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Twin-Stage Procedure. Part 2:A Clinical Evaluation Test

Surriiya Hobo. DDS. MSD, PtiD'Hisao Takayama. Pi^D'*

The Twin-Stage Procedure was used fo meosure fhe amount af disacdu-sion reproduced on the articutator (in vitro) and in the mouth (in vivo). Theresuits almost coincided, even witnout measurement af the condylar path.By using this new procedure, auantitative control of ttie amount of disoc-clusion. which was previously impossible, iios become a reality in daily ciini-cai procedures. The amaunf of disocciusion occurring on an articuiatorcoincided witti tfie amount of disocclusion intraorally: therefore, the Twin-Stage Procedure is considered to be highiy reiiable. (int J Periodont RestDent 1997:17:457-463.)

'Founding Director, internotionoi Dentai Acodemy, Tokyo, Japan;and Protessor, University ot Southern Californio Scnooi ot Dentistry,Los Angeles, Caiitornia.

'Senior Reseorotier, internationoi Dental Academy, Tokyo. Japon;and Leoturer. Konogav^a Institute ot Technology, Kanagawa, Jopan.

Reprint requests: Dr Sumiyo Hobo, international Dentoi Acodemy,25-18 1 -Ctiome Shotoh, Shibuya-ku, Tokyo. 150-24 Japan.

in Part 1, the authors introducedtine "Twin-Stage Procedure"developed to control thetooth-contact condition of oresforafion during eccentricmovements witiiout measuringthe condylor poth.^ To evoiuatethe accuracy ot this procedure,the amount of disocoiusion wasexomined both in vitro ond invivo.

For the in vitro test, posteriorocciusion was woxed on anarticuiator under Condition 1(standard cusp angie). Then theamounts of disocciusion wereexomined under Condition 2(anterior guidance), it the pro-grommed amount ot disocciu-sion appeared, it indioated thevalidity of adjustment vaiues ofon orticuiator computed basedon the mathematical model otmandibuiar movement.

in the in vivo test, the restor-ations made on an articuiatorwere cemented in the pa-tient's mouth ond the amountsot disocciusion were examined.Disocciusion that occurredas it had on the articuiator

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suggested that the condyiarpath of the potient was con-troiled by the anterior guid-ance of fhe restoration to coin-cide with the adjusted voiueson the articuiator

In vitro test

The in vitro test wos conductedon an articuiator. The castswere mounted on on artioula-tor under Condition 1. Affer thecompletion of posterior oociu-sol waxing, on articuiafor underCondition 2 was used to meo-sure the specific omount of dis-occiusion that occurred duringvarious eccentric movements.

Method

Eight sets of typodont castswere mounted on Twin HobyArtioulators (3M Dental). Theooolusal surfaces of the maxil-lory and mandibular posteriorteeth areas were scraped witha scalpel. While an articuiatorwas adjusted under Condition1, the anterior segment ot themaxiliary cast was removed.The occlusal surfaces of thecast were waxed to a bal-anced articulation so that themaxillary and mandibuiar pos-terior teeth contacted eveniyduring eccentric movements, inthis woy, the stondard cuspangie would be reproduced, Astrict check wos conducted soa registration strip of 12.5-|jm

thickness was held in positionfor each posterior tooth at max-imum intercuspation and duringeach eccentric movemenf.

The orticulator was re-adjusted under Condition 2. Thecondyles were moved 3 mmfrom centric occlusion using opositioning index, and protrusiveand right and ieft iateraleccentric positions were repro-duced. Then, using a leafgauge (3M Health Care) con-sisting of 0.1-mm-thick strips, theamount of disooclusion wasmeasured at each eccentricposition. For each measure-ment, the leaf gauge wosinserted between the moxillaryond mondibulor posterior teeth.

While pushing the top of themounting screw on the upperframe of fhe arficulator withone hand, the leaf gauge wasremoved using the other handwith the same degree of forceas on the screw. If the foroe pro-duced by pushing on top of thescrew with one hand was toostrong, fhe condyles were sepa-rated from the fossa box. Carewos taken to avoid causing theupper frome of the articuiatorto tilt when the ieaf gauge wasremoved. It must be verifiedthat the tip of the anteriorguide pin that touches the sur-face of ths anterior guide tableat maximum intercuspationdoes not separate from the sur-face of the anterior guide tableat the eccentric position. Forexample, a 1,0-mm disocciusionwas creoted when nine strips

but not 10 strips of the leafgouge couid be removed. Theamount of disocoiusion wasmeasured at the first and sec-ond molars and the premolarsat the same time. The data oneach side during each eccen-tric movement was averaged(Figs 1 to 9),

Results

The omount ot disocclusionmeosured on eight sets of casts(totai of right and left sides =16) was 1.01 ± 0.03 mm for bothright and leff sides during pro-trusive movement, and 0.92 ±0,07 mm on the nonworkingside and 0.50 ± 0,00 mm on theworking side during lateraimovement. The resuits of the invitro test are shown in Table 1.

Table 1 Results of the in vitrotest (mm)

Standard Meosuredvalue dafo

Protrusive 1,0 1.01 ±0.03Nonworking l.O 0,92 + 0.07Working 0.5 0.5G + 0.00

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Fig I Stane casts of Typodant aremounted on an articulator underCondition I. The malar occiusal sur-kices are eliminated and a graove Isfarmed to provide mechanical reten-tion for wax.

Fig 2 Anterior tooth segment isremoved fram the maxiiiary cost.

Fig 3 Completed wax-up of the maxil-lary posterior teeth.

Fig 4 Compieted wax-up of themandibular posterior teeth.

Fig 5 Balanced articulation is creotedana the standord cusp angle is formedunder Condition I. The tooth contactconditions during various eccentricmovement are examined.

Fig 6 Sagittal condyiar path and theonterior guide table ot an articulatorare adjusted to Condition 2 and themoxiiiary anterior segment is replacedon the cast. Wax is added to establishthe anterior guidance.

Fig 7 Occiusal view ot anterior guid-ance.

Fig 8 Plastic indexes are placed on thefosso bax to advance the condytes 3.0mm from centric occiusian. When tiieyare used on the right and left sides, pro-trusive movement is reproduced, Whenthey are used on one side, a lateralmovement is reproduced.

Fig 9 With the articuiotor in the rightiaterai positian. strips of leaf gauge areinserted to measure the amount oí dls-acciusion.

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Table 2 Comparison of fhe results of in vitro and in vivo tesfs

in vitro I h vive Difference

ProtrusiveNonworkingWorking

1.06 ±0.130.98 ± 0.050.52 ± 0.07

1.01 ±0.110.94 ±0.120.52 ± 0.07

-0.04 ± 0.0Ó-0.04 + 0,100.00 ± 0.03

Discussion

These results indicate that byusing the Twin-Stage Procedure,the standard amount of disoc-clusion ocours within theamount of error less than 0,1mm, which is the thickness of astrip of a ieat gauge. As men-tioned in Part 1, an articuiator isused as a simuiator of mandib-uiar movement and not as oreproducer. The above resultsindicate that the adjustmentvoiues for an articuiator arevalid to reproduce the stan-dard omount of disooclusion.

The standard values of thecusp angle used as the maindeterminant can be applied tomanufacture occlusal morphol-ogy in computer aided design-computer aided manufacture(CAD/CAM). Use of a universallyaccepted fixed value versusdirect measurement of thecondylar path is significant infurther development of fheCAD/CAM system, Theretore theTwin-Stage Procedure wiil pro-vide a solution in the produc-tion of three-dimensionol oc-clusal dynamics by CAD/CAM,

In vivo test

The in vivo test tested restora-tions made on an artiouiatorond oemented in the patient'smouth. The test was performedto determine if the exactamount of disooclusion was re-produced as had occurred onthe articuiator. Since the occur-rence of a predictable amountot disocclusion on the articuia-tor was veritied, the amount oidisooclusion occurring intrao-rally was investigated to see if itwas simiiar.

Method

Eight oociusai reconstructionoases were selected. The restor-ations were fabricated on anarticuiator using Conditions 1and 2. The amount of disocciu-sion to on accuracy of 0,1 mmvyas measured during eaohGocentric movement using aleaf gauge in the same manneras in the in vitro test.

The condyie of the articuia-tor was moved 3 mm using apositioning index to reproduoeprotrusive and right and left iat-eral movements, it was, verified

that the standard amount ofdisocclusion was reproduced.An anterior siiicone core wasthen fabricated using heavybody silioone impression mater-iai for each ecoentric position.

After the restarations werecemented in the patient'smouth, the patient was askedto reproduoe eaoh eocentrioposition using the silicone coreas a positioning guide. Theamount of disocclusion wasmeasured using a ieaf gauge.

Retractors were used andthe leaf gauge v̂ ias insertedbetween the maxiilary andmandibular molars, obliquely ifnecessary to avoid the lips. Forexample, the 1.0-mm disocclu-sion oorresponded to 10 stripsleaf gauge that couid be re-moved under gentie biting con-ditions but not under heavy bit-ing conditions. The amount ofdisocclusion was measured atthe first and second molars andpremolars, individually. Themean data tor each side duringeaoh eccentric movement wascalculated (Figs 10 to 15).

Results

The in vivo results of the amountot disocclusion of the eightocciusai reconstruction cases(n = 1Ó) are shown in Table 2compared with the data on thearticuiator. The data measuredintrooraliy coincided with thedata on the articuiator, with anerrorof less than 0.1 mm.

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Fig W Occiusai view of the mqxiilarycrown restorations.

Fig 11 Occlusal view af the mandibu-lar orawn restoration.

Fig 12 index was made using siiiconeimpression moterioi for eoch 3.0-mmeccentric position. Disacciusion can besimultaneausiy canfirmed using a leafgauge on the nonworking side.

Fig ¡3 Occiusai view at the campieteiyrestored maxillary teeth.

Fig 14 Occlusal view afthe campieteiyrestored mandibular teeth,

Fig 15 Siiicone inaex is inserted at a3.0-mm eccentric mavement ta mea-sute the amount of disocclusian.

Discussion

Although the condylar path isnot measured using the Tvi/in-Stage Procedure, the amount ofdisocclusion produced oocurswithin 0,1 mm accuracy. Thisievei of accuracy is attributedto a shift from complete depen-dence of the main determinantof ooolusion on the condyiarpath to the cusp angle. Theconoept that focuses on con-

dylar path, incisai poth, andcusp angle, ali treated as oneunit simultaneously, becomesmore significant,

in the Twin-Stage Procedure,the amount of disocclusion wosprogrammed and reproducedaccurately. The influence of thecondylar path on the amountof disocclusion was iess thonwhat has been assumed, i-iow-ever, since the angie of thecondyiar path of a patient

should differ from the angieadjusted on an ortiouiator (40degrees in Condition 2), the dif-ference was supposed toappear clearly as differentamounts of disocciusion on thearticulator and introoraiiy,

in spite of possible conjeo-ture, significant ditferences werenot found in the eight casesevaluated. This can be ex-plained by the fact that theinfluence of the oondylor path

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on the amount of disocciusion isvery smail and aimost negligi-ble.

The reasons the influence ofthe condylar path is almostnegiigible ore discussed. Thereare noticeabie deviations inthe oondyiar path representedby the difference of eccentricand returning condyior poths.^Furthermore, the condylar pothis influenced greotly by anteriorguidance.^'2 These observationscan be attributed to the ioose-ness of the TMJ structure in rela-tion to the surrounding sott tis-sues. This indicates that the twoposterior apexes of the mandib-ular triangle (the right and ieftcondyles) are "fiexibie."

What would happen it in-stead of fhe traditionai mandib-uiar triangie a smail triangleexisted at the onterior guidancearea that guided the man-dible? If there are three contactpoints in the anterior portionbecause the maxiiiary andmandibuior posterior teeth dis-occiude during eccentricmovement, the mandibie moybe guided by this smaii triangie.As a resuit, the condyiar pathmoy be infuenoed by the ante-rior guidance, if the above istrue, the foilowing is the mostproboble expianation: The an-terior guidance of restorationsWCS made by adjusting thecondylar path ot the articuiatorto the standard value (40 de-grees) in the obove tests.

Accordingiy, when the res-torations were cemented in thepatient's mouth, the condyiorpath of the patient might hovebeen guided to the 40-degreevaiue. As a result, the standardomount of disocciusion oc-curred. The favorable coinci-dence ot the data trom anarticuiator and data measuredintraorally suggests the validityot the above-mentioned hy-pothesis.

According to computationbased on the mathematicoimodei of mandibuiar move-ment, the rotes of influence ofsagittoi protrusive and iateraicondyiar path inclinations onthe amount of disocciusion are0.020 mm per degree and0.015 mm per degree, respec-tiveiy. This means, converseiy,that when an error ot 0.1-mmmaximum exists in the amountot disocciusion on the restora-tions cemented in the patient'smouth, the sagittai condyiorpath wiii deviate by 5.0 de-grees during protrusive move-ment and 6.1 degrees duringiaterai movement.

Since the amount of disoc-ciusion produced by the Twin-Stage Procedure occurs within0.1-mm accuracy, it is consid-ered that the condyiar pathshouid be controiied within 40 ±5 to 7 degrees, occording to theabove-mentioned hypothesis.

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Conclusion

In the past, ta produce disoc-clusion in restorations it hasbeen necessory to use a pan-tograph, a fully adjustablearticulatar, and to make cor-rective occlusoi adjustmentsusing a remount technique bythe trial-and-error method. Toverify the disocclusion in thepatient's mouth, an occiusairegistrotion strip of 12.5-|jmthickness has been used, if thestrip oan be removed as soonas eocentrio movement stortswhen positioned between themoxiilary and mandibular pos-terior teeth, disocclusion is veri-fied. However, use of thismethod does not oilow o con-trol of the amount of disocclu-sion.

It restorations were madeusing a pantograph ond a fullyodjustable arficulator ondplaced in the patient's mouth,the occlusion on the orticuiatordid not coincide exactly withthat presented intraorally. How-ever, when the Twin-Stage Pra-oedure was applied, the resuitsshowed that the amount of dis-occiusion on the articuiatorond in the mouth were almostthe same. By using an entireiydifferent appraach (the Twin-Stage Prooedure), quantitativecontrol of the amount of disao-clusion previously regarded asimpossibie has become o reoi-ity in daily clinical procedures.

in conclusion, the Twin-Stage Procedure is consideredto be highly reliable becousethe amount of intraoral disoc-clusion coincided with the pro-grammed amount of disocclu-sion.

References

1. Hobo S. Tokayoma H. Twin-StageProoedure. Part l : A new method toreproduce precise eccentric oc-ciusol reiations. int J Rericdont RestDent 1997:17:113-123.

2 Hobo S, Takayama H. Réévaluationof the condyiar poth as the refer-ence for occlusion. J Gnathol 1995:1d:3l-40.

3. Hobo S, Takayama H. Effect of ca-nine guidance on working condyiarpath, int J Prosthodont 1969;2:73-79.

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