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 Table of Contents  
ORIGINAL ARTICLE
Year : 2009  |  Volume : 9  |  Issue : 2  |  Page : 84-87

Electronic survey: An in-vitro study on reliability of the technique


Department of Prosthodontics, V S Dental College and Hospital, Bangalore, India

Date of Web Publication28-Aug-2009

Correspondence Address:
P Rohit
#20/2, Sougandika Nilaya. 8th main, M.C.Binny layout, Vijayanagar, Bangalore - 560 040
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0972-4052.55250

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  Abstract 

Aim: The main purpose of this study is to determine the dimensional accuracy of digital models as compared to that of study models and the reliability of electronic surveying. The study aims to measure and compare distances between selected landmarks on study models and digital models, angles of cervical convergence and undercut depth at predetermined points on the abutment teeth. Material and Methods: The study used partially edentulous study models of each of Kennedy's class I, II and III classifications. All the three models were scanned using multislice helical computerized tomography (CT) scan and digitized on the computer. The physical model and digitized models were surveyed and measured using various parameters and study designs. Measurements were conducted directly on study models and computer models using Altair Hyper mesh network version 7.0. The differences between the two sets of measurement were statistically analyzed using a suitable method. (t- Test). Results: The differences between measurement of distance on study models and digital models were found to be within the precision of 0.17mm, 0.08, o for the angle of cervical convergence and precision of 0.04mm for undercut depth of the abutment teeth. All these measurements were statistically slightly significant and well within the range of intraoperator variations. Conclusion: This study shows that dental cast can be scanned with the help of an accurate and sophisticated scanner and the scanned data can be transferred to the computer and used for the purpose of digital or electronic surveying.

Keywords: Computer aided design, dental cast, electronic surveying, study model, three dimension


How to cite this article:
Rohit P, Shetty S, Satish B. Electronic survey: An in-vitro study on reliability of the technique. J Indian Prosthodont Soc 2009;9:84-7

How to cite this URL:
Rohit P, Shetty S, Satish B. Electronic survey: An in-vitro study on reliability of the technique. J Indian Prosthodont Soc [serial online] 2009 [cited 2020 Dec 5];9:84-7. Available from: https://www.j-ips.org/text.asp?2009/9/2/84/55250


  Introduction Top


Computer aided design and manufacturing technique have shown some promising applications in the fabrication of crown, [6],[14] inlays and maxillofacial and oral surgery, [16],[18] the field of removable prosthodontics has not embraced this technology so far. [2] If electronic surveying programs can be developed, digitized surveying of a three-dimensional (3D) scanned dental cast may be almost instantaneous. Due to remarkable development in medical CT in recent years, 3D computer graphic presentation using a Voxel model has become possible. Also, 3D image reconstructions canbe completed in a matter of minutes, which is a strong advantage. [11],[14],[15]

The purpose of this study was to verify the dimensional accuracy of the digitized 3D dental cast and reliability of surveying the same, which may eventually enable the prosthodontic procedures to be carried out digitally.


  Methodology Top


Partially edentulous study models of each of Kennedy's class I, II and III classifications were obtained from Frasaco partially edentulous rubber moulds using type III dental stone.

These casts were scanned using a Siemens Somatom sensation 16 and a multislice CT scan with slice thickness of 0.75 mm. Image acquisition was carried out with 120kV of X-ray tube voltage and 80 mA tube current. The scanned 3D computer data was obtained in DICOM format. The data points obtained from the scan were aligned and combined using CAD software (Mimics) to provide single coherent data set of the entire object. The resultant data points were used to generate a 3D surface model using Mimics software. Graphic pre processing software Altair Hyper mesh version 7.0 was used in a workstation computer with hard ware Pentium processor with a speed of 2.1 GHz and 512 MB RAM for creating hyper mesh models. This computer model provided similar geometry of study models.

The following three criteria were selected for the purpose of determining the dimensional accuracy and reliability of electronic surveying:

  1. Measurement of distance between predetermined points on the surface of the models
  2. Determination of angles of cervical convergence of the abutment teeth
  3. Determination of under cut depth of the abutment teeth
Measurement of Distance Between Specific Points

On the study models and digital models, following specific points were located for measuring the dimensions.

Point A: mid point between the incisors

Point B: midpoint at the posterior boundary of the cast

Point C: anterior mid point on the base of the cast

Point D: posterior mid point on the base of the cast

Point E: located at 15mm from the top most point on the posterior base on the left side of the cast

Point F: located at 15 mm from the top most point on the posterior base on the right side of the cast

Point G: located at the buccal cusp tip of

14 of Kennedy' class I

44 of Kennedy's class II and class III

Point H: located at the buccal cusp tip of the

24 of Kennedy's class I

34 of Kennedy's class II and class III.

These points were carefully chosen to accurately locate them on both study models and digital models. The distance between these points were measured both on the study and digital models, five times each, by the same operator to avoid the intraoperator error.

Measurement I: distance between Point A and Point B

Measurement II: point C and Point D

Measurement III: point E and Point F

Measurement IV: point G and Point H

A Vernier Caliper was used to measure the distance between the above-mentioned points on the study model. Measurements between the points on the digital model were read and recorded by the computer.

Measurement of Angle of Cervical Convergence

As it was not feasible to measure the precise angle of cervical convergence on the study models, photographic images of the study models with the analyzing rod placed against the proximal surfaces of the abutment teeth were obtained. The procured images were transferred to the computer and angles were measured. On the hyper mesh digital model a node was located at the most convex point on the proximal surface of the abutment teeth (premolars and molars). A tangent was drawn to this particular node. Angle of cervical convergence was measured between this node and the tangent.

Measurement of Undercut Depth

For the purpose of standardizing and comparing the undercut depth in the study models as well as digital models, photographic images of the study models with analyzing rods placed against the proximal, buccal and lingual surfaces of the abutment teeth were obtained. A point, 2 mm apically, was located on the analyzing rod from the contact point of the analyzing rod with the tooth surfaces. Distance in mm was measured between these points to the tooth surface to determine the undercut depth. On the hyper mesh digital model, a node was located at the most convex point on the proximal, buccal and lingual surfaces of the abutment teeth (premolars and molars). A tangent was drawn to this particular node. A point, 2mm apically, was located on the tangent from the contact point of the tangent with the tooth surfaces.

All the measurements were repeated five times each to eliminate intra operator errors. All the values are recorded and analyzed with a suitable statistical method.


  Results Top


The results show the,

  • measurement of the distances AB(I), CD(II), EF(III) and GH(IV) in Kennedy's Class I , class II and class III in both the study models and digital models.

    {in class I Measurement I, the mean distance measured is 61.14 mm in study model and 61.04 mm in digital model with a mean difference of 0.10 mm and standard deviation of 0.0089. P less than 0.001}.
  • measurement of angles of cervical convergence.

    {In class I - first pre molar: the mean degree of angle of convergence is 4.01 degree on study model and 4.09 degree on digital model with mean difference is angle measured of 0.08 degrees and SD is equal to 0.008. p less than 0.001}
  • measurement of under cut depth on the proximal surface.

    {in class I - first pre molar, the mean distance of under cut depth measured is 0.81mm on the study model and 0.61 mm on the digital model with a mean difference of 0.2 mm and SD is equal to.0.0008 .p less than 0.001.}
  • measurement of under cut depth on the buccal/lingual surface

    {In class I - first pre molar , the mean distance of under cut depth measured is 0.120 mm on the study model and 0.101mm on the digital model with a mean difference of 0.090 mm and SD=.0.0012. p less than 0.001.}

  Discussion Top


Computer aided design and computer aided machining techniques have been successfully introduced in the field of fixed partial denture [6] and maxillofacial technology [11],[14],[16] over a number of years. The development and evaluation of these advances continue. Advantages of such system have been well documented, and may well eventually become the "next generation" method of fabrication. [1] Electronic surveying of scanned cast enables a pattern for a metal framework to be fabricated on computer screen according to the principles of RPD. [1] Various methods were used for imaging dental hard structure such as holography, Moiré topography, Ppotostereometry, laser scanner etc. For electronic surveying, the digitized 3D image should be an accurate/reliable reproduction of the dental cast. If not, the clinical results will be affected. [11],[14],[16],[17]

This study, therefore, was undertaken to determine the dimensional accuracy of digital models as compared to conventional study models. Study models and digital models of Kennedy's class I, class II and class III partially edentulous situations were used. The main reason behind using Kennedy's classification is that, it permits immediate visualization of partially edentulous arch. It also permits a logical approach to the problem of design and makes possible the application of sound principle of partial denture design. [3],[4],[9],[10] The triangular faceted models termed as stereo lithography or STL files are commonly used in transferring CAD model to rapid prototyping technologies.

A multislice helical CT scan with 0.75mm collimation width was used to produce these images later which are converted to digital models by the use of Mimic software. A collimator regulates the tomographic slice thickness and can be varied to produce different slice thicknesses typically from 1mm to 10-15mm. DICOM format. (Digital imaging and communication in medicine) is supported by CT scanner to transfer the scanned images. [11]

For the purpose of determining the dimensional accuracy and the reliability of electronic surveying the following three criteria were used:

  • measurement of the distance between pre determined points on the surface of the models
  • determination of angle of cervical convergence of the abutment teeth
  • determination of under cut depth of the abutment teeth
Measurement of distance was done by locating specific points, which were carefully chosen as they could be repeated both on the study models and the digital models The size of the angle of cervical convergence determines the amount of retention. To be retentive the teeth must have angle of cervical convergence cervical to the height of contour. [3],[4] As it was not feasible to measure the precise angle of cervical convergence and undercut depth on the study models, photographic images of the study models with the analyzing rod placed against the proximal surfaces of abutment teeth were obtained and transferred. The angle of cervical convergence and the depth of undercut were also measured on the hyper mesh digital model. These values were recorded and analyzed with a suitable statistical method.

Some minor differences in measurement of the distance between predetermined points on both study model and digital model were seen. The difference in the measurement of the distance ranged from 0.01 mm to 0.17 mm with a mean of 0.104 mm and standard deviation (SD)= 0.011. But these differences were statistically significant (p less than 0.05). When the angles of cervical convergence were measured on the proximal surface of the abutment teeth, it was found that, there was a difference of 0.07 0 to 0.09 0 with a mean difference of 0.078 0 and SD is equal to 0.0047. The p value was significant.(p less than 0.05). The under cut depths were measured on the proximal surfaces as well as buccal and lingual surfaces of the respective abutment teeth. The differences in measurement of under cut depth varied from 0.03 mm to 0.21 mm with a mean of 0.09mm and SD of 0.0059. Statistically, these differences were significant.

The differences between the manual measurements and measurements made on the 3D image were slightly statistically significant and were, in fact, well within the range of intra operator variation. When the measurements are made on 3D digital image operators variation plays a role because the operator has to click on the points to be measured.

Limitation

There are difficulties in measuring the undercut depth in mesiobuccal and distobuccal line angles because of the superimposition of adjacent tooth structure and also; the junction of the tooth surface and the gingival tissue is not delineated on the digital model. These limitations can, however, be overcome with further research in software programs.


  Conclusion Top


From this study we can conclude that the dental cast can be scanned with the help of an accurate and sophisticated scanner such as multislice helical CT scan and the digital model used for the purpose of diagnosis, treatment planning and storage for future use. The dimensional accuracy of the digital models when compared to the study models was statistically significant; and measurement can be made to a precision of 0.17mm.Therefore, digital/electronic surveying is a reliable method which can be accomplished by locating the height of contour, guide planes and undercut depth measurements as the digital model can be viewed from various angles and positions.

Information of the cast can be utilized more effectively if the 3D shapes of the cast are digitized for medico legal purposes as the consumer protection act (1987) states that it is necessary to retain all patient records for not less than 11 years. [5] Compared with hand-made set-up models, the computed diagnostic cast has advantages such as high speed processing and quantitative evaluation on the amount of 3D movement of the individual tooth relative to the craniofacial plane. [21]

Future developments may enable the computer to automatically determine a suggested path of insertion and with further research unwanted undercut could be eliminated and useful undercuts identified. At another stage, components of removable partial denture could be stored in a computer library and "dragged and dropped" in place on a scanned and surveyed cast from icons appearing on the screen. This would allow virtual patterning to be carried out much faster than by using current techniques. [1]

 
  References Top

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Abstract
Introduction
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