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Root Canals and Crowns – Innovative Techniques

Background
There has been a general consensus that endodontically treated teeth are brittle and subjected to
fracture [1-4]. These teeth are usually associated with extensive loss of tooth structure due to
caries,, trauma, or further endodontic treatment. Similar results were reported in relation to the
removal of dentin during root canal preparation for metal posts;the increase in diameter of posts
did not provide a significant increase in retention, however, it could increase the stiffness of posts
at the expense of the remaining dentine and the fracture resistance of the root[5-12].
Retention of posts often requires tooth structure removal; a procedure that may reduce the strength
of roots. Cast tapered posts were found the least retentive in all post designs [8]. These
comparisons are relevant only if the post fits the root canal accurately, because retention is
proportional to the total surface area. When the canal shape is ovoid, the walls of prefabricated
posts are unlikely to adapt well along their entire interface with the canal walls. As a result, the
post may not fit the preparation closely, and the luting agent may not totally fill the interface. This
makes parallel sided posts only effective in the most apical portion of the post space because of the
considerable flare of the post space in the coronal portion [13].
Pettiette et l.,[14] evaluated the effect of various tapers of canal preparation on the retention of
posts. The canals were prepared using NI TI instrument of tapers ranging from 0.04 to 0.12-mm
instrument taper and were compared for retention against canals prepared using 0.02-mm taper
instruments. Canals prepared with 0.04 taper instruments provided the best retention, while 0.02
taper size showed the worst retention. The authors concluded that the difference in post retention
was related to the differences in film thicknesses. The thinner the film thickness the better the
retention of the post; however, too little cement film thickness did not adequately maintain proper
adhesive properties. On the other hand, tapers that produced too divergent canals allowed thick
cement film to be present, which resulted in intercement bond failure.
Several methods for post preparation were investigated, including rotary instruments, heated
instruments and solvents. The literature is equivocal on post space preparation and no method has
been found consistently superior. When the mechanical method is preferred, it has been established
that Gates Glidden® drills and P-type® reamers used on low-speed are the safest instruments [3].
It was stated that round canals, particularly in maxillary central incisors, can be prepared to
provide a post space with parallel walls or minimal taper [13]. Conversely, canals with elliptical
cross sections must be prepared with a restricted amount of taper. Taper of 6-8 degrees were
reported to be necessary to ensure adequate retention while eliminating undesired undercuts [13].
This is analogous to extracoronal preparations; retention increases rapidly as vertical wall taper is
reduced.
Although several advances in canal preparation were introduced recently, canal preparation using
Gates Glidden® drills technique is still the standard technique taught at the dental school of the
Jordan University of Science and Technology, and is still common in Jordan and other parts of the
world.
There is no documentation in the literature on the taper of post space. This study was conducted to
measure the degree of post space taper of cast posts, which were prepared using Gates Glidden®
drills technique, using innovative image program. Also to evaluate the relation and suitability of
this technique based on the anatomy aspects of the common candidate teeth for post
reconstruction.
METHODS
The research was performed with the approval of the relevant research committees in the
department and faculty of dentistry in the Jordan University of Science and technology.
Working casts of post crown cases, which were treated by 5th year dental students, were collected
from the dental laboratory at the Dental Teaching Center of JUST. These cases included only the maxillary anterior teeth group and the maxillary and mandibular premolar teeth group. Dental
students followed strictly the instructions on post preparations given by two clinical supervisors;
who met and agreed on the technique before starting the study. The technique involved post
preparations using simultaneous Gates Glidden® drills to one size beyond the largest file size used
for endodontic treatment. Only working casts for patients who received successful endodontic
treatment by dental students at the dental health centre were selected. A sectional impression of
each post space was taken using a polyvinyl siloxan® putty/wash impression technique (3M
ESPE®). The wash impression material was injected into the canal using A lentulo spiral® drill
along with a stainless steel wire. The wire was modified by making a loop at one end and the wire
was roughened and tray adhesive (3M ESPE tray adhesive®) was added. The part of the
impression, which was related to the post space, was cut out of the impression and then was
labeled with the correspondent tooth, Figure (1). Sony® XCL-U1000 camera with a 10× special
high quality attachment lens was used for capturing two digital images of each canal space
specimen; one for Facio-Lingual dimension (FL) and the other for the Mesio-Distal (MD)
dimension. Each canal impression was placed on a radiology viewer box facing the digital camera,
which was held on a stand in a position perpendicular to the line of sight from the canal impression
position. Two experienced dentists watched and supervised the image acquisition process. Two
specimens were randomly selected to serve as a control group; during the process of capturing the
images of the specimens, the control group specimens were remounted for imaging in every 10
studied specimens. The images collected for the control group were processed independently to
validate the accuracy and reproducibility of the image acquisition process. Duplicate
measurements were obtained to measure the reliability of the examination using percent
agreement, Kappa test, which revealed more than 95% agreement in the measurements of the
control group.
The size of the captured images was 1365×741 pixels with a pixel spacing of 0.0125mm/pixel.
Figure (1) presents an example of an acquired image.
Image Processing [15]
The first step in the process was to convert the colored image into grayscale image. After that, the
grayscale image was converted to binary image by thresholding in order to isolate the canal
impression from the background. The output binary image had values of 0 (black) for all pixels in
the input image with luminance less than a threshold value, T, and 1 (white) for all other pixels.
The threshold value T was selected based on the histogram of the grayscale image, which was
bimodal in this case. Therefore, T was selected halfway between the main two peaks in the
histogram, as shown in Figure (2). To eliminate the effect of any unwanted pixels, morphological
cleaning was performed on the binary image. According to this operation, any 1-pixel that was
surrounded by zero pixels was wiped out. As a result of this process, the binary image object
representing the canal impression was identified and isolated from the background. The next step
in the process was to allow interaction with the user, who had to identify the region on the tapered
canal impression at which the taper angle was to be measured. This included the area between the
start of the canal coronally to the end of the apical area. The user drew a polygon surrounding the
tapered region using the computer mouse. Based on this selection, a mask image was created to
exclude everything in the binary image except for the region of interest (ROI). The boundary of the
object enclosed by the ROI was found using two morphological operators; namely erosion and
dilation. An eroded image was subtracted from a dilated image resulting in a boundary image
containing only the boundary of the region of interest. The ROI was then scanned and the
coordinates of each boundary point representing each side of the canal impression was recorded.
Two coordinate vectors, V1 and V2 were extracted, in which each entry in these vectors
represented the row and column position of the corresponding boundary point in the image. Each vector was used to fit its corresponding entries to the best linear curve using auto-regression
techniques.
Linear Regression Curve Fitting [15]
For fitting a set of data points to a straight line, an expression of the form y = mx + b was used,
where m was the line’s slope and b was its y-intercept. Linear regression techniques were used to
estimate the slope and y-intercept in such a way to minimize the Mean Square Error (MSE). The
MSE was defined as the sum of the squares of the deviations from the mean. Assume that the xi
values were precise and all uncertainty is in yi . The deviations in yi were given by
e y (mx b) i i i = − + (1)

RESULTS
A total of 84 working dies for cast post fabrications were collected: 48 dies for the maxillary
anterior teeth group, 18 dies for the maxillary premolar groups and 18 dies for the mandibular
premolar groups. The maxillary anterior group was subdivided into central, lateral, and canine
subgroups. Premolar groups were also subdivided into first and second premolars subgroups in
both the maxillary and mandibular premolar groups. Two images were acquired for each canal
specimen in the FL and MD dimensions for a total of 168 images. The acquired images were then
processed, as discussed before, and the taper for each impression was evaluated. The data were
analyzed using Statistical Package for Social Sciences software (version 15.0: SPSS Inc., Chicago,
IL, USA.) and One way Analysis of Variance (ANOVA) to draw statistical inferences. The means
and standard deviations for each group and subgroup were calculated in the MesioDistal (MD) and
FacioLingual (FL) dimensions, as presented in Table (1). The total mean of taper for all
preparations was 10.7 degree. The highest mean of total taper was registered in the maxillary
premolar group, followed by the mandibular premolars, and last by the maxillary anterior teeth
group.
The validity of assumptions underlying ANOVA was verified, in order to draw statistical
inferences on the evaluated tapers. Normal quantile plot was carried out in order to check for
normality of the measured taper with respect to teeth group and imaging dimension. Figure (3)
showed the normal quantile plot when the calculated taper was set as the dependent variable while
the teeth group (Figure 3a), subgroup (Figure 3b), imaging dimension (pose) (Figure 3c), as well
as teeth group and pose simultaneously (Figure 3d) were set as independent variables, respectively.
The plots in the lower left corner of each figure depict the ordered standardized residuals on the
vertical axis and the normal quantile values on the horizontal axis. The residuals are fairly linear
with respect to quantile values, which in turn validate the normality of the data. To check for
homogeneity, Levene’s test of equality of Error Variance was performed. This test checks the
validity of the null hypothesis that the error variance of the dependent variable is equal across
different groups. When performed, it was found that there was no significant difference between
the variances. Henceforth, the data was reasonably assumed to be homogeneous. Finally, each
specimen represents a different working die that was prepared independently from other working
dies. Furthermore, each image was processed independently to measure the taper providing no clue
as to the likely value of other tapers. Therefore, it was practically assumed that taper values were
statistically independent.
In order to study the effect of the three main independent variables namely; tooth group, subgroup,
and image dimension (pose), one-way ANOVA was conducted several times taking into account
the effect of these variables in the taper value. Table (2) presented the value of the significance (p)
for each case. As can be seen from the table, ANOVA for total taper revealed no significant
differences among the groups (p = 0.256).
With regard to the maxillary anterior teeth, the central incisors had the highest mean of total taper
(9.25), followed by the lateral incisor (8.80), and the canine group (8.65). The mean of total taper for maxillary and mandibular second premolars were higher than the maxillary and mandibular
first premolars, Table (1). The differences between the subgroups were not statistically significant,
Table (2).
The maxillary central incisors had the highest MD an vd FL taper in the maxillary anterior group.
The lowest MD taper was registered in the lateral incisor group, while the canine group had the
lowest FL taper. No significant differences were detected between the subgroups in both the MD
dimension (p = 0.905) and the FL dimension (p= 0.789), as reported in Table (2).
On the other hand, the mean taper for the maxillary first premolars was lower than the maxillary
second premolars in both the MD and FL dimension. According to Table (2), the difference in the
FL dimension was statistically significant (p = 0.003). The same was true in the mandibular groups
in both dimensions, but with no statistically significant differences. Finally, The FL taper for all
teeth was higher than the MD taper except in the maxillary premolar group.
DISCUSSION
This study included three groups for investigation: the maxillary anterior teeth group (including 3
subgroups for the central, lateral, and canine teeth), and the maxillary and mandibular premolar
groups ( including first and second premolar subgroups). Molars and mandibular incisors were not
included in the study. Cast posts are not routinely used in these teeth; mandibular anterior teeth
have thin roots which make it difficult to prepare a post, and molars usually do not require posts
because they have more tooth structure and large pulp chambers to retain a core.
The amount of remaining dentin and the nature of root morphology are important before
attempting to prepare any canal space for post installation. Root diameter may differ in the FL and
MD dimension. Maxillary central and lateral incisors usually have sufficient bulk of roots to
accommodate post restorations. However, care must be exercised with post of excessive length if
the roots taper rapidly to the apex. The outline and pulp cavities of these teeth are similar. Central incisors are larger, and it is extremely rare for these teeth to have more than one root or one canal.
Where abnormalities do occur they seem to affect the maxillary lateral incisor, which may present
with an extra root, second canal, dens invaginatus, germination, or fusion. The canal is tapered
with an oval or irregular cross section cervically that becomes round only very near the apex. The
root canal differs greatly in outline when viewed in FL or MD dimension. The former generally
shows fine straight canals, while the MD dimension shows a wider canal [16]. After post
preparation, the taper in the FL dimension for these teeth was larger than the MD pose, but with no
statistical significance. This might indicate an over tapering and over preparation in the FL
dimension.
The FL taper in the canine subgroup was smaller than the maxillary central and lateral incisors,
and was larger, but not statistically significantly, than the taper of the canine group in the MD
dimension. These results did not reflect the anatomy of the roots and root canals; the maxillary
canines have wide faciolingual roots and root canal spaces. The root canal is oval and does not
begin to become round until the apical third [16].
The maxillary first premolars normally have two separate canals, which are usually straight with a
round cross section. The root canals are wide buccopalatally but narrow mesiodistally. This was
not consistent with the results after post preparation, in which, the FL dimension was significantly
smaller than the taper in the MD dimension. Increasing the taper is highly risky in such roots,
because they present a variety of problems for post retained restorations. Root walls are commonly
thin and roots taper rapidly to the apex. Proximal invaginations and canal splitting are common.
Parallel sided posts might be more suitable in this group of teeth. The same observations are true
for the maxillary second and mandibular premolars, but these teeth tend to have single canals and
greater bulk of tooth structure. One area of concern with the maxillary first premolar is the angle of
the crown to the root, often the root will be lingually inclined and active drilling of a post space
perpendicular to the occlusal surface will result in a perforation along the facial wall of the root.
There was no documentation in the literature of similar studies for the purposes of comparisons.
Pettiette et al., [13] used NI-Ti instruments to produce post space preparations of controlled tapers
ranging from 0.04 to 0.12-mm. It was to not possible to compare the tapers reported in this study
with those reported by Pettiette et al., [13] because teeth investigated in this study were root treated
using the traditional step back technique, and the canals were prepared for posts using Gates
Glidden® drills.
The aim of this study was not to evaluate the performance of dental students due to lack of
references for comparisons. The data reported were only descriptive and helped to draw
conclusions on the safety of the technique used, based on the anatomy aspects of the teeth.
Only cases treated by undergraduate students were used in this study in order to provide the same
conditions and technique as much as possible. The differences in canal tapers among the groups
might be attributed to the differences in the skills of students rather than the technique used. All
cases were performed in strict academic atmosphere and under close supervision in order to
minimize this effect. Further studies using cases treated by professionals are recommended.
Post space preparation taper for all groups were higher than the recommended 6-8 degree taper.
This recommendation was not based on in vitro or in vivo studies of post crown restorations;
instead, it was solely made analogous to the extra coronal preparations. Recently, the
recommendations for extra coronal preparations have been subjected to scientific scrutiny. It has
been determined that dental students, general practitioners, and prosthodontists do not routinely
create such minimal angles [17-21]. More recently, resistance to lateral forces and not retention
along the path of insertion has been advocated as the determining factor in a crown’s resistance to
dislodgment. Resistance testing was more sensitive than retentive testing to changes in taper [22-
24]. Shillingburg et al., [25] recently suggested that the taper of crown preparation should be
between 10-22 degree. Similar recommendations were also suggested by Goodacre et al., [26].
Based on the available evidence; it was difficult to decide whether or not the results reported for post taper were satisfactory. Root canals have natural taper before endodontic treatment and when
root treated, the taper is further increased. Despite the technique used, the final taper of the post
space will be influenced by the taper produced after root canal therapy. The advent of rotary
nickel-titanium instruments led to the possibility of rounder canal profiles and more controlled
taper than the hand files [27, 28]. Further investigations in this area are required.
Resistance to fracture is another important factor that must be achieved with post and core retained
restorations. The mechanism of root fracture is still not fully understood. Root canal treatment was
suggested as a factor influencing the incidence of vertical root fracture [29-31]. It has not been
established whether fractures occur at the time of filling or manifest themselves at later time [32].
Rundquist & Versluis [33] studied the influence of different canal tapers on radicular stress
distributions and reported that during root canal obturation, root stress decrease as the canal taper
increase, while the relation is reversed after root filling is complete and occlusal load is applied.
The authors also reported that vertical fractures initiated at the apex are a result of filling force,
whereas vertical root fractures initiated cervically are a manifestation of subsequent masticatory
events on the root filled tooth [33].
Oval-shaped root canals, which are found in approximately 25% of roots [34], pose problems with
regard both to the effectiveness of canal preparation and to fracture susceptibility. The narrow
radius of curvature at the buccal and lingual extensions of the canal means that these locations
serve as sites of stress concentration [35]. A finite element study indicated that when an internal
load was applied in models with a round canal, the stress distribution was low and relatively
uniform. The thickness of the surrounding root dentine hardly affected this distribution. In
contrast, the oval canal showed much higher stresses and a very uneven stress distribution [36].
There were no significant differences among the mean tapers of the groups despite the vast
anatomic differences. This could be related to the fact that standard deviations of many
groups/subgroups were too big compared to their respective means, which probably made it difficult to obtain statistical significant differences between teeth with different anatomy aspects.
This could be related to the post preparation technique, step back technique used in endodontic
therapy, or both. Intuitively, it could be still reasonable to speculate that the lack of statistical
differences between the different subgroups was and indication of over tapering and over reduction
of tooth structure, especially in the maxillary first premolar teeth. Increasing the taper of the canal
preparation by removing more dentine from the canal wall would diminish the structural durability
of the root and make them more susceptible to fracture [10, 36].
To minimize failures, the optimum diameter for the tapered post of cast alloy relative to root
diameter was reported to be approximately 1:4. [37]. Post fitting in oval canals was affected by
different drill/tips used for canal preparation. A fine grit oval tip combined with oval posts was
reported to provide the best post fitting [38].
Potential fracture might be reduced by practitioners being aware of risk factors such as the post
preparation technique, post selection, coronal restoration, and inappropriate selection of tooth
abutment for prosthesis [39].
This study provided a descriptive data on the taper of post space prepared using simultaneous
Gates Glidden® drills. No attempt was made to measure the remaining tooth structure, the retention
/ resistance to dislodgment, or the resistance of the posts to fracture. These are important features
and required further studies.
The methodology for measuring taper was based on image processing techniques.. Unlike other
studies, in which microscopic visual perception was utilized, this study used a fully automated
innovative process to locate the taper region and hence measured the amount of taper with minimal
human interaction. Such an approach guaranteed minimal inter- and intra-inspector variation.
CONCLUSIONS
The technique used for post preparation did not follow the anatomy of the roots and root canals.
No differences in post taper were found between the maxillary anterior teeth, maxillary premolars,
or mandibular premolars despite the vast differences among the anatomy of these teeth. While
using this technique might be satisfactory in the maxillary anterior teeth, the same could not be
said for the maxillary first premolars.
The Innovative Image processing technique used in this study was valid for data processing in this
field.

The taper of cast post preparation measured using innovative image processing
technique
BMC Oral Health 2010, 10:19 doi:10.1186/1472-6831-10-19
Khaled Q Al Hamad (khaled.m5@lycos.com)
Faruk A Al-Omari (fomari@yu.edu.jo)
Ahmad S Al Hyiasat (hyiasat@just.edu.jo)
ISSN 1472-6831
Article type Research article
Submission date 20 February 2010
Acceptance date 4 August 2010
Publication date 4 August 2010

© 2010 Al Hamad et al. , licensee BioMed Central Ltd.
This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0),
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Root Canals and Dental Crowns is a post from: Phoenix Dentists

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