Comparison of the accuracy of a 3-dimensional virtual method and the conventional method for transferring the maxillary cast to a virtual articulator.

STATEMENT OF PROBLEM: The currently available virtual articulators fail to locate the digitized maxillary cast at the exact position in the virtual environment. Some locate the casts on a mechanical articulator with a facebow, and this position is then digitized for the virtual environment. PURPOSE: The purpose of this study was to compare the location of the maxillary cast on an articulator by using 2 different procedures: the conventional method and a virtual method. MATERIAL AND METHODS: With the conventional procedure, the kinematic axis of the participant was determined with an axiograph. The location of the maxillary cast in reference to this axis was then physically transferred to a Panadent mechanical articulator. By a virtual procedure, the same kinematic axis and the maxillary cast were transferred directly from the participant to the Panadent virtual articulator by means of reverse engineering devices. The locations obtained with both procedures were compared in a virtual environment with an optical scanner. By calculating the deviation at every point of the occlusal surface, the results obtained with this procedure were then compared with those of the conventional method. RESULTS: The mean deviation on the occlusal surface was 0.752 mm, and the standard deviation was 0.456 mm. CONCLUSIONS: The deviation between the procedures was sufficiently small to allow the methodology for orthodontic purposes. However, the accuracy of the virtual procedure should be improved so as to extend its use to other fields, such as orthognathic surgery or dental restorations, in which the clinical technique requires an articulator.

An evaluation of face-bow transfer for the planning of orthognathic surgery.

PURPOSE: The purpose of this study was to evaluate the error magnitude in the clinical application of face-bow devices. Technical and methodologic inaccuracies, as well as deviations from reference planes, were determined. MATERIALS AND METHODS: The presented method is part of a 3-dimensional virtual planning procedure for orthognathic surgery and included 15 patients with dentoskeletal deformities. Cone beam computed tomography datasets obtained from patients with a referenced face-bow plane and a centric registration splint were matched with cone beam computed tomography datasets of the registered plaster model of the maxilla mounted in an articulator. To assess potential sources of methodologic errors, angulations were measured between the virtual face-bow plane and the horizontal cross bar of the virtual articulator. To evaluate the reproducibility of the anatomic reference plane, angulations between the Frankfort plane and the horizontal cross bar of the articulator were measured. Statistical significance was set at P < .05 and tested by univariate analysis of variance. RESULTS: Technical and methodologic errors showed a mean deviation of 3.5°, with a median of 3.6° and SD of 2.7°. The values did not reach statistical significance (P = .1). However, there was a significant error (P < .05) in determining the position of the anatomic reference plane by face-bow transfer. The mean deviation was 7.7° (values ranged between 1.2° and 18.9°), with a median of 6.7° and SD of 5.3°. CONCLUSIONS: In this study the traditional use of face-bow devices showed inaccuracies in model mounting as well as in assignment of anatomic reference planes. Three-dimensional virtual computer-assisted planning seems to be more accurate than conventional methods.

Evaluation of an improved orthognathic articulator system. 2. Accuracy of occlusal wafers.

The errors produced by occlusal wafers constructed on casts of the teeth mounted on a standard articulator and an improved orthognathic articulator were investigated by carrying out simulated orthognathic surgery on plastic skulls. The wafers were used to relocate the position of the maxillae of the skulls. The vertical and horizontal displacements of the maxillae were determined from measurements of the positions of markers on the skull and teeth. Comparison of the magnitudes of the actual and intended movements showed that wafers constructed on the standard articulator had systematic prediction errors of up to 5mm, but the improved orthognathic articulator showed much smaller random errors. There was a statistically significant improvement in overall accuracy in predicting maxillary Le Fort I position with the use of the improved orthognathic articulator which the authors recommend for clinical use.

Evaluation of an improved orthognathic articulator system: 1. Accuracy of cast orientation.

A systematic study was carried out using plastic model skulls to quantify the accuracy of the transfer of face bow registration to the articulator. A standard Dentatus semi-adjustable articulator system was compared to a purpose built orthognathic articulator system by measuring the maxillary occlusal plane angles of plastic model skulls and of dental casts mounted on the two different types of articulators. There was a statistically significant difference between the two systems; the orthognathic system showed small random errors, but the standard system showed systematic errors of up to 28°.

The use of articulators in UK dental schools.

The increasing complexity of many restorative procedures often involves articulation of study and working casts to ensure accurate fabrication of restorations. Correct selection and use of articulators can be crucial to successful restoration. The aim of this paper is to determine which articulators are recommended for various restorative procedures in UK dental schools, for use by undergraduate students. A questionnaire-based study of all UK dental schools was carried out, with a 100% response rate. Recommended articulator application for specified procedures was established from the literature and questionnaire results were compared with this. The results indicated that dental schools in the UK generally teach appropriate articulator use for most procedures. However, there are some limited areas of what may be argued to be inappropriate recommendation in some establishments.

A survey on the utilization of disposable quadrant articulators.

This study evaluated utilization of the six most popular disposable quadrant articulators presently used in crown fabrication. Most general practitioners and crown and bridge laboratory technicians reported that they frequently or always used disposable quadrant articulators for single units and simple three-unit fixed partial dentures. All evaluated articulators demonstrated an inability to maintain and repeat the centric stop position. The disposable quadrant articulator, used in combination with dual-arch impressions, is a viable method for fabricating single and short-span restorations. While a nonrepeatable centric stop was present in all six articulators evaluated, the method was considered a time-efficient and cost-saving alternative to traditional all-metal simple-hinge articulators.

The use of specific dental school-taught restorative techniques by practicing clinicians.

In 1995, a survey requesting information about the utilization of certain prosthodontic techniques was mailed to 3,544 graduates of a midwestern dental school. Responses were received from 1,455 alumni, representing a 41 percent return rate. In general, the results are consistent with international and national trends and show significant disparity in the utilization rates of certain procedures between general dentists and prosthodontists, as well as a disconnect between what is taught in the undergraduate dental educational program and what is applied in practice. For example, while prosthodontists typically apply what was taught in their educational program, utilization rates of general dentists for the facebow was 29.64 percent; the custom tray 68.48 percent; border molding 58.67 percent; altered casts 24.10 percent; custom posts 49.29 percent; prefabricated posts 67.54 percent; and semi-adjustable articulators 50.64 percent. While no solutions to this disconnect are offered the authors do pose important questions that must be addressed by the dental educational community.

Condylar repositioning in mandibular retrusion.

STATEMENT OF PROBLEM: There has been debate about which anteroposterior maxillomandibular relationship should be used in fixed and removable prosthodontics for patients without a definite maximal intercuspation. The choice has important implications for complete denture fabrication in which any guidance to the previous dentulous maximal intercuspal position is missing. PURPOSE: This study was designed to estimate the posterior displacement that takes place at the mandibular condyles and occlusal surfaces when the mandible is moved from maximal intercuspal position to the most retruded mandibular position. MATERIAL AND METHODS: Articulated occlusal models of 18 subjects with a natural dentition and well-defined maximal intercuspation were studied. Models were rearticulated in retruded contact position, and the original vertical dimension of occlusion was restored by eliminating all interfering occlusal contacts on the retruded path of closure. Measurement of change in condylar position was estimated using a SAM2 articulator with condylar position indicator. Posterior displacement at the occlusal surfaces was measured with a traveling microscope. RESULTS: Condylar analog retrusion in the horizontal plane at the initial retruded contact position varied from 0.6 to 2.4 mm (mean 1.0 +/- 0.4 mm); however, after occlusal adjustment to restore the original vertical dimension of occlusion, retrusion was reduced to a range of 0 to 0.4 mm (mean 0.2 +/- 0.1 mm). Retrusion at the occlusal surfaces was found to vary from 0.4 to 1.5 mm (mean 0.7 +/- 0.3 mm) at retruded contact position; however, after occlusal adjustment, retrusion was reduced to a range of 0 to 0.5 mm (mean 0.2 +/- 0.1 mm). CONCLUSION: Posterior displacement at both the occlusal surfaces and the condyles was small when interfering occlusal contacts on the retruded path of closure were removed.

The effect of posture and anesthesia on the occlusal relationship in orthognathic surgery.

PURPOSE: This study investigated the differences in centric occlusion (intercuspal-CO) and centric relation contact position (CR) in patients in an upright and supine posture when conscious and when supine under general anesthesia to rationalize planning for complex bimaxillary osteotomies. PATIENTS AND METHODS: Twenty-four men and 37 women, with a mean age of 24.7 +/- 4.5 years were included in this study. Occlusal records on each patient included 1 in CO and 3 in CR in each of the following postures: conscious upright, conscious supine, and anesthetized supine. Casts were mounted in the Denar Mark II articulator and transferred to the Denar Vericheck (Denar Corp, Anaheim, CA) recording tool for measurements. The centric relation displacement vectors (CO-CR) were projected in various axes by 3D graphics. RESULTS: The upright conscious CO-CR (mm) vectors were significantly different from both the supine conscious and anesthetized positions in the anteroposterior plane. The difference in the vertical plane was only significant in Class II division 1 patients. CONCLUSION: The posterior change in centric relation explains the apparent occlusal discrepancies created by rigid fixation of the mandible applied in the supine anesthetized position. By taking the CR records in the supine conscious position, the "loss" of maxillary advancement can be avoided.

Use of articulators in the United Kingdom by consultant orthodontists in planning orthognathic surgery.

This questionnaire survey aimed to investigate articulator use in orthognathic surgical planning by consultant specialists in the United Kingdom (UK). A total of 205 questionnaires was sent to all consultant orthodontists in the UK. One hundred thirty questionnaires were completed, representing a 63.4% response rate. Consultants had been in their post for a median of 10.5 years (range 0 to 30 years) with a mean of 11.3 (SD 8.4) surgical cases per year. When asked which cases had been planned on a semiadjustable articulator, two thirds of consultants (67.7%) planned maxillary single-jaw procedures, 45.4% planned mandibular single-jaw procedures, and 77.7% planned bimaxillary procedures. While 98.5% of consultants reported access to at least one type of articulator, 14.6% of maxillary single-jaw and 5.4% of bimaxillary surgery was not planned on any articulator. Twice as many consultants who had been in their post fewer than 11 years had been trained on a semiadjustable articulator, compared to those who had more than 11 years of experience. Semiadjustable articulators were the most popular for planning orthognathic surgery. Consultants with less than 11 years of experience completed more surgical cases each year and were more likely to have been trained on a semiadjustable articulator than consultants with more than 11 years of experience. However, no link exists between the age of the consultant and the type of articulator selected for planning; this suggests that more mature consultants have received further training on contemporary articulator systems since receiving their accreditation.

Description of mandibular finite helical axis pathways in asymptomatic subjects.

Despite wide use of systems to record jaw motion with six degrees of freedom, most studies have analyzed only the movement of a single mandibular point. The finite helical axis (FHA) is a mathematical model which can be used to describe comprehensively the movements of a rigid body. The aim of this investigation was to describe the FHA of the mandible during habitual jaw movements. Thirty subjects (13 females, 17 males; mean age, 26 years; range, 18 to 34 years) without myoarthropathies of the masticatory system participated in the study. Opening and closing movements, performed at 1-Hz frequency, were recorded with the optoelectronic system Jaws-3D. Three opening and closing movements were recorded from the right side and three from the left side of the jaw. The movement data were low-pass-filtered for noise reduction prior to the computation of the finite helical axis by means of a software program developed in our laboratory. The following parameters were calculated: the rotation of the FHA, its spatial orientation, and the translation along it, as well as its position and distance relative to an intracondylar point. In addition, methodological errors of the model were calculated. During opening and closing, the group mean FHA rotation was 24.3 degrees +/- 4.2 degrees. The group mean of the maximum total translation along the FHA was 0.9 +/- 0.7 mm. The group mean distance between the FHA and the intracondylar point was 48.9 +/- 9.9 mm. The FHA pathways were smooth and varied between individuals. Furthermore, the finite helical axes were never localized within the condyle, and often were located outside of the mandible. The analysis of the FHA pathways yields more information on whole mandibular movements than simply the movements of a single condylar point.

Movements of the mandibular condyle kinematic center during jaw opening and closing.

Little is known about the detailed kinematics of the human temporomandibular condyle during jaw opening and closing. According to the rotate and swing model by Osborn (1989), the condyle is kept in close contact with the articular eminence during opening. Whether the condyle is in closer contact with the articular eminence during opening than during closing is unknown. Another consequence of the model is that the opening condylar movements are less variable than the closing movements. In this study, the hypothesis that the opening condylar path is closer to the articular eminence and also less variable than the closing condylar path is tested. Twenty subjects (10 males and 10 females with a mean age of 22) without signs or symptoms of a craniomandibular disorder performed 2 series of 4 protrusive movements and 2 series of 4 empty opening-closing movements. The movements were recorded by a six-degrees-of-freedom opto-electronic jaw movement recording system (OKAS-3D). The kinematic center of the condyle was used as a reference point for the reconstruction of condylar movement paths. Characteristics of the opening and closing paths were investigated by means of a displacement index (DI). This index is the quotient between the three-dimensional path length and the three-dimensional path distance between the start and the end point of an opening or closing movement of the kinematic center. The DI was smaller (p < 0.0001) and also less variable (p < 0.0001) during opening than during closing. The smaller DI value, in combination with the concave nature of the movement path, indicates that the opening path of the kinematic center lies above the closing path and thus closer to the articular eminence.

Wear in the upper and lower incisors in relation to incisal and condylar guidance.

The incisors of 85 patients were examined for tooth wear with the aim to evaluate possible association between the degree of wear and anterior guidance. Apart from clinical functional analysis electronic axiography using the system CADIAX was carried out. Anterior guidance in protrusive movement was marked on individually mounted plaster casts and scanned using a pantograph. The cephalometric analysis program CADIAS was used for the metric determination of the anterior guidance angle to the axis-orbital plane, and in order to relate the anterior guidance to the horizontal condylar inclination. The interincisal angle was included in the evaluation as a further functional angle and according to overjet and overbite there were classified typical incisor relations. Where both anterior guidance and incisor inclination and relation were in the normal range, there was an increase in incisal tooth wear in the group investigated. With reduced functional space, frequently found together with anterior deep bite, incisor wear was less extensive. While the occurrence of parafunction was shown not to be related to anterior guidance and incisor inclination and relation parafunctions were the decisive factor in the occurrence of severe incisor wear. Where anterior guidance and incisor inclination and relation are normal, the area of enamel under load is smaller and the space for bruxism larger. Parafunctional forces thus appear to result in a higher degree of wear than is found with the reduced functional space of a steep incisor position. Anterior guidance could not be evaluated as a parafunctonal inducing factor, but only as determining the wear pattern in the individual.

[The intraoral determination of the sagittal condyle path inclination during face-bow recording]. / Intraorale Bestimmung der sagittalen Kondylenbahnneigung während der Gesichtsbogenübertragung.

In a clinical trial involving 40 fully dentate persons, three different intraoral procedures to determine the sagittal condyle path inclination were checked for accuracy. In each test person the condyle path inclination was determined three times, i.e. by the central bearing point method, the protrusive check-bite method and by a protrusive bite record integrated in the bite fork. At 5 mm of protrusion the reproducibility of the results obtained with the central bearing point method (mean value 2.1 +/- 2.0 degrees) were far better than that with either of the two other methods; the difference was statistically significant. The protrusive check-bite records and the protrusive bite fork records did not differ from each other as far as their accuracy was concerned (mean values 3.6 +/- 2.1 degrees and 3.4 +/- 1.6 degrees, respectively). The mean value of the condyle path inclination to the hinge axis infra-orbital plane in the sample was about 50 +/- 8 degrees regardless of the method used. Thus, protrusion records can be integrated in the face bow transfer.

[The problem of the reproducibility of occlusal contacts]. / Untersuchungen zur Frage der Reproduzierbarkeit okklusaler Kontakte.

To perform target orientated therapy simulators of condyle movements (CPS) have been introduced recently. It is believed to improve analysis of jaw by computer aided registration and evaluation systems (i.e. transformation of the true axis and jaw movements for use in articulators). Although the electronic evaluation systems helped to improve our understanding of the movements of the jaw, up to now the reproduction of physiological and pathological movements and their therapeutic changes have not been solved satisfactorily. The reason is sometimes miscorrection of system inherent projection errors. Additionally, numerical analysis and changes of the registered pathological movements are restricted to certain limits within the articulators for therapeutical purpose.

[Statistical analysis of functional interrelations between anterior guidance and posterior determinants]. / Analyse statistique des interrelations fonctionnelles entre guide antérieur et déterminant postérieur.

During the last decade, the concept of an effective, even steep anterior guidance did evolve toward the description of a "kinder" or "softer" anterior guidance which, while ensuring the disclusion of the posterior teeth, leaves some anterior functional freedom. It then becomes necessary to determine more precisely the criteria for diagnosis, orthodontic treatment, prosthetic restoration of anterior functional surfaces. In order to reveal some functional agreement between an anterior and posterior guidance, the analysis of the functional morphology of antero-maxillar teeth in relation to their skeletal context was proposed, in a study of 33 cases, so as to correlate the condylar steepness measured by axiography. It should be noted that a decreasing gradient exists between the mean respective values of the functional slopes of the central incisors (S1 = 64.3 degrees), and lateral incisors (SF1 = 53.5 degrees) and the canine (Cf = 51.8 degrees). Although the cingular surface S1 does not appear to have any influence on the posterior guiding factor (r = 0.1), in fact the anterior functional surface which is stategic in mastication, is located on the occlusal surface of the palatol side (S2) of the antero-maxillary teeth. The correlation is significant (r = 0.325) when the anterior guidance (incisives and canines) is associated to the mean condylar value more than being correlated to the mean functional value of the central incisor alone. It thus appears logical that the functional modeling of the articular cavities be influenced by the overall anterior guidance and not by the single central incisor. It has also been possible to emphasise the influence of the orientation of the occlusal plane in the anterior functional relationship (r = 0.547); thus, the anterior guidance appears to have to be assessed over a "relative overall analysis": overall: meaning the overall anterior guidance, i.e. the six anterior maxillary teeth; relative: meaning measured in relation to the occlusal plane. This concept of relative overall analysis of the anterior guidance, developed using a simple computerised program, will allow the diagnosis and anterior prosthetic or orthodontic restoration to be facilitated.

Does one articulator meet the needs of both fixed and removable prosthodontics?

A survey to determine the type of articulators used in teaching fixed and removable prosthodontics in the 59 United States dental schools was conducted. Of 115 questionnaires mailed, 104 responses were received. The following observations resulted from analysis of the questionnaire. Of the 81 articulators used in fixed and removable prosthodontics, 65 (76.5%) were of the arcon design. The most common articulators used were the Whip-Mix (16 schools), the Hanau 158 (14 schools), the Hanau 96 H-2 (13 schools), and the Denar Mark II (11 schools). The most common articulators used in fixed prosthodontic programs were the Whip-Mix (16) and the Denar Mark II (11), while the most common articulators for removable prosthodontics were the Hanau 158 (14) and the Hanau 96 H-2 (13). Twenty-two schools issued two articulators of different design, and three schools issued two articulators of the same design. The outstanding difference in requirements for fixed and removable prosthodontic teaching programs was that those in fixed prosthodontics desired an articulator in which the maxillary and mandibular members could be separated, while those in removable prosthodontics did not.