Computer simulation of occlusal discrepancies resulting from different mounting techniques

The Accuracy of Transferring Casts in Maximal Intercuspal Position to a Virtual Articulator

PURPOSE

To analyze the accuracy of transferring casts in maximal intercuspal position to a virtual articulator by using transfer plates in the laboratory scanner before and after occlusal optimization.

MATERIAL AND METHODS

Five sets of standard dental casts were mounted on a mechanical articulator in maximal intercuspal position. The number and position of occlusal contacts were determined with 12-μm articulating foil. After a calibration process according to the manufacturer's instructions, the mountings were transferred to a virtual articulator using the transfer plates in a laboratory scanner. The occlusion of the digital casts was determined before and after the occlusal optimization process. Then, the sensitivity and positive predictive value were determined by comparing the occlusal contact points in the virtual articulator with those in the mechanical articulator. To evaluate trueness, the occlusal surface of the maxillary teeth in the mechanical articulator was recorded by polyvinyl siloxane occlusal record in maximal intercuspal position and retained on the mandibular arch. The trueness was calculated as the deviation between the occlusal surface of the maxillary teeth in the mechanical articulator and the virtual articulator. To evaluate precision, one set of the casts was scanned 10 times. And the deviation of the interarch position of the maxillary arches when superimposing the mandibular arches of every 2 different scans was calculated.

RESULTS

The sensitivity before occlusal optimization (0.14 ± 0.15) was significantly lower than that after occlusal optimization (0.82 ± 0.10) (p = 0.003). However, there was no significant difference between the positive predictive value before (0.80 ± 0.45) and after (0.81 ± 0.09) occlusal optimization (p = 0.952). The trueness before (91.0 ± 10.7 µm) and after (75.4 ± 25.2 µm) occlusal optimization had no significant difference (p = 0.249). The precision before occlusal optimization (11.6 ± 3.8 µm) was significantly superior to that after occlusal optimization (75.6 ± 39.2 µm ) (p < 0.001).

CONCLUSIONS

The accuracy of transferring casts in maximal intercuspal position to a virtual articulator using transfer plates in the laboratory scanner could be improved after occlusal optimization and can meet the clinical needs for occlusal design and analysis of prostheses.

Evaluation of a Technique for Reducing Chairside Occlusal Adjustment of Crowns

PURPOSE

To assess whether the occlusion of metal-ceramic crowns, as received from the laboratory, and the time taken to adjust the occlusion of crowns not deemed acceptable, can be reduced by fabricating the crowns with controlled amounts of infra-occlusion during the laboratory phase.

MATERIAL AND METHODS

An opposing set of typodonts, articulated in maximum intercuspal position served as the patient in an in vitro simulation. Seventy-five metal-ceramic crowns were fabricated for the mandibular right second molar with three different occlusal contact specifications: A, control group had occlusal contacts shared evenly by the crown and the neighboring teeth (n = 25); B, first experimental group had the occlusion relieved until 2 shimstock foils were able to be pulled from between the crown and the opposing tooth without tugging (n = 25); C, second experimental group had the occlusion relieved until 4 shimstock foils were able to be pulled from between the crown and the opposing tooth without tugging (n = 25). The occlusion of each crown, as received from the laboratory, was assessed using one of three categories (Excellent, Acceptable, and Poor). Chi-square analysis was used to test the differences in occlusal outcomes between the three study groups. For all of those rated "Acceptable," the time taken to adjust each crown to proper occlusion was recorded. One-way analysis of variance (ANOVA) and Bonferroni tests were carried out to compare the adjustment times across the three study groups.

RESULTS

The 2-shim group had the best outcome, with 56% of the crowns rated as "Excellent" (p = 0.001). In addition, there were statistically significant differences in adjustment times between the control group (A) and the 2-shim (B) and the 4-shim (C) groups (p = 0.0001), but not between the 2-shim (B) and 4-shim (C) groups (p = 0.08).

CONCLUSIONS

Metal-ceramic crowns fabricated with controlled interocclusal relief of 2- and 4-shims each required less time for chairside occlusal adjustment than crowns fabricated in the laboratory to conventional occlusal contact. However, the overall superior outcome, in terms of the possibility for immediate insertion as received from the laboratory as well as favorable chairside adjustment time, for the 2-shim prespacing suggests that this dimension is the preferred option over 4-shim prespacing to reduce occlusal inaccuracies of indirect restorations.

Reproduction accuracy of articulator mounting with an arbitrary face-bow vs. average values-a controlled, randomized, blinded patient simulator study

OBJECTIVES

The benefit from positioning the maxillary casts with the aid of face-bows has been questioned in the past. Therefore, the aim of this study was to investigate the reliability and validity of arbitrary face-bow transfers compared to a process solely based on the orientation by means of average values. For optimized validity, the study was conducted using a controlled, randomized, anonymized, and blinded patient simulator study design.

MATERIAL AND METHODS

Thirty-eight undergraduate dental students were randomly divided into two groups; both groups were applied to both methods, in opposite sequences. Investigated methods were the transfer of casts using an arbitrary face-bow in comparison to the transfer using average values based on Bonwill's triangle and the Balkwill angle. The "patient" used in this study was a patient simulator. All casts were transferred to the same individual articulator, and all the transferred casts were made using type IV special hard stone plaster; for the attachment into the articulator, type II plaster was used. A blinded evaluation was performed based on three-dimensional measurements of three reference points.

RESULTS

The results are presented three-dimensionally in scatterplots. Statistical analysis indicated a significantly smaller variance (Student's t test, p < 0.05) for the transfer using a face-bow, applicable for all three reference points.

CONCLUSIONS

The use of an arbitrary face-bow significantly improves the transfer reliability and hence the validity.

CLINICAL RELEVANCE

To simulate the patient situation in an individual articulator correctly, casts should be transferred at least by means of an arbitrary face-bow.

Does a face-bow lead to better occlusion in complete dentures? A randomized controlled trial: part I

OBJECTIVES

In a double-blinded randomized controlled clinical trial, the impact of face-bow registration for remounting complete dentures (CDs) on the occlusal parameters (part I) was evaluated.

MATERIALS AND METHODS

New CDs of 32 patients were duplicated and mounted after intraoral pin registration according to mean settings (group 1) and (group 2) using a face-bow (arbitrary hinge axis). The vertical dimension was reduced to the first occlusal contact point, and a bite record was fabricated in the articulator. The number of contacts and the number of teeth in contact were evaluated by a computer program (laboratory result). After randomization, half of the CDs were adjusted according to protocol of group 1 and group 2 and delivered to the patients. After 3 days (T1) and 84 days (T2), clinical static contact points and teeth in contact were counted. Contact points and teeth in contact of both groups (laboratory results) and at different moments (clinical results) were analyzed statistically with the F test and bootstrapping.

RESULTS

Laboratory: No. 2 (face-bow) showed more occlusal contact points than no. 1 (mean setting), p > 0.05. The number of teeth with at least one occlusal contact was significantly higher in no. 2 (p = 0.027). Clinic: The mean number of teeth with at least one clinical contact point was significantly higher in no. 1 (no. 1 = 7.13, no. 2 = 5.31; p = 0.042). Extent of the vertical shift poorly correlated with number of laboratory occlusal contact points (R ² = 0.017).

CONCLUSIONS

Considering the complex multistep study design, a limited number of participants, and referring to one specific arbitrary face-bow, the following conclusion could be drawn: no substantial difference by the use of the arbitrary face-bow compared to a mean setting could be determined, when changing the vertical dimension in the articulator within a remounting procedure of complete dentures.

CLINICAL RELEVANCE

Further research is necessary to determine the effects of different arbitrary face-bows on the fabrication and adaptation of removable dentures.

Evaluation of the inclination of maxillary occlusal plane on mounted casts using a fixed value and customized nasion indicator in artex articulator: Facebow system - An in-vivo comparative study

Aim:

To evaluate and compare the variations in the inclination of occlusal plane of casts mounted on Artex articulator using a facebow with a fixed value and customized nasion indicator.

Materials and Methods:

Twenty two subjects were selected for this investigation. Two maxillary impressions were made, and casts poured. For each of the twenty two subjects, the facebow records were made with, Artex face-bow using a fixed value nasion indicator and customized nasion indicator and mounted. The angle between the occlusal plane of upper cast and the upper articulator arm was measured with a Universal bevel protractor and compared with the gold standard cephalometric value.

Results:

It shows that, when angle was measured between maxillary occlusal plane and upper member of the articulator, on the mounted cast using a customized nasion indicator and fixed value nasion indicator against the gold standard cephalometric value as a whole, it was found to be not significant. But, if each patient were evaluated individually, there found to be the difference in the angle.

Discussion and Conclusion:

Variation in occlusal plane was very minimal and close to the cephalometric value when using customized nasion indicator compared to fixed value nasion indicator on the Artex.

Impact of arbitrary and mean transfer of dental casts to the articulator on centric occlusal errors

When fabricating dental restorations, casts are usually transferred to the articulator based on arbitrary hinge axes or mean values instead of true hinge axis points. Using arbitrary hinge axis points or mean values can lead to occlusal errors if the vertical relation is changed in the articulator (e.g., when a centric record is used). This study predicted the probability of occlusal errors occurring in a group of subjects when casts are mounted based on arbitrary hinge axis points or mean values. In 57 healthy volunteers, true hinge axis points, arbitrary hinge axis points, right infraorbital point, maxillary incisal point, and the palatal cusps of the second molars were determined. Mean hinge axis points were established based on Balkwill angles between 17° and 25°. Occlusal errors evoked by cast mounting in relation to arbitrary or mean axes compared to true hinge axes were calculated. Errors were determined for vertical relation settings of 2 and 4 mm. With 2 mm vertical relation, occlusal errors ≥340 µm occurred with a 10% probability with arbitrary hinge axis mounting. At the same probability level, the error increased moderately to ≥440 µm with mean value mounting and a Balkwill angle of 17°. With a Balkwill angle of 25° occlusal errors ≥1,120 µm occurred with 10% probability. Occlusal errors increased considerably with a vertical relation setting of 4 mm. If vertical relation shall be altered, a transfer of the casts according to arbitrary hinge axes is recommended. If casts are transferred according to mean values, errors are bigger depending on the articulator used.

A variation on split-cast mounting for complete denture construction

This article describes a variation of the split-cast mounting technique wherein the border of the definitive cast is wrapped with masking tape to form a container for the dental plaster normally used to affix the cast to the articulator. The entire inferior surface of the cast is coated with a thin film of petroleum jelly, and the cast is mounted in the articulator. After the dental plaster has set, the cast is retained by means of the masking tape. The cast is separated from the dental plaster simply by removing the masking tape.

Observation of horizontal mandibular positions in an edentulous patient using a digital gothic arch tracer: a clinical report

Dentures were fabricated for a 73-year-old woman using porcelain 20-degree maxillary posterior teeth and acrylic resin flat planes in the mandibular posterior region. A digital gothic arch tracing device was used to observe the horizontal mandibular positions before insertion and to evaluate the therapeutic effect of the diagnostic dentures at 1 and 3 months after insertion.

Occlusal plane discrepancies generated by transverse horizontal axis deviations

STATEMENT OF PROBLEM

Any deviation from the patient's rotational axis of the mandible during closure and the arc of closure on an articulator has been reported to produce occlusal discrepancies. These discrepancies can affect diagnostic planning procedures and the occlusal relationships of restorations.

PURPOSE

This study investigated the magnitude and types of occlusal errors produced by deviations between the patient's transverse horizontal axis and the axle on the articulator.

MATERIAL AND METHODS

A simplified monoplane model was diagrammatically analyzed for deviations from the model's true transverse horizontal axis. Axis deviations of 5 mm were used in 8 different directions from the true transverse horizontal axis of the model. Changes in the orientation of the mandibular plane to the maxillary plane were illustrated diagrammatically. These changes were quantified by geometric and trigonometric assessments.

RESULTS

All axis deviations produced mandibular plane orientation errors with angular plane malalignment to the maxillary plane on complete closure. These axis deviations also produced anterior-posterior shifting of the mandibular plane relative to the maxillary plane.

CONCLUSION

Several different types of planar shifting and angular discrepancies were demonstrated from deviations away from the true transverse horizontal axis.

Three-dimensional computer morphometry of the maxilla and face in infants with Pierre Robin sequence--a comparative study

OBJECTIVE

To analyze the morphology of the maxillary crest in infants with Pierre Robin sequence using an anthropometric coordinate system and to compare the data with those of healthy infants.

SETTING

The study was performed at a craniofacial center servicing a large geographic area.

PARTICIPANTS

The study involved eight infants aged 1-28 days (average, 7 days) with an established diagnosis of Pierre Robin sequence and six healthy infants aged 1-43 days (average, 22 days).

MAIN OUTCOME MEASURES

Physical models of the maxilla and face obtained by alginate replication were analyzed by computer morphometry yielding the three-dimensional topology of the maxillary crest.

RESULTS

The maxillary crest of children with Pierre Robin sequence shows an increased inclination relative to the transverse plane (30 +/- 3.9 degrees) as compared with that of healthy infants (20 +/- 2.9 degrees). The maxillary crest of the patients is shortened in the sagittal direction by comparison with healthy controls.

CONCLUSIONS

The increased inclination of the maxilla in infants with Pierre Robin sequence may aggravate the retroposition of the mandible and may thus be a pathogenetic factor contributing to the severe respiratory problems.

Use of personal computers for Gothic arch tracing: analysis and evaluation of horizontal mandibular positions with edentulous prosthesis

STATEMENT OF PROBLEM

Determining mandibular position for an edentulous patient raises the question of whether to emphasize centric relation or muscular position. This challenge results from the lack of a convenient procedure for quantifying the horizontal mandibular position, which can be determined by a variety of methods.

PURPOSE

This study analyzed and evaluated the horizontal mandibular positions produced by different guidance systems.

MATERIAL AND METHODS

Twenty-six edentulous subjects with no clinical evidence of abnormality of temporomandibular disorder were selected. Horizontal position data for the mandible obtained by gothic arch tracing was loaded into a personal computer by setting the sensor portion of a digitizer into the oral cavity to serve as a miniature lightweight tracing board. By connecting this with a digitizer control circuit set in an extraoral location, each mandibular position was displayed in a distinguishable manner on a computer display in real time, then recorded and analyzed.

RESULTS

The gothic arch apex and tapping point varied, depending on body position. In the supine position, the gothic arch apex and the tapping point were close to the mandibular position determined by bilateral manipulation.

CONCLUSION

This system provides effective data concerning mandibular positions for fabrication of dentures.

Guided versus unguided mandibular movement for duplicating intraoral eccentric tooth contacts in the articulator

STATEMENT OF PROBLEM

The extent to which intraoral excursive tooth contacts are duplicated correctly in the articulator is constantly being investigated.

PURPOSE

This study evaluated whether data from unguided or guided hinge axis movement are superior in duplicating excursive tooth contacts.

MATERIAL AND METHODS

Intraoral records of tooth contacts from intercuspal position to 4 mm protrusion and laterotrusion were obtained from 50 subjects by using occlusogram wax. These movements were simulated in a SAM2 "P" articulator using horizontal condylar inclination and Bennett angle from recorded guided and unguided mandibular movements. Occlusograms were compared with contacts generated by the articulator.

RESULTS

On average, the articulator duplicated approximately 73% of intraoral protrusive and 81% of intraoral laterotrusive contacts for up to 4 mm of movement, using either data from unguided or guided movements. Both movements yielded identical means and no significant difference of duplicated contacts. However, individual differences scattered within approximately +/-32%.

CONCLUSION

Exclusive recommendation of guided movement can be questioned for accurate imitation of dynamic tooth contacts. In addition to the type of movement, other factors must be detected to enhance articulator adjustment and workings.

Three-dimensional analysis of cleft palate topology in newborn infants with reference to the cranial skeleton

OBJECTIVE

To describe a method of determining the three-dimensional topology of the palatal crest relative to a reproducible anthropomorphic coordinate system in newborn infants with unilateral cleft palate. For this purpose, physical models of the maxilla and face were analyzed by computer morphometry.

DESIGN

The study was limited to infants referred to the craniofacial center during the first 11 days after birth.

SETTING

The study was performed at a craniofacial center servicing a large geographic area.

PARTICIPANTS

The method was applied to 12 infants with unilateral cleft lip, alveolus, and palate (eight patients with left-side clefts and four with right-side clefts).

MAIN OUTCOME MEASURES

The three-dimensional topology of the palatal crest referenced to an anthropometric coordinate system was the primary outcome measure. The anthropometric reference system is defined by the tragus points and the midpoint of a line connecting the endocanthia.

RESULTS

The topology of the maxillary crests of the patients was characterized by considerable variability. The center of the premaxilla as defined by the attachment of the frenulum was frequently displaced by several millimeters from the midsagittal plane. The displacement was to the left in infants with right-side clefts and to the right in infants with left-side clefts. The premaxilla can be rotated by more than 30 degrees relative to the normal position. No significant retroposition of the minor segment as determined by the location of the tuber points was found. Several morphometric anomalies were found to be correlated linearly.

CONCLUSIONS

We propose that the morphologic deviations are in part caused by the neuromotor activity of the tongue and of the interrupted M. orbicularis oris. The data can serve as the starting point for a longitudinal study of craniofacial development in children with cleft palate and for studies on the efficacy of different therapeutic approaches.

Geometric analysis of occlusal plane orientation using simulated ear-rod facebow transfer

PURPOSE

Ear-rod facebow techniques may position casts high or low between the upper and lower members on the articulator when using orbitale or nasion as anterior reference positions. This study assessed the effect of changing the anterior reference position on simulated mountings utilizing simulated ear-rod facebow.

MATERIALS AND METHODS

Tracings from latoral cephalographs made on seven subjects were superimposed with an outline model of a semiadjustable articulator. Three simulated mountings were performed for each subject in which the plane of occlusion was positioned (1) high, (2) midway, and (3) approaching the lower member (low). Maxillary and mandibular occlusal planes representing intercuspal position were determined from the cephalographs and positioned on the articulator model in a simulated centric relation position for each mounting. Condylar guidance was determined from a simulated protrusive position.

RESULTS

Analysis of the three mounting positions demonstrated no change in intercuspal position within subjects; however, angles formed between upper member and condylar guidance were 14.9% smaller for the mid-distance mounting, 42.9% smaller for the high mounting, and 13.4% higher for the low mounting positions when compared with a standard Frankfort horizontal plane reference.

CONCLUSIONS

The cephalographs showed extreme variability in the position of the ear piece to bony structures of the skull, but this deviation appeared to be compensated by a change in the horizontal condylar guidance relative to mounting. Results also suggest that casts may be mounted in a convenient mid-position for routine articulation.