surface at inclined loading (Fig. 4) were consistent with the findings of other studies8, g, l1 The surface in contact with the underside of the core showed a high concentration and magnitude of stress that extended down into the middle surface. The concentration was very pronounced for the cast post and core group as a result of sharp internal line angles that are known to concentrate stress and should therefore be av0ided.l’ Accentuated rounding of the line angles in the samples tested in this study might have favored better distribution of the load. It appears that increased plasticity of the amalgam and the composite resin cores helped distribute the stress to the surface underneath the cores and consequently created less cervical stress. All other groups

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showed an even transfer of stress to the middle surface, raising the magnitude of the stress over that observed at vertical loading. The increased support from the cervical surface did to some extent protect the posts from intrusion. Apical stresses were lower than at vertical loading but the nature of the stress was the same, as was the interrelationship between the groups.

SUMMARY Two-dimensional photoelastic stress analysis was used to evaluate stress produced by posts and post-core restorations. Forty models were used for four equally sized groups-one control and three experimental groups. The models were prepared using PSM-1 photoelastic plastic and a prefabricated serrated post for the control and for two of the experimental groups. One experimental group received composite resin cores and the second group received amalgam cores. The third experimental group was made up of cast gold posts and cores. All models were evaluated at installation; five samples in each group were evaluated at 30 lb vertical loading and the other five were evaluated at 30 lb inclined loading. The developed fringe patterns were photographed, and data were registered and analyzed statistically for the cervical, the middle, and the apical areas. Considering the limitations of this study, it can be concluded that: 1. Cylindrical cast and prefabricated posts are passive on installation, demonstrating minor stresses around the lower half of the post, especially at the apex. 2. With vertical and inclined loading, cylindrical posts are intruded and create high apical stresses. Stiffer core materials diminish this intrusion but increase the cervical stresses. REFERENCES 1. Kahn H, Fishman I, Malone WF. A simplified method of constructing a core following endodontic treatment. 3 PROSTHETDENT 197?;37:32-6. 2. Steele GD. Reinforced composite resin foundation for endodontically treated teeth, J PROSTHETDENT 1973;30:816-9.

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3. BarabanDJ. Immediate restoration of pulpless teeth. J PROSTHETDENT

1972;28:607-12. 4. Frederick DR. An application of the dowel and composite resin core

technique. J PROSTHETDENT 1974;32:420-4. 5. Hirasawa T, Hirano S, Hirabayashi S, Harashima I, Aizama M. Initial dimensional change of composite in dry and wet conditions. J Dent Res 1983;62:28-31. 6. Oliva RA, Lowe JA. Dimensional stability of composite used as a core material. J PROSTHETDENT 1986;56:554-61. 7. Shillingburg HT, Fisher DW, Dewhirst RB. Restoration of endodontitally treated posterior teeth. J PROSTHETDENT 1970;24:401-9. 8. Standlee JP, Caputo AA, Collard EW, Pollack MH. Analysis of stress distribution by endodontic posts. Oral Surg 1972;33:952-60. 9. Standlee JP, Caputo AA, Holcomb JP. The Dentatus screw: comparative stress analysis with other endodontic dowel designs. J Oral Rehabii 1982;9:23-33. 10. Henry PJ. Photoelastic analysis of post core restorations. Aust Dent J 1977;22:157-9. 11. Cooney JP, Caputo AA, Trabert KC. Retention and stress distribution of tapered-end endodontic posts. J PROSTHETDENT 1986$5:540-6. 12. Assif D, Oren E, Marshak BL, Aviv I. Photoelastic analysis of stress transfer by endodontically treated teeth to the supporting structure using different restorative techniques. J PROSTHETDENT 1989;61:535-43. 13. Reinhardt RA, Krejci RF, Pao YC, Stannard JG. Dentin stresses in post-reconstructed teeth with diminishing bone support. J Dent Res 1983;62:1002-8. 14. Pao YC, Reinhardt RA, Krejci RF. Root stresses with tapered-end post design in periodontally compromised teeth. J PRO~THETDENT 1987; 57:281-6.

15. Caputo AA, Hokama SN. Stress and retention properties of a new threaded endodontic post. Quintessence Int 1987;18:431-5. 16. Standlee JP, Caputo AA, Holcomb J, Trabert KC. The retentive and stress distributing properties of a threaded endodontic dowel. J PROSTHET DENT 1980;44:398-404. 17. Caputo AA, Standlee JP. Biomechanics in dentistry. Chicago: Quintessence Publishing Co, 1987:201-2. 18. Gross MJ, Turner CH. In&a-radicular pressure changes during the cementation of post-retained crowns. J Oral Rehabil 1983;10:237-49. 19. Craig RG. Restorative dental materials. 8th ed. St Louis: CV Mosby Co, 1989~261-2.

20. Shillingburg HT, Hobo S, Whitsett LO. Fundamentals of fixed prosthodontics. 2nd ed. Chicago: Quintessence Publishing Co, 1981:150-5. 21. Barkhordar RA, Radke R, Abbasi J. Effect of metal collars on resistance of endodontically treated teeth to root fracture. J PROSTHETDENT 1989;61:676-8. Reprint requests to: DR. PETERYAMAN SCHOOLOF DENTISTRY UNIVERSITYOF MICHIGAN 1011 N. UNIVERSITY ANN ARBOR,MI 48109-1078

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