The occlusal surface of restoration should allow opposing teeth to pass each other in harmony with mandibular movements and their determinants. ¹ Horizontal condylar guidance (HCG) is the posterior determinant of eccentric jaw movements. ² Steep HCG allows more cuspal inclination. Flatter teeth are in harmony with less steep HCG. ¹

Two major types of articulators have been introduced to mimic temporomandibular joints and jaw movements: arcon and nonarcon. The angle between the occlusal plane and condylar track changes during the hinge movement of the upper member of nonarcon articulators while in the case of arcon articulators, this angle would not be affected. ³

When the HCG of the nonarcon articulator is adjusted using an extrusive interocclusal record, closing movement after removing the interocclusal record results in a less steep angle between the upper member and the condylar track (flatter HCG). Less steep HCG produces teeth with less cusp height, fossa depth, and cusp inclines to prevent occlusal interferences. ¹ , ² , ³ This is an acceptable compromise but not an ideal outcome. Because changes produce flat teeth which may prevent posterior occlusal interferences in extrusion but also reduce masticatory efficacy. ¹

Comparative studies on the accuracy of arcon and nonarcon articulators have yielded controversial results. There are controversies regarding the difference and accuracy of arcon and nonarcon articulators. Condylar guidance angles obtained using arcon articulators are relatively similar to nonarcon instruments in some studies, ⁴ , ⁵ , ⁶ and also significantly different in other studies. ⁷ , ⁸

The “rotation coordinating device” (RCD) is supplementary for nonarcon articulators to maintain the correct HCG during hinge movements. ⁹ The device provides a tight connection of the articulator condylar axis to the condylar track to transmit rotations of the articulator condylar axis to the condylar track. Hence, components rotate coordinated by the same degree and direction ⁹ Figure 1. Figure 1

Rotation coordinating device consists of components that tightly engage with condylar axis and condylar track of articulator to provide a mechanical connection of these parts. 1 and 2: Components that engage condylar track. 3: Component that engages condylar axis.

The study was designed as a comparative in-vitro investigation a fully adjustable articulator served as hypothetical patients. Irreversible hydrocolloid (Alginoplast Alginate; Heraeus Kulzer GmbH) was used to obtain impressions from the maxillary and mandibular dental arch of a volunteer with an intact masticatory system. Stone casts were duplicated three times using additional silicone ( Silatec, DMG) molds to fabricate three pairs of stone casts. One pair was mounted on the fully adjustable articulator (Artex-Reference, Amann Girbach Dental GmbH) in an average arbitrary position in the maximum intercuspation. Face bow registrations were done to mount the remaining pairs of casts on two semi-adjustable articulators (Artex CPR [Amann Girbach] as arcon articulator and Dentatus ARH [Dentatus AB]) as nonarcon articulator. For face bow registration, condylar rods of the face bow were exactly positioned on the hinge axis of the fully adjustable articulator (hypothetical patient's hinge axis). A small round hole was made on the lateral surface of the maxillary cast as the third point of reference. The vertical distance between the hole and the center of the hinge axis of the articulators was measured by a height gauge with 0.02 mm accuracy (Mitutoyo 506-208 Vernier Height Gauge, Mitutoyo) and used for positioning the casts in the semi-adjustable articulators.

Protrusive interocclusal records were obtained from the hypothetical patients.

HCG of the fully adjustable articulator was set on five different angles for both sides (20°, 30°, 40°, 50°, and 60°). Sixteen serial protrusive records were obtained using additional silicon bite registration material (Futar D, Kettenbach GmbH) in each HCG (80 protrusive records). The sample size was calculated statistically which showed that 16 measurements per group were needed (α =0.05 and Power = 0.80, d = d = 3.4). All records were obtained in the same eccentric position (edge-to-edge position) by an anterior auto-polymerizing resin jig (DuraLay Inlay Pattern Resin, Reliance Dental Manufacturing).

Each protrusive record was used to adjust the HCG of arcon and nonarcon articulators. The angle between the longitudinal axis of the upper member and the condylar path of the articulators was measured as HCG using the method described by “Manshaee and Shakerin.” ⁹ Using the same manner, the hinge movement of the upper member and rotation of the condylar tracks also were determined by measuring the difference between their orientations before and after record removal ⁹ Figure 2. Figure 2

Horizontal condylar guidance and hinge movements in nonarcon articulator. AB and CD line: Plane of upper member before and after record removal, respectively. E and F: Orientation of condylar track before record removal and after record removal without using rotation coordinating device (RCD). G and H: Orientation of condylar track after record removal by using RCD. 1: The angle between AB line and CD line represents the hinge movement of upper member. 2: The angle between EF and GH represents the rotation of condylar track. The angle between AB and EF was measured as horizontal condylar guidance (HCG) of nonarcon articulator before record removal (NARC-R). The angle between CD and EF was measured as HCG of nonarcon articulator after record removal without using RCD (NARC-NR). The angle between CD and GH was measured as HCG of nonarcon articulator after record removal by using RCD (NARC-RCD).

Table 1shows that the mean HCG of all semi-adjustable articulators was different from actual HCG of arcon articulator (ARC), nonarcon articulator after record removal without using RCD (NARC-NR) and nonarcon articulator after record removal by using RCD (NARC-RCD) in 20.17° test value. One-way analysis of the variance of studied groups showed that there are significant differences between the registered HCG of studied groups Table 2. The registered HCG of nonarcon articulator after removing the interocclusal records was significantly less than all studied groups Table 1and Figure 3. Figure 3

Horizontal condylar guidance of studied groups. ARC: Arcon, NARC-R: Nonarcon before record removal, NARC-NR: Nonarcon after record removal without using RCD, NARC-RCD: Nonarcon after record removal by using RCD.

The null hypothesis was rejected because the HCG without RCD was significantly different from other groups Table 1. Christensen ¹⁰ proposed adjusting HCG using protrusive interocclusal records. The interocclusal record method is simple and practical for HCG adjustment directly on the articulator with no sophisticated equipment. However, the HCG of the nonarcon articulator would change after the interocclusal record is removed from the articulator. ³

Manshaee and Shakerin ⁹ introduced a novel RCD to maintain the correct HCG of nonarcon articulators. The current study was conducted to evaluate the effect of this device on the accuracy of the HCG setting.

In clinical studies, pantographic values ⁵ , ¹¹ , ¹² , ¹³ , ¹⁴ , ¹⁵ or radiographic inclination of articular eminence ⁷ , ¹⁶ , ¹⁷ have been compared with the HCG of articulators. In this study, a fully adjustable articulator was used to simulate patients according to previous studies. ¹⁸ Hence, the measured HCG of the fully adjustable articulator was the actual posterior determinant of movements. In addition, confounding factors were eliminated and there was no concern about the patient's neuromuscular control that can interfere with jaw movement. ¹⁹ Additional silicone was used as check bite material due to high accuracy and dimensional stability. ²⁰ , ²¹ , ²² Accuracy of HCG adjustment would be affected by the degree of protrusion. ⁶ , ²³ To provide identical protrusion, the same anterior jig was used to make records in edge-to-edge teeth position for all the specimens.

HCG of the arcon articulator was significantly flatter than the actual HCG (4.6° ±4.3° on average, P < 0.001). There was also a significant difference (4.7° ±3.9° in average, P < 0.001) between the actual HCG and conventional nonarcon instrument before record removal which reached 10.0° ±3.8° when the record was removed Table 1. This is in agreement with previous studies that imply HCG adjustment (in both arcon and nonarcon articulators) through interocclusal records tends to result in flatter inclination in comparison with patient-attributed HCG obtained from radiographs or pantographic tracings. However, some differences were found in results regarding HCG test values: mean HCG of ARC, NARC-NR, and NARC-RCD groups were similar to actual HCG values in the 20.17° test value. In steeper HCG, the differences between fully adjustable values and other groups were increased. This may be related to thicker elastomeric interocclusal records in steeper HCG that result in more compressibility of the record under hand pressure which produces flatter HCG. In contrast, Gross et al. ¹³ found 4°–17.8° higher HCG of 3 arcon instruments than pantograph records. The extreme length of the pantographic protrusive path (8 mm) may be the reason for Gross et al. results. ¹³ Hence, both arcon and nonarcon instruments are prone to exert significant errors although errors are higher in nonarcon articulators. Overlooked record-cast maladaptation, record compression due to the force applied on the articulator, and sensitivity of HCG components of the articulator may be reasons for repeated flatter HCG in instruments. ⁶ , ¹³ , ²³

No significant difference was detected between the arcon and nonarcon instruments before record removal Table 1. HCG of the nonarcon instrument was reduced significantly after record removal (5.2° ±0.7° in average) which was also significantly lower than the HCG of the arcon articulator (5.2° ±4.2°, P < 0.05). The findings of the current study about the difference in HCG between arcon and nonarcon instruments are in accordance with Zabarović et al. ⁸ and Shillingburg and Sather ²⁴ but are not in concurrence with other previous studies. ⁵ , ⁶ , ⁷ , ²⁵ Shillingburg and Sather ²⁴ state that the HCG of the nonarcon articulator is reduced by 8° after record removal. Other authors have not found significant statistical differences between the two types of instruments. ⁵ , ⁶ , ²⁵ In the current study, the upper member of the nonarcon articulator rotate s by 5.2° ±0.6° that is significantly correlated with the mean HCG difference found between ARC and NARC-NR groups (r = 0.449, P < 0.001) Figure 5. Hinge movement is the source of the HCG difference between arcon and nonarcon articulators and the controversial results of studies may be due to a different amount of hinge movement.

The result of the current study showed that the use of RCD can significantly increase the HCG of nonarcon articulators in comparison with NARC-NR (5.4° ±0.8°, P < 0.05). The recorded HCG with RCD (NARC-RCD) was not significantly different from the ARC group Table 1. Furthermore, using the RCD, the difference between ARC and NARC-RCD was not affected by rotation the of articulator upper member (r = 0.099, P = 0.386) Figure 4.

Previous studies found that even 5° alteration of HCG has a certain effect on cusp height and steepness that may lead to occlusal errors. ²⁶ , ²⁷ , ²⁸ However, according to the current study, the possibility of protrusive and nonworking occlusal interferences might be reduced because regardless of articulator type, registered HCG was lower than actual HCG. On the other hand, using arcon and nonarcon articulators with RCD greater HCG was registered than nonarcon articulator without RCD resulting in steeper occlusal inclinations that can lead to improved masticatory function, however, conventional nonarcon articulators should not be considered invalid instruments because the functional significance of flatter occlusal surface due to reduced HCG following record removal is not documented.

One of the limitations of the current study is that hand pressure was used for seating the cast on the protrusive record during HCG adjustment and this may vary between adjustments which can cause some inaccuracies. Standardized method for applying a fix amount of pressure is proposed for future studies. The current study was performed in the relatively same hinge movement of mandible for all specimens. The RCD effectiveness may be different when the degree of rotation changes. Relatively controlled conditions of the current study may yield optimistic results for HCG of arcon and RCD-modified nonarcon articulators comparing with actual values. Results may vary in clinical situations.

Within the limitation of this study, the following conclusions can be drawn regarding HCG registered by protrusive interocclusal records:

HCG of both arcon and nonarcon is significantly flatter than the patient's actual condylar guidance

Acknowledgment

This study was supported by Isfahan University of Medical Sciences Research Grant # 397641. The manuscript was based on a thesis submitted to the undergraduate school of Isfahan University of Medical Sciences in partial fulfillment of the requirement for the Doctor in Dental Surgery Degree.

Financial support and sponsorship

Supported by Dental Research Center, Isfahan University of Medical Sciences Research Grant # 397641. Ethical code: IR.MUI.RESEARCH.REC.1397.415.

Conflicts of interest

The authors of this manuscript declare that they have no conflicts of interest, real or perceived, financial or nonfinancial in this article.