CR and Protrusion

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    Preliminary study of electromyographic characteristics fordistinguishing centric relation and protrusion in edentulouspatients

    Shi Sheng-gen, MD,a Ou-Yong Guan, DDS,b and Zhang Cheng-fancCollege of Stomatology, Fourth Military Medical University, Xian, ChinaThe mean voltages of the masseter and anterior temporal muscles of 30 Chineseedentulous patients were recorded in centric relation and protrusive positions(from 1 mm to 4 mm anterior to centric relation) under two given occlusal forceconditions. It was found that the mean ratio o f individual voltages of the anteriortemporal muscles to the masseter muscles was equal to or greater than 1.2 incentric relation, but the ratio was less than 1.2 in protrusive positions in most ofthe subjects. The results demonstrated that the ratio of voltages of the anteriortemporal muscles to masseter muscles may be a reliable parameter for distinguish-ing centric relation from protrusion in Chinese edentulous patients. (J PROSTHETDENT 1993;69:171-5.)

    R egistration of the horizontal relation of the man-dible to the maxillae is an important procedure n makingcomplete dentures. If an eccentric relation is mistaken forcentric relation, a confused occlusion will follow. Manymethods used o obtain the centric relation of the mandi-ble to the maxillae fail to give the patient the ability tomove the mandible to the needed position.1-4Shpuntoffand Shpuntoe introduced a visual feedback echnique andindicated that this method could allow patients to place hemandible at a specialposition and maintain it during reg-istration by monitoring electromyographic (EMG) pat-terns. Becauseof the lack of a reliable EMG parameter fordifferentiating centric from eccentric relations, this tech-nique is still not in use n the clinic.In this study, the EMG activities of the masseterandanterior temporal muscles n centric relation (CR) andprotrusive positions (PP) under two given occlusal forcesat a constant vertical dimension were recorded and ana-lyzed to obtain the reliable EMG parameter for identifyingthe maxillomandibular relationship anteroposteriorly inedentulous patients.MATERIAL AND METHODS

    This study involved 30 Chineseedentulouspatients, 12womenand18 men n the age angeof 50 to 70 years. Theyhad gooddenture-supporting tissuesand were free of clin-ical signsand symptoms of temporomandibular oint dys-function.

    Presented in part before The First National Prosthodontics Aca-demic Conference, Xian, China.aLecturer, Department of ?rosthodontics.bProfessor, Department of Prosthodontics.CAssociate Professor, Department of Prosthodontics.Copyright @ 1993 by The Editorial Council of THE JOURNAL OF

    PROSTHETIC DENTISTRY.0022-3913/93/$1.00 + .lO. 10/l/38837

    L4wer baseplate

    Transducer

    Fig. 1. Apparatus for controlling occlusal orce and pro-trusive distance.

    Autopolymerizing acrylic resin baseplatesand wax oc-clusion rims were madeon dental stone casts or each pa-tient by useof standardized techniques.After the occlusalvertical dimensionwasestablishedby the methodsof facialmeasurements nd use of the physiologic rest position ofthe mandible, the CR was determined by use of theextraoral graphic tracing method. When the CR was rans-formed to a Dentatus AR0 (AB Dentatus, Stockholm,Sweden)articulator, the wax occlusion ims were removed.Next, a central bearing plate, with five parallel superficialgrooves1 mm apart on its surface, wasattached to the cen-ter of the maxillary baseplatewith autopolymerizing acrylicresin. In addition, a metal beam with an occlusal forcetransducer mounted on it was attached to the mandibularbaseplate across the first molar regions, The tip of thetransducer, like a central bearing point, contacted with agroove of the central bearing plate on the maxillary base-plate. These components (1) maintained the occlusal ver-

    FEBRUARY 1993 171

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    STUDY OF EMG CHARACTERISTICS THE *JOURNAL OF PROSIIiLTI( IJENTISTRY

    Protrusive distance:of the mandible (mm)Fig. 3. Changes of mean voltages of masseter (Mm) and anterior temporal muscles (Tarwith increasing protrusive distance of the mandible. (Occlusal force was at the levei of 4kg.1

    Table II. Maximum, minimum, mean, and standard deviation of individuals ratios of voltages of anteriortemporalis to masseter muscles(Mm) with two levels of occlusal force in five mandibu lar positionsMandibularposition

    Occlusal force 2 kg Occlusal force 4 kgN Min Max ?r SD Min Max z SD t

    CRAnterior toCRlmm2mm3mm

    4mm

    30 1.2 19.0 5.33 4.11 1.3 30.7 4.51 5.46 0.657*

    30 0.5 1.4 0.82 0.17 0.5 1.7 0.86 0.26 -0x1*30 0.2 0.7 0.47 0.11 0.2 0.9 0.49 0.1:3 -0.801*30 0.1 0.4 0.32 0.09 0.1 0.6 0.33 0.12 0.004*30 0.1 0.3 0.16 0.08 0.1 0.4 0.19 0.09 -1.125

    CR, centric relation.*p > 0.05, P-tailed test (NS)

    After 1 hour of Myo-monitor instrument treatment, theaverage myoelec tric amplitude was simultaneously re-corded with the EM-2 instrument from the Mm and the Tawith the two levels of occlusal force in five mandibu lar po-sitions. There was a l-minute rest between each recordingto avoid muscular fatigue.

    A paired t-test was used to compare the EMG activitiesof the muscles and the mean ratio of the individual voltagesof the Ta to the Mm (Ta/Mm ratio) in different conditions.A 2 x 2 chi-square test was chosen to analyze the relation-ship between the mandibu lar position and the magnitudeof the individual Ta/Mm ratio with two levels of occlusalforce. Probability levels less than 5 5%were accepted as sta-tistically significant.RESULTS

    Because there was no significant difference in the EMGactivities of the Ta and Mm observed between the right and

    left sides from the initial statistics, the average amplitudeof the two sides was used as a parameter for further anal-ysis.

    The mean and standard deviation of the voltages for theMm and the Ta with two levels of occlusal force in fivemandibu lar positions are shown in Table I. Figs. 2 and 3express the relationship between the activities of the mus-cles and the amount of protrusive distance (employedspline interpolation). The mean voltage of the Mm waslower in CR and rose with increasing protrusive distance.On the contrary, the Ta activity was higher in CR but be-came lower when the mandible moved forward. All of themuscle activities with 4 kg of occlusal force were signifi-cantly greater than those with 2 kg of occlusal force. Com-parison of the activities between the Mm and the Tashowed that, in CR, the voltage of the Ta was significantlyhigher than that o f the Mm; however, in protrusion (whenthe mandible was 2 mm or more anterior to CR), the vdt-

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    THE JOURNAL OF PROSTHETIC DENTISTRY SHENG-GEN, GUAN, AND CHENG-FAN

    Table III. Distribution of mandibu lar position* by individual Ta/Mm ratio and protrusive distanceProtrusive distance

    IndividualTalMm ratioOmm lmm 2mm 3mm 4m m

    N % N % N % N % N 5%Occlusal orce 2 kg

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    STUDY OF EMG CHARACTERISTICS THE JOURNAL OF PROSTHETIC DENTISTRY

    gradually with increasing protrusive distance in approxi-mately 80% of patients. When the mandibles were 1 mmanterior to CR, the Ta/Mm ratio was between 0.6 and 1.2,while the protrusive distance was 2 mm, the ratio was from0.4 to 0.8, and as the protrusive distance increased to 3 mmor more, the ratio was less than 0.4; and (3) the value wasnot affected by the levels of occlusal forces (Table II).These results demonstrate that the individuals Ta/Mmratio may be capable of distinguishing centric relation fromprotrusion and, if the mandible is in a protruded position,the value may be used for estimating the protrusivedistance as well.

    Studiess-lo have shown that the maximum intercuspa-tion is anterior to the centric relation in most dentitions,and the distance between intercuspation and centric rela-tion is approximately 1 mm. Although the maximumintercuspation is regarded as a more functional position forprosthetic purposes, after all teeth are extracted there is noscientific method to locate it. Many research projects havebeen done on this subject.

    An interesting finding of the present study was the crosspoint of the curves, which expressed the relationshipbetween the change of the muscles activities and theamoun t of protrusive distance. The distance between thecross point and the zero point (CR) was approximately 0.8mm (Figs. 2 and 3), which is approximate to the distancefrom maximum intercuspation to centric relation in natu-ral dentitions.8 In addition, at the cross point the meanvoltages of the Ta and the Mm were equal; the numberswere similar to those observed in natural dentitions,lOcomplete dentures wearers,l and immediate completedentures wearers12 n maximum intercuspation. This effectled to the conclusion that when the Ta/Mm value equals 1or is near 1, the mandible may be in the maximum inter-cuspation. Therefore it can be presumed that if the Ta/Mm

    ratio that equals 1 or is near 1 is used as an index duringthe registration, the original maximum intercuspation inthe edentulous patient may be discovered.

    The EMG characteristics of the masticatory musclesunder freely tapping movement performed by edentulouspatients in centric and eccentric relat-ions are not includedand are considered for further study.REFERENCES

    1.2.3.4.5.6.I.8.9.

    10.

    11.

    12.

    Furnas IL. Centric relation in full denture constructton. J Am Dent As-sot 1938;25:718-25.Myers ML. Centric relation records-historical review. J P~osr~krDENT 1982;47:141-5.Beresin VE, Schiesser FJ. The neutral zone in complete and partialdentures. 2nd ed. CV Mosby Co, 1978:109-16.Heartwell CM, Rahn JA. Syllabus of complete