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Twin-tables tecthnique for occlusal rehabilitation:I-Mechanism of anterior guidancePart

Sumiya Hobo, DDS, MSD, PhD*International Der;tal Academy, Tokyo, Japan, and University of California. School of Dentistrv.Los Angeles, Cali:.

Anterior guidance and the condylar path have been considered independent factors.In a recent study, it was revealed that the anterior guidance influenced theworking co:ndylar path and even changed when the lateral incisal path deviatedfrom the optimal orbit. This supports the hypothesis that anterior guidance andcondylar path are dependent factors. When setting anterior guidance, it is recom-mended to set the working condyle so that it moves straigh t outward along thetransverse horizontal axis. The angle of hinge rotation created by the angulardifference between anterior guidance and condylar path assists the posteriordisclusion, but is not solely responsible. The anatomy of the cusps is created byestablishing the appropriate form of the posterior cusps aligned to the condylarpath so that it also contributes to posterior disclusion. Posterior disclusion iscrucial in controlling harmful lateral forces. The molars must disclude slightlymore than the deviation in the condylar path to avoid occlusal interferences. (JPROSTHET DENT 1991;66:299-303.)

M .rtdlbular movement. has been studied kinemat-ically through advanced research technology.1-3 Conse-quently, anterior iguidance, which was previously a vagueocclusal concept, has heen ana.yzed and it is now possibleto compute anterior guidance f rom the condylar path.4 Acomputer system developed for this purpose includes theC,yberhoby computer pantograph with the capability tomeasure the condylar path and produce digital and analogdata, the Cyberhoby fully adjustable articulator, and theAnteroputer device, which computes three-dimensionaladjustments for the incisal table.5 However, this system isexpensive, complicated, and unsuitable for daily practicedespite its merit for research. A disparity then existsbetween advances in research and daily clinical dentalpractice.

Part I of this ar title reviews the mechanism of anteriorguidance to provide a foundation for understanding thetwin-tables technique. This technique is an uncomplicatedpractical method to register anterior guidance from thecondylar path. Part II presents the procedures of thistechnique for daily clinical practice.IMPORTANCE OF ANTERIOR GUIDANCE

The orbit of the incisal point during protrusive move-ment is guided by the lingual surfaces o f the maxillary in-cisors and is termed incisal guidance.j The lateral move-ments are guided hy the lingual surfaces of the maxillary--Presented at .:he Amerir.an Acadc,my of Fixed Prosthodontics

meeting, Chicago, 111.aDirector. International Dental Academy; Visiting Professor, Uni-

llersity of California, Sc.hool of Dentistry.10/l/29420

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canines, referred to as canine guidance. The term anteriorguidance has replaced the two terms. Although anteriorguidance only ranges from 2 to 6 mm, it greatly influencesocclusion, In healthy patients, anterior guidance is approx-imately 5 degrees steeper than the cnndylar path in thesagittal plane.8 Therefore, when a patient protrudes themandible, the anterior teeth guide it downward, creatingspace between the posterior teeth refe rred to as posteriordisclusion. The same phenomenon occurs during lateralmovement because the lingual inclination of the maxillarycanine is steeper than the condylar path.

The mandible can be compared to a tripod configurationfor discussion purposes.g The posterior legs are representedby right and left condyles whose shape helps to generate thecondylar path, and the anterior leg is represented by theincisal point that influences anterior guidance. The three-dimensional position of the mandible is determined bythese three elements so that if the condylar path alone ismeasured without consideration of anterior guidance, man-dibular movement is not accurately reproduced. Becausecondylar path and anterior guidance are elements of oneanatomic unit, the mandible, they borh affect total man-dibular movement.ANTERIOR GUIDANCE AS ANINDEPENDENT FACTOR

Early gnathologic concepts focused primarily on thecondylar path. The concepts were based on the theory thatthe condylar path does not change during adulthood andthat the determination of anterior guidance is at thediscretion of the dentist. McCollum and StuartlO believedthat anterior guidance was independent ot the condylarpath. In clinical practice, the condylar path is measured

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Fig. 1. Effect of anterior guidance on working condylarpath. Neutral line represents imaginary incisal path (dot-ted line) when working condyle moves along transversehorizontal axis. Actual incisal path is steeper than neutralline. Working condyle then moves below transverse hori-zontal axis or detrusion (A). If actual incisal path is flatterthan neutral line, working condyle moves above axis orsurtrusion (B) .

with a mechanical pantograph using clutches without theinfluence of anterior guidance. When the condylar path isadjusted on the articulator and the diagnostic casts aremounted, both condylar path and anterior guidance arecombined to simulate total mandibular movement. Cur-rent prosthodontic procedures follow this premise.

There are sparse studies that support this concept, andDawsons statementl is particularly well-known. He statedthat the condylar path was not a determination of anteriorguidance, and that it does not matter whether the ante-rior path is flat or curved, concave or convex or parabolic,the rotating condylar sliding down the unchanged condy-lar path permits the lower anterior teeth to follow anynumber of path variations without interference. Heconcluded that anterior guidance could be freely changedby the dentist.

To verify whether condylar guidance and anterior guid-ance were independent factors, the lateral condylar pathsin 10 patients were measured with an electronic mandibu-lar recording device.12 The measurements were recordedunder two test conditions: tooth-contact, or without clutchcondition, and nontooth contact, or with clutch condition.The results demonstrated that the working-side condylarpath changed remarkably while the nonworking-sidecondylar path did not reveal noticeable changes. Theworking condyle, under the nontooth-contact condition orwith a clutch, exhibited small sagittal displacements andshowed a tendency to move straight laterally along thetransverse horizontal axis. Conversely, under the tooth-contact condition, the working condyle moved laterally anddeviated sagittally in various directions. From this move-ment, it was observed that the condylar path was affectedby anterior guidance and condylar guidance and anteriorguidance were dependent, not independent, factors.

Fig. 2. During protrusive movements, condyle rotatesalong horizontal axis if anterior guidance (/?) s steeper thancondylar path ((Y).Angle of hinge rotation compensates forthis angular difference.

Takayama and Hobo 13-15 erived kinematic formulae tocalculate anterior guidance from the condylar path. Ante-rior guidance computed from these formulae confirmed astatistical correlation to the data of anterior guidance onthe same patients at p < 0.01 level of significance. With theidentical formulae, an imaginary incisal orbit was com-puted for each patient under the assumption that theworking condyle translates straight laterally along thetransverse horizontal axis. Then this orbit, or the neutralline, and the actual lateral incisal paths under tooth-con-tact conditions were compared. The results demonstratedthat the working condylar path deviated superiorly whenthe actual lateral incisal path was above the neutral line,and the working condylar path deviated inferiorly when theactual lateral incisal path was below the neutral line (Fig.1). A similar tendency was noted in the anteroposterior di-rections. Therefore there was a correlation between devia-tion of the lateral incisal path from the neutral line andsagittal deviation of the working condylar path at p < 0.01level of significance.12

Sagittal deviation of the working condylar path-eithersurtrusion, detrusion, retrusion, or protrusion-is evidentwhen anterior guidance and the condylar path are in dis-cord. This correlation also indicated that anterior guidanceand condylar guidance were dependent factors.

According to the study of 50 test patients, the meanworking condylar path is a straight lateral movement alongthe transverse horizontal axis.2*3 If this movement is phys-iologic, then the incisal path deviation from the neutral linemay imply that the working condyle compensates for dis-crepancies by deviating within the sagittal plane. It is thenrecommended to adjust the anterior guidance so that theworking condyle moves straight outward along the trans-verse horizontal axis during lateral movement.ANGLE OF HINGE ROTATION

Posterior disclusion occurs when anterior guidance issteeper than the condylar path. Early gnathologic concepts300 SEPTEMBER 1991 VOLUME 66 NUMBER 3

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TWIN-TABLES TECHKIQI E: PAHT I

Table I. Degree of disclusionAngle cusp

Me;lSUrlXl of hinge shape\ .due rotation factor (mm)---Protrusiw .l 0.2 0.9Korking J.5 0.5 0Nonworkir~p I.0 0.5 0.5- -__-. ___

suggested a fully balanced occlusion as the ideal occlusionwith anterior guidance paralle. to the condylar path. Ante-rior guidance was considered an extension of the condy-lar path. Setting anterior guidance for balanced occlusionwas simple because it was parallel to the condylar path.However, when the concept of edisclusion was introduced byI)Amico,7 the relationship between anterior guidance andcondylar path was questioned because the degree to whichanterior guidance was set, to the condylar path was un-known.

The mandible rotates around the intercondylar axisduring eccentric movements when anterior guidance issteeper than the condylar path (Fig. 2). The factor thatcompensates for the difference in steepness is the angle ofhinge rotatron. The dentist should determine the influencethat the angle of hinge rotation contributes to disclusion toestablish an ideal anterior guidance.

Takayama and Hobo15 analyzed disclusion relative tothe angle cd hinge rotation by using kinematic formulae.The results indicated that the angle of hinge rotation con--tributed to posterior disclusion by approximately 0.2 mm-for protrusive movement and 0.5 mm on average for lateralmovement on both working and nonworking sides. Accord-ing to an investigation of molar disclusion during eccentricmovements when the right and left condyle moves 3 mm inprotrusive movement and the nonworking condyle moves3 mm in lateral movement, the amounts of disclusion were1.1 t 0.6 mm during protrusive movement, and 0.5 i O.:imm on the working side and 1.0 + 0.6 mm on the non-working side during lateral movement measured at themesiobuccal cusp tip of the mandibular first molar.16 Theactual disclusion on the working side (0.5 mm) was equalto the amount created by the angle of hinge rotation (0.5mm). Howlever, t.he actual disclusion during protrusive andlateral movements on the nonworking side differ from theangle of hinge rotation. This leaves residual amounts ofdisclusion unaccounted for, namely, 0.9 mm in protrusiveand 0.5 mm on the nonworking side (Table I), thussuggesting that the angle of hinge rotation was not solelyresponsible for disclusion. The residual amounts can alsobe attributed to another determinant of disclusion, thecusp shape factor.CUSP SHAFE FACTOR

When the slopes of posterior cusps are parallel to thecondylar path inclination and anterior guidance is parallelto the condylar path, the opposing cusps slide during pro-

Fig. 3. During protrusive movement, condyle translateswithout rotation when anterior guidance (~3)and condylarpath (fi) are parallel.

Fig. 4. When cusp inclination of molars is parallel to an-terior guidance, there is no posterior disclusion despitesteeper anterior guidance (fi) than condylar path ((Y).

I ,/

Fig. 5. Posterior disclusion is evident when cusp inclina-tion of molars is parallel to condylar path and anteriorguidance (8) is steeper than condylar path ((Y).

trusive movement without discluding, despite the degree ofsteepness (F ig. 3). If anterior guidance is steeper than thecondylar path, the posterior teeth disclude. However, if thecusp inclination of the molars is parallel to anterior guid-

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Fig. 6. Cusp inclination (A) decreases (is more shallow) ifit is semicircular (B). Producing a convex cusp diminishescusp inclination.

ante, there is no posterior disclusion even though anteriorguidance is steeper than the condylar path (Fig. 4). Theposterior teeth disclude only when the cusp inclination ofthe molar is parallel to the condylar path and anteriorguidance is steeper than the condylar path (Fig. 5). Thecusp inclination (A) also becomes more shallow with asemicircular shape (B) as illustrated in Fig. 6. The shape ofthe cusp has great influence on the disclusion of posteriorteeth. To produce fully balanced occlusion, it is then nec-essary to make the cusp with a straight edge, whereas fordisclusion the cusp requires a convex semicircular shape.

If the shapes of the posterior cusps are less steep than thecondylar path, the posterior teeth disclude even if anteriorguidance is parallel to the condylar path. The residualamounts of disclusion that were not accounted for by theangle of hinge rotation can be attributed to this mecha-nism, regulated by the cusp shape factor. The semicircularshape of the cusps affects posterior disclusion and is thefactor contributing to the residual disclusion. This is calledthe cusp shape face. In practice it is critical that cusp in-clination, including the semicircular shape of the cusp,combined with the angle of hinge rotation contributes toposterior disclusion. The molar cusp inclination should beparallel to the condylar path, not parallel to anterior guid-ance, in establishing posterior disclusion.FACTORS THAT DETERMINEDISCLUSION

Molar disclusion during eccentric movements is effectivein eliminating harmful lateral occlusal forces. However, acomprehensive theory identifying factors that determine aspecific amount of disclusion is unavailable in the litera-ture. Mechanically, the maxillary and mandibular teethshould be in contact during eccentric movements for opti-mal chewing efficiency. Maximal shear force is observedwith a fully balanced occlusion. However, the condyle mustfollow one orbit precisely during eccentric movements for

HOBO

Table II. Comparison of buffer space with disclusionBuffer space Disclusion(mm) (mm)

Protrusive 0.8 1.1Working 0.3 0.5Nonworking 0.8 1.0

optimal function in a fully balanced occlusion. If thecondyle deviates slightly, it directly influences the relationbetween the teeth, resulting in occlusal prematurities anddeflective occlusal contacts.

McCollum and StuartlO described the condylar path asa fixed entity in an adult. In a recent study, when repeti-tive lateral movements were compared with the respectivecondylar paths, no movement traced the same line.17 Thedeviation in the condylar path during eccentric movementswas attributed to the shock-absorbing nature of the artic-ular disk. This study refers to this deviation in condylarpath as a buffer space. The average buffer spaces are 0.2mm in centric relation, 0.3 mm in laterotrusion, and 0.8 mmalong the protrusive and nonworking sagittal condylarpath. Molar disclusion should be greater than the bufferspace to avoid occlusal interferences during eccentricmovements.

When the average amount of disclusion is compared withbuffer space, the amounts closely match (Table II). In pro-trusive movement, the amount of buffer space is 0.8 mmand disclusion on the nonworking side is 1 mm. Theamount of disclusion should be slightly more than thebuffer space to prevent deflective occlusal contacts provid-ing separation between the opposing posterior dentition, sothat when the condyle is displaced during articular diskcompression , harmful occlusal forces can be controlled.SUMMARY

The mechanism of anterior guidance was reviewed fromrecent mandibular movement studies to provide a basis forunderstanding the twin-tables technique, which is a prac-tical method for establishing anterior guidance from thecondylar path. Anterior guidance and the condylar pathpreviously were considered independent factors. In arecent study, it was revealed that anterior guidance influ-ences the working condylar path and even changes whenthe lateral incisal path deviates from the optimal orbit.This supports the hypothesis that anterior guidance andthe condylar path are dependent factors. In setting anteriorguidance, it is recommended to set the working condyle sothat it moves straight outward along the transverse hori-zontal axis.The angle of hinge rotation produced by the angular dif-ference between anterior guidance and the condylar pathassists posterior disclusion but is not solely responsible.The anatomy of the cusps is created by establishing theappropriate form o f the posterior cusps aligned to the

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TWIN-TABLES TECHNIQV'? PART I

condylar path; thus it also con1 ributes to posterior disclu-sion.

Posterior disclusion is crucial in controlling harmful lat-eral forces but the factors that determine the preciseamount of disclusion nave not been established. The dis-elusions recorded in healthy adults were 1.1 k 0.6 mm inprotrusive movement and 0.5 t 0.3 mm on the workingside and 1.0 z 0.6 mm on the nonworking side during lat.-era1 movement. T hesr number; correlated with the devia-tion in the condylar path. The molars must discludeslightly more than the deviation in the condylar path toavoid occlusal interferences.

REFERENCESI. Hobo S. Mmeans of ar electronic measurmg s!~stem. Part III. Rotational center otlateral movement. J PHOSTHET DEUT 1984;52:66-72.

4. Hobo S. Taknyamrt H. Kinematic iwestigation of anterior guidance asa basis for ww gnathol,)gical conwpts. J Gnathology 1989;8:14-48.

5. Hobo S, Takayama H. \ new system for measuring condylar path andcomputing anterior g llidance. Part I. Measuring principles. Int .IProsthodon t 1988;1:99 06.

A compa:rison of the abrasiveness of six ceramic surfaces andgoldRichard Jacobi, DDS,a Herbert T. Shillingburg, Jr., DDS,b andManville (G. Duncamon, Jr., DDS, PhDCUniversity of Oklahoma, College of Dentistry, Oklahoma City, Okla.A type III gold alloy and six different ceramic surfaces were secured in an abrasionmachine opposing extracted teeth to determine their relative abrasiveness andresist,ance to wear. The rankings of restorative materials from least abrasive tomost abrasive were: gold alloy, polished; cast ceramic, polished; porcelain, polished;cast ceramic, polished and shaded; porcelain, polished and glazed; cast ceramic,cerammed skin shaded; and cast ceramic, cerammed skin unshaded. The ranking ofmaterials from most wear-resistant to least wear-resistant was: gold alloy, castceramic cerammed, cast ceramic cerammed and shaded, porcelain polished,porcelain glazed, cast ceramic polished and shaded, and cast ceramic polished. (JPROSTHET DENT 1991;66:303-9.)

G ld has long been a preferred dental restorativernaterial because of biocompstibility, durability, and low

Supported b:f Presbyterian Healt 2 Foundation grant PHF No . 45BAssociate Professor, Departmen-; of Fixed Prosthodontics.tProfessor and Chairman, Depar ;ment of Fixed Prosthodontics .CProfessor and Chsirman, Department of Dental Materials.10/l/24177

abrasiveness against natural teeth; however, the primarydisadvantage is an unnatural metallic appearance. Porce-lain crowns, inlays, and veneers were introduced more than100 years ago to serve as more natural-appearing restora-tions. Ceramic materials have become increasingly popu-lar because of improvements in materials and techniques.

The excessive wear of natural teeth opposing porcelainhas stimulated authors to discourage the use of porcelain

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