Data collection

The measurements included seven categories.

• Mesiodistal widths—As described by Hunter and Priest,2 the greatest mesiodistal diameter from the anatomic mesial contact point to the anatomic distal contact point of each tooth was measured parallel to the occlusal plane. Measurements were recorded to the nearest 0.1 mm.

• Arch length—The arch length was measured by the segment arch approach. Segment A is the distance from the mesial contact point of the right first permanent molar to the mesial contact point of the right canine. Segment B is the distance from the mesial contact point of the right canine to the mesial contact point of the right central incisor. Segment C is the distance from the mesial contact point of the left central incisor to the mesial contact point of the left canine. Segment D is the distance from the mesial contact point of the left canine to the mesial contact point of the left first permanent molar. The segments were summed to the nearest 0.1 mm to equal the arch length for both the maxillary and mandibular arch.

• Arch width: Intermolar width was measured as the distance between the mesiobuccal cusp tips of the permanent first molars. Intercanine width was measured as the distance between the crown tips of the permanent canines. These measurements were made on both maxillary and mandibular casts. Measurements were recorded to the nearest 0.1 mm.

• Overjet: The greatest amount of horizontal overlap was recorded from the labial surface of the mandibular central incisors to the lingual surface of the maxillary central incisor. Overjet was recorded to the nearest 0.5 mm on plaster models and 0.1 mm on computer-based models.

• Overbite: The greatest amount of vertical overlap of the maxillary central incisors to the mandibular central incisors was measured with the casts occluded. Overbite was recorded to the nearest 0.5 mm on plaster models and 0.1 mm on computer-based models.

Calculated values

• Space available was the summation of the maxillary and mandibular arch length segments (A + B + C + D) to the nearest 0.1 mm. This was done on both the plaster models and the computer-based models.

• Space required was the summation of the maxillary and mandibular mesiodistal tooth widths including right and left second premolar, first premolar, canine, lateral incisor, and central incisor. This was done on both the plaster models and the computer-based models.

Instrumentation

Digital calipers (Digimatic Caliper: 700-113 MyCal Lite, Mitutoyo America Corp, Plymouth, Mich) were used to measure the plaster study casts. A standard computer mouse was used to draw the distances from point to point on the computer models. The monitors used were 19-inch CRT (18-inch, 45 cm viewable Optiquest z90, ViewSonic, Walnut, Calif) with a monitor resolution of 1280 × 1024 pixels and 0.22-mm horizontal dot pitch (0.26 mm diagonal dot pitch) with 32-bit color. All measurements were made to the nearest 0.1 mm, except for the plaster overbite and overjet measurements that were measured to the nearest 0.5 mm. The software used to display and measure the computer-based models was Version 1.17 (OrthoCad, Cadent Inc, Fairview, NJ).

Examiners were second and third year orthodontic residents at The Ohio State College of Dentistry. To test the accuracy of measurements made on computer-based models, one examiner (Dr. Quimby) measured the 10 plaster models and the 10 computer models made from the standard dentoform. All the measurements were performed for each set of models and then repeated two weeks later. The dentoform was measured twice, and the mean of these values was considered the “gold” or “truth” standard.

Efficacy has been defined as the extent to which a procedure produces the intended result under ideal conditions, and effectiveness has been defined as the extent to which a procedure produces the intended result under normal conditions.9 The distinction between efficacy and effectiveness may be compared with the differences between a laboratory test of a procedure and a field test of a procedure or between a controlled experiment and everyday experience.

To evaluate the efficacy and reproducibility of measurements made on computer-based models, two examiners, experienced in the measurement of computer-based models, measured the 50 plaster models and the 50 computer models made from the orthodontic patients. All the measurements were performed for each set of models and then repeated after an interval of at least two weeks.

To evaluate the effectiveness of measurements made on computer-based models under normal conditions, a random set of 10 models was selected from the sample group of 50 models. Ten examiners measured the 10 plaster models and the 10 computer-based models at two time intervals, at least two weeks apart. All the measurements were performed for the 10 models.

Statistical tests

The statistical methods used in this study were the repeated measures analysis of variance for multiple repeated measurements and the intraclass correlation coefficient for reliability (JMP version 4.0; SAS version 8.2, SAS, Cary, NC).10

Dentoform

Table 1 presents the mean values for measurements made directly on the dentoform and the same measurements made on the plaster and computer-based models. There were no significant differences between measurements made on the plaster models and those made on the dentoform. Measurements made on the computer-based models differed significantly from measurements made directly on the dentoform for mandibular and maxillary space available (Table 1). The variance of measurements made on the computer-based models was significantly greater than the variance of measurements made on the plaster models for all measurements except mandibular intercanine distance.

TABLE 1. Mean (standard deviation) for Measurements Made on Gold Standard (dentoform), Digital, and Plaster Models (n = 10)

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Two examiners, 50 model pairs

Intrarater reliability (reproducibility) for the two examiners measuring the 50 plaster and 50 computer-based models on two separate occasions was high. Intraclass correlation coefficients were 0.90 or greater for all measurements on each system (data not shown).

The repeated measures analysis of variance showed significant differences between computer-based models and plaster models (Table 2). There were significant differences in all categories measured except lower canine arch width and mandibular arch length required. Measurements were greater for the computer-based models except for overbite and overjet. However, these measurements were only recorded to the closest 0.5 mm for the plaster models.

TABLE 2. Mean Values (SD) of Two Measurements From Plaster and Digital Models Prepared from Duplicate Impressions of 50 Patients and the Mean Difference between the Same Measurements on Digital and Plaster Models and System Significance

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Ten examiners, 10 model pairs

Table 3 presents the data generated from the 10 examiners and 10 patients. There were significant differences between the two systems, computer-based and plaster models, and among the examiners for all measurements except mandibular intercanine width.

TABLE 3. Mean Value (SD) for 10 Examiners at Two Time Intervals Measuring 10 Models. Digital and Plaster Models and the Difference between Measurements and System and Examiner Significance. (n = 10)

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Dentoform

The direct measurements of the plastic teeth in the dentoform served as the gold standard control to evaluate the accuracy of measurements made on plaster and computer- based models. Measurements made on plaster casts poured within one hour of the impression did not differ from those made directly on the gold standard dentoform. Measurements made on computer-based models produced from plaster casts poured the day after the impressions were taken did not differ from those made on the gold standard except for mandibular and maxillary space available. Thus, for measurements made directly between two anatomical points on the computer-based models the accuracy was equal to that of plaster models.

The process of measuring segments apparently introduced error into the measurements. In the present investigation, plaster models poured within one hour after the irreversible hydrocolloid impressions were made were accurate representations of the original, master dentoform teeth. If we accept the plaster model as the “gold standard” of current clinical practice, where measurements are seldom made directly in the patient's natural dentition, the measurements made on the two systems did differ statistically except for overbite and overjet. In the majority of measurements, the magnitude of the differences was less than 0.5 mm.

Two examiners, 50 model pairs

Measurement taken on both the plaster and computer- based models by experienced examiners showed a high degree of reproducibility. All categories showed excellent reliability, with intraclass correlation coefficients >0.90 (data not shown). In comparing the two systems, measurements made from computer-based models were larger than those made on plaster casts except overbite and overjet (Table 2). For the most part, the differences between measurements on the two model systems were less than 1 mm. The measurements for overbite and overjet were made to 0.1 mm on the computer-based models and 0.5 mm on the plaster models. Therefore, smaller values for the computer-based models are not unexpected. Thus, when applied by experienced examiners, the process of making measurements on computer-based digital models is reproducible and efficacious.

Ten examiners, 10 model pairs

For the data from the 10 examiners (Table 3), significant examiner differences existed for all categories measured. Additionally, statistically significant differences between the two model types occurred in all categories except the mandibular intercanine width. The measurements made on computer-based models showed greater variation in all categories except overbite and overjet. The mean difference between the two systems for 10 examiners ranged from 0.19 to 1.9 mm, and this was similar to the mean difference between the systems for two examiners, which ranged from 0.15 to 2.9 mm. Most measurements differed by less than 1 mm. Thus, when applied in a more realistic setting, the computer-based measurement system was as effective as plaster models.

The conditions of this study probably overestimate the effectiveness of the plaster model. While it is unlikely that irreversible hydrocolloid impressions are poured within one hour in all orthodontic practices, it is equally unlikely that they are stored for 24 hours. The choice of one hour was on the basis of published reports that showed that plaster models poured within one hour were accurate representations to test bodies.1112 This study does not demonstrate that plaster models, as they are commonly prepared in orthodontic practices, are effective representations of the natural dentition.

There was a significantly greater variance associated with measurements made on the computer-based models compared with the same measurements made on the plaster models (Tables 2 and 3). We hypothesize that the larger values for measurements made on the computer-based models may have several possible sources: (1) the increased time that elapsed before the irreversible hydrocolloid impressions were poured in plaster, (2) the process of producing the plaster casts by the manufacturer, (3) the process of scanning and recording data points from the plaster model, (4) the display and measurement algorithms of the manufacturer's proprietary software, and (5) the examiners' lack of familiarity with the computer-based measurement of computer-based models.

Other investigators have reported that storage of irreversible hydrocolloid in a wet paper towel for 24 hours before pouring plaster models produced significant dimensional changes compared with models poured immediately or after storage for one hour in a paper towel.11 There is a further effect of the disinfection protocol that can, in turn, be anticipated to change with the length of storage of the impression before the cast is poured.12–14 The manufacturer (OrthoCad, Cadent Inc) does not release detailed, technical information either on the process of producing the casts ie, type of dental stone, powder-water ratios, water temperature etc, or on the accuracy and reproducibility of the scanning method by which data points are produced from models. Finally, the relative inexperience of the 10 examiners with the computer-based models and the measurement software compared with the measurement of plaster models may have increased both measurement error and the variability of the measurement values.

The differences between the plaster and computer models, though generally small, were statistically significant; the question remains open if they are clinically significant. The computer models are reasonably reliable and accurate. They can provide the clinician with adequate information to develop a treatment plan and thus eliminate the need for storing plaster casts. The true test of clinical significance would be to determine whether treatment plans produced with computer-based models differed significantly from treatment plans produced with plaster models. In turn, the results of the treatment from the two different sets of models would determine the true value of computer-based models. In the opinion of the authors, it is questionable whether the differences demonstrated in this investigation would lead to significantly different clinical results.

The computer-based model software is routinely being updated to provide more features and to improve the accuracy of the measurements. More recent versions of the software have new features that theoretically could improve accuracy. It is the opinion of the authors that the accuracy and reliability of the computer-based models is acceptable, and it will be the relative convenience and total cost of the computer-based model that determines its acceptance. Models can be viewed chair-side in seconds, and thousands can be stored in the space of a moderately sized hard cover novel. The model can be shared over a network within an office or offices of a practice or with another party without it ever leaving the practice or without the danger of the models being damaged by handling. A copy of the model can be secured at a second site for minimal or no cost. All these benefits are based on networked chair-side computers with their associated benefits and costs. However, computer failure, software failure, or manufacturer insolvency can possibly mean that the models may become inaccessible for a time or forever.