Reproducibility and Variability of Centric Relation Point in Completely Edentulous Patients

Sushil Kumar Kar, MDS; Arvind Tripathi, MDS; and Praveen Rai, BDS

ABSTRACT The purpose of the study was to evaluate the reproducibility and deviation from the centric relation point with time in completely edentulous patients.

Providing harmony between the occlusion and the condylar paths has a significant role for edentulous patients to restore oral function, preserve the stomatognathic system and prevent oral diseases and has a great influence on mandibular movements. [1–3] Factors that govern the condylar paths are mainly bony fossae, muscle tone, attached ligaments, shape and movements of the menisci and anterior guidance. [4–11] Among the methods that have been performed to establish a stable maxillomandibular relationship, gothic arch tracing has been broadly accepted clinically for determining the horizontal mandibular position of an edentulous jaw. The gothic arch (needle point) tracing is not only effective to record maxillomandibular relationships in completely edentulous patients but is helpful during the diagnosis and treatment of restorative problems. The main aim of the tracing needle is to record the 3D movement of the temporomandibular joint (TMJ) onto a 2D flat plate on a certain level. [12] Most of the patients could not reproduce the centric point due to long-term edentulism, uncontrolled muscle movement and some symptoms of temporomandibular disorder. [13] The purpose of this study is to evaluate the reproducibility of centric point and changes in vertical dimension of occlusion (VDO) by cone beam computed tomography (CBCT) at different intervals in completely edentulous patients.

MATERIALS AND METHODS

The study was conducted on 60 completely edentulous subjects (male and female) aged 45 to 70 from our outpatient department. All subjects were informed in detail about the nature of the study and a written consent was obtained. Ethical clearance was obtained from the ethical committee of our institution before starting the study. For better standardization of all the study subjects and to avoid bias in the study, the inclusion criteria applied were edentulous subjects with healthy residual ridges showing Angle’s class I ridge relations, a period of edentulism of more than one year and no preprosthetic surgery. The exclusion criteria were history of any craniofacial surgery or trauma, signs and symptoms of TMJ disorders, facial asymmetries, history of uncontrolled systemic disorders, poor neuromuscular control, highly resorbed ridges, previous denture wearers and poor mental attitude.

Preparation of Work Field

Occlusal rims were fabricated following all impressions-making protocols and lab procedures and were adjusted in the patient’s mouth to register the maxillomandibular relations. Using a facebow transfer (Spring-Bow, Water Pik Technologies, Fort Collins, Colo.), the maxillary cast was mounted on the articulator (Hanau Wide-Vue, Whip Mix, Louisville, Ky.). Tentative centric relation was then registered at the conventionally established vertical dimension and the mandibular cast was mounted. Four millimeters of freeway space was adopted as a standard in subsequent appointments. As the objective of the present study was to evaluate the reproducibility of centric relation point over a period of two years, four sets of occlusal rims were fabricated from four definitive impressions at an interval of six, 12, 18 and 24 months respectively. In order to use a standard occlusion rim for the tracers at subsequent appointments, the following procedure was adopted. A line was drawn on the mounting plaster parallel to the occlusal plane at a known distance. The line was used to verify the height of the occlusion rim on future replication. The base with the occlusion rim was then embedded in putty silicone to form an index that enabled the replication of occlusion rims with an assumption that no changes had occurred in the stomatognathic systems in subsequent appointments.

The tracer assemblies (Hight extraoral tracers, Teledyne Technologies, Thousand Oaks, Calif.) were secured to the respective maxillary and mandibular occlusal rims following all standardize protocols to assure that they maintained the absolute parallelism. The tracing table was covered with permanent marker and the subjects were asked to perform centric, protrusive and lateral movements repeatedly until satisfactory tracings were obtained. The whole procedure was repeated after six, 12, 18 and 24 months respectively following the denture insertion. Deviation from the centric point, protrusive, left and right lateral track was recorded and measured for any changes from the earlier tracings. The errors in the measurements were minimized by maintaining the same dimensions of the occlusal rims and attachment of tracer assemblies at the fixed reference positions. The mean of five readings was calculated by a digital Vernier caliper with an error of 0.01 mm (FIGURE 1).

FIGURE 1. Deviation from centric relation position calibrated with Vernier caliper

Preparation of Subjects

Two metallic balls (0.2 mm diameter) were embedded on the labial and buccal notch of the maxillary and mandibular denture respectively (F I G U R E 2). CBCT (KaVo Dental, Biberach, Germany) of subjects were taken and the images were then reconstructed and converted into the DICOM format (Cybermed Inc., Daejeon, Korea) (F I G U R E 3). The vertical dimension of occlusion (VDO) was recorded at the time of insertion and changes in VDO were measured at intervals of six, 12, 18 and 24 months respectively.

FIGURE 2. Complete dentures with the metallic balls.

FIGURE 3. CBCT of a subject with the dentures. Yellow line = 64.50mm

Statistical Tools Employed

The statistical analysis was done using SPSS Version 15.0 statistical analysis software (IBM, Armonk, N.Y.). The values were represented in number (%) and mean ± SD. Student’s t-test was used to test the significance of two means and a paired t-test was used to compare the change in a parameter at two different time intervals. The level of significance was designated by p.

RESULTS

A total of 60 edentulous subjects aged 45 to 70 were included in the study based on the inclusion and exclusion criteria from a patient pool of 200. Reproducibility of centric relation was evaluated at six, 12, 18 and 24 months, and it was found that 18 subjects (30.0%) gave nonreproducible records (designated as A), while in 42 subjects (70.0%), the centric point was reproducible (designated as B). The proportion of B subjects was higher until the age group 60 years (47.61% versus 33.33%), while in the age group 60 to 70 years, the proportion of A subjects was more (38.8% versus 23.8%). The difference in the age of group B and group A subjects was not found to be statistically significant (p > 0.005). Further, in females (n = 16), the percentage of reproducible subjects was slightly greater than nonreproducible subjects and this difference was not significant statistically (TABLE 1).

TABLE 1 Group Comparison of Age and Gender SEE FULL VERSION OF JOURNAL FOR TABLE

The VDO for the reproducible group was relatively unchanged during the study period, but the VDO was reduced from 66.69 ± 1.02 mm to 65.39 ± 1.01 mm at the end of 24 months for the nonreproducible subjects. The difference in VDO among subjects in groups B and A was found to be statistically significant during the study period (p < 0.001) (TABLE 2). In both the groups, loss in VDO from baseline (at insertion) was observed with time. In group A subjects, the VDO was reduced by 1.30 ± 0.09 mm at the end of 24 months with a percentage change of 1.95%, while in group B, subjects’ decline in VDO was only 0.71 ± 0.17 mm at the end of 24 months (TABLE 2).

TABLE 2 Intergroup Comparison of Vertical Dimension of Occlusion (mm) in Groups A and B and Different Intervals SEE FULL VERSION OF JOURNAL FOR TABLE.

TABLE 2A Intragroup Comparison of Vertical Dimension of Occlusion (mm) within Groups A and B at Different Intervals SEE FULL VERSION OF JOURNAL FOR TABLE.

For group A subjects, no diversion from the centric was found in the majority of the subjects (55.56%) at the end of six months. Subsequently, the deviation during the excursive movements was 38.89% on the left side and 33.33% on the right side, while during protrusion the deviation was 27.78% of the subjects (TABLE 3). At six months, diversion from the centric point was observed in only eight subjects with a mean value of 0.46 ± 0.12 mm, but progressively the numbers of subjects were increased with mean diversion of 1.13 ± 0.20 mm at the end of 24 months. Diversion from the centric point was found to be statistically significant (p < 0.001) in all the succeeding months (TABLE 3).

TABLE 3 Direction of Diversion at Different Intervals in Group A (n = 18) FULL VERSION OF JOURNAL FOR TABLE

TABLE 3A Details of Group A at Different Intervals FULL VERSION OF JOURNAL FOR TABLE

DISCUSSION

In the fabrication of a complete denture, the most important step is the establishment of a stable maxillomandibular relationship at the centric relation position. Numerous methods have been advocated to determine the centric relation position, and gothic arch tracing is one such method. In this method, the stylus traces the path of the excursive movements of the mandible and converts 3D movement of the bilateral TMJ onto the 2D flat plate in the shape of a gothic arch or an arrowhead. But most of the patients face difficulty in reproducing the centric relation point because of long-term edentulism, uncontrolled muscle movements or temporomandibular disorder symptoms, the state of underlying supporting tissue, the accuracy of the final impression, the size of the residual ridges, movement of the tongue, cooperation by the patient, stability of the trial bases, errors in mounting and processing of the final denture. Hence, the present study was conducted in order to compare the reproducibility of the centric point by gothic arch tracing and its possible correlation with changes in the VDO by CBCT at intervals of six, 12, 18 and 24 months respectively. CBCT was preferred over orthopantomograph and lateral cephalometry in the present study, because it is a diagnostic tool that has revolutionized diagnosis and treatment planning in the dental field. It is an image scanning and volumetric reconstruction technique that allows us to obtain linear measurements in 3D using computer software. The advantages of CBCT are reduced size of irradiation, minimized scattered radiation that would degrade image quality, provided isotropic voxels, i.e., equal in all three proportions and rapid scanning time.

Ever since Gysi proposed this method in 1910, needle point tracing has been accepted as the most accurate method for location of the centric maxillomandibular relation at a given degree of jaw separation. [14] El-Aramany et al. showed that determination of the centric point by the gothic arch tracing method could be accurately reproduced in the same appointment. [15] Grasso and Sharry found a variability in the apex positions of the needle point tracings, which were recorded over a period of time, and consequently recommended that a 1 mm area of freedom be given in the centric occlusion in order to accommodate this variability. [16] Pyott et al. proved that the measurements at vertical dimensions may be checked before, during and after completion of the restorations in order to verify the vertical dimension. [17–19] In his experiment, Myers found that the forefinger- and thumb-pressure method of recording the occlusal reference position did not consistently give a more posterior position than the needle point tracing method. [20] Atashrazm said that there was no influence of the fast-processing technique on the number of the occlusal contacts and occlusal vertical dimension of complete dentures. [21] Several authors have investigated the use of radiographic images for gnathological studies, but very few studies have incorporated CBCT to assess the vertical relation of occlusion. Panoramic images are often not accurate for measuring the vertical dimension due to several structures being superimposed. The use of CBCT has recently become a versatile tool because more reliable results can be expected. Although the CBCT are subdivided into two types, the authors have used the cross-section CBCT to measure and compare the VDO.

Results of the present study showed that in 70% of the patients, centric relations were reproducible and the probability of reproducibility of centric relation was relatively higher up to the age group 51 to 60 in comparison to the age group 61 to 70. Further, the proportion of group A subjects was higher among females as compared to group B subjects and the difference was found to be statistically significant. Douglas reported 20 years’ of findings after he researched the changes on the craniofacial complex in complete denture wearers. [22] He concluded that there was a loss in the vertical dimensions and the mandible was shifted in a counterclockwise manner and slightly forward. These changes were completely unaffected by the gender of the patient or the dental technique employed. In the present study, the differences in VDO among subjects with reproducible and nonreproducible centric relation subjects at six, 12, 18 and 24 months after insertion was found to be statistically significant (p = 0.071). However, a loss in the VDO was observed from the baseline in both nonreproducible and reproducible centric relation subjects with time.

Hadjieva investigated the changes in the measurement of the VDO with different durations of full denture treatments and found that the difference between the VDO in the old and the new dentures was from 2 mm to 3.99 mm in subjects who had the old denture treatment for a period of three to six years. [23] All patients undergoing the new treatments had their vertical dimension of occlusion increased by 2 mm to 6 mm. This demonstrated how there were significant changes in the dimensions of VDO even after only three years of wearing dentures. He concluded that the changes that occurred in patients with complete denture treatment were due to a variety of reasons, such as resorption of the residual alveolar ridges and wear of the occlusal surfaces of the artificial teeth leading to shortening of the length of the lower third of the face, making the patient appear older than their actual age. Serrano et al. studied centric relation change during therapy with corrective occlusion prostheses and found that corrective occlusion prosthesis therapy for 24 hours a day did not improve the reproducibility of the centric relation of asymptomatic patients within three months. [24]

At the end of six months, 55.56% of group A subjects showed no diversions from the gothic arch tracing, but the remaining subjects produced a diversion of 16.67% on the left lateral path, 16.67% toward protrusion and 11.11% on the right lateral path. Subsequently, the diversion was increased to 38.89% on the left lateral path and 33.33% on the right lateral path while toward protrusion it was increased to 27.78% at the end of 24 months. At six months, the diversion from the centric point was seen in only eight cases. The mean diversion was found to be 0.46 mm ± 0.12 mm and was statistically significant (p < 0.001). At 12, 18 and 24 months, the diversions from centric were progressively increased with a value of 0.62 mm ± 0.13 mm, 0.83 mm ± 0.13 mm and 1.13 mm ± 0.20 mm respectively and were found to be significant statistically (p < 0.001).

Breitner put forward a report on the transformation of the TMJ in rhesus monkeys when the mandible displaced anteriorly. [25] In the report, he concluded that by raising the bite, excessive stress was transmitted to the extra-alveolar bone when forces were applied parallel to the long axis of the tooth. In the present study, diversions from the centric point were more common in nonreproducible subjects as compared to reproducible subjects with the duration of time. Hence, it can be assumed that the longer the duration of wearing a complete denture, the more difficult reproducing the centric relation point and the diversion from the mandibular paths will be.

CONCLUSION

Within the limitation of this study, it can be concluded that the centric relation point can be reproducible with age and duration of wearing a complete denture irrespective of gender. Changes in the vertical dimension of a complete denture patient is due mainly to the resorption of the residual alveolar ridges and wear of the occlusal surfaces of the artificial teeth, which result in shortening of the lower onethird of the face, making the patient look older, which leads afterward to hypertonus of the muscles and changes in the TMJ. The loss of the VDO was found to be comparatively higher among nonreproducible subjects.

ACKNOWLEDGMENT The authors acknowledge the technical support provided by all faculty members and postgraduate students of the department. The authors also acknowledge the management of the institution for its valuable support.

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AUTHORS

Sushil Kumar Kar, MDS, is a professor in the department of prosthodontics at Saraswati Dental College and Hospital in Lucknow, India. Conflict of Interest Disclosure: None reported.

Arvind Tripathi, MDS, is a professor and head of the department of prosthodontics at Saraswati Dental College and Hospital in Lucknow, India. Conflict of Interest Disclosure: None reported.

Praveen Rai, BDS, is a junior resident in the department of prosthodontics at Saraswati Dental College and Hospital in Lucknow, India. Conflict of Interest Disclosure: None reported.

THE CORRESPONDING AUTHOR, Sushil Kumar Kar, MDS, can be reached at drsushil_kar@yahoo.co.in.