How a Discrepancy Between Centric Relation and Maximum Intercuspation Alters Cephalometric and Condylar Measurements

In natural dentition, the mandible can assume two different positions that have been a source of controversy in orthodontic planning and treatment: the centric relation (CR), defined as the maxillary-mandibular relationship in which the condyles articulate with the thinnest avascular portion of their respective discs, with the condyle being in the anterior-superior position against the slopes of the articular eminence¹; and the maximum intercuspation (MIP), which refers to the maximum occlusal relationship independent of the condylar position.^1,2

A discrepancy between CR and MIP is created when any occlusal contact changes the mandibular closing arch in CR, altering the condyle-fossa relationship to permit the maxillomandibular relationship in MIP.^3,4 Most patients have this closing reflex, which is determined by a proprioceptive mechanism.⁵

In orthodontics, the MIP is the most common mandibular position used in cephalometrics, which along with dental casts and photographs of the patient stands unquestionably as the proper criteria for diagnosis. Also, for a proper treatment plan, the condylar position must be evaluated and a determination of CR is a prerequisite for analyses of occlusion and jaw relationship. When a CR-MIP discrepancy exists, there might be a difference between the cephalometric results carried out in these two occlusal relationships, which could interfere in the evaluation of the maxillomandibular discrepancy and possibly lead to a misdiagnosis.

The aim of this study was to determine, with the use of cone-beam computed tomography (CBCT), if there is a significant difference between the cephalometric and the condylar measurements obtained in MIP and those obtained in CR, which would interfere in orthodontic diagnosis and planning.

This retrospective study was independently reviewed and approved by the University Ethics Committee (registered under the number CAAE-0417.0.213.000-12) of the School of Dentistry, Pontifical Catholic University of Minas Gerais in Brazil. Informed consent was obtained from all individuals, and subjects' rights were protected at all times. Based on the standard deviation of 2.5 degrees reported by Lim et al for the primary outcome of current research (ie, ANB angle),⁶ an alpha significance level of 0.05, and a power of 0.80 to detect differences between groups greater than 3 degrees, a sample size of 30 patients was adopted.

The sample consisted of 30 patients randomly selected from the records of the Department of Prosthodontics (15 men and 15 women, mean age 14.2 years and standard deviation 2.7 years, CR ≠ MIP between 1 mm to 2 mm) using opaque envelopes divided by sex. All patients had CBCT scans in MIP and in CR as part of a past clinical evaluation protocol in the Department. Exclusion criteria were individuals with preliminary orthodontic treatment, endocrine-metabolic diseases, hyperthyroidism or hypothyroidism, autoimmune diseases, juvenile rheumatoid arthritis, and/or signs and/or symptoms of temporomandibular disorders (TMD), such as pain, clicking, noises, and articulatory shifts during the mandibular movements of opening and closing.

The CBCT scans, one in MIP (T1) and the other in CR (T2), were obtained from a CBCT scanner (i-CAT^™ Next Generation, Imaging Sciences International, kavo.com) using a typical setting for orthodontic records: 23 cm x 17 cm field of view (height/width), 0.3 mm voxel size (scan time, 17.8 seconds), 37.10 mAs, and 120 kVp. All patients were instructed to bite into MIP during the T1 scan. For the T2 scan, the patients wore a Lucia jig to achieve CR.⁵ All jigs were made by the same investigator (RPF) using pattern resin (DuraLay, Reliance Dental Manufacturing LLC, reliancedental.net).

Lateral cephalograms (Figure 1) were obtained from the CBCT data using the orthogonal projection option, and the following cephalometric measurements were made: ANB: A point-Nasion-B point angle; SNB: Sella-Nasion-B point angle; SN.OCC: Sella-Nasion to occlusal plane angle; SN.GoGn: Sella-Nasion to mandibular plane angle; FMA: Frankfort horizontal plane to Gonion-Menton angle; Y Axis: Sella-Gnathion to Frankfort horizontal plane angle.

Sagittal, axial, and vertical condylar variations between the CR and MIP positions were analyzed in a tridimensional (3D) software (Dolphin 3D Imaging Software 11.5 Premium, Dolphin Imaging & Management Solutions, dolphinimaging.com) by the same investigator (RPF). Scans at T1 and T2 were oriented 3-dimensionally: coronal (by the median sagittal plane), axial (by the two frontozygomatic sutures), and sagittal (in the right side view, by the Frankfort plane).⁷ The sagittal (red), coronal (green), and axial (blue) lines were standardized and activated through the equal slice-volume layout. The following lines were used as a reference for condylar measurements: in the sagittal section, the blue and green lines; in the coronal section, the red and blue lines; and in the axial section, the red and green lines (Figure 2).

For sagittal and vertical condylar measurements, lines were drawn as follows: red tangential lines to the side edges of the right and left orbits, a green tangential line to the nasion point, and a blue line passing by the frontozygomatic sutures. The distance from the highest point of each condyle to the blue line (SRV and SLV) and from the most anterior point of each condyle to the green line (SRH and SLH) were obtained by the option "digitize measure - line" and provided the vertical and sagittal condylar measurements (Figure 2).

For coronal and vertical condylar measurements, lines were drawn as follows: a green tangential line to the vertebral column, a red line positioned on the median line, and a blue line passing by the frontozygomatic sutures. The distance from the highest point of each condyle to the blue line (CRV and CLV) and from the most medial point of each condyle to the red line (CRH and CLH) were obtained by the option "digitize measure - line" and provided the vertical and coronal condylar measurements (Figure 3).

For axial and vertical condylar measurements, a blue tangential line to the lower edge of the articular eminence, a red median line, and a green tangential line to the nasion point were drawn. The distance of the most anterior point of each condyle to the green line (ARV and ALV) and of the most medial point of each condyle to the red line (ARH and ALH) were obtained by the option "digitize measure - line" and provided the vertical and axial condylar measurements (Figure 4).

Descriptive statistics, including means and standard deviations, were calculated for all evaluated dimensions. The Kolmogorov-Smirnov test showed a normal distribution of the data. The paired t-test was used to evaluate differences between T1 and T2 for each dimension. The level of significance was set at 5%. The analyses were performed using analysis and graphing software (GraphPad Prism Software, GraphPad Software, graphpad.com).

The same examiner made all measurements. To test intra-examiner reproducibility, the same examiner remeasured 15 random images for each evaluated dimension a week later. The results were then compared with the original measurement. The intra-examiner reliability values were determined by the intraclass correlation coefficient (ICC), which varied between 0.88 and 0.99, indicating high reproducibility. The analyses were performed using MedCalc Statistical Software (MedCalc Software BVBA, medcalc.org).

Sagittal effects in the mandible-There were statistically significant differences between T1 and T2 (P< .05) in the ANB (-0.96°) and SNB (+1.21°) angles (Table 1).

Vertical effects in the mandible-The results showed statistically significant differences (P< .05) in the SN.OCC (-0.58°), SN.GoGn (-1.38°), FMA (-0.88°), and Y Axis (-1.45°) angles, which suggests inferior and posterior rotation of the mandible (Table 1).

Linear sagittal effects of the condyles-SRH and SLH presented mean increases of 0.45 mm and 0.29 mm, respectively, when compared T1 with T2. The SRV and SLV presented a reduction of 0.48 mm and 0.36 mm, respectively (Table 2). None of these differences was statistically significant (P> .05).

Linear coronal effects of the condyles-When comparing T1 with T2, the CRV, CLH, and CLV measurements presented a mean decrease of 0.45 mm, 0.12 mm, and 0.06 mm, respectively. The CRH presented an increase of 0.55 mm (Table 2). None of these differences was statistically significant (P> .05).

Linear axial effects of the condyles-ARH, ARV, and ALV presented mean increases of 0.54 mm, 0.41 mm, and 0.48 mm, respectively. The only statistically significant difference (P< .05) was noted in the ARH measurement. The ALH presented a decrease of 0.15 mm, also not statistically significant (Table 2).

Orthodontic treatment seeks to correct patient skeletal and dental malocclusions. Orthodontic treatment should accurately diagnose skeletal and dental disharmonies to define the treatment plan. Orthodontists should be sure that their orthodontic records represent the true anatomical position of all the parts of the stomatognathic system and the relationships between them.

The first objective of this study was to evaluate if cephalometric measurements performed in CR and MIP were significantly different, and if so, to analyze if those differences could impact orthodontic diagnosis and planning. The second goal was to verify if there was any 3D modification in the condyle-fossa relationship. Only a few studies in the literature have evaluated the interrelationship between CR, MIP, and cephalometry.^6-9 Their results tried to demonstrate that cephalometric analysis should be carried out with patients in CR, suggesting that the centrically related cephalometric analysis offers greater accuracy of information. However, there is much criticism in their findings due to the lack of experimental evidence and the assumed older CR position of posterior and superior condyle-glenoid fossa. Moreover, one of these studies was descriptive rather than experimental or observational, and there was no temporomandibular joint (TMJ) imaging to confirm subjects' condylar positions, nor was there a reference to validate their method of CR and MIP registrations.⁹ Furthermore, they used 2-dimensional (2D) x-rays, which are not accurate enough to define a 3D relationship in a 2D medium.

The present study used a Lucia jig, a worldwide-accepted method to obtain the CR position, and CBCT imaging to evaluate the condyle-fossa relationship. Moreover, CBCT allows accuracy in the visualization of craniofacial structures, such as the TMJ, with minimal distortion at low cost and lower radiation dosage when compared to conventional tomography.¹⁰

Previous studies have shown that patients with a large CR-MIP discrepancy had specific dentofacial characteristics, such as backward positioning and rotation of the ramus and mandible.⁶ In the present study, patients with a small CR-MIP discrepancy (<2 mm in the sagittal plane) had a larger ANB and overjet and a steeper mandibular plane in the CR position than in the MIP position. Although these changes might not be clinically significant, this implies that even in patients with a small CR-MIP discrepancy, the skeletal morphology in the MIP position might change to a more retruded mandible and a more hyperdivergent skeletal pattern in the CR position. As a result, a class II malocclusion in patients with a larger CR-MIP discrepancy may be exacerbated in the CR position, which may in turn increase treatment difficulty.

A CR-MIP discrepancy of less than 1 mm in the vertical or horizontal plane is epidemiologically normal and apparently not a risk factor for TMD.¹¹ However, a more than 2 mm slide from CR to MIP in the vertical or horizontal plane is a critical factor that needs to be considered when assessing relative risk factors for TMD.⁶ The present study evaluated the condyle-fossa relationship, and a significant CR-MIP discrepancy was found in the axial plane between the condylar measurements (ARH P < .05). Further studies are necessary to evaluate the condyle-glenoid fossa relationship in patients with larger CR-MIP discrepancies.

Nevertheless, this study has some limitations. When the cephalogram is taken with the Lucia jig with the condyles seated superiorly in the fossa, a premature contact frequently occurs between maxillary and mandibular molars or premolars, leading the mandible to a clockwise rotation. This backward rotation increases the lower anterior facial height and places the pogonion in a more retrognathic position, as can be observed in the CR records where the vertical cephalometric measurements increased, which is in accordance with a previous study.⁶

All told, the present study suggests that the cephalometric analysis and the evaluations of the condyle-fossa relationship should continue to be carried out in MIP when the anteroposterior CR-MIP discrepancy is less than 2 mm, because it is an easy procedure to perform with a reasonable cost-benefit ratio for the patient. Further CBCT studies are necessary to evaluate if CR-CO discrepancies greater than 2 mm would impact the cephalometric measurements and the condyle-glenoid fossa relationship, as the discrepancy between the CR and MIP positions is physiologically acceptable when it is 1 mm to 2 mm in the anteroposterior direction and less than 0.5 mm transversally and vertically at the condylar level.¹²

Moreover, the clinical applicability of the MIP and CR mandibular positions is subject to different opinions because of contradictory results in the literature.^7,13 Many authors support the use of CR in restorative occlusal therapies and orthodontic planning,^14-16 while others suggest an individual analysis for each clinical situation to determine the ideal mandibular position.^17,18 Therefore, the present authors suggest that orthodontic treatment should be planned in CR when the patient presents extensive prosthetic restorations, occlusal adjustment, unstable MIP with the presence of signs and/or symptoms of disharmony in the stomatognathic system, and large CR-MIP discrepancies.

Based on this CBCT study, several conclusions can be drawn. First, patients with a small CR-MIP discrepancy (<2 mm) present significant differences between the cephalometric analyses carried out in CR and MIP, although they might be of low clinical significance. Second, the cephalometric analysis and the evaluations of the condyle-fossa relationship should continue to be carried out in MIP when the anteroposterior CR-MIP discrepancy is less than 2 mm, because it is an easy procedure to perform with a reasonable cost-benefit ratio for the patient. Lastly, a class II malocclusion in patients with a large CR-MIP discrepancy may be exacerbated in the CR position, which in turn may increase treatment difficulty. In these cases, not only must cephalometrics be registered in CR, but dental casts and photographs of the patient are needed to permit orthodontists to plan an appropriate treatment.

This study was partly financed by Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES) - Finance Code 001.

Raquel Parreiras Ferreira, DDS, MSD
Prosthetic Resident, Department of Restorative Dentistry, School of Dentistry, Pontifical Catholic University of Minas Gerais, Belo Horizonte, Minas Gerais, Brazil

Paulo Isaias Seraidarian, DDS, MSD, PhD
Associate Professor, Department of Restorative Dentistry, School of Dentistry, Pontifical Catholic University of Minas Gerais, Belo Horizonte, Minas Gerais, Brazil

Giordani Santos Silveira, DDS, MSD
PhD Student, Department of Orthodontics, School of Dentistry, Pontifical Catholic University of Minas Gerais, Belo Horizonte, Minas Gerais, Brazil

Martinho Campolina Rebello Horta, DDS, MSD, PhD
Associate Professor, Department of Oral Pathology, School of Dentistry, Pontifical Catholic University of Minas Gerais, Belo Horizonte, Minas Gerais, Brazil

Juan Martin Palomo, DDS, MSD
Program Director of Orthodontics and Craniofacial Imaging Center, School of Dentistry, Case Western Reserve University, Cleveland, Ohio

Ildeu Andrade Jr., DDS, MSD, PhD
Associate Professor, Department of Orthodontics, School of Dentistry, Pontifical Catholic University of Minas Gerais, Belo Horizonte, Minas Gerais, Brazil