Qualitative computer aided evaluation of dental impressions in vivo

Summary

Objectives

Clinical investigations dealing with the precision of different impression techniques are rare. Objective of the present study was to develop and evaluate a procedure for the qualitative analysis of the three-dimensional impression precision based on an established in-vitro procedure. The zero hypothesis to be tested was that the precision of impressions does not differ depending on the impression technique used (single-step, monophase and two-step-techniques) and on clinical variables.

Methods

Digital surface data of patient's teeth prepared for crowns were gathered from standardized manufactured master casts after impressions with three different techniques were taken in a randomized order. Data-sets were analyzed for each patient in comparison with the one-step impression chosen as the reference. The qualitative analysis was limited to data-points within the 99.5%-range. Based on the color-coded representation areas with maximum deviations were determined (preparation margin and the mantle and occlusal surface). To qualitatively analyze the precision of the impression techniques, the hypothesis was tested in linear models for repeated measures factors (p<0.05).

Results

For the positive 99.5% deviations no variables with significant influence were determined in the statistical analysis. In contrast, the impression technique and the position of the preparation margin significantly influenced the negative 99.5% deviations.

Significance

The influence of clinical parameter on the deviations between impression techniques can be determined reliably using the 99.5 percentile of the deviations. An analysis regarding the areas with maximum deviations showed high clinical relevance. The preparation margin was pointed out as the weak spot of impression taking.

Introduction

Different techniques have been used investigating dental impression procedures. First, procedures assessing the impressions directly based, for instance, on the ADA Specification No. 19 [1], [2], [3], [4] or secondly, indirect procedures measuring casts manufactured from impressions of reference dies [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18] have been used. In general in vitro procedures have used simplified measurements (e.g. calculating distances) [1], [2], [3], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21]. Most of these measurement procedures have been designed for quality assessment during the manufacturing process [22]. These procedures are often based on standards and specifications and are approved for measuring material properties (shrinkage, plastic deformation, elastic recovery). Results of the measurements can hardly been transferred to clinical properties [5], [7], [22], [23]. Measurements based on master gages with perfect fit on the reference model [24], [25] whose fit on duplicate models is estimated, are limited to enlarged dies. A differentiation between perfect fit and undersized dies is not possible. Procedures manufacturing cast objects for each duplicate die are closely related to the clinical situation [24], [26], [27], [28]. However, errors given by the complex procedure of impression taking, manufacturing casts, and casting of the restorations have to be considered [22], [24]. A more realistic approach is offered by procedures estimating three-dimensional changes [29], [30], [31]. A linewise mechanical digitizing of tooth models and calculation of the differences to the original die has been used to determine the impression precision [30], a holographic test procedure has been introduced for measuring the dimensional stability [32], and the use of optical measurement procedures for assessment of the reproduction of soft tissue by different impression techniques has been described [33]. The comparison of impression techniques required a measurement in an identical spatial adjustment and only allowed a relative comparison of the techniques [33]. For the three-dimensional evaluation of multiple measurements information is required regarding the alignment of the data-sets [29], [34], [35]. In most cases this requirement is fulfilled by the reproducible mechanical positioning of the measured objects [34]. Wear tests on filling materials or abrasion or attrition tests on teeth or restorations are also based on the comparison of multiple measurements of the surface of the teeth to be examined [30], [34], [36], [37], [38], [39], [40], [41]. These processes are based on the comparison of direct measurements [34], impressions [38], [39] or models of the tooth surface [30], [36], [37], [38], which were gained at different times. The analysis of the differences is done after digitizing the models with subsequent mathematical alignment of the data-sets [42] or by identical positioning of the models [43] with mainly mechanical measurement at certain points [37]. A measurement for the precision of the alignment is the Root-Mean-Square (RMS) error [39], [42]. RMS-errors below 0.010 mm are rated as excellent whereas values above 0.050 mm determined as a poor correspondence [39]. Thus, the RMS-error can be regarded as a measure for the quality of the model.

Clinical investigations dealing with the precision of different impression techniques are rare. Wöstmann introduced a procedure based on teeth, which were prepared for crowns prior to extraction [44], [45]. Impressions were taken before extraction, while the fit of the cast crowns were estimated after extraction. However, these investigations were limited to measurements of the marginal gap at four locations on a tooth. Böning investigated impressions taken from permanent molars regarding the gingival sulcus reproduction ability and a classified quality ranking [46]. However this procedure has been limited to non-prepared teeth.

There have been no in vitro and in vivo studies, investigating dental impression procedures, which consider the complex, three-dimensional geometry of prepared teeth [5], [47]. To-date, there is a lack of studies which consider the influence of the impression technique and clinical factors (e.g. position of the preparation margin, gingivitis, oral hygiene) on the precision of the reproduction of the prepared teeth and the assessment of the amount of error. Analyzing clinical conditions with a 3D procedure seems to be adequate to derive recommendations with regard to the choice of the impression technique according to the indication, and to develop procedures for the correction of imprecise impressions in subsequent steps. It was the objective of the present study to develop and evaluate a procedure for the qualitative analysis of the three-dimensional impression precision based on an established in vitro procedure [48]. The zero hypothesis was that the in vivo precision of the reproduction of the prepared teeth does not differ in a qualitative analysis depending on the impression technique used (single-step, monophase and two-step-techniques) and clinical variables.