A Discussion of Some Factors of Relevance to The

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A discussion of some factors of relevance to theocclusion of complete denturesPeter J. Mack, MDS, FDSRCS, DRDRCS*

Key words: Complete dentures, occlusion,prosthodont ics.AbstractThere exists general agreement that in theconstruction of complete dentures the accuratepositioning of the plane of occlusion is essential forcorrect denture function. Yet rarely does aprosthodontist give detailed instructions concerningthe positioning of this plane to the technician whois to set the teeth.

In this paper the three-dimensional location andform of the occlusal plane is discussed.

For both anatomical and mechanical reasons theauthor favours the use of the mandibular rather thanthe maxillary record rim as the clinical determinantof the level of the artificial occlusion. A change inthe method of setting the facebow is recommendedto allow for the difference between the cranialFrankfort plane and the axis-orbital plane of thearticulator.

Arguments are advanced to support the proposalthat artificial teeth should be set to an intercuspallocation forward of centric relation; and that the formof the antero-posterior compensating curve of theartificial dentition should be determined by theclinician before the teeth are set to the registrationrims.Received for publication May 1987. Revised May1988.Accepted August 1988.Introduction

In the intact dentition the natural plane ofocclusion of the teeth may be de scribed and defined,assessed and analysed. Fo r the edentulous patientthe same plane must be accurately located in three

Senior Lectu rer in Res torative Dentistry, The University ofWestern Australia.

dimensions by a clinician who often has littleaccurate information concerning the spatial positionof the lost natural teeth. T h e intention of this paperis to re-appraise some of the clinical techniquesemployed to determine th e ideal form, positioningand transference to an articulator of the occlusalplane of a dental arcade intended to suit a completedentu re patient.Four aspects will be discussed:1. T h e clinical orientation of the occlusal plane.2. Transference of the maxillary cast to an3. De ntu re intercuspation.4. The compensating curve.

articulator.

1. The clinical orientation of the occlusalplane for complete denturesThe registration of the maxillo-mandibular jaw

relationship is commonly achieved by use of tworegistra tion or bite blocks: close-fitting dental baseson which are set rims made up from e ithe r a stablewax or impression compound.4 Initial adjustments to the registration blocks

Ti m e and effort may be reduced if the reg istrationrims approximate the required dimensions beforethe patient attends. Over a period of years it hasbeen found tha t when measured close to the m id-line, an upper sulcus to rim depth of 18 mmapproximates in many to the required finaldimension. O thers have found that in th e anteriorregion a registration rim formed 12 mm above thetissue surface of the edentulous ridge to be anequally acceptable dept h. At th e posterior border,a rim built up 5 0 mm from th e tissue covering thepterygo-palatine notch w ould appear t o be a usefulheight from which to commence adjustments Fig.

Most prosthodontists recommend that thesupport to the maxillary lip be determined a t an1).

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Fig. .-Registration rim: recomm ended vertical dimen sion.

Fig. 2.-Fox occlusal plane guid e.

early stage. Later redefinition may be required, butan approximate contour will aid the sequence ofregistration rim adjustments that are to follow.Gross alterations to upper rim length should alsobe made at this time.

ii) The alignment in the incisal or transverse planeIn the clinical situation it is pe rha ps easiest firstto adjust the incisal plane on the maxillary regis-tration rim . O nce established, the level will indicatethe required alignment of the incisal edges of theup per teeth an d also the visible horizontal junctionbetween the occluding dentures.T h e coronal aspect of the plane is commonly setto lie parallel to an imagined interpupillary line. Fo rmost this will prove to be in harmony with theaesthetics of the face. Because people are rarelybilaterally symmetrical, it is important that theclinician should finalize the transverse registrationfrom a distance. Features such as sloping eyebrowsor spectacles which attract attention at 300 mm are

lost in the more general appraisal of a more distantview. The tendency for both the transverse planeto be raised and the centre line offset to the side

from which the clinician operates must also becounteracted.One useful technique is to employ an occlusalplane guide Fig: 2) which may be held against thesurface of the maxillary registration rim. Anassistant can be asked to confirm th e angulation ofthe plane from a distance of a t least two metres,directly in front of the patient.T h e centre line may be determined a t the sametime and inscribed on to the registration rim.

iii) The alignment in the sagittal planeT h e sagittal antero-posterior) aspect of theartificial occlusion has in the past been the subjectof considerable discussion. It is the single axis mostoften referred to as the occlusal plane OP).Although ther e exists considerable variation in thedescription of this plane, most recently summ arizedby Williams, one generally accepted definition isthat in the natural dentition this plane contactsa) anteriorly the midpo int of lines bisecting theoverbite of the central incisor teeth, andb) posteriorally th e midp oint of lines bisectingthe overbite of the occluding buccal cusps of thefirst molar teeth of each side.

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For complete dentures it has been commonpractice to set this sagittal plane parallel to the ala-tragus line. M a n y t have questioned the accuracyof this extra-oral guide; an d the angular divergenceof the ala-tragus line and occlusal plane has beendetermined by several workers Ta ble 1).Lundquist and Luth er3 in a survey of over 3000dental patients, found th at a forward projection ofthe occlusal plane in the fully dentate would passthrough the lateral commissures of the lips. Thisanterior guide to the positioning of the plane wouldappear to be logical he teeth ope ning at the samelevel as the external entry to the mouth.

Table 1 Deviation between ala-tragalline and the occlusal planeReference Angular deviationin degreesGysi 1929* -7Gysi 1958* -7Nevakari 1958* -6Isaacson 1959* 4Lindblom 1960* -7Posselt, Nev stedt 1961 -6Olsson 1961 -7Chick, Payne 1962 -2Gonzales, Kingery 1968 4Divergence -5.55*In Olsson A, Posselt U.6

Fig. 3.-Occlusal plane aligned to the retromolar pad.

Pound4 stated that a posterior extension of theocclusal plane would pass through the centres ofthe retro-molar pads. In a series of long termanalyses of th e origin of the retro-molar tissues inthe edentate, M cCrorie5 was able to co nfirm thatthe retromolar pads were derived directly from thegingival mucoperiosteum surrounding the crown ofthe last molar tooth. It would therefore appearsensible to use the retro-molar pad as a n indicator

T h e multiple publications on this topic are not individually referencedin this paper.

of posterior tooth height: an intra-oral guide toreference an d intra-oral plane Fig. 3 .T o utilize this posterior intra-oral indicator theocclusal plane must be determined on themandibular record rim. A plane so located tends

to be lower in the molar region than that formedby reference to the extra-oral ala-tragus line. Shouldany divergence occur between the new artificialplane and that of the lost natural line of occlusion,it would seem preferable for denture stability thatthe posterior border should be lower rather thanhigher. On firm occlusal pressures the effect of aplane inclined slightly down at the posterior border

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OCCLUSAL PRESSURE

ANTERIOR \ POSTERIORFig. 4.-Effect of an inclined occlusal plane.

would be to drive the up per prosthesis backwards,the lower forwards. Th is is the preferred option forcomplete dentures Fig. 4 .iv) The vertical location

Positioning the now an gled occlusal plane in thecorrect vertical location seems perhaps a simpleprocedure, but denture problems originating fromn ncorrect occlusal face height are com mon. W ithage, those who retain their natural dentition tend

to exhibit considerably less maxillary and moremandibular tooth during speaking and at rest. Th ischanging display is in part a consequence ofincreased alveolar eruption as documen ted byTa llgre d), and in part due to natural age changeswhich result in a reduced muscle tone in the softtissues of the lips and lower face. The action ofgravity further aids this relentless process ofcovering more upp er tooth and allowing the lowerlip to fall away from its higher and more y outhfulposition against the mandibular teeth. Robert andBrunde record an almost equal increase in lowerand decrease in upp er too th display of about 1 mmfor every 10 years of adult life.It seems possible that an over-low mandibularincisor position set by a clinician who has com pen-sated for a lack of forward lip contour by low eringthe level of the maxillary incisors may be aggravatedby too closely following the oft-advocated routineof setting the mandibular incisal edges to the planeof the mandibular registration rim. This issometimes justified as a routine technique byarguing that minimal incisor overbite and overjet

reduces prosthesis-displacing anterior leverages. Bethat as it may, such an artificial setting is oftenincorrect, for in the anterior part of the mou th the

level of the occlusal plane is set as a reference forthe co rrectly placed artificial maxillary teeth. In thenatural den tition the no rmal vertical overb ite of theincisor teeth permits the mandibular incisal edgesto lie naturally superior and palatal) to the tips ofthe maxillary teeth an d hence superior to thelevel of the correctly adjusted registration rims.Patients requesting a natural appearance tounaesthetic complete dentures often ask for theirlower teeth to be raised and made more visible. Mylowers dont show enough is a common complaint.The length of the lower incisal teeth should beset to harmonize with th e soft tissue aesthetics ofthe lower face, and only be indirectly dependentupon the position of the maxillary teeth and theseparately considered vertical height of themandibular premolar and molar occlusion.Inevitably this means lifting the incisal edges of thelower anterior teeth above the level of themandibular rim . T h e angle of the incisal guidancecan therefore only be established a t a late stage inthe registration procedure.In summary, for ideal denture aesthetics theclinician shou ld initially trim the maxillary recordblock anteriorally for aesthetics against the upp erlip, refining any gross adjustments carried out a tthe first stage of registration block modification.The upper rim should be correctly extendedforwards, and then set to lie just below the levelof the resting lip in the young, perhaps slightlyabove that level in the elderly. The mandibularrecord block should be trimmed posteriorally tomatch the intra-oral landmarks. Th ese ad justments

should be confirmed by other methods, includingphonetic tests. T he molar region of the upp er blockshould finally be trimm ed to give light con tact onAustralian Dental Journal 1989;34:2. 25


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to the lower rim at the now confirmed correctocclusal face height.T hu s the height of the posterior maxillary molarteeth should be determined secondarily to that ofthe occlusal plane and after that o f the occlusal faceheight. T h e vertical overlap overbite) of the

anterior teeth is determined independen tly of either.2. Transference of the maxillary cast tothe articulator

Transference of this clinically determined planeof occlusion to an articulator may be accom plishedrapidly and accurately by the use of a dentalfacebow. S om et consider that the use o f an arbitary-axis facebow introduces inconsistent variationswhich detract from its value as an instrument forcast transference, whilst others are of the opinionthat facebow transfe rs are acceptable if the referencepoints are determ ined w ith accuracy. All agree thatthe kinematic bow is the instrum ent of choice forhinge-axis location.

Many articulators use the Frankfort planePorion-Infra-orbitale) as a reference. Thoughconvenient, this anatom ical plane differs by approx-imately 5.5 degrees from that of the axis-orbitalplane determined by a kinematic facebow TableIf an articulator is constructed to theFrankfort plane, a correcting adjustment to theangulation o f the facebow registration is required.Th is should be achieved by the addition of a spacerto the orbital plane guide of the up per arm of thearticulator as has been proposed by Gonzales andKingery.83 Denture intercuspation at centricrelation or centric occlusion

There is general agreement that in the naturaldentition the path traced by the m andible on closingfrom the postural to th e intercuspal position is theresult of reflex activity in th e attached muscu lature.Centric occlusion is the location to which theclosing mandible expects to return. As such itwould also appear to be the natural location for theedentulous intercuspal position.

There is, however, also general agreement thatin determining jaw relations in the edentuloussubject there is only one jaw po sition that is consis-tently reproducible and that is to be fou nd on theretruded arc of closure. Reproducibility of closingtranslation is the goal of many advocated clinicaltechniques of jaw registration.If teeth on com plete dentures are m ade to occludeat centric relation they will do so each time themand ible closes on the reproducible) retruded arc.

It is this degree of certainty, reproducibility andconvenience12 that has resulted in th e position ofcentric relation being advocated as the ideal inter-cuspal location for com plete de ntures even thoug hthere is now little doubt that such mandibularpositioning is an entirely artificial concept.13T o develop optimal mechanical a nd biologicalefficiency in a n artificial dentition for a completelyedentulous patient, it would seem to the au thor thatcentric occlusion and centric relation should notcoincide, but be separated in both the horizontaland vertical plane. The arguments advanced tosupport this proposal are:a) In the natural dentition of man the intercuspaland ligamentous occlusal positions rarely coincide.It is usually possible to record a definite jaw

movem ent or occlusal slide between theb) Retrusive facets have a protective fu nction inaccidental or forced ligam entous closure.16Due tothe angulation of retrusive inclines, any potentialmovement of the jaw resultant from a suddenrearward displacing force will be dissipated throughthe interlocked cusps of the teeth to th e suppo rtingbone, and will not be transferred directly to thecondyle heads.c) All comp lete dent ure s exhibit slight vertical

a nd ho r i z on t a l move me n t s due t o t i s s uecompression. Even an extremely well-fittingcomplete den ture is able to move in an an terior orposterior direction a minimum of 0.5 mm beforetissue compression resists the movement. If theintercuspal position is registered at a retruded jawlocation, posteriorally directed pressures on to themandible be they external or self-generated) w illnot be resisted by alveolar bone via rigid naturalcuspal contacts, but by compression of the condyleson to the posterior tissues of the glenoid fossae.d) If the occlusion of complete dentures is setso that full intercuspation can only occur on aretruded arc of closure, wear of the dentures a n d o rslight forward posturing of the m andible must resultin initial cuspal contacts being established betweenprotrus ive facets . The resul tant horizontalcomponent h as the potential to unseat the denturesby movement of the uppe r den ture in an anteriorand the lower in a posterior direction.e) Should the jaw be deliberately retruded tocentric relation, co mplete dentures set to give full

intercuspation at a slightly forward centric occlusionwill provide initial intermaxillary contacts onretrusive facets. The horizontal component of theforces developed between these facets will poten-126 Australian Dental Journal 1989;34:2.


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-TANGENTB CHOR

Fig. 5.--Tangential and chorded occlusal planes.

tially m ove the up per denture backwards, the lowerdenture forw ards. Such mo vements would enhancethe correct seating of the den tures upon the alveolarridges as the teeth slide into full intercuspation.

f ) T he reduced area of tooth contact that occurswhen the mandible adopts a ligamentous positionpermits greater intercuspal point loadings todevelop, hence possibly facilitating the commi-nution of harder foods.g) During mastication a shearing com minutionof food takes place between working side cusp facetsduring the closing phase of the chewing cycle, inwhich action laterotrusive facets may be utilized asshearing surfaces.

T o reproduce this p articular action in an artificialdentition, the mandible m ust be capable of a slightrearward movem ent on the working side, from thein te rcuspa l pos i t ion to a more r e t rude dligamentous position. Such mov ement is onlypossible if there exists a spatial separation of centricocclusion and centric relation.T he ore t i c a l d i s c us s i ons c onc e rn i ng j a wrelationships can only be of clinical utility if valuescan be assigned to the concepts discussed.Th e length of the occlusal slide in ad ults with fu llnatu ra l den tition is s ta te d by P ~ s s e l t ~ . ~ ~o be1.25 k 1 mm . Jacksonlg suggested tha t themovem ent path traced between the intercuspal anda retruded occlusal position generated by a patientsown musculature is often greater than had previ-ously been considered normal. He recordedhorizontal retrusive m ovements averaging 2.5 mmrange 0.5 to 5 0 mm), with a simultaneous vertical

shift averaging 3 mm; and lateral shifts of 2 mm.The average inclination of retrusive facets inrespect to the o cclusal plane has been determinedas 30.6 with a standard deviation of 12.6.20

T h e result from com bining Posselts values withthose of ang ulation determin ed by A rstad20 givesa value for the horizontal t rans la t ion of1.08 0.9 mm , and for the vertical displacement of0 64 r 0.5 mm.

T h e au thor suggests that these figures might formthe basis for convenient and clinically acceptablevalues: that th e translation betwe en centric relationand centric occlusion in the artificial occlusionshould be represented by a mandibular movementof 0.5 mm in bo th the vertical and horizontal plane.It is therefore proposed that for all patients theintercuspal position of the ideal artificial occlusionshould be established 0.5 mm anterior to and at

0.5 mm reduced occlusal face height from theposition registered as centric relation.4. The compensating curve

In th e fully dentate, th e occlusal surfaces of thenatural teeth are aligned to a helicoid curve whichhas been well described by many authors, possiblyin the greatest detail by Brown and co-workers.21This natural three dimensional curve has histori-cally been dignified by the names of those w ho firstdescribed each separate part of the overall form:Spee, Monson, Wilson, and so on. In theedentulous patient these natural forms do not ex ist.The compensating curves set to the teeth of acomplete denture prosthesis serve but one hn cti on ,that being to p ermit by artificially balancing thearticulation a reasonable degree of denture stabilityduring excursive chewing actions.

Tw o techniques have been proposed to achievein the complete denture patient the accurateformation of these essential but artificial antero-posterior and lateral compensating curves. Thecurves may be derived from the clinically adjustedflat occlusal registration rims plane) Fig. 5) so that:a) T he flat m andibular rim is considered to forma tangent to the required curvature, orb) T he flat mandibular rim may be consideredto form a chord of the required curvature.

These techniques will be considered to turn.a) The tangential concept

A com pensating curve set to contact the clinicallyorientated man dibular occlusal rim at a tangent maybe orientated upon a single contact on each side ofthe arch.T he cusp tips of the second premolar teeth aremost comm only employed as this point of contact.Posterior to these contacting teeth the compen-sating curve is conventionally set to rise above the

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Fig. 6.-E ffect of a posteriorally raised occlusal plane.

level of the trimmed record block. The degree ofrise is ideally arranged to establish a balancedantero-posterior articulation in harmony with theclinically established incisal and condylar gu idanceangles, but all too often it is set by a dentaltechnician with no further considera tion than to suitthe exigencies of the occlusion during the wax-upof the denture. Because of the mechanics of anytangent-type articulation it is not possible todetermine the level of the posterior denture teethuntil the construction of the trial denture is wellestablished.Shou ld the ala-tragus line be used as the referenceto set the occlusal plane as discussed at l.iz , it ispossible for the posterior teeth of the artificialocclusion to rise well above the correct na tural level.Th is effect of raised maxillary m olar teeth will beparticularly noticeable for those patients whodemonstrate a steep condylar guidance angle.The consequences are that the upper denturebecomes too thin for the simple placement ofnormal sized molar teeth, th e lower dentur e comesposteriorally too high for p atient com fort, and theresultant inclined p lane effect tends to u nseat bothprostheses Fig. 6 .

b) The chorded conceptIf, however, the occlusal rim is taken to be a chordof the eventual occlusal curve, a four-point con tactmay be clinically established by the clinician at theregistration stage, and transferred as fixed pointsfor the attention of the technician. The points tobe indicated on the registration rims are as previ-

ously recommended at l.iv) bilaterally thecommissures of the lips and the retromolar pads.The level and inclination of the occlusal plane is

thus fixed by the clinician and need no t thereafterbe changed.T h e radius of the compensating curve may nowbe planned by the technician to lie between fixedpoints and to harmonize with the clinically deter-mined incisal and condylar guidance angles.The practical effect is to lower the midpoint ofthe com pensating curve that is, in the premolar andfirst molar region) below the plane of the regis-tration rims. The resultant form simulates closely

the curvature of the natural den tition in which theplane of occlusion passes from incisal tip to themolar teeth, bu t does not necessarily contact lowerlevel intermediate cusps.Further advantages are apparent in this chordedarrangement. T h e occlusal plane curves to followmore closely the contour of the edentulousmandibular ridge, and therefore loadings tend tobe directed normally to th e underlying supportingtissues and bone. T he most dependent point of thecurve is at the site of denture balance in thepremolarlfirst molar region where the greatestocclusal forces are commonly concentrated.T h e advantages of a chorded den tal arrangementmay be summarized as those of accuracy in deter-mining the position of the teeth and in establishingthe plane of occlusion.The technician has both anterior and posteriorguides for setting procedures: teeth may be set morequickly and correctly in the intervening inter-alveolar space.

ConclusionW hen taking the bite for an ede ntulous patientit is perhaps salutary for clinicians to consider this28 Australian Dental Journal 1989;34:2.


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time-honoured yet often ill-understood term as anacronym for the clinical procedures undertaken. Forthe edentulous patient the clinician registers abilateral inter-alveolar transferable evaluation.Possibly the greatest emphasis should be placed onthe last word, for in the eden tulous it is indeed theclinicians evaluation of a patients occlusal needsand jaw relations that determines the ultimatesuccess or failure of the complete dentureprosthesis.References

1. Mack0 Full dentures. Dental Practitioner Handbook No.2. Williams DR. Occlusal plane orientation in complete3. Lundquist DO, Luther WW. Occlusal plane determination.4. Pound E. Conditioning of denture patients. JAm Dent Assoc5. McCrorie JW. Origin of the pear-shaped pad. Dent Pract

Dent Rec 1963;13:517-9.6. Tallgren A. Changes in adult face height due to ageing, wear

and loss of teeth and prosthetic treatment. A Roentgencephalometric study mainly on Finnish women. ActaOdontol Scand 1957;15:Suppl 24:l-122.

7. Robert G, BrundeGC The kinetics of anterior tooth display.J Prosthet Dent 1978;39:502-4.8. Gonzalez JB, Kingery RH. Evaluation of planes of reference

for orientating maxillary casts on articulators. J Am Dent9. Olsson A, Posselt U. Relationship of various skull reference

13. Revised reprint. Bristol: John Wright, 1978.dentures. Dent 1982;10:311-6.

J Prosthet Dent 1970;23:489-98.1962;64:46 1-8.

ASWC 968;76:329-37.lines. Prosthet Dent 1961;11:1045-9.

10. Chick AO, Payne AGL. A note on the occlusal plane andthe inclination of anterior teeth. Br Dent J 1962;112:159-60.

11. Winstanley RB. The hinge-axis: A review of the literature.J Oral Rehabil 1985;12:135-59.12. Lucia VO. Gnathological concept of articulation. Dent Clin

North Am 1962;6:183-97.13. Moss ML. A functional cranial analysis of centric relation.

Dent Clin North Am 1975;19:431-42.14. Posselt U. Studies in the mobility of the human. ActaOdontol Scand 1952;lO:Suppl 10:19-160.

15. Gibbs CH, Lundeen HC. Advances in occlusion. Section1. Postgraduate Dental Handbook Series No. 14.Massachusetts: John Wright, 1982:2-32.

16. Anderson JN, Storer R. Immediate and replacementdentures. Oxford: Blackwell Scientific, 1973:295-319.17. Devlin H, Wastell DG. T he mechanical advantage of biting

with the posterior teeth. J Oral Rehabil 1986;13:607-10.18. Posselt U. Physiology of occlusion and rehabilitation. oxfordBlackwell Scientific, 1969:Ch5: 107-73.19. Jankelson B. Neuromuscular aspects of occlusion: occlusal

articulation. Dent Clin North Am 1979;23:57-168.20. Arstad T. The capsular ligaments of the temporomandibularjoint and retrusion facets of the dentition in relationship tomandibular movement. Oslo: Akademisk Forlag, 1954:l-95.

21. Brown WAB, Whittaker DK, Fenwick J, Jones DS.Quantitative evidence for the helicoid relationship betweenthe maxillary and mandibular occlusal surfaces. J OralRehabil 1977;4:91-6.

Address for correspondenceheprints:Division of Restorative Dentistry,The University of Western Australia,Dental School,179 Wellington Street,Perth, Western Australia, 6000.

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A Discussion of Some Factors of Relevance to The

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