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Journal of Orthodontics, Vol. 35, No. 3, 191-201, September 2008 doi:10.1179/146531207225022626
© 2008 British Orthodontic Society
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Scientific Section

A comparison of plaster, digital and reconstructed study model accuracy

Andrew P. Keating

Regional Orthodontic Unit, Waterford Regional Hospital, Waterford, Ireland

Jeremy Knox

Morriston Hospital, Swansea, UK

Richard Bibb

Department of Design and Technology, Loughborough University, Loughborough, UK

Alexei I. Zhurov

School of Dentistry, Cardiff University, UK

Address for correspondence: Jeremy Knox, Orthodontic Department, Morriston Hospital, Swansea SA6 6NL, UK. Email: jeremy.knox{at}swansea-tr.wales.nhs.uk

Received 23 November 2006; accepted 6 May 2008


   Abstract
Top
 Abstract
Introduction
 Materials and methods
 Results
 Discussion
 Conclusions
 Future work
 Contributors
 References
 
Objectives: To evaluate the accuracy and reproducibility of a three-dimensional (3D) optical laser-scanning device to record the surface detail of plaster study models. To determine the accuracy of physical model replicas constructed from the 3D digital files.

Design and setting: A method comparison study using 30 dental study models held in the Orthodontic Department, School of Dentistry, Cardiff University.

Materials and methods: Each model was captured three-dimensionally, using a commercially available Minolta VIVID 900 non-contact 3D surface laser scanner (Konica Minolta Inc., Tokyo, Japan), a rotary stage and Easy3DScan integrating software (TowerGraphics, Lucca, Italy). Linear measurements were recorded between landmarks, directly on each of the plaster models and indirectly on the 3D digital surface models, on two separate occasions by a single examiner. Physical replicas of two digital models were also reconstructed from their scanned data files, using a rapid prototyping (RP) manufacturing process, and directly evaluated for dimensional accuracy.

Results: The mean difference between measurements made directly on the plaster models and those made on the 3D digital surface models was 0.14 mm, and was not statistically significant (P = 0.237). The mean difference between measurements made on both the plaster and virtual models and those on the RP models, in the z plane was highly statistically significant (P <0.001).

Conclusions: The Minolta VIVID 900 digitizer is a reliable device for capturing the surface detail of plaster study models three-dimensionally in a digital format but physical models of appropriate detail and accuracy cannot be reproduced from scanned data using the RP technique described.

Key words: Orthodontics, study models, three-dimensional imaging


   Introduction
Top
 Abstract
 Introduction
Materials and methods
 Results
 Discussion
 Conclusions
 Future work
 Contributors
 References
 
Orthodontic treatment outcome and treatment change have traditionally been recorded with gypsum-based study models, which are heavy and bulky, pose storage and retrieval problems, are liable to damage and can be difficult and time consuming to measure.1Go–5Go Legislation relating to the retention of patient records after the completion of treatment6Go has led to huge demands on space for storage that has prompted the development of alternative methods of recording occlusal relationships (Table 1Go) and electronic storage of records.7Go–13Go


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  		Table 1 Alternative methods of recording occlusal relationships.
 
The replacement of plaster study models with virtual images has several advantages including ease of access, storage and transfer,14Go and the accuracy of image capture techniques has been reported.1Go,3Go,15Go–20Go However, if the physical restoration of a digital occlusal record is needed, possibly for medico–legal reasons, an accurate method of three-dimensional (3D) reconstruction is required.

Rapid prototyping (RP) systems, such as stereolitho-graphy, generate 3D models from a digital file through incremental layering of photo-curable polymers.21Go The dimensional accuracy of physical replicas reproduced using the stereolithography technique has been evaluated by a number of authors. Barker et al.22Go found a mean difference of 0.85 mm between measurements made on actual dry bone skulls and physical replicas of the skulls produced by stereolithography from three-dimensional computed tomography (3D-CT) scans of the original dry bone skulls. They concluded that RP models could be confidently used as accurate 3D replicas of complex anatomic structures. Using similar techniques, Kragskov et al.23Go and Bill et al.24Go found mean differences of – 0.3 to 0.8 mm and ±0.5 mm between measurements on 3D-CT images and stereolithographic models.

The objectives of this study were:

Null hypotheses


   Materials and methods
Top
 Abstract
 Introduction
 Materials and methods
Results
 Discussion
 Conclusions
 Future work
 Contributors
 References
 
Manual measurements
The local REC chairman confirmed that no ethical approval was required for this study. A minimum of 7–10 models per group were calculated to be required to allow a 90% chance of detecting a 0.3 mm difference in related sample means (SD = 0.2) at the 5% level of significance (alpha = 0.05, power = 0.90).25Go Thirty randomly selected plaster study models, held in the Orthodontic Unit of University Dental Hospital, Cardiff, were used in the study. Each study model was cast in matt white Crystal R plaster (South Western Industrial Plasters, Chippenham, UK) and conventionally trimmed.26Go To be included in the study the plaster study models had to completely reproduce the arch, show no surface marks, loss of tooth material, voids or fractures and demonstrate varying degrees of contact point and buccolingual tooth displacements.

A hand held digital calliper (series 500 Digimatic ABSolute Caliper, Mitutoyo Corporation, Kawasaki, Japan), was used to manually measure the plaster models. This calliper had a measurement resolution of 0.01 mm, was accurate to ±0.02 mm in the 0–200 mm range and automatically downloaded data eliminating measurement transfer and calculation errors.

All plaster models were measured in a bright room without magnification. The plaster models were not prepared in any way prior to measuring and the anatomical dental landmarks used in the measurements were not pre-marked. A single examiner conducted all the measurements after an initial training period.

Twenty linear dimensions were measured, on each model, in each of the three planes (x, y and z) with all measurements being recorded to the nearest 0.01 mm. The following dimensions were selected for measurement:

x plane:

  1. intercanine distance – measured as the distance between:
    1. the occlusal tips of the upper canines;
    2. the occlusal tips of the lower canines.

  2. interpremolar distances – measured as the distance between:
    1. the buccal cusp tips of the upper and lower first and second premolars;
    2. the palatal cusp tips of the upper first and second premolars;
    3. the lingual cusp tips of the lower first premolars;
    4. the mesiolingual cusp tips of the lower second premolars.

  3. intermolar distances – measured as the distance between:
    1. the mesiopalatal cusp tips of upper first and second molars;
    2. the mesiobuccal cusp tips of the upper and lower first and second molars;
    3. the mesiolingual cusp tips of lower first and second molars;
    4. the disto-buccal cusp tips of the upper and lower first molars.

y plane:

  1. in the upper arch the distance from the mesiopalatal cusp tip of the upper second molar to:
    1. the mesiopalatal cusp tip of the upper first molar;
    2. the palatal cusp tip of the upper first and second premolar;
    3. the cusp tip of the upper canine;
    4. the mesio-incisal corner of the upper lateral incisor.

These dimensions were measured on both sides of the upper arch.

2. in the lower arch the distance from the mesiolingual cusp tip of the lower second molar to:
  1. the mesiolingual cusp tip of the lower first molar and second premolar;
  2. the lingual cusp tip of the lower first premolar;
  3. the cusp tip of the lower canine;
  4. the mesio-incisal corner of the lower lateral incisor.

These dimensions were measured on both sides of the lower arch.

z plane:

  1. The clinical crown height of all the teeth, in both upper and lower arches, from the second premolar to second premolar inclusive, measured as the distance between the cusp tip and the maximum point of concavity of the gingival margin on the labial surface. Measurements were made on two occasions separated by at least one week.

Virtual measurements
A non-contact laser-scanning device (Minolta VIVID 900) was used to record the surface detail of each of the 30 study models using a telescopic light-receiving lens (focal distance f = 25 mm) and rotary stage (ISEL-RF1, Konica Minolta Inc., Tokyo, Japan). The rotary stage facilitated the acquisition of multiple range maps by moving the plaster study models in sequence by a controlled rotation as they were being scanned, thus ensuring the entire visible surface of each plaster model was captured. The stage was controlled by a computer software program (Easy3DScan Tower Graphics, Lucca, Italy) and integrated controller box (IT116G, Minolta Inc., Osaka, Japan).

Easy3DScan was used to align, merge and simplify the range maps acquired at different angles to produce a composite surface dataset that was then imported into the RapidForm 2004 software program (INUS Technology Inc., Seoul, Korea) as a triangulated 3D mesh (Figure 1Go). An automated measuring tool was used to record the same measurements that had been conducted manually on the plaster study models. The 3D digital surface models were magnified and rotated on screen to aid identification of the anatomical landmarks as necessary. Linear distances between landmarks were calculated automatically to five decimal places (Figure 1Go). Replicate measurements were made on all digital model images with a time interval of at least one week.


Figure 1
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  		Figure 1 On-screen 3D virtual model image. Intercanine width measured

 
Measurement of reconstructed models
One pair of upper and lower plaster models were scanned individually using an identical protocol, adhering to the inclusion criteria listed previously. Only one set of models was evaluated due to the current cost of stereolithography. The scanned data for both upper and lower plaster models were saved as binary STL files and imported into the Magics RP software (Materialise Inc., Leuven, Belgium).

A 3D Systems stereolithography machine (SLA-250/ 40, 3D Systems Inc., Valencia, CA, USA) containing a hybrid epoxy-based resin (10110 Waterclear, DSM Somos, New Castle, DE, USA) was used to construct replica (RP) models from the digital files using a build layer thickness of 0.15 mm (Figures 2Go and 3Go).


Figure 2
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  		Figure 2 Original upper plaster model (left) and stereolithography model (right) as seen from (A) frontal, (B) occlusal, (C) right buccal and (D) left buccal directions

 

Figure 3
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  		Figure 3 Original lower plaster model (left) and stereolithography model (right) as seen from (A) frontal, (B) occlusal, (C) right buccal and (D) left buccal directions

 
Identical measurements, in x, y and z planes, were made on the reconstructed stereolithography models to those recorded on the original plaster study models and virtual models. Replicate measurements were made one week later.

Statistical analysis
A Bland–Altman analysis27Go was undertaken to determine agreement between repeat model measurements. Intra-rater reliability was assessed by visually comparing the difference in repeat measurements and performing non-parametric, Wilcoxon signed rank hypothesis tests. This is described in the Results section.


   Results
Top
 Abstract
 Introduction
 Materials and methods
 Results
Discussion
 Conclusions
 Future work
 Contributors
 References
 
Data analysis demonstrated a non-normal distribution of results, and non-parametric tests (Wilcoxon signed rank test) were therefore employed in the statistical analysis.

No significant difference (P > 0.2) was demonstrated in measurements at initial time (T1) and one week later (T2) for the manual measurement of plaster study models (Table 2Go, Figure 4Go), 3D digital surface model measurement (Table 3Go, Figure 5Go) or manual measurement of the stereolithography, reconstructed models (Table 4Go, Figure 6Go). Almost all points were clustered around the mean difference of zero, within two standard deviations of the mean difference (Figures 4Go–6GoGo) indicating good intra-rater reliability.


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  		Table 2 Variation in repeat measurements of plaster models – 20 measurements in each plane repeated on 30 models.
 

Figure 4
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  		Figure 4 Bland Altman plot for repeat measurements plaster model. Twenty measurements in each plane repeated on 30 models (reference lines showing 2SD)

 

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  		Table 3 Variation in repeat measurements of virtual models – 20 measurements in each plane repeated on 30 models.
 

Figure 5
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  		Figure 5 Bland Altman plot for repeat virtual model measurements. Twenty measurements in each plane repeated on 30 models (reference lines showing 2SD)

 

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  		Table 4 Variation in repeat measurements of the reconstructed model – 20 measurements in each plane repeated on one model.
 

Figure 6
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  		Figure 6 Bland Altman plot for repeat stereolithography model measurements. Twenty measurements in each plane repeated on one pair of models (reference lines showing 2SD).

 
A comparison of linear measurements made on the plaster study models and 3D digital surface (virtual) models is presented in Table 5Go and Figure 7Go. The mean difference in all planes was 0.14 mm (SD = 0.10 mm) and was not statistically significant (P > 0.2).


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  		Table 5 Difference between plaster and virtual model measurements (means of 20 measurements in each plane compared).
 

Figure 7
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  		Figure 7 Bland Altman plot for differences in plaster and virtual model measurements (reference lines showing 2SD)

 
Measurements made in x and y planes were not significantly different for reconstructed models and plaster models (P > 0.3) or 3D digital surface models (P > 0.5). However, in the z plane, measurement differences were significantly different (P <0.001, Tables 6Go and 7Go; Figures 8Go and 9Go). All z plane reconstructed model measurements were significantly smaller than the corresponding plaster and 3D digital surface model measurements.


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  		Table 6 Difference between plaster and reconstructed model measurements (means of 20 measurements in each plane compared).
 

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  		Table 7 Difference between virtual and reconstructed model measurements (means of 20 measurements in each plane compared).
 

Figure 8
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  		Figure 8 Bland Altman plot for differences in plaster and stereolithography model measurements (reference lines showing 2SD)

 

Figure 9
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  		Figure 9 Bland Altman plot for differences in virtual and stereolithography model measurements (reference lines showing 2SD)

 

   Discussion
Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
Conclusions
 Future work
 Contributors
 References
 
This study has demonstrated a simple and reproducible method of study model measurement. The excellent reproducibility of plaster, digital and reconstructed model measurements reported compares favourably with Zilberman et al.20Go and Bell et al.28Go who reported mean intra-operator errors of 0.18 and 0.17 mm respectively, when the same points were measured by the same operator at different times on plaster study models, and Stevens et al.14Go who reported a concordance correlation coefficient of 0.88 for the measurement of digital models using emodel software (GeoDigm, Chanhassen, MN, US). The reconstructed stereolithography model measurements in this study demonstrated a greater range in repeated absolute measurement differences (0.02–0.41 mm) compared to those for the other two methods (0.01–0.34 mm) reflecting the greater difficulty in measuring these models.

This study has also demonstrated the validity of digital (virtual) models derived from the laser scanning process described. The problems of trying to acquire dimensionally accurate images using structured light scanning methods have been reported by Bibb et al.29Go and Mah and Hatcher.30Go The light beam from structured light scanners travels in straight lines so any object surfaces that are obscured or are at too great an angle to the line of sight of the light source will not be scanned. This results in ‘voids’ or ‘holes’ in the scanned surface data. To overcome this problem the object or the scanner needs to be moved to different angulations and the scan process repeated at each angle. For irregular objects multiple scans of the same object from different angles may need to be acquired. The data from each of these scans can then be ‘stitched’ (registered and merged) together using special software programs to produce a single composite surface model of the object.29Go,31Go,32Go Compounding these difficulties are the errors introduced during computer processing of the acquired data that are necessary to reduce artefacts and yet retain detail, while errors can also be introduced during the merging together of the multiple perspectives to form the single composite surface model of the object being scanned.30Go

A number of authors who have evaluated alternative ways of measuring study models have suggested what they consider a clinically significant measurement difference. Schirmer and Wiltshire33Go regarded a measurement difference between alternative measurement methods of less than 0.20 mm as clinically acceptable. Hirogaki et al.11Go suggested the accuracy required with orthodontic study models to be about 0.30 mm while Halazonetis32Go reported that an accuracy of 0.50 mm was sufficient for head and face laser-scanning but would be inadequate for scanning study models. Bell et al.28Go investigating the accuracy of the stereo-photogrammetry technique for archiving study models decided a mean difference of 0.27 mm (SD = 0.06 mm) between this technique and measurements made by hand on plaster models was unlikely to have a significant clinical impact.

The accuracy of the on-screen virtual models as reported in this study compares favourably with some studies but less favourably than with others. These studies varied greatly in their 3D capture techniques and software analysis systems (Table 8Go).


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  		Table 8 Accuracy of various 3D capture techniques (Average measurement error is the mean difference between measurements made on virtual models and the original plaster models).
 
The statistically significant difference between measurements made directly on the plaster models and those made on the reconstructed models was largely due to errors in the z plane. The stereolithography models were built in 0.15 mm layers from a clear resin. Model translucency made landmark identification difficult and layering resulted in some loss of surface detail particularly at the cervical margin (Figures 2Go and 3Go). In addition, errors in data conversion and data manipulation generated while converting digital surface models to stereolithography file format can result in some distortion30Go,34Go and the RP technique can also introduce errors due to model shrinkage during building and post-curing.22Go However, clinical significance of these errors will depend on the intended purpose of the reconstructed model. The models may not be sufficiently accurate for appliance construction but may be suffi-cient to demonstrate pre- or post-treatment occlusal relationships. Unfortunately, the current prohibitive cost of stereolithography limited this study to the evaluation of only one pair of reconstructed models.

This study has presented a novel method of digitally recording study model data, offering the profession a valid alternative to the use of conventional plaster models and the potential to significantly reduce the burden of model storage. In addition, the potential for physical reconstruction of a model from the digital archive has been demonstrated which may go towards addressing medicolegal concerns.


   Conclusions
Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 Conclusions
Future work
 Contributors
 References
 


   Future work
Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 Conclusions
 Future work
Contributors
 References
 
A stereolithography process employing thinner layers or other RP technologies that use significantly thinner build layers may address the deficiencies of the reconstructed models used in this study. Techniques that should be investigated include digital light processing based machines (EnvisionTEC GmbH, Gladbeck, Germany) and the various printing based processes such as poly-jet modelling (Objet Geometries Ltd, Rehovot, Israel), multi-jet modelling (3D Systems Inc. Rock Hill, SC, USA) and single head jetting (Solidscape Inc., Merrimack, NH, USA). These processes are all capable of producing physical models with a layer thickness of up to 10 times thinner than the stereolithography models described in this paper (layer thicknesses range from 0.013 to 0.150 mm).


   Contributors
Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 Conclusions
 Future work
 Contributors
References
 
Andrew Keating, Richard Bibb and Jeremy Knox were responsible for study design. Richard Bibb contributed to this work while at the National Centre for Product Design and Development Research, University of Wales Institute Cardiff, which supplied the stereolithography models. Andrew Keating was responsible for data collection. Andrew Keating and Alexei Zhurov were responsible for data manipulation and processing. All authors were responsible for data analysis and preparation of manuscript. Jeremy Knox is the guarantor.


   References
Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 Conclusions
 Future work
 Contributors
 References
 
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24 Bill JS, Reuther JF, Dittmann W, et al. Stereolithography in oral and maxillofacial operation planning. Int J Oral Maxillofac Surg 1995; 24: 98–103.[CrossRef][Medline]

25 Altman DG. Practical Statistics for Medical Research. London: Chapman and Hall, 1991, 455–58.

26 Adams PC. The design and construction of removable Orthodontic appliances, 4th Edn. Bristol: John Wright and Sons, 1976.

27 Bland JM, Altman DG. Measuring agreement in method comparison studies. Stat Methods Med Res 1999; 8: 135–60.[Abstract/Free Full Text]

28 Bell A, Ayoub AF, Siebert P. Assessment of the accuracy of a three-dimensional imaging system for archiving dental study models. J Orthod 2003; 30: 219–23.[Abstract/Free Full Text]

29 Bibb R, Freeman P, Brown R, Sugar A, Evans P, Bocca A. An investigation of three-dimensional scanning of human body surfaces and its use in the design and manufacture of prostheses. Proc Inst Mech Eng [H] 2000; 214: 589–94.[Medline]

30 Mah J, Hatcher D. Current status and future needs in craniofacial imaging. Orthod Craniofac Res 2003; 6(Suppl.1): 10–16.[CrossRef][Medline]

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