J Dent Hyg Sci Vol. 14, No. 1, 2014, pp.29-34 RESEARCH ARTICLE 저온열화현상이지르코니아코어와전장도재의전단결합강도에미치는영향 김기백ㆍ김재홍 고려대학교일반대학원보건과학과치의기공전공 Influence of Low Temperature Degradation on Bond Strength of Yttria-Stabilized Tetragonal Zirconia Polycrystal Core to Veneering Ceramic Ki-Baek Kim and Jae-Hong Kim Department of Health Science Specialized in Dental Laboratory Science and Engineering, Graduate School, Korea University, Seoul 136-703, Korea The purpose of this study was to evaluate the influence of low temperature degradation (LTD) on the bonding strength of yttria-stabilized tetragonal zirconia polycrystal (Y-TZP). The push-shear bond test method was used to investigate the core-veneer bonding strength of industrially manufactured Y-TZP core ceramic and manufacturer recommended veneering ceramic. Four groups from ceramic-zirconia specimens (n=28; n=7 per group) were assigned into four experimental aging conditions, namely storage in an autoclave at 134 o C for 0, 3, 5, 10 hours. Bonding strength was obtained using a universal testing machine with crosshead speed 0.5 mm/min. Data were statistically analyzed using one-way ANOVA and Tukeyʼs test (α=0.05). In bonding strength test, the group which was treated with LTD showed lower bonding strength than no treated group. The ceramic-zirconia bonding strength was affected by LTD (p<0.05). Digital microscope examination of the fracture surface showed mixed failures with adhesive and cohesive types in LTD with treated Y-TZP groups. Key Words: Autoclave, Bonding strength, Low temperature degradation, Yttria-stabilized tetragonal zirconia polycrystal 서론 치아가상실된경우과거에는단순히저작과발음기능의회복과같은기능적인측면이주된관심사였지만, 최근에는그러한기능적인회복만이아니라심미적인측면에서도만족할만한결과를요구하고있다. 이러한환자들의요구와재료및치료술식의발달로금속수복물보다는금속-도재수복물이나금속사용이배제된도재수복물의사용이증가하고있다 1). 하지만금속-도재수복물의경우에는도재소 성과정을거치는동안금속하부구조의변형및이에따른적합성문제, 금속의불투과성에의한변연치은부의비심미성및금속으로인한과민반응이일어날수있는가능성등이문제점으로지적되고있다 2,3). 1980년대후반부터치의학분야에는심미성이높은전부도재수복시스템들이소개되어상용화되기시작했으나, 낮은파절강도로인해시장성을충분히확보하지못했다. 이를해결하기위한대안으로다양한재료의술식이개발되었고, 2000년대초반부터지르코니아를이용한보철물이소 Received: January 14, 2014, Revised: February 19, 2014, Accepted: February 19, 2014 ISSN 1598-4478 (Print) / ISSN 2233-7679 (Online) Correspondence to: Jae-Hong Kim Department of Health Science Specialized in Dental Laboratory Science and Engineering, Graduate School, Korea University, 161, Jeongneung-ro, Seongbuk-gu, Seoul 136-703, Korea Tel: +82-2-940-2762, Fax: +82-2-909-3502, E-mail: noreason07@korea.ac.kr Copyright 2014 by the Korean Society of Dental Hygiene Science This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/ by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
J Dent Hyg Sci Vol. 14, No. 1, 2014 개가되었는데, 지르코니아는산화지르코니움 (zirconium oxide, ZrO 2) 의총칭으로화학적안정성과체적안정성을갖는다형 (polymorphic) 구조로되어있으며, 상전이시발생하는체적확장에의해균열의진행을억제하여기존의도재에비해높은굴곡및파절강도를갖고있다 4). 외부에서가해지는응력또는기공과정에서발생하는응력에의하여정방정계결정상이단사정계결정상으로상변이 (phase transformation) 가일어날수있다. 이때 3 5% 의부피팽창을수반하여내부압축응력이형성되면미세균열의진행을억제하여강화효과를얻을수있고, 이러한복잡한메커니즘을상변이강화 (transformation toughening) 라고한다 5). 하지만지르코니아의내구성에대한연구에서높은강도와인성에도불구하고정방정지르코니아를저온에서장시간사용하면정방정상에서단사정상으로의자발적인상변이가일어나, 이에따른부피팽창이시편내에미세균열을유발하여강도의급격한저하가일어나는저온열화 (low temperature degradation) 현상이생긴다는연구보고가있다 6). Chevalier 등 7) 의연구에서는 250 o C 이하의습윤환경 ( 물, 증기 ) 이지르코니아상변이에미치는영향을보고하였다. 연구결과불순물함량, 결정크기, 밀도, 미세구조뿐만아니라습윤환경과온도가정방정계에서단사정계의상 (phase) 으로불안정화를유도한다는것을보고하였다. 이러한불안정화는지르코니아표면의거침과정방정계상태의결정을단사정계의상으로상변이를유도하여미세잔금을야기한다고한다. 지르코니아전장용상부도재의파절원인에대해서는많은연구 8) 가진행되고있는데, 지르코니아코어와상부도재간의결합력이낮고파절되는원인으로는열팽창계수차이에의한응력집중과지르코니아에대한상부도재의낮은젖음성, 상부도재의소성에따른수축, 열이나응력부하에의한지르코니아와상부도재계면에서의지르코니아결정변태와제작과정에발생한결합등으로연구되고있다 9). 또한장기적인임상연구에서도지르코니아전부도재관에서의전장도재가과도하게탈락 (delamination) 되는현상에대한연구보고가있다. Sailer 등 10) 의연구에서는 3년후에약 13% 정도파절되었다는임상연구가있으며, 이와같은결 과는금속도재관의실패율과비교했을때상당히높은것이라고보고되고있다 11). 구강내환경은치과재료의물리화학적변화를유발할가능성이매우높다. 특히수분이있는환경에서온도변화는저온열화발생을위한조건을제공한다. 따라서치의학분야에서지르코니아보철의성공적인적용을위해서는이러한구강내환경에서피로의특성을파악하는것이매우중요하다. 본연구는지르코니아의저온열화가보철물의안정성및내구성에미치는영향을확인하기위해구강내조건을재현한고온증기멸균소독기에장시간시편을처리한후전단결합강도를측정하여지르코니아전부도재관의열에의한변화를알아보고자하였다. 또한지르코니아코어와전장용상부도재간의계면에서발생하는파절양상을분석하여기초자료를제공하고자하였다. 연구대상및방법 1. 연구대상 1) 실험재료본연구에서사용한 yttria 안정화된지르코니아는 computer-aided design/computer-aided manufacture (CAD/ CAM) 가공용으로제작된 Rainbow block (Dentium Co., Inc., Seoul, Korea) 을사용하였고, 전장도재는 Vintage Hale ZR (Shofu Inc., Kyoto, Japan) 을사용하였다. 실험에서사용된치과용지르코니아와전장도재의조성은 Table 1과같다. 2) 시편제작구강환경에서의열변화가상부도재세라믹과지르코니아코어와의전단결합강도에미치는영향을알아보기위하여전용의지르코니아블록 (Rainbow) 을 International Organization for Standardization (ISO) 6872 규격 11) 에의거하여두께 1.2 mm (±0.2 mm), 직경 12 16 mm의디스크형태시편으로제작하였다. 가소결된시편은 water-cooled diamond disc (Metsaw; R&B Inc., Seoul, Korea) 를이용하여 Table 1. Types of Core and Veneering Ceramic, Composition and Manufacturing Company Names (n=28) Brand Composition Manufacturer Coefficient thermal expansion Rainbow ZrO 2 88 96%, Y 2O 3 4 6%, Dentium Co., Inc., Seoul, 10.0 10 6 K 1 Al 2O 3<1% Korea Vintage Hale ZR SiO 2<73.6%, Al 2O 3<11.9%, CaO<0.7%, MgO<0.5%, K 2O<7.0%, Na 2O<7.8%, Li 2O<0.4%, B 2O 3<0.8%, Ce 2O 3<0.7% Shofu Inc., Kyoto, Japan 9.4 10 6 K 1 30
김기백ㆍ김재홍 : 저온열화현상이지르코니아전단결합강도에미치는영향 Fig. 1. Illustration of specimen preparation. Table 2. Definitions of Different Failure Modes Failure mode Adhesive Cohesive Mixed Definition Complete delamination of veneering porcelain from core material Fracture occurs completely and only within veneering porcelain or within core material Fractured surfaces are within veneering porcelain with areas of core material exposed indicating localized adhesive failure Fig. 2. Schematic diagram of bond strength test. M: metal zig, C: zirconia core, V: porcelain veneer, L: load (cross-head speed 0.5 mm/min). 디스크형태로제작하고전용소환로 (Zirkonofen 600; Zirkonzahn GmbH, Gais, Italy) 에서최종소결하였다. 지르코니아표면의거칠기를일정하게만들기위해 400, 600 grit silicon carbide paper (Buehler Ltd., Lake Bluff, IL, USA) 로연마한후증류수에넣어 5분간초음파세척한다음건조하여사용하였다. 시편마다동일한크기를갖는도재를축성하기위해실리콘몰드를사용하여두께 4 mm, 직경 5 mm의상부도재 (Vintage Halo ZR) 를제조사의소성스케줄에따라완성하였다. 한명의숙련된치과기공사가작업하고, 도재의수축량을감안하여동일한과정을모든시편에 2회에걸쳐서시행하였다. 각실험군당 7개의시편을제작하여 4개의실험군으로총 28개의시편으로실험하였다 (Fig. 1). 2. 연구방법 1) 저온열화실험구강내지르코니아코어의내구성에대한실험을단기간에하기어렵기에수년간구강안에서코어를장착한것과같은인위적노화환경을조성하고자, ISO 13356 규격 12) 에따라고온증기멸균소독기 (1L-NS-AS; Ilshin Autoclave, Seoul, Korea) 134 o C의온도에서 0.2 MPa의압력의수열조 건에서시행하였다. Chevalier 등 7) 은약 5시간동안 134 o C 에서, 증기살균은 37 o C에서 15 20년의열화를재현한것과같음을보고한바있다. 선행연구에의거하여저온열화의정도를조절하기위해동일한방법에서 3, 5, 10시간으로나누어진행하였다. 2) 전단결합강도측정완성된시편들은만능시험기 (Model 8871; Instron, Canton, MA, USA) 에전단장치를부착하여 ISO/TR 11405 규격 13) 에의거하여 cross-head speed 0.5 mm/min로압력을가하여시편의지르코니아표면에결합된상부도재가파절될때의힘 (load) 을측정하였다. 지르코니아표면에결합되어있는전장도재가힘에의하여파절되는값과단면적을이용하여산출한전단력을지르코니아-전장도재간의결합력으로평가하였다 (Fig. 2). 3) 파절면관찰전단결합강도측정후접착계면의파절양상은 Table 2와같은기준 14) 으로실체현미경 (KH-7700; Hirox, Kyoto, Japan) 으로관찰하여분류하였다. 4) 통계분석전단결합강도의평균과표준편차를계산하고, 측정된각군의전단결합강도간에유의할만한차이가있는지를검증하기위해 SPSS 12.0 통계처리프로그램 (SPSS Inc., Chi- 31
J Dent Hyg Sci Vol. 14, No. 1, 2014 cago, IL, USA) 을사용하여일원분산분석 (one-way ANO- VA) 을통해시행하였고, 집단간의차이를다중비교검정의하나인 Tukey's honestly significant difference test 방법을통해사후분석하였다. 본논문에서사용된모든통계는 95% 의유의수준에서검정되었다. Table 3. Mean and Standard Deviation (SD) of the Bond Strength (MPa) according to Autoclave Time Group Autoclave time (h) n 결과 1. 저온열화시간에따른전단결합강도변화각실험군에대한전단결합강도의평균과표준편차는 Table 3과같으며, 각실험군간의전단결합강도차이를검정하기위해일원분산분석을시행한결과각실험군별로전단결합강도는유의한차이가있었다 (p<0.05, Table 4). 지르코니아코어와전장도재간의평균전단결합강도는저온열화를시행하지않은그룹에서는 24.29±2.42 MPa, 10 시간저온열화를시행한그룹에서 20.17±2.79 MPa로, 가장장시간저온열화를처리한시편이유의하게전단결합강도가낮아진것으로나타났다 (p=0.031). 2. 파절양상분석저온열화처리하여지르코니아코어와전장도재간의전단결합강도측정후파절양상을살펴본결과, 저온열화처리를하지않은시편에서는표면에전장도재내에파절이 Mean±SD (MPa) Min Max Ⅰ 00 7 24.29±2.42 a 20.53 27.86 Ⅱ 03 7 21.83±3.04 b 18.38 24.31 Ⅲ 05 7 21.23±1.95 c 17.54 26.02 Ⅳ 10 7 20.17±2.79 a 16.04 22.85 Data with the same letters are significantly different at 0.05 significance level. 발생되는응집성파절이비교적많이관찰되는것을알수있었으며, 저온열화처리가길게진행될수록접착성, 응집성파절이혼합하여동시에발생하는혼합형파절이대부분관찰되었다 (Fig. 3). Table 4. One-Way ANOVA Results on the Bond Strength of All Groups Sum of squares 고찰 지르코니아는높은파절강도와투과성으로인해심미치과영역에서첨단신소재로개발되기시작했으며, 치과 CAD/ CAM 술식에서사용되는지르코니아는안정화제 Y 2O 3 첨가물이사용되어 yttria-stabilized tetragonal zirconia polycrystal로알려져있다 15). 안정화된지르코니아는외적으로가해지는충격이나스트레스를받게되면미세균열이발생되고균열주변에자발적인상변이를일으키며 3 5% 정도의팽창을하며국부적인응력을형성하여균열이더진행및전파되지않도록한다. 그리하여다른세라믹과는다른상변이를통한인성증가 (transformation toughening mechanism) 로인하여높은파절저항성을갖게된다. 하지만상변이에따른이러한기구의인성이증진되는장점과는다르게파절의원인으로단사정으로서의상전이에의한표면의조도화, 입계의파괴, 미세균열의발생등에강도와경도의감소가거론되고있다 6). 일반적으로상전이에따른열화는 100 o C 400 o C에서장시간열처리에의해일어나지만물이나수증기하에서열화속도가가속되는것으로알려져있다 16). 상온에서안정화된정방정상의지르코니아의균열주 Degree of freedom Mean squares F-ratio p-value Inter-group 064.154 3 21.385 3.504 0.031 Intra-group 146.467 24 06.103 Sum 210.620 27 Fig. 3. Representative images of the zirconia core surface (microscope images, 160). (A) Cohesive failure, (B) mixed failure. 32
김기백ㆍ김재홍 : 저온열화현상이지르코니아전단결합강도에미치는영향 변으로발생된단사정의분포가많게되고이로인하여전체적인강도의저하와정방정상의지르코니아열팽창계수 (coefficient thermal expansion) 인 8.5 10.5 10 6 에서단사정상에서는 6.5 7.5 10 6 로변하게되어지르코니아를하부구조로사용하고그위에전장도재를축성후소결시킨도재관의경우파절및박리를야기시킬수있는원인이될수있다 17). 본연구에서는구강내환경에서지르코니아심미보철물의전장도재결합력실험을직접적으로수행하기어렵기에, 수년간구강안에서보철물을장착한것과같은인위적인노화 (aging) 환경을조성하였다. 그리하여시판중인 Rainbow block을가공한시편을준비하여 autoclave에서저온열화실험을하였다. 이연구의목적은구강내환경에서의저온열화가전장도재와지르코니아의결합력에어떠한영향을미치는지알아보고자하는것이다. 본실험결과저온열화전과처리시간에따른전단결합강도결과는저온열화처리전은 24.29 MPa, 저온열화처리시간이길수록결과값은감소되는경향을보였다. 10시간저온열화처리후의전단결합강도는 20.17 MPa로나타나통계적으로유의한차이가있었다 (p<0.05). 기존연구에서수분과열에의한저온열화현상으로인해지르코니아의결정에미세균열이발생되는것으로판명되었으며, 수분의양이많고온도가높을수록미세균열이가속화됨을조사한바있다 18,19). Michael 등 20) 의연구에의하면전장도재축성과정에서도재에함유된수분이도재와지르코니아간의결합력을감소시킨다는연구가보고되었다. 전장도재축성과정에서사용되는수분은소성전건조과정에서수증기로증발하게되고지르코니아결정은수증기에노출된다. 그러므로전장도재내부의수분이증발하는동안정방정계의상에서단사정계의상으로전환되고, 균열현상을유발하여전장도재와지르코니아코어의결합력이떨어지는결과를낸것이라는결론을내었다. 이와같은결과를미루어볼때본실험의결과와유사함을확인할수있었다. 또한 Kohorst 등 21) 은고압증기멸균기를이용하여 134 o C 열수상태 (hydrothermal condition) 에 5시간보관한후전장도재를소성한실험군의전단결합강도가유의하게낮게측정된결과도본실험에결과를뒷받침할수있겠다. 파절양상은항상결합강도의결과값과일치하지않지만저온열화처리를하지않는높은전단결합강도에서는응집성파절양상이많이발생되었으며, 저온열화처리를한시편에서는혼합형파절양상을이루었다. 지르코니아와전장도재간의결합력이전장도재파절강도보다높은경우응집성파절양상을보이며, 전장도재의강도가낮다면접착성, 혼합형파절이나타난다고한다 22). 이러한경우를볼때파절양상과결합강도값의관계성을정확히논하기어렵지만, 저온열화현상으로인한상전이가기계적인강도를저하시켜서이와같은파절경향을보였다판단할수있다. 치과용지르코니아는심미보철물의수복재료로본격적으로사용된지얼마되지않았으나구강안에존재하는타액이나음식물과같은수분과저작압, 측방압등기계적마찰력으로의해저온열화현상이일어날수있는가능성이산재한다. 또한구강안에완성된심미보철물을장착한후오랫동안가해지는응력들이표층에서심층으로축적이될것이라짐작되기에상전이를일으키게되고저온열화현상이진행될것이다. 이에본실험설계의지르코니아코어와전장도재의전단결합강도평가는심미보철물의내구성을예측하는데매우유용할것이라생각된다. 하지만전단결합강도에영향을주는하중속도, 응력분산유형, 두께비율, 결합면의길이와넓이및파절시작점의위치와균열확산경로등 23) 다양하기때문에절대적인임상적결과로확대해석하기에는다소무리가있음을고려해야할것이다. 본연구는치과용지르코니아와전장도재간의전단결합강도를실험하였고, 저온열화현상중에전장도재와의결합력이약화되는양상을나타냈다. 그러나본실험에서사용한시편은임상적인치과수복물의형태를반영하지못하였고, 단일제품으로실험을진행하였기에심미보철물로사용하는치과용지르코니아에대한일부연구로서의한계를갖는다. 그렇지만, 치과용지르코니아를이용하여제작되는보철물의내구성및재료에대한안정적인사용을위한시사점을제시했다고본다. 요약 본연구는최근심미보철물제작에널리사용되는지르코니아의저온열화가수복물의안정성및내구성에영향을미치는여부를확인하기위하여전단결합강도를측정하였다. 디스크형태의시편을각각 7개씩제작한후전장도재를축성하여, ISO 13356 규격에의거한조건으로고압증기멸균기에저온열화의정도를조절하기위하여 3, 5, 10시간동안수열처리를진행하였다. 제한된조건하에시행된실험을통해전단결합강도를확인하였으며, 측정후파절양상을관찰하였다. 저온열화처리전후의시편에대한전단결합강도의변화는각실험군별유의한차이가있었다 (p<0.05). 10시간을저온열화처리한시편이가장낮은전단결합강도로나타났으며, 처리시간이길어질수록전단결합강도가낮아지는경 33
J Dent Hyg Sci Vol. 14, No. 1, 2014 향을보였다. 파절양상으로저온열화처리를하지않은시편에서응집성파절을보였으며, 저온열화처리가길어질수록혼합형파절경향으로전환되었다. 결론적으로본연구는저온열화현상이진행될수록지르코니아코어와전장도재간의결합강도가낮아지는경향을확인하였다. 본실험결과를토대로치과용지르코니아를이용한보철물제작과정과환자에게장착후안정적인사용을위하여저온열화현상에대한주의가필요할것으로생각된다. References 1. Denry I, Kelly JR: State of the art of zirconia for dental application. Dent Mater 24: 299-307, 2008. 2. Kim KB, Kim WC, Kim HY, Kim JH: An evaluation of marginal fit of three-unit fixed dental prostheses fabricated by direct metal laser sintering system. Dent Mater 29: e91-e96, 2013. 3. Tara MA, Eschbach S, Bohlsen F, Kern M: Clinical outcome of metal-ceramic crowns fabricated with laser-sintering technology. Int J Prosthodont 24: 46-48, 2011. 4. Piconi C, Maoccauro G: Review zirconia as a ceramic biomaterial. Biomaterials 20: 1-25, 1999. 5. Hannink RH, Kelly PM, Muddle BC: Transformation toughening in zirconia containing ceramics. J Am Ceram Soc 83: 461-487, 2000. 6. Chevailer J, Cales B, Drouin JM: Low-temperature aging of Y-TZP ceramics. J Am Ceram Soc 82: 2150-2154, 1999. 7. Chevalier J, Gremillard L, Deville S: Low-temperature degradation of zirconia and implications for biomedical implants. Annu Rev Mater Res 37: 1-32, 2007. 8. Sundh A, Sjogren G: A comparison of yttrium-oxidepartially-stabilized zirconia ceramic crowns with varying core thickness, shapes and veneer ceramic. J Oral Rehabilitation 31: 682-688, 2004. 9. De Jager M, Pallav P, Feilzer AJ: The influence of design parameters on the FEA-determined stress distribution in CAD/CAM produced all-ceramic dental crowns. Dent Mater 21: 242-251, 2005. 10. Sailer I, Feher A, Filser F, et al.: Prospective study of zirconia posterior fixed partial dentures: 3-year follow up. Quintessence Int 37: 41-49, 2006. 11. Hammerle CHF: Success and failure of fixed bridge-work. Periodontal 4: 41-51, 1994. 12. International Organization for Standardization: Ceramic materials, ISO 6872. International Organization for Standardization, Geneva, Switzerland, 2008. 13. International Organization for Standardization: Implant for surgery ceramic materials based on yttria-stabilized tetragona zirconia (Y-TZP), ISO 13356. International Organization for Standardization, Geneva, Switzerland, 2008. 14. International Organization for Standardization: Dental Materials-guidance on testing of adhesion to tooth structure, ISO/TR 11405. International Organization for Standardization, Geneva, Switzerland, 1994. 15. Kern M, Wegner SM: Bonding to zirconia ceramic: adhesion methods and their durability. Dent Mater 14: 64-71, 1998. 16. Evans AG, Heuer AH: Review-transformation toughening in ceramics: martensitic transformations in crack-tip stress fields. J Am Ceram Soc 63: 241-248, 1980. 17. Kosmac T, Oblak C, Jevnikar P, Funduk N, Marion L: The effect of surface grinding and sandblasting on flexural strength and reliability of Y-TPZ zirconia ceramic. Dent Mater 15: 426-433, 1999. 18. Sato T, Ohtaki S, Shimada M: Transformation of yttria partially stabilized zirconia by low-temperature annealing in air. J Mater Sci 20: 1466-1470, 1985. 19. Deville S, Chevalier J, Gremillard L: Influence of surface finish and residual stresses on the aging sensitivity of biomedical grade zirconia. Biomaterial 27: 2186-2192, 2006. 20. Michael JT, Christoph B, Michael VS, Norbert T: XRD 2 micro-diffraction analysis of the interface between Y-TZP and veneering porcelain: Role of application methods. Dent Mater 26: 545-552, 2010. 21. Kohorst P, Borchers L, Strempel J, et al.: Low-temperature degradation of different zirconia ceramics for dental applications. Acta Biomater 8: 1213-1220, 2012. 22. Ai-Dohan HM, Yaman P, Dennison JB, et al.: Shear strength of core-veneer interface in bi-layered ceramics. J Prosthet Dent 91: 349-355, 2004. 23. Kim KB, Kim JH: A study on the shear bond strength of veneering ceramics to the lithium disilicate (IPS e.max CAD) core. J Dent Hyg Sci 13: 290-295, 2013. 34