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Revista Facultad de Odontología Universidad de Antioquia

Print version ISSN 0121-246X

Rev Fac Odontol Univ Antioq vol.25 no.1 Medellín July/Dec. 2013

 

ORIGINAL ARTICLES DERIVED FROM RESEARCH

 

FACE BOWS IN THE DEVELOPMENT OF MICHIGAN OCCLUSAL SPLINTS1

 

 

Jesús Gámez C.1; Alejandro Dib K.2; Irene Aurora Espinosa de S.3

 

1 BSc in Stomatology, DDS, Benemérita Universidad Autónoma de Pue- bla (BUAP), Puebla, Mexico. Intern at the Specialization in Oral Pros- thesis and Implantology, Universidad Nacional Autónoma de México (UNAM). Email address: jgamcal@gmail.com

2 MSc in Biological Sciences. Comprehensive Dentistry Specialist. DDS, MS. Professor at the School of Stomatology, Benemérita Univer- sidad Autónoma de Puebla (BUAP), Puebla, Mexico. Email address: adibk@unidentis.com

3 Ph.D. en Ciencias Médicas y especialista en Cirugía Maxilofacial, DDS, MS, docente de la Facultad de Estomatología, Benemérita Universidad Autónoma de Puebla (BUAP), Puebla, México. Correo electrónico: ireneesp@buffalo.edu

 

SUMBITTED: FEBRUARY 19/2013-ACCEPTED: JULY 30/2013

 

Baldión PA. Influence of post-bleaching time on the adhesion of a composite resin to enamel. Rev Fac Odontol Univ Antioq 2013; 25(1) 92-116.

 

 


ABSTRACT

INTRODUCTION: Michigan occlusal splints (MOS) are frequently used for the management of patients with bruxism. The literature mentions the use of face bows for mounting models in semi-adjustable articulators, but its benefit in the development of MOS is still controversial. Therefore, the objective of this study was to compare the record of number of contact points and mounting time between MOS made with and without face bows in patients diagnosed with bruxism. METHODS: a total of 90 splints were made and distributed among 45 patients diagnosed with bruxism at the Oral Rehabilitation Clinic of Benemérita Universidad Autónoma de Puebla (BUAP). The two splints (one made with a face bow mounted model and the other one without it) were compared at the articulator and clinically. The number of obtained contact points was recorded in both splints as well as the time needed for mounting. The comparisons were made with Wilcoxon statistical test and a significance level lower than 0.05. RESULTS: the splints with face bows showed a greater average of contact points in the mouth (11.67) compared with the ones without face bows (11.58), with no significant difference (p = 0.799). Mounting time was higher in the splints made without face bows (51 s) compared with the ones with face bows (33 s), with no significant difference (p = 0.332). CONCLUSIONS: there are no significant differences in using face bows for developing MOS in bruxism patients.

Key words: face bow, Michigan occlusal splint, semi-adjustable articulator, bruxism, occlusal plane.


 

 

INTRODUCTION

Bruxism is a disorder characterized by teeth clenching and grinding in a parafunctional and unconscious manner.1 Approximately 85 to 90% of the population has tightened their teeth at some point in their lives.2 Although the etiology of bruxism is attributed to peripheral factors such as occlusal interferences and key factors such as neuro-physiopatological processes, personality, and stress, it is widely accepted that the genesis of this parafunction is multifactorial.3 The treatment for bruxism patients is therefore varied. One of the treatments suggested for managing bruxism is reversible and irreversible occlusal therapy, defined as any treatment directly altering the mandibular position, the occlusal pattern, or both.1 Reversible occlusal therapy with occlusal splints (OS) was introduced by Karolyi in 1901 for the treatment of bruxism. MOSs were developed at the University of Michigan between the 1950's and the 1960's.4 However, the authors never suggested using face bows to mount upper models in articulators.5

Face bows were first introduced by Snow in 1900, with the intention of locating the rotation axis of the jaw hinge. A face bow is a gauge-like instrument used to record the spatial relationship of the maxilla with one or several points of anatomical reference in order to later transfer this relationship to an articulator. It guides the dental model in the same direction of the articulator's opening and closing axis. The anatomical references traditionally used are the transverse horizontal axis of the mandibular condyles and another selected point.6,7 Rosenstiel et al point out that in order to be accurate in reproducing the patient's movements, it is necessary to transfer the upper model by means of the face bow, the interocclusal record in a centric relation to the lower model articulation, and condylar elements appropriately adjusted (interocclusal records of protrusion and lateral movements).7

Arbitrary face bows are less accurate than the kinematic ones but they are sufficient in most routine dental treatments. They are based on average anatomical values that provide an estimate of the transverse horizontal axis. Manufacturers have designed face bows so that their relationship with the real axis offer an acceptable level of error. The bow is usually stabilized by using an easily identifiable reference such as the external auditory meatus, and for this reason they are provided with intra-auricular devices. A minimum error of 5 mm may be expected at the point where the axis located, and it may lead to a lack of inclination in the occlusal plane.7

The studies that evaluate OS effectiveness mention the use a face bow as one of the procedures to develop OS in order to mount models on the articulator.3, 8-11 However, studies of a similar nature do not mention the use of face bows to mount models on the articulator.12-16 Moreover, some other studies that directly evaluate the use of face bows for developing OS and full dentures argue that it provides no clinical benefits.17-20

The objective of this study was therefore to compare the number of contact points and the time used to mount Michigan OS made with and without face bows on patients diagnosed with bruxism.

 

METHODS

First, a pilot study was expressly made to identify differences between the amount of contact points obtained in splints made with face bows and those without them (with a mean of 0.8 points of contact). Based on this difference and with a confidence level of 95% and a power of 80%, a sample of 47 units per group was calculated. In consequence, a total of 94 Michigan occlusal splints were made, two for each patient (one with each method). The sample included patients treated at the outpatient clinic of the Master of Science in Stomatology and Oral Rehabilitation at Benemérita Universidad Autónoma de Puebla (BUAP), who reading and voluntarily signing informed consent and meeting the following inclusion and exclusion criteria:

Inclusion criteria:

  • 18-70 years of age.
  • Either sex.
  • Voluntarily participating in the study, signing an informed consent.

Exclusion criteria

  • Patients with more than 8 lost teeth without replacement (except third molars).

Each patient was given a screening questionnaire on signs and symptoms of bruxism, containing ten questions (table 1). Later, a clinical examination was performed to confirm the diagnosis of bruxism. This assessment included: presence and type of tooth wear using Pergamalian classification,21 and presence of masseter muscle hypertrophy through standardized painless palpation greater than 2 pounds, based on Dworkin's criteria of 1992,22 according to which, in a healthy patient, 2 pounds of pressure on muscle should be painless. Hence if the pressure is higher and there is no pain response, masseter hypertrophy can be considered.

The bruxism diagnosis was confirmed by analyzing both the screening questionnaire and the clinical evaluations. If patients responded affirmatively to at least one of the first five questions or to three of the last five questions, and they also presented ei- ther tooth wear or masseter hypertrophy, then they were considered as having bruxism.

A total of 47 patients were included, obtaining models from their both arches: 4 impressions with irreversible hydrocolloid (Alginoplast, Heraeus Kulzer, Hanau, Germany), two of the upper area and two of the lower one, later obtaining type IV gypsum models (Elite rock, Zhermack, Rovigo, Italy), with a period of approximately ten minutes. Each patient was prepared two Michigan FOs, one with face bow record and transfer and another one with the use of an occlusal plane device. 17, 20 In order to mount the models in both techniques, we used a semi-adjustable articulator "Whip Mix 8500" (Whip Mix, Kentucky, United States), which contains the "Quick Mount # 8645" face bow. The interocclusal record was made in centric occlusion and maximum intercuspidation,4, 10 with a bite registration material (Bite Imprint, 3M ESPE, Seefeld, Germany). To obtain face bow records we followed the manufacturer's instructions described in the owner's manual for this face bow model.

The Michigan-type OSs were made on self-curing transparent acrylic (Opti-CryI, New Stetic, Antioquia, Colombia) according to the specifications of Ramfjord and Ash, with a flat occlusal surface, occlusal contact for all opposing teeth, and completely free from interference in mandibular excursions. 4, 5

Out of the 47 patients initially enrolled, two were discarded: one did not return for his second ap- pointment (on the grounds of lack of time) and the other one presented substantial changes in his den- tal condition, which prevented splinting.

Once the OSs were completed, we observed and recorded the contact points of the lower arch cusps, free of interference obtained in the articulator with articulating paper (Articulating Paper 200 µ Bausch; Nashua NH, United States). Eccentric movements were also observed and posterior organs disocclusion was evaluated when making protrusion and lateral movements. A photo of the contact points obtained was also taken. The splints were later placed on patients, observing the contact points obtained with the articulating paper and comparing them with those of the photograph. In case of differences in the number of contact points, the mounting time was recorded in order to match the number of points obtained in the patient and in the articulator. Adjustments were made with a carbide bur (NTI-Kahla GmbH Rotary Dental Instruments, Kahla, Germany) attached to a low speed handpiece. The researcher recorded the differences between contact points and the required adjustment time, which remained blinded to the group processing the splints to avoid bias. A database was made in SPSS 19 and descriptive/inferential statistics was used. Comparison of variables, points of contact, and the adjustment time of both procedures were made with Wilcoxon test statistics, given the distribution of the variable, with a significance level lower than 0.05.

 

RESULTS

 

The screening results are shown in figure 1, which shows that both night clenching and stress occurred in almost all of the study population regardless of gender. (figure 2) shows the results observed in the clinical examination; masseter muscle hypertrophy was the sign most frequently occurring, consistent with the reports of nighttime clenching, in more than 80% of patients —and more than a third part of these presented fractured restorations—. The teeth most severely worn were the canines (both upper and lower), which had the highest percentages of considerable flattening, dentin exposure and contour loss, followed by the right upper and lower incisors (table 2) .

The results of the contact points obtained and the adjusting time with both methods are shown in tabla 3 . Development of the two Michigan occlusal splints showed similarity in the number of registered contact points (11 points). No significant differences were found in comparing the methods, both in the articulator (p = 0.124) and the mouth (p = 0.799) and in comparing the difference between these two methods (p = 0.101). Similarly, adjustment time was similar between the two methods (<1 min). The statistical analysis showed no significant differences (p = 0.330).The intragroup comparison was later performed (mouth contact points vs. articulator contact points) for each group [(splints made with face bow (p = 0.462) and splints made without it (p = 0.078)]. No significant differences were found in both groups, but a tendency is observed in the group of splints made without face bow (table 4).

 

DISCUSSION

 

The results of this study showed that the use of face bows in manufacturing Michigan occlusal splints does not provide clinical benefits. The statistical ana- lyzes yielded no significant differences between the splints made with face bows and those without them at contact points obtained in both the articulator and the patient's mouth. Similarly, the required time to adjust the splints on patients showed no differences.This research study agrees with that of Shodadai et al, who did not either find differences with and without the use of facial bow, both in contact points and in adjustment time required for each of the methods used, 17 although they only performed a pilot study using statistical analysis in one direction only.In a similar study that included the three different mounting techniques to prepare OS models: with face bow, with Camper's plane, and with a plane parallel to the floor, Cunha et al found no differences among the three methods. Similarly, the researchers used the Wilcoxon test and their statistical analyzes resulted in no difference (p = 0.2580 at the articulator's contact points and p = 0.640 at the points obtained after the three adjustments).20

The use of face bows has been questioned in fields other than OS development. In 2004, Fernandes et al compared dentures made with two different techniques: with and without face bows. Both groups presented balanced prosthesis, but the technique without face bow showed better cosmetic results, as well as more comfort and stability, besides a balanced occlusion even without using the face bow. 18 Similarly, Heydecke et al identified some differences between dentures made with and without face bows. In general, patients preferred the prostheses with no face bow as they provided the best results in terms of esthetics (p = 0.026), stability (p = 0.021), and overall satisfaction (p = 0.044). These results showed that the use of face bows is also questionable in other fields as it does not improve the conditions of satisfaction, stability and masticatory functions of full dentures.19

Neither the present study nor the aforementioned articles found relevant differences between using or not a face bow, but it is hard to determine the results accurately. According to Shodadai et al, the difficulty in finding differences is due to a combination of factors: a change in vertical dimension due to occlusal bite registration; poor evidence of a pure condylar rotation, and the existence of an axis of rotation of the condylar hinge at mandibular opening; unpredictable and variable condyle movements at mandibular opening; use of a fixed hinge rotation axis at the articulators, and the presence of temporomandibular pain.17

Even maxillofacial surgery uses conventional face bows for mounting models before an orthognathic surgery, but the effectiveness of face bows is questionable due to their poor reproducibility and accuracy—besides all the factors that may be present in the laboratory affecting its performance.23The present study allows us to conclude that one of the most significant differences between the face-bow mounted models and those mounted with occlusal plane parallel to the floor is simply occlusal plane inclination. No additional difference was found in all the other features involved in the development of Michigan OS. Therefore, since no statistically significant differences have been found in terms of points of contact and adjusting time between OS made with and without face bows, it might be inferred that this difference in occlusal plane inclination does not affect the abovementioned variables.

The fact that occlusal plane inclination does not affect the collection of points of contact was confirmed by Adrien and Shouver, who examined the errors occurring when mounting mandibular models. Error location was calculated by four flat occlusal planes at 0, 10, 20 and 30° with wax of 1, 2, and 3 mm in thickness. They found out that an error in occlusal plane orientation has a small influence on the location of mandibular models since for every 10° in difference the error may increase to a maximum of 0.0730 mm with wax of 3 mm in thickness, and this error drops to 0.0235 mm with wax of 1 mm in thickness. Therefore, they conclude that the occlusal plane has relatively little influence on error location. 24 Gateno et al also compared the occlusal plane inclination of mounted models, but they used three different face bow systems measuring the occlusal plane with a cephalogram for orthognathic surgery. These authors found big differences when comparing the inclination of models mounted with conventional face bow and the measurement obtained on the cephalogram. This finding is due to the difference between the axis-orbital plane and the Frankfort plane, since conventional face bows are usually referenced to the axis-orbital plane and not to the Frankfort plane as previously thought. Therefore, face bows that have the Frankfort plane as a reference point performed better when compared with the cephalogram measurements. The authors conclude that conventional face bows do not accurately reproduce the occlusal plane inclination.25

Thus, as noted in the literature and in the present study, we may conclude that there are three main factors to claim that the use of an arbitrary face bow provides no clinical benefit in the development of Michigan OS. The first one is the rotation axis of the jaw hinge, since arbitrary face bows cannot accurately transfer the precise location of this rotation axis; in addition, the semi-adjustable articulator has a fixed rotation axis. Therefore, it cannot accurately reproduce mandibular opening movements since such movements comprise a rotational and translational movement of the condyle, and therefore the rotation axis of the jaw changes position unpredictably. This variation in rotation axis position may have a consequence: regardless of the technique for mounting models in the articulator, it must be adjusted in the mouth in order to have the same result obtained in the articulator.17, 24 The second factor is occlusal plane inclination—the most easily visible difference between models mounted with face bows and without them—. Occlusal plane inclination may vary from 0 to 30° and have a difference of about 0.0730 mm at the most, which is clinically irrelevant.24 This is probably the reason why there was no significant difference between OSs made without a face bow with an inclination of 0° and the ones mounted with a face bow and a possible variable inclination degree depending on each patient. Finally, the last factor is the relationship of the face bow with the Frankfort plane and the axis-orbital plane. Both share the lowest point of the orbit but the difference lies in the second point, since the Frankfort plane goes to the porion and the axis orbital plane goes to the rotation axis of the jaw hinge. Since the back portion of face bow arms go into the ear canal, these arms would pass over the rotation axis of the jaw hinge instead of the porion. It is therefore wrong to say that arbitrary face bows are referenced to the Frankfort plane. Even if the occlusal plane inclination were relevant, the arbitrary face bow would not accurately reproduce this inclination.25, 26However, this does not mean that arbitrary face bows are useless; they are rather an instrument which, as mentioned, are good to transfer the spatial relationship of the maxilla to the articulator and are an excellent choice for diagnosis and a great teaching method in universities. The problem is that it is not a reproducible, accurate mechanism. Replacing it by a simpler mechanism generates no significant drawbacks, at least for developing Michigan OS. This means that using a face bow or not is about the same or, viewed another way, OS present the same errors whether made with a face bow or without it, and at the end, in some cases, both must be adjusted in the mouth. Our suggestion is then to do some research on the use of kinematic face bow, which is much more accurate. This would let us determine whether or not the use of face bows provides benefits. According to our results, we may conclude that arbitrary face bows for the Whip Mix 8500 articulator provides no significant clinical benefit in terms of number of contact points or adjusting time at the clinic, i.e., there are no significant differences between using or not arbitrary face bows for developing Michigan OS in patients diagnosed with bruxism.

 

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