ENDODONTOLOGY Volume: 25 Issue 2 December 2013 Original Research Effects of composite resin core thickness overlying the head of a prefabricated post on the strains produced in apical portion of a root - A parametric evaluation. Kumar Tarun # * Bhola Meenu ## * Beena Rani Goel # ** Sanjay Patil # *** ABSTRACT Aim - The present study was conducted in vitro to evaluate the relationship of composite resin core thickness to the apical strain produced in the root surface of the teeth. Methodology - A parametric evaluation method using strain gauges was applied to study the effect of composite resin thickness overlying the head of a prefabricated post on apical root strain produced in an endodontically treated tooth. 20 extracted premolars were selected. A common clinical occurrence was simulated by performing endodontic treatment on each tooth and then restoring the teeth with prefabricated posts and composite resin cores over which Ni- Cr alloy crowns were cemented. Each tooth was tested at three separate stages. Stage I After endodontic treatment but before post insertion. Stage II The teeth were randomly divided into 2 groups of 10 teeth each after baseline testing. Group I Post buried within the cores. Group II The head of the post was exposed on the occlusal surface of the core. Stage III After a cast crown was cemented. Results - of the study indicated a significant reduction in apical strain produced with 1mm of composite resin overlying the head of post at post and core stage whereas complete coronal coverage with 2mm on healthy tooth structure. Conclusion - Within the limitation of the experimental design the present study reported a significant reduction in the apical strain produced when the post head is covered with composite resin. Introduction The restoration of endodontically treated teeth has become a great challenge to restorative dentistry since the effectiveness and predictability of endodontic therapy has increased. Many teeth treated endodontically need posts for their rehabilitation. Various factors have to be considered during selection of a post and core system, like the choice of core materials and stress producing characteristics in relation to the size and shape of the selected posts. Composite resin cores are popular for core build up because of simplified mixing and insertion procedures and they polymerize in minutes. Crown reduction can be done at the same visit. Studies have indicated that prefabricated posts with composite resin cores are the most widely used post and core system. Conflicting opinions have been expressed about the ideal thickness of the composite resin that should overlie the head of prefabricated posts to prevent transmission of stresses on the restoration through the post in to the root. # Dept. of Operative Dentistry and Endodontics, ## Dept. of Pedodontics and Preventive Dentistry, * Dashmesh Institute of Research and Dental Sciences, Faridkot, ** PMNM Dental College and Hospital, Bagalkot. Karnataka, *** Bhojia Dental College and Hospital, Baddi, H.P. 46
EFFECTS OF COMPOSITE RESIN CORE THICKNESS OVERLYING THE HEAD OF A PREFABRICATED POST ON THE STRAINS PRODUCED IN APICAL PORTION OF A ROOT - A PARAMETRIC EVALUATION. Nathanson 1996 suggested that approximately 1mm of composite should overlie the head of a prefabricated post after the crown preparation has been completed to prevent the transmission of stresses on the restoration through the post into the root. On the other hand, Christenson 1996 advised that the head of the post should extend through the composite resin core to the surface of the preparation to prevent thin, unsupported areas of resin from fracturing during provisional restoration fabrication or on the removal of final impression. Despite the fact that composite resins used in conjunction with prefabricated posts are the most widely used system these days, there are very few studies which outline the effect of varying thickness of composite resin core overlying prefabricated post on stress distribution in roots of teeth. A few studies, which have been conducted in this regard, have expressed divergent clinical opinions. The present study was carried out to evaluate the effect of composite resin thickness overlying the head of prefabricated post on apical root strain measurement. Materials & Methods The present study was conducted in the post graduate clinic, Department of Conservative Dentistry and Endodontics at PMNM Dental College and Hospital. Bagalkot. The list of various materials and instruments used in this study is given in Table No.1. From a pool of 25 unrestored, freshly extracted mandibularfirst premolars, 20 intact teeth, similar in length and diameter were selected. All the teeth were free of decay and fractures. Each tooth was radiographed from the facial and proximal views to confirm that the dentin thickness was comparable. The plan was to simulate a common clinical occurrence by performing endodontic treatment on each tooth and then restoring the teeth with prefabricated post and composite resin cores, over which Ni - Cr alloy crowns were cemented. Although the teeth were selected to be as similar to each other as possible, each tooth was able to serve as its own control by use of staged testing, which started with teeth which had been prepared for passive insertion of a prefabricated post. Entrance to pulp chamber was effected from the occlusal surface by a round bur using a high speed handpiece. The working length of each tooth was evaluated radiovisiographically and recorded. All the teeth were endodontically instrumented to a No. 40 file, 0.75mm short of the apical foramina using traditional reaming and filing. The teeth were obturated with No. 40 gutta percha master cones, AH 26 sealer and lateral condensation technique. The gutta percha was removed to a level 5mm above the apical foramen using hot pluggers and canal was prepared with peeso reamers No (1-4). After canal preparation, prefabricated 16mm x 1.3mm Filpost titanium alloy anatomic form post was checked for passive seating. A radiograph was taken to ensure proper seating and a gap of at least 1mm between post and gutta percha 47
KUMAR TARUN, BHOLA MEENU, BEENA RANI GOEL, SANJAY PATIL Bonding of strain guages A reference point was marked 6mm above the root apex. Each tooth was lightly roughened on the proximal surface 6mm above the root apex using disc to facilitate bonding. The backing of strain guage and tooth surface were cleaned with a cotton pellet soaked in acetone to ensure removal of all impurities. The guage and tooth was kept under 60 watt bulb to ensure a moisture free surface. The strain guage was then bonded to tooth structure using cyanoacrylate cement. The guage was held under finger pressure until proper bonding was ensured. To allow flexion of root at the level of apical third of the post under testing conditions, a layer of medium body polyvinyl siloxane was placed over the strain guage and around the circumference of each root. Each tooth was embedded parallel to its long axis till CE junction in acrylic self cure resin to stabilize it during testing procedure. The occlusal surfaces of the teeth were slightly flattened with a straight fissure stainless steel carbide bur to make the tooth perpendicular to the long axis of the tooth for subsequent compressive testing and to facilitate seating of the platen of universal testing machine. I Stage I - The strain guage wires were then attached to a strain read out device and the acrylic resin block containing the tooth was placed in a water vessel on the universal testing machine. Each tooth was subjected to 200 N compressive load and strain in the root at the apical portion of the post was measured in microstrain units (ìå). This magnitude of compressive load is consistent with average biting forces in the bicuspid region. Each measurement was replicated 10 times to ensure that the measurement error was negligible. Readings were made on the strain readout device as soon as the maximum of 200 N load was reached. These measurements were then averaged for each tooth and the standard deviation was calculated to establish baseline strain data. A matrix of quadra functional putty material was placed over the crown of each tooth and then cut in half to allow to allow measurement of subsequent post and core heights and to permit restoration of the teeth to baseline dimensions. II Stage II ( Post/Core ) - After establishing baseline strain data, the crowns of the teeth were removed to a level 2mm above the facial cementoenamel junction with a carborandum disc. The teeth were randomly divided into two groups Group I ( post buried) - One of the previously selected post was fitted to finish 2mm below the planned occlusal surface as measured against the putty matrix. The post was cemented into the canal using glass ionomer luting cement. A visible light cure core was fabricated so that 1 mm of the resin covered the top of the post and the occlusal surface of the core was, in turn 1mm below the planned occlusal surface of the crown. This group was referred to as buried. Group II (post exposed) - Teeth were treated in the same manner as those in group I with the 48
EFFECTS OF COMPOSITE RESIN CORE THICKNESS OVERLYING THE HEAD OF A PREFABRICATED POST ON THE STRAINS PRODUCED IN APICAL PORTION OF A ROOT - A PARAMETRIC EVALUATION. exception that the head of the post was 1mm below the planned occlusal surface when measured against putty matrix. Thus the head of the post was flush with the occlusal surface of the composite resin core. This group was referred to as exposed. In both groups i.e post exposed and post buried, each tooth was subsequently prepared for complete metal crown. The crown preparation was extended 1mm apical to the tooth and core junction to provide 1mm collar of tooth structure with a chamfer bur. The teeth were then tested on the Instron machine to measure root strain at the apical end of the post when a 200 N compressive force was applied at the occlusal surface of the core. A series of readings were made and the mean and standard deviation were calculated for each tooth. Stage III (crowned) - Full crown wax patterns were prepared on teeth in both groups upto baseline dimensions as measured against putty matrix. These patterns were then invested and cast in Ni-Cr alloy. The crowns were checked for marginal fit. Finishing and polishing was done using non-precious alloy finishing kit. These crowns were cemented using glass ionomer luting cement (Shofu). Testing was then conducted through 10 trials for each crowned tooth with the same compressive forces previously used. The strain readings were averaged and the standard deviation was calculated. Results The present study was conducted to evaluate the effect of composite resin thickness overlying the head of prefabricated post on apical root strain measurements. Out of a pool of 25 mandibular 1 st premolars, 20 intact teeth, which were free of fractures and other defects, were selected and randomly divided into two groups of 10 each. The strain measurements for each sample are summarized in Table No.2. Each measurement is average of ten replications. The medium difference between two groups was tested with a two sample non-parametric rank procedure, Wilcoxon rank sum test and Mann Whitney U test. Strain values at base line testing - group 1 and group II The average strain value for group I was 361.2ìå and group II was 376.3ì å. When difference between both the groups was compared it was statistically non-significant. Strain values at post and core stage group I (a) and group II (a) The average strain value for group I (a) was 375.5ìå and group II (a) was 423.3ìå. There was increase in strain values for the group II (a) and the difference was statistically significant (P<0.001) when compared with values of group I (a). Strain values after crowning group I (b) and group II (b) Average strain values for group I (b) was 268.4ìå and group II (b) 381.4ìå. There was definite increase in values in group II (b) and the difference was statistically significant (P<0.001) when compared with those of group I (b). Discussion The successful restoration of an endodontically treated tooth is an on going challenge to the restorative dentist. A major component of such a restoration is often the endodontic post or dowel. A post and core restoration has many aspects for the dentist to consider. Among these are the choices of materials and the stress producing characteristics 49
KUMAR TARUN, BHOLA MEENU, BEENA RANI GOEL, SANJAY PATIL in relation to the size and shape of the selected post. These stresses may produce strains in teeth, which can prove detrimental to the success of post and core system. According to Standlee and Caputo 1987, the application of occlusal function is the most severe stress that can be applied to a post and core or crown. The most dangerous place for these stresses to cause strains is I the apical portion of the root, where fracture often renders a tooth non-restorable. The present study was carried out to evaluate the effect of composite resin thickness covering the head of prefabricated core on apical strain produced in root. Studies have documented that the use of composite resin in combination with prefabricated post is highly successful but still there are no studies concerning the ideal thickness of composite resin that should cover the head of a prefabricated post. In previous studies conducted by Standlee et al 1972 and by Standlee, Caputo and Holcomb 1982 methods like photoelastic models have been used extensively to study stress distribution in a tooth. Ho M H et al 1994 and Ko C C et al 1992 used finite element analysis to study the stress distribution in a tooth. In the present study strain gauges were used to measure the strain. Standlee and Caputo 1987 have stated that strain gauges give an accurate assessment of strain being produced. In the present study strain gauges were bonded and experimental models were prepared according to criteria outlined by Ross, Nicholls and Harrington 1991 and Walton, Ruse and Glick 1996. In the present study, at the baseline stage, there was no difference between the two groups as same procedures were performed for each tooth in both groups. There was a wide variation between strain gauges used and inherent differences in anatomy of each tooth. Walton, Ruse and Glick reported similar findings from their study. The mean strain recorded at baseline was significantly more than that recorded by Walton, Ruse and Glick. This difference may be due to difference in sensitivity and resistance of strain gauges used. At post and core stage the present study reported a significant difference between two groups (P<0.01). Strain measurements recorded in post exposed group were significantly greater than those recorded for post buried group. Studies by Chang and Millstein 1993 reported that compressive failure loads of post and core systems were affected by the core thickness that covers the post head. A core of 3mm thickness resists the compressive forces better than core with 1mm thickness. Walton, Ruse and Glick 1996 conducted a similar study and reported no significant difference between post buried and post exposed groups. If changes in strain measurements from baseline to post and core stage are taken into consideration, for group I there was no significant difference but for group II there was significant increase in strain measurements. Peter Yamen and Thorsteinsson 1992 from their study concluded that with vertical loading, posts are intruded and create high apical stresses. As there was no composite core thickness covering the head of the post, the stresses were being directly transferred to root through the post. This findings endorses Nathanson s 1996 study which suggested that approximately 1mm of composite resin should overlie the head of a prefabricated post after crown preparation has been completed. This prevents the 50
EFFECTS OF COMPOSITE RESIN CORE THICKNESS OVERLYING THE HEAD OF A PREFABRICATED POST ON THE STRAINS PRODUCED IN APICAL PORTION OF A ROOT - A PARAMETRIC EVALUATION. transmission of stresses on the restoration through the post into the root. Yaman and Thorsteinsson 1992 have also reported decrease in apical strain when using stiffer core materials. The present study also supports the findings that post/core has no reinforcing effect on a tooth as there was no decrease in strain measurements for both groups. Sorensen and Martinoff 1984 have shown that placement of posts in mandibular premolars does not significantly improve the At stage III after crown placement, there was significant difference recorded between two groups. Strains produced in group I were much lesser as compared to group II. This finding is in accordance with findings of Walton, Ruse and Glick 1996. If changes from baseline to crowning stage are compared, strains produced in group I at stage III were much lesser than baseline stage whereas for group II, strains were almost same as recorded for baseline stage. Studies by Assif. D et al. 1989,1993 success rate. TABLE NO. 1 Item Description Manufacturer Location Peeso reamers & #1-6 Mani. Inc Japan Gates glidden drills Strain gauge * 350 Micro- Releigh, New York measurement group Bonding material Cyano acrylate cement Super glue Japan Filpost Titanium alloy, 1.3 x 16mm Filhol Dental Baltimore USA product POS-121 Impression material Aquasil soft, putty quadra Dentsply Detrey Germany functional siloxane Universal Instron Ltd Canada testing machine Strain readout device Digital strain Instruments & controls Bangalore, India meter model-543 Acrylic Self cure DPI Mumbai, India Alloy Verasoft, type III, Ni-Cr alloy Aalba dent Cardelia, USA Composite resin Curex-lite DPI India Luting cement Hy-bond glass ionomer CX Shofu. Inc. Kyoto, Japan * Electrical strain gauges, or more commonly strain gauges, use the principle that certain electrical resistance elements undergo a change in resistance when they are subjected to strain. 51
KUMAR TARUN, BHOLA MEENU, BEENA RANI GOEL, SANJAY PATIL TABLE NO. 2 RESULTS OF STRAIN MEASUREMENTS (in microstrains ìå) * S.No. Baseline Post and Core After Crown Stage I Stage II Stage III Group I Group II Post Post Post Post Buried ( Ia) Exposed ( IIa) buried ( Ib) exposed(iib) 1 370.8 380.2 381.4 423.7 320.5 438.8 2 312.4 316.4 352.8 412.4 265 385.4 3 350.2 370.4 332 435.2 285 420.4 4 370.1 385.2 412.5 470.4 235.8 356.9 5 385.2 365.4 405.2 455.2 240.7 360.8 6 373.4 380.2 353.2 413.4 261.3 381.3 7 380.2 395.4 401.5 445 220.4 360.5 8 356.2 390 380.7 405.7 304.1 324.1 9 345.2 389.1 370.2 382.3 280.6 400.8 10 368.6 390.6 356.1 390.1 270.4 385.2 Mean 361.2 376.3 375.5 423.3 268.4 381.4 SD 21.3 23 24.4 28.2 30.9 33.3 * Each measurement given is the average of 10 replications for each tooth have shown that cores covered by a complete cast crown with a 2mm margin on healthy tooth structure exhibit reduced stress concentration at the apex despite the presence of dowel or its design. Sorensen and Martinoff 1984 have also reported increased success rate for mandibular premolars when coronal coverage was provided. Clinical significance Although the statistical difference between two groups is apparent, it is questionable whether the observed findings have clinical significance. It is documented in literature as cited by Walton, Ruse and Glick that maximum strain required for fracture of dentin is 2787 microstrains. The maximum strain recorded in this study was almost 6 times lesser than maximum strain required for fracture of dentin. The present study had its own limitation as no consideration was given to periodontal ligament, bone support and cement thickness which has significant effect on distribution of stresses. The clinical significance of present study is questionable because for both the groups the strain was equal to or lesser than that recorded at baseline. 52
EFFECTS OF COMPOSITE RESIN CORE THICKNESS OVERLYING THE HEAD OF A PREFABRICATED POST ON THE STRAINS PRODUCED IN APICAL PORTION OF A ROOT - A PARAMETRIC EVALUATION. Strain guage Strain guage bonded to root surface Root covered with polyvinylsiloxane material Head of post covered with 1mm composite Head of post exposed Further studies on strain in endodontically treated teeth restored with post and cores may be facilitated- 1. Establishing baseline apical strain value of unrestored teeth before endodontic treatment to provide a more clinical direction in assessing restored versus intact strain. 2. Conducting additional testing at an angle to the long axis of the tooth and fatigue to create a more complete picture of the strains generated in the dentin of teeth restored with posts, cores and crowns. Conclusion The present study evaluated and compared the amount of strain measured in the root of an extracted tooth at the apical end of a prefabricated post, when a composite resin core completely covers the head of the post and when the post is exposed at the occlusal surface of the core. Within the limitations of the experimental design, the following conclusions were drawn- 1. Covering the head of prefabricated post with 1mm of composite resin core, under the crown, produces significantly less apical strain compared to when post is flush with the occlusal surface of the core. 2. 1mm of composite thickness overlying the head of prefabricated post significantly decrease strain values at post and core stage. 3. Complete coronal coverage with 2mm margins on healthy tooth structure decreases the strains I apical area irrespective of dowel/core design. Coronal coverage of an endodontically treated mandibular premolar significantly reduces the amount of strain in the apical portion of root. 53
KUMAR TARUN, BHOLA MEENU, BEENA RANI GOEL, SANJAY PATIL References : 1. Assif D. (1989) Photoelastic analysis of stress transfer by endodontically treated teeth to the supporting structure using different restorative techniques Journal of Prosthetic Dentistry 61, 535-545. 2. Assif D. et al. (1993) Effect of post design on resistance of fracture of endodontically treated teeth with complete crown. Journal of Prosthetic Dentistry 69, 36-40. 3. Caputo A.A., Hokoma B.N. (1984)Retention and stress distribution characteristics of new dowel system. Journal of Prosthetic Dentistry 51, 652-655 4. Caputo A.A., Standlee J.P. (1987) Biomechanics in clinical dentistry. Chicago. Quintessence Publishing Co. Inc, pp185-203 5. Chang W.C., Millstein P.L. (1993) Effect of design of prefabricated post heads on core materials. Journal of Prosthetic Dentistry 69, 475-482. 6. Davy D.T., Dilley G.L., Krejci R.F. (1981) Determination of stress patterns in root filled teeth incorporating various dowel designs Journal of Dental Research 60, 1301-1310. 7. Frederick D.R., Serene T.P. (1975) Secondary intentional dowel and core Journal of Prosthetic Dentistry 34, 41-47. 8. Ho M.H. et al. (1994) Three dimensional finite element analysis of the effects of posts on stress distribution in dentin Journal of Prosthetic Dentistry 72, 367-372. 9. Ko C.C. et al. (1992)Effects of post on dentin stress distribution in pulpless teeth. Journal of Prosthetic Dentistry 68, 421-427. 10. Millstein P.L., Nathanson D. (1991) Relation between serrated steel dowel and different core materials Journal of Prosthetic Dentistry 65, 480-482. 11. Oliva R.A, Lowe J.A. (1986) Dimensional stability of composite used as a core material Journal of Prosthetic Dentistry 56,554-561. 12. Ross R.S., Nicholls J.I., Harington G.H. (1991) A comparison strains generated during placement of five endodontic posts Journal of Endodontics 17, 450-456. 13. Sorensen J.A., Martinoff J.T. (1984) Intra-coronal reinforcement and coronal coverage: A study of endodontically treated teeth Journal of Prosthetic Dentistry 51, 780-784. 14. Standlee J.P. et al. (1972) Analysis of stress distribution by endodontic posts. Oral Surgery, Oral Medicine, Oral Pathology 33, 952-960 15. Standlee J.P., Caputo A.A., Holcomb J.P.(1982) The dentatus screw: comparative stress analysis with other endodontic dowel designs. Journal of Oral Rehabilitation 9, 23-33. 16. Torsteinsson T.S., Yaman P., Craig R.G. (1992) Stress analysis of four prefabricated posts Journal of Prosthetic Dentistry 67, 30-33. 17. Walton J.N., Ruse N.D., Glick N. (1996) Apical root strain as a function of post extension into a composite resin core. Journal of Prosthetic Dentistry 75, 499-505. 18. Yaman P., Thorsteinsson T.S. (1992) Effect of core materials on stress distribution of posts. Journal of Prosthetic Dentistry 68, 416-420. 54