ABSTRACT
Accuracy of int raoral and ext raoral digit al dat a
acquisit ion for dent al rest orat ions
+HLNH58'2/3+1+DUDOG6$/0(12, Matthias MOLDAN1.DWKDULQD.8+119LNWRU6,&+:$5'73, Bernd WÖSTMANN3,
5DOSK*XQQDU/87+$5'71
1- Universität Ulm, Zentrum Zahn-, Mund-, und Kieferheilkunde, Klinik für Zahnärztliche Prothetik, Ulm, Germany. 2- Private Practice, Salach, Germany.
3- Justus-Liebig Universität, Poliklinik für Zahnärztliche Prothetik, Giessen, Germany.
&RUUHVSRQGLQJDGGUHVV Heike Rudolph - Department of Prosthetic Dentistry - Center of Dentistry - Ulm University - Albert-Einstein-Allee 11 - 89081 - Ulm - Germany - Phone: +49 731-500 64240 - Fax: +49 731-500 64247 - e-mail: [email protected]
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he com put er- aided design ( CAD) and com put er- aided m anufact uring ( CAM) process chain for dent al rest orat ions st art s w it h t aking an im pression of t he clinical sit uat ion. For t his purpose, eit her ext raoral digit izat ion of gypsum m odels or int raoral digit izat ion can be used. Despit e t he increasing use of dent al digit izing syst em s, t here are only few st udies on t heir accuracy. Obj ect ive: This st udy com pared t he accuracy of various int raoral and ext raoral digit izing syst em s for dent al CAD/ CAM t echnology. Mat erial and Met hods: An experim ent al set up for t hree- dim ensional analysis based on 2 prepared ceram ic m ast er dies and t heir corresponding virt ual CAD- m odels was used t o assess t he accuracy of 10 ext raoral and 4 int raoral opt ical non- cont act dent al digit izing syst em s. Depending on t he clinical procedure, 10 opt ical m easurem ent s of eit her 10 duplicat e gypsum dies ( ext raoral digit izing) or direct ly of t he ceram ic m ast er dies ( int raoral digit izing) w ere m ade and com pared w it h t he corresponding CAD- m odels. Result s: The digit izing syst em s show ed differences in accuracy. How ever, all t opical syst em s w ere w ell w it hin t he benchm ark ofP7KHVHUHVXOWVDSSO\WRVLQJOHWRRWKPHDVXUHPHQWV&RQFOXVLRQV6WXG\UHVXOWV
are lim it ed, since only single t eet h w ere used for com parison. The different preparat ions represent various angles and st eep and parallel opposing t oot h surfaces ( incisors) . For m ost
GLJLWL]LQJV\VWHPVWKHODWWHUDUHJHQHUDOO\WKHPRVWGLI¿FXOWWRFDSWXUH8VLQJ&$'&$0
t echnologies, t he preparat ion angles should not be t oo st eep t o reduce digit izing errors. Older syst em s m ight be lim it ed t o a cert ain height or t aper of t he prepared t oot h, w hereas new er syst em s ( ext raoral as w ell as int raoral digit izat ion) do not have t hese lim it at ions.
Keyw ords: Dent al t echnology. Dent al equipm ent . Syst em s analysis. Dent al inform at ics.
I N TRODUCTI ON
Com put aided design ( CAD) and com put er-aided m an u f act u r in g ( CAM) t ech n ologies allow t he use of a range of m anufact uring m et hods and m at erials for dent al rest orat ions. Sophist icat ed high-st rengt h m at erials, such as zirconium oxide, w hich can only be processed using CAD/ CAM t echnologies, and generat ive ( addit ive) t echnologies, such as rapid prot ot yping, require digit al dat a as t he key t o apply any CAD/ CAM process29.
The CAD/ CAM process chain st art s w it h t aking an im pression of t he clinical sit uat ion, for exam ple, by capt uring a pat ient ’s prepared t oot h or t eet h.
Convent ionally, high- precision condensat ion- curing or addit ion- curing im pression m at erials are used t o m ake dent al im pressions. Subsequent ly, dent al st one is poured int o t hese im pressions. The result ing gypsum m odel can be digit ized ext raorally23. An
ever m ore w idely applied alt ernat ive is int raoral d i g i t i za t i o n7 , 2 8, w h i ch a l l o w s p r a ct i t i o n e r s t o
dispense wit h convent ional im pressions and gypsum m odel fabricat ion. Bot h m et hods of dat a capt ure h av e ad v an t ag es an d d i sad v an t ag es. Cl i n i cal param et ers have t he great est im pact on t he qualit y of dent al im pressions21. These fact ors, which include
also im pede int raoral digit izing. Because of space r est r ict ion s, in t r aor al digit izin g dev ices h av e a sm aller m easuring area t han ext raoral digit izers. As a result , several digit al dat a set s m ust be obt ained and com bined t o capt ure m ore t han t w o t o t hree neighboring t eet h w hen using int raoral digit izing devices. This process, t erm ed m at ching, will always int roduce a sm all syst em at ic error t o t he dat a of any m easu r in g sy st em . How ev er, conv en t ion al im pr essions of t he full ar ch ar e also subj ect t o g r eat er d ev iat ion t h an t h ose of sin g le t eet h . Furt herm ore, ext raoral digit izat ion always includes t he errors int roduced by im pression and gypsum m odel- m aking in addit ion t o t he error result ing from digit izat ion27. Regardless of t he digit izing t echnology
applied, digit izat ion qualit y affect s precision in t he CAD/ CAM process2. CAM precision also depends on
t he qualit y of t he m achining com ponent s1,31.
Every st ep in t he CAD/ CAM process chain can ad d t o t h e r esu lt in g m an u f act u r in g er r or. Th e individual errors can cancel or m ut ually reinforce each ot her. Mixed effect s are also possible18. Wit h
t hr ee- dim ensional ( 3D) analy ses, ever y st ep of t h e CAD/ CAM p r ocess ch ain can b e assessed , from int raoral digit izing or im pression- m aking t o t he r esult ing dent al r est orat ion. Such analy ses com pare a real m odel wit h it s corresponding virt ual CAD m odel. Pr eferably, t he r eal m odel is m ade of a direct ly digit izable m at erial. Pow der use can EH DYRLGHG WR UHGXFH WUDQVOXFHQF\ RU UHÀHFWLRQ w hich w ill always increase error in t he digit izing process10,24.
Dat a from every st ep in t he CAD/ CAM process chain, as w ell as from result ing digit ized t em plat es or r est orat ions, can be aligned w it h t he v ir t ual CAD m odel and analyzed in 3D w it h suit able CAD soft ware3,9,12,19.Very high st andards for alignm ent
m ust be set for t hese analyses. Wit h alignm ent or r egist rat ion, dat a set s ar e posit ioned in one com m on coordinat e syst em w it h t he least possible m ean deviat ion25. This deviat ion is usually given
in t er m s of t he r oot m ean squar e ( RMS) er r or and should be docum ent ed for t he assessm ent of syst em at ic error in 3D analysis5.
Repeat ed m easur em ent of r efer ence m odels has been used t o analyze t he precision of digit izing sy st em s3 , 9 , 1 3 , 1 5. How ev er, geom et r ically def in ed
reference m odels are of lim it ed value for analyzing freeform surfaces. Because t he surface of prepared t eet h will render freeform surfaces aft er digit izat ion DQGEHFDXVHWRRWKVKDSHKDVDVLJQL¿FDQWLPSDFW on achievable precision w it hin a CAD/ CAM process chain6,15, syst em precision should be analyzed w it h
appropriat ely shaped reference m odels.
According t o I SO 12836: 201213GH¿QLWLRQ
accuracy descr ibes t he closeness of agr eem ent bet w een t h e r esu lt of a m easu r em en t an d t h e t r u e v a l u e o f t h e m e a su r a n d . Accu r a cy i s a
qualit at ive concept . I t s quant it at ive count er par t is t rueness13. Precision is calculat ed as a st andard
dev iat ion indicat ing t he closeness of agr eem ent am ong t he r esult s obt ained w hen apply ing t he ex p er i m en t a l p r o ced u r e sev er a l t i m es u n d er prescribed condit ions10,16.
Despit e t he increasing use of dent al digit izing syst em s, st udies on t heir accuracy are lacking6. The
current st udy com pared t he accuracy of various different int raoral and ext raoral digit izing syst em s f o r d en t a l CAD / CAM t ech n o l o g y. Th e st u d y ’s hypot heses w ere t hat :
( 1) Opt ical non- cont act digit izing syst em s for int raoral and ex t raoral dat a acquisit ion in CAD/ CAM t echnology have differences in accuracy; and ( 2) The shape and t ype of t oot h t o be digit ized affect t he achievable accuracy, depending on t he digit izing syst em used.
M ATERI AL AN D M ETH ODS
An experim ent al set up for 3D analysis was used t o assess and com pare t he accuracy of different dent al digit izing sy st em s, 10 for ex t raoral dat a acquisit ion ( plus one syst em t est ed a second t im e using a zoom lens) and four for int raoral dat a acquisit ion ( Figur e 1, Figur e 2) . Analy ses w er e based on t w o prepared ceram ic m ast er dies ( m olar and incisor) and t heir corresponding virt ual CAD m odels.
Re fe r e n ce digit iz in g sy st e m , a n a ly sis of precision
Figure 1- Experimental setup (reverse-engineering of the ceramic master dies and corresponding virtual computer-aided design (CAD) models excluded)
System name Type of system
Light source/ technology
Manufacturer Data provided by (Germany)
D250, D640 E RLL 3Shape, Copenhagen, Denmark WIELAND Dental + Technik, Pforzheim
digiSCAN E WFP AmannGirrbach, Pforzheim,
Germany; Zoom lens: IVB, Jena, Germany
Ulm University
Scan 900 E WFP ce.novation, Riedenburg, Germany Fraunhofer IKTS, Hermsdorf/Thuringia
es1 E RLG HWNRQ*UlIHO¿QJVXFFHVVRU
Straumann, Freiburg, Germany
Ulm University
Everest Scan Pro E WFP KaVo Dental, Biberach/Riss, Germany
KaVo Dental, Anwendungstechnik CAD/
CAM, Warthausen
Lava Scan, Lava Scan ST
E WFP 3M ESPE, Seefeld, Germany 3M ESPE, Seefeld
ODKM97 E WFP Fraunhofer IOF, Jena, Germany Ulm University
ZENO Scan S100 E RL WIELAND Dental + Technik, Pforzheim, Germany
WIELAND Dental + Technik, Pforzheim
cara TRIOS I CM, p- Heraeus Kulzer, Hanau, Germany University of Giessen
CEREC Bluecam I BL, p+ Sirona Dental Systems, Bensheim, Germany
University of Giessen
Lava C.O.S. I AWS, pd 3M ESPE, Seefeld, Germany University of Giessen
iTero I CM, p- Align Technology Inc., San Jose, California, the United States of
America
University of Giessen
Reverse engineering of ceram ic m aster dies and corresponding virt ual CAD m odels
A m olar and an incisor w ere prepared according t o t he rules for all- ceram ic rest orat ions ( 3M ESPE, Seefeld, Bavaria, Germ any) . The result ing gypsum dies w er e digit ized w it h a high- pr ecision opt ical w h it e- lig h t f r in g e p r oj ect ion d ig it izin g sy st em ( ODKM97) w it h a m easurem ent uncert aint y of ~ 8 PDFFRUGLQJWRWKHPDQXIDFWXUHU8VLQJD&$' soft ware program ( ce.novat ion, ilm cad, I lm enau, Thuringia, Germ any) , duplicat e ceram ic dies w ere co n st r u ct ed an d CAD / CAM- m ad e ( Fr au n h o f er I nst it ut e for Ceram ic Technologies and Syst em s, I KTS, Herm sdorf, Thuringia, Germ any) by m illing from part ially sint ered blocs ( dispersion ceram ic: zirconia 80% , alum ina 20% ) . To elim inat e surface UHÀHFWLRQRIWKHGHQVHO\VLQWHUHGGLHVWKH\ZHUH sandblast ed w it h corundum abrasive ( grain size 50 PDWEDU
The acquired ( ODKM97) digit al dat a set s ( point FORXGV IURP WKH FHUDPLF GLHV ZHUH ¿OWHUHG WR rem ove out liers and st ray point s. Corresponding virt ual CAD m odels w ere generat ed from t he point clouds by t riangulat ion ( “ Point s t o Polygon” feat ure, Geo m ag i c St u d i o ) . Th ese r ev er se- en g i n eer ed CAD m odels of t he incisor and m olar w ere used t o analyze t he m easuring dat a from all digit izing syst em s evaluat ed ( Figure 2) .
Ext raoral digit izing
I n accordance w it h com m on clinical and dent al ODERUDWRU\SURFHGXUHVJ\SVXPGLHV¿YHPRODUV DQG ¿YH LQFLVRUV ZHUH PDGH IURP LPSUHVVLRQV for each of t he sy st em s/ var iat ions for ex t raoral digit izing. Each gypsum die was digit ized once.
I m pr ession s w er e m ade w it h t h e on e- st age t w o- phase im pr ession t echnique using addit ion-curing high- precision polyet her ( I m pregum Pent a H DuoSoft and I m pregum Garant L DuoSoft ; 3M ESPE, Seefeld, Bavaria, Germ any) as well as individualized cust om st ock t ray s ( KKD im pr ession t ray s w it h r et en t i o n , si ze 3 ; Ken t zl er - Ka sch n er D en t a l , Ellwangen/ Jagst , Baden-Würt t em berg, Germ any) , w hich w ere covered w it h a t hin layer of adhesive an d dr ied for 1 0 m in u t es ( Poly et h er - Adh esiv e; 3M ESPE, Seefeld, Bavar ia, Ger m any ) . Because all im pressions w ere m ade at room t em perat ure rat her t han body t em perat ur e, t he set t ing t im e recom m ended by t he m anufact urer was ext ended by 5 m in.
Wit hout using any surfact ant s, im pressions were poured wit h vacuum - m ixed class 4 st one ( est het ic-rock 285, apricot ; dent ona DENTAL, Wipperfürt h, Nor t h Rh in e- West ph alia, Ger m any ) f ollow in g a st andar dized pr ot ocol20,30 2QO\ ÀDZOHVV J\SVXP
dies w ere used. Dies w it h bubbles or pearls w ere discarded and rem ade ( im pression and die) . The bases of t he dies w ere reduced by dry t rim m ing
only, t o avoid wat er cont am inat ion of t he gypsum . $OO ¿YH PRODU DQG ¿YH LQFLVRU J\SVXP GLHV w e r e d i g i t i z e d w i t h i n 7 2 h o f m a n u f a c t u r e w it h t h e r espect iv e ex t r aor al digit izin g sy st em ( Figure 2) . Follow ing com m on clinical and dent al ODERUDWRU\ SURFHGXUH V\VWHPVSHFL¿F VWDQGDUG provisions w ere used t o im prove dat a qualit y aft er digit izing. Dat a ex por t for m at was eit her binar y * . st l ( st er eo l i t h o g r ap h y f i l e f o r m at ) o r * . asc ( ASCI I : Am erican St andard Code for I nform at ion I nt erchange) , depending on t he digit izing syst em .
I nt raoral digit izing
For all int raoral digit izing syst em s, t he ceram ic m ast er dies w ere used direct ly for dat a acquisit ion, again according t o clinical procedure, w hich does not include im pression or gypsum m odel m aking. 7KHLQFLVRUDQGWKHPRODUZHUHGLJLWL]HG¿YHWLPHV w it h each syst em . Whereas iTero and cara TRI OS ( Figure 2) did not require t he applicat ion of powder ont o t he ceram ic m ast er dies, CEREC Bluecam ( VI TA CEREC Pow der; VI TA Zahnfabrik, Bad Säckingen, Ba d e n - W ü r t t e m b e r g , Ge r m a n y ) d i d r e q u i r e powdering of t he ceram ic dies before digit izing. The Lava C.O.S. required neighboring and ant agonist ic t eet h for t he digit izing process. A slight dust ing of pow der ( Pow der for Chair side Oral Scanner ; 3M ESPE| LAVA, Seefeld, Bavaria, Germ any) was ap p lied f or p at t er n r ecog n it ion an d m at ch in g , according t o t he m anufact urer ’s inst ruct ions. Again, V\VWHPVSHFL¿FVWDQGDUGSURYLVLRQVZHUHXVHGWR im prove dat a qualit y aft er digit izing. Dat a export form at was binary * .st l.
Dat a handling and alignm ent
Each dat a set was orient ed in it s own m easuring coordinat e syst em . Before analysis, dat a set s w ere aligned t o t he respect ive virt ual CAD m odel ( m olar RULQFLVRUVHUYLQJDVUHIHUHQFHIHDWXUH³EHVW¿W regist rat ion”, no point reduct ion chosen, Geom agic Qualify) . The error in t his alignm ent was recorded as 506HUURUSULRUWRWKHDQDO\VLVRIWKHTXDOLW\RI¿W
lens: 25,841; iTero: 21,179; cara TRI OS: 21,218; digiSCAN: 34,207; Lava C.O.S.: 24,310; Lava Scan: 40,550; Lava Scan ST: 39,854; ODKM97: 119,301) . For bet t er com parabilit y, all dat a set s were sam pled DWDFRQVLVWHQWLQWHUSRLQWGLVWDQFHRIPZKLFK r educed t he num ber of point s, especially in t he larger dat a set s. Aft er t he procedure, incisor dat a set s consist ed of 2,800 t o 29,000 dat a point s; m olar dat a set s com prised 3,500 t o 39,000 dat a point s. Apart from t his sam pling, all dat a set s ( point clouds or STL- surfaces) w ere used unchanged and exact ly as export ed by t he respect ive digit izing syst em .
Quant it at ive and qualit at ive analysis
For qu an t it at iv e dat a an aly sis, t h e sh or t est dist ance bet w een each point in a m easur em ent point cloud and t he reference was com pared for HDFKV\VWHPYDULDWLRQIRUHDFKRIWKH¿YHLQFLVRU DQG¿YHPRODUPHDVXUHPHQWV0D[LPXPDQGPHDQ posit ive and negat ive deviat ions from t he virt ual CAD m ast er m odel w er e calcu lat ed ( Geom agic Qualify) . Color- coded graphs ( Geom agic Qualify) w ere used t o show how t he calculat ed differences w ere spread over t he com plet e die surface, t hus
r ev eal i n g st r en g t h s an d w eak n esses o f each digit izing syst em .
St at ist ical analysis
Dat a w ere analyzed descript ively w it h one- way analysis of variance ( ANOVA; I BM SPSS 21.0; I BM Corporat ion, Arm onk, NY, USA) and t he St udent– New m an–Keuls post - hoc procedure, at a level of VLJQL¿FDQFHRID= 0.05. Fact ors for t he m ult ivariat e analysis of variance ( MANOVA) were “ t oot h” ( incisor v er su s m olar ) an d “ sy st em ” ( t h e 1 5 digit izin g syst em s/ variat ions) .
RESULTS
Re fe r e n ce digit iz in g sy st e m , a n a ly sis of precision
Co m p a r i n g ea ch o f t h e 1 0 m ea su r em en t s obt ained by ODKM97 w it h one anot her result ed in DPHDQGHYLDWLRQRIP6'DQG P6'7KHPD[LPXPSRVLWLYHGHYLDWLRQZDV PDQGWKHPD[LPXPQHJDWLYHGHYLDWLRQZDV P7KHFRQ¿GHQFHLQWHUYDO&,IRUWKHPHDQ SRVLWLYHGHYLDWLRQZDV&,WR&,P
and t he CI for t he m ean negat ive deviat ion was 5% &,WR&,P
The value for Cronbach’s alpha was 0.999.
Precision of dat a alignm ent
For m ost digit izing syst em s, an RMS error of PZDVDFKLHYHGIRUGDWDDOLJQPHQWZLWKWKH UHIHUHQFH&$'PRGHOPD[LPXPPPLQLPXP P2QO\'506PPD[LPXPP PLQLPXPPdigiSCAN w it h zoom lens ( RMS PPD[LPXPPPLQLPXPP DQG6FDQ506PPD[LPXPP PLQLPXPP had less exact alignm ent values.
Quant it at ive analysis
Quant it at ive dat a analysis rendered t he posit ive a n d n e g a t i v e d e v i a t i o n s f o r e a ch o f t h e 1 0 m e a su r e m e n t s f o r e a ch o f t h e 1 5 d i g i t i zi n g sy st em s/ var iat ions. The effect of differ ent t oot h shape ( incisor versus m olar) was also det erm ined ( Figure 3) .
Qualit at ive analysis
Am ong ext raoral digit izing syst em s, ODKM97, Lava Scan, Lava Scan ST, es1, and Everest Scan Pr o h ad t h e m ost p r ecise r ep r od u ct ion of t h e ceram ic m ast er m olar ( Figure 4a, left and m iddle) . Rat h er t h an im pr ov in g t h e digit izat ion r esu lt s, t he addit ion of t he zoom lens t o t he digiSCAN sy st em r esu lt ed in d ecid ed ly p oor er d ig it izin g qualit y ( Figure 4a, right ) . The zoom lens affect ed t he m olar m easur em ent s m or e t han t he incisor m e a su r e m e n t s. Th e i n si g n i f i ca n t d i f f e r e n ce s bet w een ext raoral ( Figure 4a, left and m iddle) and int raoral digit izing syst em s ( Figure 4b) in m easuring t he ceram ic m ast er m olar can be assessed at a glance w it h t he color- coded graphs.
The ceram ic m ast er incisor proved t o be m ore challenging t han t he m olar for m ost of t he digit izing syst em s exam ined. The only except ions w ere t he D640, ZENO Scan S100, ODKM97, and t he int raoral syst em cara TRI OS ( Figure 5b, left ) . Most syst em s had higher deviat ions in t he incisor dat a set s t han in
t he m olar m easurem ent s ( Figures 5a and b, m iddle and right ) . The pow der, w hich had t o be applied before using t he CEREC Bluecam syst em , clearly show ed in t he result ing higher posit ive deviat ions ( Figure 5b, m iddle) .
While t w o generat ions of t he sam e digit izing syst em ( D250 and D640) did not render different result s for t he m olar m easurem ent s, t here was a clear im provem ent w it h use of t he new er m odel for t he incisor m easurem ent s ( Figure 5a) .
St at ist ical analysis
2QHZD\ $129$ DW D OHYHO RI VLJQL¿FDQFH RI
D= 0.05, follow ed by t he St udent –New m an–Keuls
post - hoc procedure using t he m ean posit ive and
negat ive deviat ion for analyses, show ed t hat t he h ig h est d ig it izat ion q u alit y r eg ar d in g p osit iv e deviat ions was achieved by t he ext raoral digit izing
V\VWHPV2'.0PHVP DQG(YHUHVW6FDQ3URP&RPSDUDEOH QRW VLJQL¿FDQWO\ GLIIHULQJ TXDOLW\ S ZDV f ou n d f or t h e in t r aor al digit izin g sy st em s Lava &26 P DQG FDUD 75,26 P 7KH H[WUDRUDO V\VWHPV /DYD 6FDQ P /DYD 6FDQ 67 P =(12 6FDQ 6P'PDQG GLJL6&$1 P DOVR KDG KLJK DQG QRW VLJQL¿FDQWO\GLIIHULQJS GLJLWL]DWLRQTXDOLW\ DVGLGWKHLQWUDRUDOV\VWHPVL7HURP DQG &(5(& %OXHFDP P 2QO\ ' ( because of t he incisor m easurem ent s, 18.9± 8.1 PSGLJL6&$1ZLWK]RRPOHQVGXHWRWKH PRODU PHDVXUHPHQWV P S DQG6FDQPSUHQGHUHG d at a set s of low er q u alit y com p ar ed w it h t h e abovem ent ioned digit izing syst em s.
Result s for t he m ean negat ive deviat ions w ere VLPLODUZLWKEHVWUHVXOWVWRP for t he ext raoral syst em s Lava Scan, es1, Everest Scan Pr o, and ODKM97 as w ell as t he int raoral V\VWHPFDUD75,26P2QO\WKHPHDQ QHJDWLYHGHYLDWLRQVRI'PDQG L7HURPZHUHIRXQGWREHVLJQL¿FDQWO\ lar ger ( p< 0. 001) com par ed w it h t he pr ev iously m ent ioned syst em s and Lava Scan ST, D640, ZENO Scan S100, and digiSCAN ( ext raoral) as well as Lava C.O.S. and CEREC Bluecam ( int raoral) .
Again, a low er qualit y in com parison w it h all ot her syst em s ( p< 0.001) was show n by digiSCAN ZLWK]RRPOHQVDQG6FDQPDQG PUHVSHFWLYHO\
0$129$UHYHDOHGDVLJQL¿FDQWLQÀXHQFHRIWKH com bined fact ors “ syst em ” and “ t oot h” ( p< 0.001) . The effect size, m easured by calculat ing t he part ial HWDVTXDUHGdž2
p) value, was 0.480 ( p< 0.001) . The
digit izing syst em it self proved t o have t he great est LQÀXHQFH RQ WKH GLJLWL]DWLRQ UHVXOWV dž2
p= 0.713,
p< 0.001) , w hile t he fact or “ t oot h” it self had lit t le HIIHFWdž2
p= 0.001, p= 0.749) .
DI SCUSSI ON
Th i s e x p e r i m e n t a l se t u p a l l o w e d a b r o a d com par ison of dif f er en t t ech n ological con cept s for ext raoral and int raoral digit izing syst em s. The m et hod of analysis was developed from a st udy design originally used t o evaluat e 3D changes in gypsum dies over t im e20.
One m aj or advant age of dispersion ceram ic dies LVWKHLUQRQUHÀHFWLQJVXUIDFHFRPSDUHGZLWKWKH m et al dies used in prior st udies15,20.
The aspect s of t he respect ive clinical procedure w ere considered im plying gypsum duplicat e dies for t he ext raoral syst em s and digit izing t he ceram ic m ast er die direct ly w it h t he int raoral syst em s. The sm all but exist ing error of im pression and gypsum die m aking of single t eet h30 raises t he syst em at ic
er r or in r elat ion t o int raoral digit izing. How ever, elim inat ing t his bias from t he experim ent al set - up w ould have m eant t o add an error source t o t he int raoral procedure t hat is not part of t he clinical procedure. At t he sam e t im e, one of t he advant ages of int raoral digit izing w ould have been precluded.
Dispersion ceram ic dies can be direct ly digit ized w it hout t he use of pow der ( unless r equir ed by t he digit izing t echnology it self ) , w hich r educes t he syst em at ic error of t he experim ent al set up8.
I t cannot be ruled out t hat t he ceram ic m at erial LQÀXHQFHGWKHUHVXOWVHLWKHUSRVLWLYHO\RUQHJDWLYHO\ com pared wit h dent ine since especially t he presence of w at er affect s t he digit izing qualit y gr eat ly1 7.
Looking at t he quit e sim ilar result s for single t eet h ( prem olars) in a clinical t rial using t he CEREC 3D int raoral digit izing sy st em27 ( a sy st em t hat was
not part of t he st udy at hand) suggest s t hat t he PDWHULDOLQÀXHQFHLQWKHin- vit ro set - up present ed here is probably of m inor im port ance.
The only syst em requiring com plet e pow dering in t his st udy was t he CEREC Bluecam . The pow der WKLFNQHVVRIDSSUR[LPDWHO\WRPKDVWREH considered24 when com paring t hese values wit h t he
ot her int raoral syst em s. I n t he clinical procedure, know ing t he approxim at e pow der layer t hickness allow s adj u st m en t s for t h ese dev iat ion s in t h e subsequent st eps of t he process chain. Applying t he pow der as evenly as possible is m andat ory for high digit izat ion precision. Pow dering is prone t o applicat ion errors8.
The dat a set s obt ained wit h t he digit izing syst em t h at n eeded sligh t pow der du st in g f or pat t er n recognit ion ( Lava C.O.S.) did not show an addit ional layer or t hickness in t his in vit ro invest igat ion.
,Q DGGLWLRQ WR WKH XVH RI QRQUHÀHFWLQJ GLHV r ever se- engineer ing t he v ir t ual CAD m odel and corresponding real m ast er die om it s m anufact uring error22 and t hus im proves t he m et hod of analysis.
A very high precision and consequent reliabilit y ( st andard deviat ion of t he m ean posit ive deviat ion/ m e a n n e g a t i v e d e v i a t i o n o f t h e r e p e a t e d m easur em ent s) of SD= 0. 0012/ 0. 0014 m m and a very good reliabilit y ( Cronbach’s alpha= 0.999) of t he repeat ed m easurem ent s was found for t he r efer ence digit izer ODKM97. Refer ence digit izer s should be of high precision for t he accuracy analysis of dent al digit izing syst em s15.
PHDQP
Becau se g r ou p sam p le sizes w er e eq u al in t his in vit ro st udy, part ial et a- square was used t o est im at e t he effect size. However, t he int erpret at ion of t hese values according t o a rule of t hum b ( sm all 0.2, m iddle 0.5, and large 0.8) should not be over-int erpret ed; t he values are only over-int ended t o give a sense of t he size of t he effect of int erest .
Ba se d o n t h e st a t i st i ca l r e su l t s, t h e f i r st h y p ot h esis ( t h at op t ical n on - con t act d ig it izin g syst em s for int raoral and ext raoral dat a acquisit ion i n CAD / CAM t e ch n o l o g y h a v e d i f f e r e n ce s i n accuracy ) can be accept ed. Topical sy st em s for bot h ext raoral and int raoral digit izat ion ar e w ell
EHORZWKHEHQFKPDUNRIPGHULYHGIURPin
vit ro analyses of convent ional im pression and
die-m aking precision21,30. I n t erm s of t he digit izat ion
t echnology, high qualit y dat a could be obt ained wit h whit e- light fringe proj ect ion, red or blue lasers, proj ect ed laser lines or grids, confocal m icroscopy, and Act ive Wavefront Sam pling. These result s are support ed by a recent review on int raoral digit izing syst em s32.
As found in previous st udies14, t he shape of a
prepared t oot h, w hich is represent ed by t he fact or “ t oot h” in t he present st udy, was found t o affect t he achievable accuracy, depending on t he digit izing sy st em used. Thus, t he second hy pot hesis w as FRQ¿UPHG
The fact or “ t oot h” should not be t aken lit erally. Rat her, t he incisor or m olar preparat ions represent different angles ( obt use or acut e) and shadow ed r egions, w hich cannot be digit ized accurat ely or st eep and parallel opposing t oot h sur faces. For m ost digit izing syst em s, st eep and parallel incisor VXUIDFHV DUH JHQHUDOO\ WKH PRVW GLI¿FXOW DUHDV t o capt ur e. This effect can be seen in Figur e 3, w hich show s t hat m ost of t he digit izing syst em s h av e clear ly g r eat er accu r acy w h en d ig it izin g m olars. Depending on t he individual preparat ion, t he pr oblem s w it h st eep pr eparat ion angles can also occur w it h m olar s15 in conv er gence angles
of 0°6. These problem s w ere less pronounced for
WKHKLJKDFFXUDF\GLJLWL]HUPDQDO\]HGLQ t he st udy of Chan, et al.6 ( 2011) t han for low
er-DFFXUDF\ V\VWHPV ± P 7KH LQÀXHQFH RI t he convergence angle becom es m ore pronounced furt her along t he CAD/ CAM process chain4.
The successful com pensat ion for such syst em at ic error int roduced during incisor digit izat ion can be seen w hen com par ing t he m or e r ecent 3Shape d i g i t i zer m o d el ( D 6 4 0 ) w i t h t h e o l d er m o d el ':HDNQHVVHVVXFKDVGLI¿FXOWLHVZLWKVWHHS and parallel sur faces can be det ect ed w it h t he experim ent al set up present ed here.
The cur r ent r esult s ar e lim it ed because only single t eet h w er e used1. Conclusions concer ning
long- span digit izat ion11, 26 and suit able digit izing
st rat egies10 cannot be draw n. How ever, t he use
of individual t eet h is an advant age w hen broadly com paring m any different digit izing syst em s and t echnologies.
Result s of previous digit izing syst em analyses have been report ed in m any different variat ions: p o si t i v e a n d n e g a t i v e d e v i a t i o n s1 2, a b so l u t e
values26, point t o point m easur em ent s, differ ent
surface areas, deviat ion percent ages, m ean values for accuracy9, and com binat ions t her eof, w hich
PDNHLWGLI¿FXOWWRFRPSDUHUHVXOWVDFURVVVWXGLHV The applicat ion of I SO 12836: 201213 could lead t o
bet t er com parabilit y of fut ure result s.
CON CLUSI ON S
St udy result s are lim it ed since only single t eet h w er e used for com par ison. Opt ical non- cont act d i g i t i zi n g sy st em s f or i n t r aor al an d ex t r aor al dat a acquisit ion in CAD/ CAM t echnology show ed differences in accuracy, but all t opical syst em s were ZHOOEHORZDEHQFKPDUNRIP
The incisor or m olar preparat ions used represent d if f er en t an g les as w ell as st eep an d p ar allel opposing t oot h surfaces in t he incisor preparat ion. For m ost digit izing syst em s, t he lat t er are generally WKH PRVW GLI¿FXOW DUHDV WR FDSWXUH :KHQ XVLQJ CAD/ CAM t ech n ologies, t h e pr epar at ion an gles should not be t oo st eep and sharp edges should be rounded off t o reduce digit izing errors. Older syst em s m ight be lim it ed t o a cer t ain height or t aper of prepared t oot h. More recent syst em s for ext raoral, as well as for int raoral digit izat ion, do not have t hese lim it at ions.
ACKN OW LEDGM EN TS
The Germ an dent al com pany 3M Deut schland AG, in Seefeld, support ed part s of t his st udy.
Som e of t he result s w ere present ed at t he 44t h
m eet ing of t he I ADR- Cont inent al European Division ( CED) , Scandinav ian Div ision ( NOF) , and I sraeli Division ( I D) in Munich, Sept em ber 9t h–12t h, 2009.
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