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Corresponding author: Dr Ming Yi Tan, Department of Prosthodontics, Faculty of Dentistry, National University of Singapore, 9 Lower Kent Ridge Rd Singapore, 119085, SINGAPORE
Some contemporary articulator systems claim to be highly precise in their interchangeability, with tolerances below 10 μm in vertical error; however, the claims have not been independently verified.
Purpose
The purpose of this study was to investigate the interchangeability of calibrated semiadjustable articulators in service over time.
Material and methods
A calibrated mounting articulator served as the master articulator, while the test groups were used articulators with a minimum of 1-year use by predoctoral dental students (n=10); used articulators with a minimum of 1-year use by prosthodontic residents (n=10); and new articulators (n=10). One set of mounted maxillary and mandibular master models was positioned in the master and test articulators. High-precision reference markers on the master models were used to determine interarch 3D distance distortions (dRR, dRC, and dRL), interocclusal 3D distance distortion (dRM), interocclusal 2D distance distortions (dxM, dyM, and dzM), and interocclusal angular distortion (dθM) relative to the master articulator. All measurements were conducted three times using a coordinate measuring machine and then averaged to derive the final data set.
Results
For interarch 3D distance distortion, the mean dRR ranged from 4.6 ±21.6 μm for new articulators to 56.3 ±47.6 μm for articulators used by prosthodontic residents; mean dRC ranged from 65 ±48.6 μm for new articulators to 119.0 ±58.8 μm for articulators used by prosthodontic residents; and mean dRL ranged from 12.7 ±39.7 μm for articulators used by prosthodontic residents to 62.8 ±75.2 μm for new articulators. For interocclusal 3D distance distortion, the mean dRM ranged from 21.5 ±49.8 μm for new articulators to 68.6 ±64.9 μm for articulators used by predoctoral dental students. For the 2D distance distortions, the mean dxM ranged from −17.9 ±43.4 μm for articulators used by predoctoral dental students to −61.9 ±48.3 μm for articulators used by prosthodontic residents; mean dyM ranged from 18.1 ±59.4 μm for new articulators to 69.3 ±115.1 μm for articulators used by prosthodontic residents; and mean dzM ranged from 29.5 ±20.2 μm for new articulators to 70.1 ±37.8 μm for articulators used by prosthodontic residents. Mean dθM ranged from −0.018 ±0.289 degree for new articulators to 0.141 ±0.267 degree for articulators used by prosthodontic residents. One-way ANOVA by articulator type revealed statistically significant differences among the test groups for dRR (P=.007) and dzM (P=.011) only, where articulators used by prosthodontic residents fared significantly poorer than the other test groups.
Conclusions
The new and used articulators tested did not fulfill the manufacturer’s claim of accuracy of up to 10 μm in the vertical dimension. Up to 1 year of time in service, none of the investigated test groups fulfilled the criterion for articulator interchangeability, even if the more lenient threshold of 166 μm were accepted.
Clinical Implications
Cumulative errors from interchanging mounting and working articulators or between working articulators are expected to be of clinical significance, particularly in the fabrication of implant-supported prostheses.
The dental articulator reproduces maxillomandibular relationships and some or all mandibular movements, thereby aiding diagnosis, treatment planning, and the fabrication of dental prostheses. Digital or virtual articulators may overcome the limitations of analog articulators, with the key advantage that dynamic movements may be more easily and accurately captured. Nonetheless, the shift toward virtual articulators requires investment in an appropriate scanner system and accompanying software program for jaw motion analyses. Recently introduced digital workflows also require substantial optimization to attain a reasonable level of clinical utility. These barriers remain significant to many end-users. Therefore, analog articulators remain important in many practices, and manufacturers have recently introduced interchangeable analog articulator systems. These offer numerous advantages, including fewer articulators being needed to accommodate multiple ongoing treatments; reduced damage to articulators as transportation is minimized; improved ease of communication with the dental laboratory technician; and reduced transmission of infectious agents between the dental office and dental laboratory.
The definition of articulator interchangeability remains imprecise because of the lack of standardization and established criteria. The present authors propose that articulator interchangeability be defined as the ability of the dental cast to be transferred from one articulator to another with repositionable accuracy. The articulators should be calibrated with the same gauge to maintain their operability and reliability in service over time.
The validity of articulator interchangeability has been examined,
but heterogeneous methodologies have led to inconsistent and conflicting findings. Most of these earlier studies were restricted by limitations in the measuring instruments, which tended to have brand-specific calibration or check systems, and data were largely limited to 2-dimensional (2D) measurement. Many of these studies on interchangeability were also conducted on new articulators, but time in service is a crucial criterion that was not generally investigated. However, Price and Mansfield
assessed 46 Whip Mix #2240 (Whip Mix Corp) articulators in use for between 1 and 6 years using the Whip Mix #2245 check system and reported that 83% were interchangeable within a 94-μm tolerance level in the horizontal dimension, while 93.5% were interchangeable within a 52-μm tolerance level in the vertical dimension. Price et al
evaluated the interchangeability of 109 new Whip Mix #2240 and #3040 articulators using the same measuring protocol, concluding that 93% of these new articulators were interchangeable. Chung et al
investigated Hanau Wide-Vue semiadjustable articulators (Whip Mix Corp) and reported significantly worse error in articulators in use for 30 months than in new and 18-month-old articulators.
Artex CR articulators (Amann Girrbach AG) claim to be highly precise in their interchangeability, with tolerances below 10 μm in vertical error.
reported the 2D repositioning accuracy of casts on to the Artex CP with black magnetic mounting plates to be below 9 μm. However, this 2D distortion value did not consider both the upper and lower members and was also limited to one articulator. Interchangeability across articulators remains unknown.
The purpose of the present study was to investigate the interchangeability of calibrated Artex CR articulators in service over time. The null hypotheses were that no significant difference would be found in the repositioning accuracy of mounted casts across articulators in service over time and that operator designation would not significantly affect the repositioning accuracy of mounted casts across articulators.
Material and methods
A set of dentate maxillary and mandibular master models with a missing maxillary right first molar and a missing mandibular right first molar were fabricated with heat-polymerized pink (Lucitone 199 Denture Base Resin; Dentsply Sirona) and tooth-colored (Dentalon Plus; Kulzer GmbH) polymethyl-methacrylate (PMMA). Wide-neck solid abutment analogs (048.165; Institut Straumann AG) simulated crown preparations at the positions of the missing maxillary and mandibular right first molars. Six Ø8.0-mm grade 5 silicon nitride reference spheres (Ceramic Balls Si3N4: 8.0 mm G5, reference 8 057 222; Tsubaki Nakashima Co, Ltd) with a manufacturer-specified sphericity of 0.13 μm were embedded in the master model at the sites of the maxillary right second molar, between the maxillary central incisors, maxillary left second molar, mandibular right second molar, between the mandibular central incisors and the mandibular left second molar (Fig. 1). The solid abutment analogs and silicon nitride spheres were incorporated into the master models because their precisely manufactured geometric features allowed for accurate measurement via coordinate metrology.
Figure 1Maxillary and mandibular master model with silicon nitride spheres at regions buccal to maxillary right second molar, maxillary central incisors, maxillary left second molar, mandibular right second molar, mandibular central incisors, and mandibular left second molars. Implant solid abutment analogs placed at positions of missing maxillary and mandibular right first molars.
The Artex CR semiadjustable articulator system has been selected for use in the National University Center for Oral Health Singapore (NUCOHS), which is a national specialty center and base for the Faculty of Dentistry. Because of the interchangeability feature of Artex CR articulators, all newly introduced articulators had been calibrated using a strict protocol with a single calibration key (Splitex Key, Artex C-Version; Amann Girrbach AG) and verified by a trained technician from the manufacturer.
The manufacturer has not specified any schedule for recalibration. Therefore, the current arrangement is for articulators to be recalibrated at the discretion of the operator.
The test groups, each with a sample size of 10, comprised used articulators with a minimum of 1-year use by predoctoral dental students, used articulators with a minimum of 1-year use by prosthodontic residents, and unused articulators. A total of 410 articulators, comprising 228 predoctoral, 24 resident, and 158 unused articulators, were available for testing. The maximum age of used articulators was 2 years. The test articulators were selected by using a random number generator applied to the three pools of articulators. The recruited test articulators were inspected based on a checklist of criteria and were excluded if a history of significant damage or mishandling had been recorded or if visible damage (such as dents, bent components, or damage to the centric latch or central axis) had been noted upon inspection. Articulators were also excluded if, on manual inspection, the two semiaxes of the centric lock mechanism could not engage the receptacles of the respective condyle analogs with sufficient precision (Fig. 2). The random selection was repeated for the predoctoral pool after excluding two articulators.
Figure 2Articulator demonstrating precise engagement between centric lock mechanism’s semiaxis and receptacle of corresponding condyle analog.
A mounting articulator (Splitex Mounting Articulator 116mm; reference 216 020; Amann Girrbach AG) served as the master articulator. This had been newly calibrated with the same calibration key immediately before the study started. Throughout the study, the master and test articulators were stored in a dry room maintained at 25 °C and away from cleaning agents, water, or heat. The master models were mounted using magnetic mounting plates (Magnetic Mounting Plates-Black; Amann Girrbach AG) onto the master mounting articulator with gypsum (Plaster of Paris; Asia Plaster Co, Ltd) mixed with an antiexpansion solution formulated with 6% potassium sulfate and 0.6% borax in water. These were stored in a room maintained at 20 °C and left undisturbed for 96 hours to allow for complete setting of the mounting plaster.
Occlusal equilibration was performed to ensure that bilateral simultaneous contacts were present on the canines, premolars, and molars. This was verified with the use of 8-μm-thick shim stock (Arti-Fol; Dr Jean Bausch GmbH & Co KG) (Fig. 3). Thereafter, randomization was conducted to determine the sequence of measurements of the test articulators. The same set of mounted maxillary and mandibular master models was removed from the master articulator and repositioned in the test articulators for measurement.
Figure 3Right lateral and frontal views of mounted master models on master articulator.
The master and test articulators were firmly secured to the granite table of a coordinate measuring machine (CMM) (Global Silver Performance 7.10.7; Brown & Sharpe) with a reported measuring precision of 1 μm. All measurements were performed with the articulators in the upright position, with the incisal pin removed to improve accessibility of the CMM probe, and using direct computer control (DCC) mode. Two types of centroid positions were derived in this study. The centroid positions of the reference spheres were derived through eight probe hits on the external surface of each reference sphere. The centroid positions of the solid abutment analogs placed at the positions of the missing maxillary and mandibular right first molars were derived via the intersection (or pierce point) between the respective central axes and flat planes of the solid abutment analogs; 4 probe hits were conducted on each coronal platform plane, while 12 probe hits spread over 3 parallel planes were conducted on each external cone. The optimal number of probe hits for each measured feature had been determined during the pilot phase of the study.
For each articulator, three independent sets of the following measurements were computed in the CMM metrology software program (PC-DMIS CAD++ Version 2021.1 Build #210 MR1; Wilcox Associates Inc): 3D distances between centroids of paired right, central, and left reference spheres (RR, RC, and RL); 3D distance between centroids of the 2 solid abutment analogs placed at the positions of the missing maxillary and mandibular right first molars (RM); 2D distance between centroids of the 2 solid abutment analogs placed at the positions of the missing maxillary and mandibular right first molars in the buccolingual, anteroposterior, and supero-inferior directions (xM, yM, and zM); and 3D angle between the central axes of the two solid abutment analogs placed at the positions of the missing maxillary and mandibular right first molars (θM). Verification checks using CMM software program tolerance criterion functions ensured data veracity. The tolerance value for each measured physical geometric feature was set at 6 μm. For the approved data set, calculations using two or more measured features did not exceed their respective thresholds of 10 μm (RR, RC, and RL) or 20 μm (xM, yM, zM, and RM) when the 3 independent measurements were compared. These stipulated tolerance values ensured that errors in individual measurement sets were sieved out and eliminated. The three independent measurement sets, if consistent with one another, were then averaged to derive the final data set. This further reduced the magnitude of any unavoidable random errors. The dependent variables were derived by subtracting master measurements from test measurements. Three interarch distortions (dRR, dRC, and dRL) were derived from the measurement of the reference spheres, while five interocclusal distortions were obtained through measurement of the implant solid abutment analogs placed at the positions of the missing maxillary and mandibular right first molars (dRM, dxM, dyM, dzM, and dθM) (Fig. 4). Measurements on the master articulator were conducted at the start of the study and repeated at the end. The investigated 3D distances on the master articulator at these two time points differed less than a preset threshold of 10 μm, implying negligible PMMA distortion or damage to the master models throughout the study.
Figure 4Measurement process for deriving dependent variables (dRR, dRC, dRL, dRM, dxM, dyM, dzM, and dθM) for each test articulator.
A statistical software program (IBM SPSS Statistics, v27.0; IBM Corp) was used for all statistical calculations. The data were initially submitted to the Shapiro-Wilk test to check for the assumption of normality, and it was not rejected. One-way analysis of variance (ANOVA) and the Tukey honestly significant difference (HSD) test were used for the comparison of the three articulator groups. Within each test group, Pearson correlation coefficients were computed to assess the linear relationships between the sample and distortion parameters (α=.05).
Results
Interarch 3D distance distortion values between the centroids of the paired reference spheres are shown in Figure 5. For the right paired reference spheres, the magnitude of mean dRR ranged from 4.6 ±21.6 μm for new articulators to 56.3 ±47.6 μm for articulators used by prosthodontic residents. For the center paired reference spheres, the magnitude of mean dRC ranged from 65.0 ±48.6 μm for new articulators to 119.0 ±58.8 μm for articulators used by prosthodontic residents. For the left paired reference spheres, the magnitude of mean dRL ranged from 12.7 ±39.7 μm for articulators used by prosthodontic residents to 62.8 ±75.2 μm for new articulators.
Figure 5Mean 3D distance interarch distortions between centroids of paired reference spheres (dRR, dRC, and dRL). Error bars indicate standard deviation.
Interocclusal distortions between centroids of the 2 solid abutment analogs placed at the positions of the missing maxillary and mandibular right first molars are shown in Figure 6, Figure 7, Figure 8. Mean dRM ranged from 21.5 ±49.8 μm for new articulators to 68.6 ±64.9 μm for articulators used by predoctoral dental students. For the 2D distance distortions, the greatest distortions were consistently found in the articulators used by prosthodontic residents. The negative values in this segment represent the direction of distortion based on the local coordinate system. Mean dxM ranged from −17.9 ±43.4 μm for the articulators used by predoctoral dental students to −61.9 ±48.3 μm for articulators used by prosthodontic residents. Mean dyM ranged from 18.1 ±59.4 μm for new articulators to 69.3 ±115.1 μm for articulators used by prosthodontic residents. Mean dzM ranged from 29.5 ±20.2 μm for new articulators to 70.1 ±37.8 μm for articulators used by prosthodontic residents. For interocclusal angular distortions between the central axes of the two solid abutment analogs placed at the positions of the missing maxillary and mandibular right first molars, mean dθM ranged from −0.018 ±0.289 degrees for new articulators to 0.141 ±0.267 degrees for articulators used by prosthodontic residents.
Figure 6Mean 3D distance interocclusal distortions between centroids of two solid abutment analogs placed at positions of the missing maxillary and mandibular right first molars (dRM). Error bars indicate standard deviation.
Figure 7Mean 2D distance interocclusal distortions between centroids of two solid abutment analogs placed at positions of missing maxillary and mandibular right first molars (dxM, dyM, and dzM). Error bars indicate standard deviation.
Figure 8Mean 3D angular distortions between central axes of the 2 solid abutment analogs placed at positions of missing maxillary and mandibular right first molars (dθM). Error bars indicate standard deviation.
One-way ANOVA by articulator group revealed statistically significant differences among the test groups for dRR (P=.007) and dzM (P=.011) only. In both instances, the distortion for the articulators used by prosthodontic residents was found to be significantly larger than that of new articulators and articulators used by predoctoral dental students, which were not different from each other. Table 1 summarizes the results of the 1-way ANOVA and Tukey HSD procedures performed. The Pearson correlation coefficients computed to assess the linear relationships between the sample and respective distortion parameters revealed no statistical significance (Table 2).
Table 1Summary of 1-way ANOVA and Tukey HSD for distortion parameters
The null hypotheses for this study were rejected. Articulator interchangeability refers to the ability to transfer mounted dental casts across articulators with repositioning accuracy while maintaining this operability and reliability in service over time. The goal of this study was to quantify articulator interchangeability through the investigation of three interarch and five interocclusal dependent variables. Distortions were calculated relative to the master articulator. The Pearson correlation coefficients computed to assess the linear relationships between the sample and respective distortion parameters revealed a lack of correlation between intragroup samples and the reported distortion parameters, indicating that inaccuracies were widespread over all articulators rather than limited to only a few defective ones.
proposed that an error of 166 μm at the occlusal level in the region of the second molar would neither result in prolonged chairside adjustment of the resultant prosthesis nor adversely affect morphology. An alternative way to justify thresholds for clinical acceptability is to consider the physiologic movement permitted by healthy periodontal ligaments. One study reported that a 0.5-N buccolingual force resulted in 40 to 80 μm molar and 100 to 120 μm incisor movement.
Therefore, a large range of 40 to 166 μm has been suggested to be acceptable. Seven distance distortion parameters (dRR, dRC, dRL, dRM, dxM, dyM, and dzM) per test group were available for analysis, giving a total of 210 distance distortion measurements. For the distance distortion measurements of the articulators used by predoctoral dental students, 37/70 (52.9%) were less than 40 μm, 29/70 (41.4%) ranged from 40 to 166 μm, and 4/70 (5.7%) exceeded 166 μm. For the distance distortion measurements of the articulators used by prosthodontic residents, 21/70 (30.0%) were less than 40 μm, 46/70 (65.7%) ranged from 40 to 166 μm, and 3/70 (4.3%) exceeded 166 μm. For the distance distortion measurements of the new articulators, 39/70 (55.7%) were less than 40 μm, 30/70 (42.9%) ranged from 40 to 166 μm, and 1/70 (1.4%) exceeded 166 μm. These results suggest that articulator interchangeability remains questionable. All 3 test groups presented with some measurements exceeding the upper limit of 166 μm. In addition, only 97/210 (46%) of the measurements were found to be less than the lower limit of 40 μm.
The 3D distortions at the interarch level were found to be greater in magnitude than the 3D distortions at the interocclusal level. This finding of more prominent distortions at the interarch level was similar to that reported by Yee et al.
The 3D and 2D distance distortions derived from the investigation of the two solid abutment analogs placed at the positions of the missing maxillary and mandibular right first molars simulated the occlusal errors that would typically be present at the molar sites. In general, the used articulator groups (articulators used by predoctoral dental students or prosthodontic residents) showed higher distortion values than the new articulators. Specific to dzM, the prosthodontic resident articulator test group was found to be significantly worse than the other two groups. This parameter relates to the need for occlusal adjustment in the supero-inferior direction and may lead to increased chair time, the alteration of occlusal morphology, and a potential effect on material thickness and strength. Mean dxM for all groups in this study were negative, resulting in the occlusal table shifting buccally and to the patient’s right side. Clinically, points A and C would be reduced while an increase in contact at point B in maximum intercuspation occurs, creating a favorable situation (Fig. 9A). Yet, the reverse may also occur, whereby positive distortion seen on the right side of the patient shifts the occlusion laterally to the patient’s left, reducing the point B contact and increasing contacts on points A and C (Fig. 9B). Such undesired occlusal loading can cause teeth to tip. When considering dental implants, horizontal movements are much more limited, at a maximum of 5 μm,
The results indicated that mean x-distortions at the interocclusal level exceeded the limited horizontal leeway permissible in implant-supported prostheses, which may increase prosthetic complications.
Technical complications and prosthesis survival rates with implant-supported fixed complete dental prostheses: a retrospective study with 1- to 12-year follow-up.
The largest mean angular distortion was found in articulators used by prosthodontic residents, but no significant difference was found among test groups.
Figure 9Frontal cross-section of right molars in intercuspal position. A, Negative dxM results in positive contact at point B. B, Positive dxM results in lack of point B contact, increasing susceptibility to tipping of teeth.
This study revealed large standard deviations in the measured parameters, even for the new articulator group. Factors that may have contributed to this error included that articulator interchangeability depends on calibration accuracy done using a specific gauge, where the Splitex mandibular adjustment plate is held to the lower member of the articulator with an adhesive. This adhesive or the process of calibration could be the weak link affecting the accuracy of the whole system. Disturbances and discrepancies of the assembly before complete setting of the adhesive may adversely affect calibration accuracy. In addition, that articulators used by prosthodontic residents displayed the highest distortions was consistent with the findings of Price at al and Chung et al.
Logically, these articulators underwent heavier usage than articulators used by predoctoral dental students as a result of the greater volume and complexity of clinical patient procedures provided by postgraduate prosthodontic residents. Poor handling of the articulators may damage the condylar heads and tracks, and close proximity to heat may also distort the adhesive. The accumulation of such errors can lead to the deterioration of articulator interchangeability. The magnetic plates are not immune to wear, physical damage, and corrosion, which compromise the magnetic strength instrumental in plate localization. The magnetic plates should be routinely examined to ensure continued operability.
Limitations of this study included that the findings may not apply to other articulator systems that claim to be interchangeable, as various systems have developed different calibration methods and definitions for interchangeability. Validation of manufacturers’ claims provides deeper insight into the mechanics, the production, and ultimately the function of these articulators. Future studies on other articulator systems may be informative. The CMM methodology used for distortion measurement is not specific to any articulator system; the methodology described may be deployed in future studies, allowing for data comparison. Articulator magnet strength could have been tested to ensure uniformity across articulators throughout the experiment, but since all tested articulators were relatively new, such a device was unlikely to reveal any meaningful differences. The relatively short observation duration may not have been sufficient to reveal clear trends, prompting future investigations.
Conclusions
Based on the findings of this study, the following conclusions were drawn:
1.
The new and used articulators tested did not fulfill the manufacturer’s claim of accuracy up to 10 μm in the vertical dimension.
2.
Up to 1 year of time in service, none of the investigated test groups fulfilled the criterion for articulator interchangeability, even if the more lenient threshold of 166 μm were accepted.
References
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Determining the accuracy of articulator interchangeability.
Technical complications and prosthesis survival rates with implant-supported fixed complete dental prostheses: a retrospective study with 1- to 12-year follow-up.
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