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166 Journal of Dental Education ■ Volume 79, Number 2

Evaluation of Occlusal Rest Seats With 3D Technology in Dental Education Manuel António Ferreira Sampaio-Fernandes, DDM; Maria M. Sampaio-Fernandes, DDM; Patrícia A. Fonseca, DDM; Paulo R. Almeida, DDM; José C. Reis-Campos, DDM; Maria H. Figueiral, DDMAbstract: The preparation of rest seats must comply with specific sizes and shapes. Various technological systems such as Kavo PrepAssistant have been used as an auxiliary method to evaluate preclinical preparations more objectively. The aims of this study were to establish an alternative system for evaluating occlusal rest seats and to compare different types of assessment. Seventy-six undergraduate students at Oporto University Faculty of Dental Medicine in Portugal were selected as a convenience sample to prepare two occlusal rest seats in Kavo teeth #45 and #46 (FDI World Dental Federation ISO-3950) and were randomly assigned to two groups. Bearing in mind the ideal characteristics of rest seats, the investigators defined ten assessment parameters, and their evaluation weights were independently estimated by three evaluators. Four of these parameters were measured in Kavo PrepAssistant. The results of the different evaluation methods and evaluators varied considerably. The classical evaluation presented final results worse than those of the evaluations using parameters. In this study, carrying out the assessment with Kavo PrepAssistant helped to achieve a more objective and less evaluator-dependent final evaluation.

Dr. Manuel Sampaio-Fernandes is an MSc student, Faculty of Dental Medicine, Oporto University, Porto, Portugal; Dr. Maria Sampaio-Fernandes is a PhD student, Faculty of Dental Medicine, Oporto University, Porto, Portugal; Dr. Fonseca is Assistant Professor, Faculty of Dental Medicine, Oporto University, Porto, Portugal; Dr. Almeida is Assistant Professor, Faculty of Dental Medicine, Oporto University, Porto, Portugal; Dr. Reis-Campos is Associate Professor, Faculty of Dental Medicine, Oporto University, Porto, Portugal; and Dr. Figueiral is Full Professor, Faculty of Dental Medicine, Oporto University, Porto, Portugal. Direct correspondence to Dr. Manuel Sampaio-Fernandes, Faculty of Dental Medicine, Oporto University, Rua Ruy Belo 86, 4450-259, Porto, Portugal; +351 919467948; [email protected].

Keywords: dental education, prosthodontics, removable partial denture, preclinical evaluation, occlusal rest seat, Portugal

Submitted for publication 4/7/14; accepted 7/16/14

Rests are components of a removable partial denture (RPD) that transfer force through the long axis of the abutment tooth. The surface

of the abutment prepared to receive the rest is called a rest seat, and it allows the stress to be absorbed by the fibers of the periodontal ligament without damaging the supportive tissues of the tooth. In addition, rests should provide vertical support for the prosthesis while keeping retentive clasps in the correct position.1-5

In order that occlusal rests can correctly per-form their functions, rest seats must comply with specific sizes and shapes.6 The outline of a rest seat should be rounded triangular with its base in the marginal ridge of the tooth and its floor inclined downwards to the center of the tooth.1 The floor is expected to have a saucer- or spoon-shaped depres-sion.1,6,7 A minimum buccolingual width of 2.0 to 2.5 mm and a maximum equivalent to one-third of the crown or half the distance between the cusp tips have been suggested. The recommended mesiodistal length varies from one-third to one-half of the crown. To allow enough metal thickness to guarantee that the rest and the minor connector remain stiff, a thickness

of 1.0 to 1.5 mm is recommended. The inclination of the occlusal rest’s horizontal axis should be less than 90 degrees, and the inner connection between the occlusal rest and the minor connector should be rounded.5,6,8

A rest seat can be prepared with various rota-tory instruments. Many professionals use round diamond burs. However, some prefer conical burs with a round head, which are less likely to create mechanical retentions, therefore providing an advan-tage for this technique.1 The finishing is performed using a low-speed handpiece with green stones (ISO 655-204-243513-025), which provides round angles and eliminates scratches produced by diamond burs. Then, a handpiece with carborundum stones is used for polishing to create smooth surfaces, thus prevent-ing dental plaque or food retention.1

At Oporto University Faculty of Dental Medi-cine in Portugal, since the subject of RPDs involves the study of RPD components, the students must pre-pare occlusal rest seats in acrylic teeth in preclinical laboratories. By the end of the course, the students’ preparations are assessed by three evaluators on a global level. This type of evaluation presents dif-

February 2015 ■ Journal of Dental Education 167

ficulties in guaranteeing inter-evaluator calibration.9

To improve learning and ensure a more objective evaluation of preclinical performance, various auxil-iary technological systems have been used, including DentSim by DenX, Virtual Reality Dental Training System by Novint, and PrepAssistant by Kavo. The main purposes of these systems are to help students identify and understand mistakes in their dental preparations, as well as to assist instructors in moni-toring and evaluating the students’ performance.7,9-12

The Kavo PrepAssistant system provides a detailed three-dimensional viewing of dental prepa-rations through a photographic scan. Therefore, it is possible to compare the image of a dental preparation performed by a student with the image of a prepara-tion performed by an instructor, or with an image of the intact tooth, by quantifying geometric variations between both at certain given points. The geometric variables measured by the system are the differences in depth of tissue reduction and in angulation. The software does not present a general result for the den-tal preparation, but simply a series of specific results, limited to the points in question. Some parameters, such as the surface roughness, cannot be objectively evaluated by this system.9,10,13 The aims of this study were to establish an alternative system for evaluating occlusal rest seats; to compare the classical evalua-tion of the rest seats with two alternative assessment methods (the modified classical evaluation and the Kavo PrepAssistant evaluation); and to compare evaluators according to the type of evaluation used.

MethodsThis study received Institutional Review Board

exemption at the Oporto University Faculty of Dental Medicine. The sample consisted of 152 rest seats prepared by 76 fourth-year undergraduate students in the five-year course in dental medicine at Oporto University. This group consisted of 25 males and 51 females, with ages ranging from 21 to 24 years.

Each student was provided with a mandibular dentoform (Kavo basic study model). Later, the students were randomly divided into two groups, which received the following instructions. Group 1 (37 students) prepared two occlusal rest seats in tooth #45 (mandibular right premolar) of the Kavo model, without including tooth #46 (mandibular right molar; FDI World Dental Federation ISO-3950). First, these students had to prepare the distal rest seat (without including the adjacent tooth) and then the mesial rest

seat (with the adjacent tooth). Group 2 (39 students) prepared two occlusal rest seats in tooth #46 of the Kavo model, without including tooth #45. First, they had to prepare the mesial rest seat (without including the adjacent tooth) and then the distal rest seat (with the adjacent tooth).

In order to define an alternative evaluation system, we established ten assessment parameters and their evaluation weights (Table 1). Classical and modified classical evaluations were entirely and independently performed by three evaluators (A, B, and C) corresponding to the three teaching assistants in the Removable Prosthodontics II course. Evaluator A had 27 years of experience, Evaluator B had 13 years of experience, and Evaluator C had only five years of experience.

The classical evaluation (CE) consists in put-ting each prepared tooth in the Kavo model and classifying it from a global perspective using a dental explorer and a dental crown gauge. In this study, a 0 to 20 grading scale was used since this is the most commonly used scale in this college. The general grade descriptors were as follows: 17.5=very good work, 13.5=good work, and 9.5=satisfactory work. Any grade below 9.5 points was deemed unsatisfac-tory or a failure. The modified classical evaluation (MCE) consisted in a weighted average of the various parameters listed in Table 1. Each parameter was evaluated in the Kavo model, using a dental explorer and a dental crown gauge on a 0 to 20 grading scale. The following parameters were considered exclusion criteria: parameter 4 (thickness on the marginal ridge larger than 2.25 mm) and parameter 7 (angle less than 50° or more than 110°). These lead to a nega-tive final mark, either of 7 points, when it was not possible to correct the parameter or more than one exclusion parameters were present, or 9 points, when the parameter was correctable. In comparison with the CE, the MCE is more objective since it divides a global evaluation into parameters.

The prep-assistant evaluation (PE) consisted in a weighted average of the various parameters listed in Table 1. Parameters 1, 5, 6, 8, 9, and 10 were assessed by three evaluators as in the MCE. Parameters 2, 3, 4, and 7 were measured by the investigator, using the Kavo PrepAssistant system, so as to compare students’ preparations with the intact tooth (a tooth without preparation). This method involved the fol-lowing steps: scanning the students’ preparations on the acrylic teeth with Kavo PrepAssistant system (Figure 1); scanning the intact tooth (#45 or #46) and marking the reference points on its external surface


(calibration step); and measuring and classifying parameters 2, 3, 4, and 7. These parameters’ classifi-cation and limits are shown in Table 1 and Figure 2. These steps were all performed by the investigator. The exclusion criteria were parameters 4 and 7, and the rules applied were the same as in the MCE.

The statistical analysis was carried out using SPSS 15.0 for Windows 7 operating system (SPSS

Inc., Chicago, IL, USA), with a significance level of α=0.05. The following statistical tests were per-formed: Shapiro-Wilk, t-Student, Wilcoxon, ANOVA, and Tukey HSD. Statistical comparisons were per-formed for each rest seat and tooth among the three evaluators using the CE and the MCE; parameters 2, 3, 4, and 7 within the MCE and the PE per evaluator; and final average scores for each evaluation.

Table 1. Assessment parameters of occlusal rest seats and their weight

Parameter Definition Evaluation Limits Weight

Parameter 1 The outline of a rest seat should be rounded trian-gular with the apex toward the center of the occlusal surface.

Very good 17.5-20 Qualitative 5%

Good 13.5-17.4

Satisfactory 9.5-13.4

High negative 7-9.5

Low negative 0-6.9

Parameter 2 The base of the triangle (on the marginal ridge) should have 2.5 mm.

Very good 17.5-20 2.25-2.75 20%

Good 13.5-17.4 2-2.25; 2.75-3

Satisfactory 9.5-13.4 1.75-2; 3-3.25

High negative 7-9.5 1.5-1.75; 3.25-3.5

Low negative 0-6.9 <1.5; >3.5

Parameter 3 (tooth #45)

The mesiodistal length should be one-third of the crown (2.5 mm).

Very good 17.5-20 2.25-2.75 20%

Good 13.5-17.4 2-2.25; 2.75-3

Satisfactory 9.5-13.4 1.75-2; 3-3.25

High negative 7-9.5 1.5-1.75; 3.25-3.5

Low negative 0-6.9 <1.5; >3.5

Parameter 3 (tooth #46)

The mesiodistal length should be one-third of the crown (3.7 mm).

Very good 17.5-20 3.45-3.95

Good 13.5-17.4 3.2-3.45; 3.95-4.2

Satisfactory 9.5-13.4 2.95-3.2; 4.2-4.45

High negative 7-9.5 2.45-2.95; 4.45-4.95

Low negative 0-6.9 <2.45; >4.95

Parameter 4 (exclusion parameter)

The thickness in the mar-ginal ridge should be of 1.5 mm.

Very good 17.5-20 1.25-1.75 20%

Good 13.5-17.4 1-1.25; 1.75-2

Satisfactory 9.5-13.4 0.75-1; 2-2.25

High negative 7-9.5 0.25-0.75; 2.25-2.5

Low negative 0-6.9 <0.25; >2.5

Parameter 5 The floor of the rest seat is expected to have a saucer- or spoon-shaped depression.


Good 13.5-17.4


High negative 7-9.5

Low negative 0-6.9

Parameter 6 The floor of the rest seat should not have steps.


Good 13.5-17.4


High negative 7-9.5

Low negative 0-6.9

(continued)


distal rest seats: A-B p=0.004, A-C p<0.001, and B-C p=0.010).

There were also statistically significant differ-ences (p<0.05) among the three evaluators regarding the MCE in both mesial and distal rest seats (Evalu-ator A, 16.05; Evaluator B, 15.10; and Evaluator C, 15.37). The Tukey tests showed that Evaluator A differed significantly from Evaluators B and C with respect to the mesial rest seats and that all evalu-ators were significantly different from each other concerning the distal rest seats (mesial rest seats: A-B p<0.001, A-C p<0.001, and B-C p=0.876; distal rest seats: A-B p<0.001, A-C p<0.001, and B-C p=0.050).

With regard to the PE (Evaluator A, 15.58; Evaluator B, 15.10; and Evaluator C, 15.25) of me-sial rest seats, no statistically significant differences were found among evaluators (p=0.096). In the PE of distal rest seats (p=0.003), Tukey tests showed that the results of Evaluators A and B were significantly different from each other, but neither of their results differed significantly from those of Evaluator C

ResultsThe results of the evaluation of 152 rest seats

according to parameter and evaluator (with the MCE and the PE) are shown in Table 2. No rest seat was classified as “Low negative” in any of the parameters by any of the evaluators using the Kavo PrepAs-sistant system.

Comparison of EvaluatorsThe comparison among the three evaluators for

each evaluation and rest seat (Figure 3) was performed with univariate analyses of variance (ANOVA). Statistically significant differences (p<0.05) were observed among all evaluators regarding the CE of both mesial and distal rest seats (Evaluator A, 14.04; Evaluator B, 13.36; and Evaluator C, 12.76). According to the Tukey tests, all evaluators were significantly different from each other (mesial rest seats: A-B p=0.002, A-C p<0.001, and B-C p=0.007;

Parameter Definition Evaluation Limits Weight

Parameter 7 (exclusion parameter)

The floor of the rest seat should be inclined down-ward to the center of the tooth.

Very good 17.5-20 80-90 5%

Good 13.5-17.4 70-80; 90-95

Satisfactory 9.5-13.4 95-100; 60-70

High negative 7-9.5 50-60; 100-110

Low negative 0-6.9 <50; >110

Parameter 8 There should be no sharp angles.


Good 13.5-17.4


High negative 7-9.5

Low negative 0-6.9

Parameter 9 There should be no sharp edges.


Good 13.5-17.4


High negative 7-9.5

Low negative 0-6.9

Parameter 10 The surface should be smooth and polished.


Good 13.5-17.4


High negative 7-9.5

Low negative 0-6.9

Note: The evaluation was based on a 0-20 grading scale. The limits of parameters 2, 3, and 4 are expressed in millimeters, and those of parameter 7 are in degrees.

Table 1. Assessment parameters of occlusal rest seats and their weight (continued)


analyses of variance (p<0.05) were performed. The results showed statistically significant differences in all cases. The Tukey tests demonstrated that the three types of evaluation were all different from each other and that the least differences were found between the evaluations using parameters made by evaluators A and C (p=0.993 and p=0.074, respectively, between the MCEs and PEs for distal rest seats). The CE had final results worse than those of the evaluations using parameters (CE, 13.38; MCE, 15.51; and PE, 15.31).

Parameters Measured with PrepAssistant

To compare the classifications of parameters 2, 3, 4, and 7 obtained through the PrepAssistant system with those proposed by each evaluator (in the MCE), Wilcoxon tests were performed. Statisti-cally significant differences were observed in 50% of the aspects evaluated in rest seat preparations regarding Evaluator A (mesial rest seats in param-eter 3, p<0.001; parameter 4, p=0.004/p=0.007; and distal rest seats in parameter 7, p<0.001); in 87.5% regarding Evaluator B (parameter 2, p=0.033/p<0.001; mesial rest seats in parameter 3, p<0.001; parameter 4, p=0.003/p<0.001; and parameter 7, p<0.001/p=0.017); and in 75% regarding Evaluator C (distal rest seats in parameter 2, p<0.001; parameter 3, p<0.001/p=0.024; distal rest seats in parameter 4, p<0.001; and parameter 7, p<0.001/p=0.041). Evaluator A’s classification was the most similar to the evaluation obtained with the Kavo PrepAssistant system. The real values measured with this system are shown in Table 3.

Discussion The rest seats evaluation was performed using

Kavo teeth, which provided standardization to the evaluation of the Kavo PrepAssistant system. The evaluation held in the Removable Prosthodontics II course by the three teaching assistants is performed according to the classical evaluation. In the evalua-tions using parameters, the teachers established the selection criteria considering the scientific literature on the specialty.

Parameter 2 (buccolingual width), Parameter 3 ( mesiodistal length), and Parameter 4 (thickness in marginal ridge) correspond to 60% of the final grade (20+20+20). These parameters’ importance is coher-ent with the fact that they were one of the aspects

(mesial rest seats: A-B p=0.108, A-C p=0.210, and B-C p=0.940; distal rest seats: A-B p=0.002, A-C p=0.141, and B-C p=0.286).

Comparison of Evaluation MethodsFor the comparison among the three evaluation

methods for each evaluator and rest seat, univariate

Figure 1. Scanner of Kavo PrepAssistant system and scanning of tooth #45


Figure 2. Measures of parameter 2 (buccolingual width), parameter 3 (mesiodistal length), parameter 4 (thickness), and parameter 7 (angle of the floor) in Kavo PrepAssistant system

Note: The red plane is a horizontal section of the tooth, which allows measuring parameter 2. The green plane is a vertical section of the tooth, which allows measuring parameters 3, 4, and 7.


Table 2. Evaluation by number of rest seats, parameters, and evaluators

Parameter Evaluation Limits

Evaluator A Evaluator B Evaluator C PrepAssistant

n % n % n % n %

Parameter 1 VG Qualitative 71 46.71% 0 0 23 15.13%G 72 47.37% 109 71.71% 109 71.71%S 9 5.92% 42 27.63% 20 13.16%

HN 0 0 1 0.66% 0 0Parameter 2 VG 2.25-2.75 91 59.87% 42 27.63% 47 30.92% 74 48.68%

G 2-2.25; 2.75-3 59 38.82% 81 53.29% 99 65.13% 70 46.05%S 1.75-2; 3-3.25 2 1.32% 29 19.08% 6 3.95% 7 4.61%

HN 1.5-1.75; 3.25-3.5 0 0 0 0 0 0 1 0.66%Parameter 3 (tooth #45)

VG 2.25-2.75 30 40.54% 27 36.49% 31 41.89% 40 54.05%G 2-2.25; 2.75-3 35 47.30% 35 47.30% 41 55.41% 19 25.68%S 1.75-2; 3-3.25 9 12.16% 12 16.22% 2 2.70% 8 10.81%

HN 1.5-1.75; 3.25-3.5 0 0 0 0 0 0 7 9.46%Parameter 3 (tooth #46)

VG 3.45-3.95 25 32.05% 11 14.10% 31 39.74% 16 20.51%G 3.2-3.45; 3.95-4.2 53 67.95% 51 65.38% 47 60.26% 17 21.79%S 2.95-3.2; 4.2-4.45 0 0 16 20.51% 0 0 26 33.33%

HN 2.45-2.95; 4.45-4.95 0 0 0 0 0 0 19 24.36%Parameter 4 VG 1.25-1.75 22 14.47% 10 6.58% 0 0 31 20.39%

G 1-1.25; 1.75-2 69 45.39% 81 53.29% 73 48.03% 61 40.13%S 0.75-1; 2-2.25 61 40.13% 59 38.82% 79 51.97% 37 24.34%

HN 0.25-0.75; 2.25-2.5 0 0 2 1.32% 0 0 23 15.13%Parameter 5 VG Qualitative 52 34.21% 0 0 4 2.63%

G 77 50.66% 112 73.68% 101 66.45%S 23 15.13% 39 25.66% 47 30.92%

HN 0 0 1 0.66% 0 0Parameter 6 VG Qualitative 121 79.61% 17 11.18% 32 21.05%

G 13 8.55% 126 82.89% 113 74.34%S 18 11.84% 9 5.92% 7 4.61%

HN 0 0 0 0 0 0Parameter 7 VG 80-90 74 48.68% 5 3.29% 1 0.66% 33 21.71%

G 70-80; 90-95 73 48.03% 97 63.82% 120 78.95% 81 53.29%S 95-100; 60-70 5 3.29% 45 29.61% 31 20.39% 32 21.05%

HN 50-60; 100-110 0 0 5 3.29% 0 0 6 3.95%Parameter 8 VG Qualitative 139 91.45% 27 17.76% 24 15.79%

G 12 7.89% 103 67.76% 128 84.21%S 1 0.66% 22 14.47% 0 0

HN 0 0 0 0 0 0Parameter 9 VG Qualitative 70 46.05% 26 17.11% 33 21.71%

G 64 42.11% 104 68.42% 119 78.29%S 18 11.84% 22 14.47% 0 0

HN 0 0 0 0 0 0Parameter 10 VG Qualitative 52 34.21% 18 11.84% 48 31.58%

G 83 54.61% 132 86.84% 104 68.42%S 17 11.18% 2 1.32% 0 0

HN 0 0 0 0 0 0

VG=very good; G=good; S=satisfactory; HN=high negative

Note: The limits of parameters 2, 3, and 4 are expressed in millimeters, and those of parameter 7 are in degrees.


Figure 3. Distribution of final results from classical evaluation, modified classical evaluation, and PrepAssistant evalua-tion for each rest seat and evaluator (A, B, and C)


To standardize the evaluation in our study, the students were urged to prepare the rest seats with a mesiodistal length of 2.5 mm for tooth #45 and of 3.7 mm for tooth #46. Only the rest seats of 40 (54%) premolars (2.25 to 2.75 mm) and the rest seats of 16 (21%) molars (3.45 to 3.95 mm) achieved the best results (classified as very good). In their study, Sato et al. concluded that changes in length had minor effects on strength (120% to 178% when 4.0 to 2.0 mm).6 Culwick et al. found that postgraduate students and teachers in removable prosthodontics prepared wider and larger rest seats than the general dental practitioners.15

The recommended thickness of an occlusal rest in the marginal ridge area (parameter 4) is be-tween 1.0 and 1.5 mm, and two studies have shown that greater thickness leads to a higher resistance to fracture.7,14 The thickness suggested in our study for rest seats is in accordance with the findings of Sato et al.,6 which reported that resistance increases with thickness (141% to 230% when 0.7 to 1.5 mm). It is important to note that only 14% (n=22) of the rest seats evaluated in our study showed a thickness greater than 1.5 mm. Rest seats should not exceed 2 mm of thickness in order not to expose the dentine. If the dentin is exposed, the tooth should be restored.16

It should be noted that, according to mathemati-cal studies by Nathan et al.,8 the width/thickness ratio varies with the material of the prosthesis framework. The minimal dimensions proposed are the following: with cobalt-chromium RPD, l.0 mm of width, which implies 1.46 mm of thickness, or 2 mm to 0.85 mm, respectively; and with titanium RPD, values are 1 mm to 1.21 mm, and 2 mm to 0.85 mm, respectively. These theoretical values are lower than those found in our study, which follows the dimensions proposed by the majority of published studies.8,14,15

Regarding the horizontal axis of the rest seat, this should be inclined towards the abutment, and the corresponding angle between the rest seat and the minor connector should be less than 90º in order

most frequently found in the literature. These three parameters, as well as parameter 7 (the angle of the rest seat floor), whose weight is 5%, were evaluated using the Kavo PrepAssistant system, thus provid-ing a more objective evaluation.6,8,14,15 The absence of angles or sharp edges and the need for the rest seat to be polished and smooth corresponded to 15% (5+5+5) of the evaluation, since the use of burs or polishing stones is enough to correct these aspects. It is important to note that none of the evaluated stu-dents used low-speed motorized instruments to finish the preparations. In fact, students could only perform the finishing and polishing of the rest seat prepara-tions using a high-speed handpiece with fine diamond burs (ISO 806-314-243513-018). This aspect should be emphasized by teachers when referring to rest seat preparations. The weights of the remaining param-eters were distributed as follows: 5% for the outer shape of the rest seat, 10% for a spoon-shaped floor, and 5% for the absence of steps. These parameters were the least referred to in the literature.

With regard to the buccolingual width of oc-clusal rests (parameter 2), their resistance was higher in wider rest seats. However, its ideal dimension has not been scientifically determined.6,14-16 In this study, a 2.5 mm width of the rest seat on the marginal ridge was suggested for both premolars and molars, taking into account the dimensions of the teeth evaluated. The average width observed in rest seat preparations was 2.57 mm in premolars and 2.71 mm in molars. Finite element analysis of occlusal rest seats in 3D showed that highest stress in the tooth is found with wide, thin, and long rest seats.6,17 Sato et al. showed that the resistance (the structural strength of the rest) increases with width (183% to 242% when varying from 2 to 4 mm).6 However, 4 mm is far from the ideal reported by most authors.14,15 Concerning the mesiodistal length of the rest seat (parameter 3), its increase reduces resistance, which can be compen-sated by increasing the rest seat’s buccolingual width or thickness.7

Table 3. Averages of real values of four parameters measured with Kavo PrepAssistant system by rest seat

Parameter

Tooth #45Standard

Deviation #45

Tooth #46Standard

Deviation #46 AverageMesial Distal Mesial Distal

Parameter 2 (mm) 2.61 2.52 0.25 2.76 2.65 0.29 2.63

Parameter 3 (mm) 2.05 2.50 0.40 3.06 3.22 0.31 2.71

Parameter 4 (mm) 0.81 0.98 0.25 1.12 1.55 0.37 1.11

Parameter 7 (degrees) 81.14 74.00 8.01 75.15 66.41 7.77 74.17


between expert and novice performance were not evaluated.12

The principal investigator has experience in measuring parameters with the Kavo PrepAssistant system, since he used this system throughout one year in the fixed prosthodontics course as an undergradu-ate student. The differences found between evalua-tors may have been caused by the different levels of experience. It should be noted that, in the context of orthodontics, the same differences were observed by Guedes et al. in a study involving cephalometric analysis.21 However, the differences between evalu-ators were all related to the gauge, since the best and the worst results were the same for all evaluators.

Although this study focused on the measurable parameters of the Kavo PrepAssistant system to com-pare evaluators, the evaluation of the non-measurable parameters could also be substantially improved. In fact, rest seats have reduced dimensions, which com-plicate the analysis even with the help of a magnify-ing glass. Therefore, if the evaluators had had access to 3D images provided by the system, the visibility would be improved, and the inter-evaluator differ-ences in the PE and MCE would probably decrease. However, this aspect needs further evaluation.

ConclusionIn this study, the classical evaluation led to final

results that were worse than those of the two other evaluations using parameters. This evaluation also differed greatly according to the evaluator (range and criteria). With the modified classical evaluation, which included the parameters mentioned and their weighting, the differences between evaluators were reduced, being more independent of the reviewer. With the PrepAssistant evaluation, four parameters were evaluated more objectively, and there were fewer differences between evaluators in the final evaluation than with the MCE. For each evaluator, statistically significant differences among the three assessments of each rest seat were observed. Evalu-ator A (the most experienced) was the one closest to achieving an objective evaluation similar to that of the Kavo PrepAssistant system. The assessment carried out with Kavo PrepAssistant contributed to a more objective final evaluation. Indeed, in compari-son with the CE, in which all parameters were evalu-ated with little objectivity, the PE allows measuring objectively four parameters of rest seat preparations that represent 65% of the evaluation. The evaluation

to prevent slippage of the prothesis.6,14-16 However, this inclination causes a higher stress; therefore, an excess of inclination should be avoided. In our study, we classified an angle of 80º to 90º between the minor connector and the rest seat with very good marks (17.5 to 20 points) and when varying from 60º to 100º with positive marks. An angle between 90º and 100° is not considered correct, but we classified it with a positive mark because this error is minimal and easy to correct. Only 2% (n=3) of the rest seats evaluated in our study showed an angle greater than 90º. Clinically, there should be rest seats with correct depth and shape, as well as rounded edges and angles between the rest seat and the axial wall in order to provide a thick metal structure.16,18

The DentSim system by DenX and the Virtual Reality Dental Training System by Novint improve learning by allowing students to virtually practice rest seat preparations. The DentSim system gives the student a broad sensitivity because it allows choosing the type of material and instrument. However, the preparation’s evaluation is not provided. Currently, there are some computerized technology systems that offer evaluation of preparations, such as E4D Compare by D4D Technologies, but PrepAssistant by Kavo has been the only one used over the years.9,19 The Kavo PrepAssistant system makes the evalua-tion of rest seat preparations more objective because it allows measurements according to parameters. Another advantage of this system is that it enables students to self-assess their preparations, helping them understand their own mistakes. However, some parameters, such as surface roughness, cannot be evaluated objectively with this system. Furthermore, the presence of the equipment (scanner and software) is required, and the whole process is time-consuming.

This study had some limitations related to the system used, so additional tests should be performed to evaluate the uniformity of the acrylic teeth. Also, the variations regarding the preparation’s axis can lead to different assessments, so studies on the influence of this aspect should be developed. The reliability of the whole system should be taken into consideration as well. In regard to this latter aspect, an investigation was conducted by Kacer et al. to determine the accuracy of the Kavo PrepAssistant system, and their results concluded that there were high levels of accuracy within 0.05 mm and 2º of the scan.20 In our study, all students had the same number of hours to practice the rest seat preparations, and no student had access to the advanced simulation systems, such as DentSim. Therefore, differences


10. Almeida TC, Fernandes PF, Sampaio-Fernandes JCA, et al. O sistema Kavo PREPassistant no ensino pré- clínico de prótese fixa. Rev Port Estomatol Cir Maxilofac 2009;50:105-9.

11. Buchanan JA. Use of simulation technology in dental education. J Dent Educ 2001;65(11):1225-32.

12. Gottlieb R, Lanning SK, Gunsolley JC, Buchanan JA. Faculty impressions of dental students’ performance with or without virtual reality simulation. J Dent Educ 2011;75(11):1443-50.

13. Kournetas N, Jaeger B, Axmann D, et al. Assessing the reliability of a digital preparation assistant system used in dental education. J Dent Educ 2004;68(12):1228-34.

14. Sato Y, Hosokawa R, Tsuga K, Kubo T. The effects of buccolingual width and position of occlusal rest seats on load transmission to the abutments for tooth-supported removable partial dentures. J Prosthet Dent 2001;14: 340-3.

15. Culwick PF, Howell PGT, Faigenblum MJ. The size of occlusal rest seats prepared for removable partial dentures. Br Dent J 2000;6:318-22.

16. Rudd RW, Bange AA, Rudd K, Montalvo R. Preparing teeth to receive a removable partial denture. J Prosthet Dent 1999;5:536-49.

17. O’Grady J, Sheriff M, Likeman P. A finite element analy-sis of a mandibular canine as a denture abutment. Eur J Prosthodont Restor Dent 1996;4:117-21.

18. Aquino AR, Barreto AO, Aquino LM, et al. Longitudinal clinical evaluation of undercut areas and rest seats of abutment teeth in removable partial denture treatment. J Prosthet Dent 2011;8:639-42.

19. Renne WG, McGill ST, Mennito AS, et al. E4D compare software: an alternative to faculty grading in dental educa-tion. J Dent Educ 2013;77(2):168-75.

20. Kacer K, Graves A, Price E et al. Assessment of the KaVo PrepAssistant scanner. J Dent Res 2004;83 (Spec Iss A):abstract number 3421.

21. Guedes PA, Souza JEN, Tuji FM, Nery EM. Estudo comparativo das análises cefalométricas manual e com-putadorizada. Dental Press J Orthod 2010;1:44-51.

of the parameters not measurable with this system (35% of subjective components) could be improved if the evaluator had access to 3D images provided by the system. This aspect requires further investigation.

REFERENCES1. Phoenix RD, Cagna DR, DeFreest CF. Major connectors,

minor connectors, rests, and rest seats. In: Phoenix RD, Cagna DR, DeFreest CF, eds. Stewart’s clinical removable partial prosthodontics. Berlin: Quintessence, 2003:19-52.

2. Carr AB, McGivney GP, Brown DT. Major and minor connectors. In: McCracken’s removable partial prosth-odontics. St. Louis: Elsevier Mosby, 2005:35-66.

3. Phoenix RD, Cagna DR, DeFreest CF. Direct retainers, indirect retainers, and tooth replacements. In: Phoenix RD, Cagna DR, DeFreest CF, eds. Stewart’s clinical removable partial prosthodontics. Berlin: Quintessence, 2003:53-102.

4. Carr AB, McGivney GP, Brown DT. Rests and rest seats. In: McCracken’s removable partial prosthodontics. St. Louis: Elsevier Mosby, 2005:67-78.

5. Rice JA, Lynch CD, McAndrew R, Milward PJ. Tooth preparation for rest seats for cobalt-chromium removable partial dentures completed by general dental practitioners. J Oral Rehabil 2011;1:72-9.

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8. Nathan KCL, Victor HFW, Bernard MHL, et al. Math-ematical analysis of occlusal rest design for cast remov-able partial dentures. J Prosthet Dent 2007;15:29-32.

9. Cardoso JA, Barbosa C, Fernandes S, et al. Reducing sub-jectivity in the evaluation of preclinical dental preparations for fixed prosthodontics using the Kavo PrepAssistant. Eur J Dent Educ 2006;10:149-56.

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