氧化锆及4种全冠修复材料磨耗性能的对比

doi:10.3969/j.issn.2095-4344.2013.25.006

摘要/Abstract

摘要：

背景：氧化锆陶瓷是近年来新兴的一种口腔修复材料，因其美观性及生物相容性好而受到患者的欢迎，但目前关于其与对颌天然牙磨耗性能方面的研究较少。
目的：观察氧化锆与其他4种临床常用的全冠修复材料与滑石瓷对磨时对其造成的体积损失量，并与天然牙比较，研究氧化锆及另外4种修复材料的磨耗差异。
方法：将氧化锆陶瓷，vita陶瓷，金钯合金，镍铬合金，钴铬合金及对照组天然牙釉质作为上试件，分别安装于磨耗机上部夹具中，将滑石瓷作为下试件，分别安装于磨耗机下部夹具中，在人工唾液环境下进行加载磨耗实验，计算上下试件的体积损失量；使用纤维硬度测试仪对上述试件进行硬度测试；扫描电镜下观察试件表面磨耗形态。
结果与结论：5种材料所致的滑石瓷体积损失量均大于对照组滑石瓷体积损失量 (P < 0.05)；氧化锆陶瓷硬度值最高，自身及对磨滑石瓷体积损失量最大；金钯合金硬度值最小，自身及对磨滑石瓷体积损失量最接近天然牙；5种材料的硬度与其导致的对磨滑石瓷体积损失量呈显著正相关 (P < 0.05)。氧化锆及vita陶瓷主要为磨粒磨损，表面沟裂突起样结构明显、磨耗犁沟深、形态不规则，磨耗面可见散在气孔结构。钴铬合金磨耗面相对光滑，表面磨耗犁沟较浅，磨耗面散在金属磨粒；镍铬合金磨耗犁沟深而宽，磨耗面存在大量金属磨粒；金钯合金磨耗犁沟均匀分布，偶见剥脱斑块，斑块内金属磨粒附着。表明氧化锆陶瓷在临床中容易造成对颌天然牙的过度磨耗，永久黏固前应对其进行精细抛光；金钯合金的磨耗性能与天然牙最接近且最优，是较好的后牙区全冠修复材料；口腔修复材料的硬度值不能代表其磨耗性能大小，应综合考虑材料表面的微观结构等因素，采取措施发挥各种材料优势。

关键词: 生物材料, 组织工程口腔材料, 氧化锆, 金钯合金, 镍铬合金, 钴铬合金, 牙釉质, 磨耗, 硬度, 耐磨性

Abstract:

BACKGROUND: Zirconia ceramics is a novel oral repair material in recent years, and it has welcomed by patients because of its aesthetics and biocompatibility, but the research on the wearing ability of zirconia ceramics on the jaw natural tooth is rare.
OBJECTIVE: To observe the volume loss of zirconia ceramics and other four kinds of clinical commonly used full crown repair materials when grinding with steatite, and to compare with the natural tooth in order to research the wearing ability of zirconia and other repairing materials.
METHODS: The zirconia ceramic, vita ceramics, gold-palladium alloy, nickel-chromium alloy, cobalt-chromium alloy and fresh natural enamel in the control group were installed in the upper fixture of wear machine respectively, and the steatite porcelain was installed in the lower fixture. Load abrasion tests were performed in artificial saliva environment, and then the volume loss of the materials in the upper and lower fixture was calculated; the hardness of the materials above was tested with fiber hardness testing machine; the specimen surface abrasion morphology was observed under scanning electron microscope.
RESULTS AND CONCLUSION: The volume loss of the five materials when grinding with steatite was larger than that of the fresh natural enamel in the control group (P < 0.05); the hardness of zirconia ceramics was highest, and the volume loss of zirconia ceramics when grinding with steatite was largest. The gold-palladium alloy had the smallest hardness, and the volume loss of gold-palladium alloy when grinding with steatite was similar to that of the natural teeth; the hardness of all the materials was positively correlated with the volume loss of grinding steatite porcelain (P < 0.05). The zirconia ceramic and vita ceramic were mainly performed as abrasive wear, and the surface was in fissure protruding-like structure with deep abrasion furrows and irregular morphology, the scattered pore structure could be seen on the abrasion surface. The abrasion surface of cobalt-chromium alloy was relatively smooth with shallow abrasion furrows, and the scattered metal abrasive particles could be seen on the abrasion surface; deep and wide abrasion furrows and a large amount of metal abrasive particles could be seen on the abrasion surface of nickel-chromium alloy; abrasion furrows was evenly distributed on the abrasion surface of gold-palladium alloy, and the exfoliative plaque could be seen occasionally with metal abrasive particles in it. The results indicate that zirconia ceramic is likely to cause excessive abrasion on the jaw natural tooth in clinical application, so polishing should be performed before permanent cements; the gold-palladium alloy has the best wearing ability which is close to that of the natural teeth, and it is considered the better posterior region full crown repair material; the hardness of the dental prosthetic material can not represent the wearing ability, the microstructure of the material surface and other factors should be considered when using in order to play the advantages of various materials.

Key words: biomaterials, tissue-engineered oral materials, zirconia, gold-palladium alloy, nickel-chromium alloy, cobalt-chromium alloy, enamel, wearing, hardness, wear resistance

中图分类号:

R318

王军，王明臻，王万春，扬梅. 氧化锆及4种全冠修复材料磨耗性能的对比[J]. 中国组织工程研究, 2013, 17(25): 4601-4608.

Wang Jun, Wang Ming-zhen, Wang Wan-chun, Yang Mei. Wearing ability of zirconia and four kinds of full crown repair materials: Comparative study[J]. Chinese Journal of Tissue Engineering Research, 2013, 17(25): 4601-4608.

图/表（结果） 3

Wear data analysis in the specimens Seen from Table 1, the volume loss of grinding steatite porcelain caused by the five metal materials was larger than that caused by natural tooth (P < 0.05), and ranked as follows: Zirconia > vita ceramic> cobalt-chromium alloy > nickel-chromium alloy > gold-palladium alloy. The volume loss of grinding steatite porcelain in the zirconia group was significantly different from that in the other groups (P < 0.05), indicating that zirconia may easily result in the excessive wear on the opposite natural teeth. The volume loss of the grinding material caused by gold-palladium alloy was lower than that in the other groups (P < 0.05) and was similar to that of the natural tooth, thus the wearing ability of the gold-palladium alloy was close to that of the natural teeth, and the wearing ability of the gold-palladium alloy was better than that of the zirconia.

Hardness values of different specimens

The densities of human enamel, zirconia, Vita ceramics, cobalt-chromium alloy, nickel-chromium alloy, and gold-palladium alloy were 3.4, 6.2, 8.6, 9.0, 10.0, 19.0 g/cm³, respectively. The hardness values of human enamel, gold-palladium alloy, nickel-chromium alloy, cobalt-chromium alloy, Vita ceramics, and zirconia were (202.00±3.83), (168.00±4.65), (301.00±10.90), (399.00±4.32), (288.00±12.27), (942.00±64.54) MPa, respectively. The hardness value of zirconia specimens was higher than that of the other specimens (P < 0.05), while the hardness value of gold-palladium alloy specimens was lower than that of the other specimens (P < 0.05). No significant difference was found in the hardness value of nickel-chromium alloy, cobalt-chromium alloy, and Vita ceramics specimens (P > 0.05).

Correlation analysis between the hardness of different specimens and volume loss of grinding steatite porcelain (Figures 1, 2)

The hardness values of gold-palladium alloy, nickel-chromium alloy, and cobalt-chromium alloy specimens were positively correlated with the volume loss of grinding steatite porcelain (r=0.897, 0.922, 0.976, P < 0.05), indicating the higher the hardness, the larger the volume loss. The hardness of zirconia and vita ceramics was also positively correlated with the volume loss of grinding steatite porcelain (r=0.893, 0.925, P < 0.05).

Correlation analysis between the wear morphology and wear performance of different specimens (Figures 3, 4)

There are abrasive wear, surface fatigue, adhesive wear, and erosive wear. By observing and analyzing the wear morphology of the specimens, we can understand the wear mechanisms of different materials and natural teeth.

Wear surfaces of vita and zirconia ceramic specimens mainly belonged to abrasive wear, abrasive grains were attached to the wear surface and presented a membrane-like structure. On the wear surface of zironia and vita ceramic specimens, fissure and protruding-like structure was visible, furrows were deep and irregular, and pores were scattered. These fissure-like protrusions exacerbated the wear on the grinding steatite.

There were regular scratches and furrow-like structures, arranging orderly, on the surface of metal materials. Exfoliative plaques scattered on the wear surface. Under the high-powered electron microscope, exfoliative plaques exhibited irregular shape and local plastic deformation, and metal abrasive grains were seen adherent to the wear surface. It could be seen that adhesive wear and abrasive wear happened on the metal materials. The protrusion structure on the wear surface led to the force deformation, and some parts with low shear strength were avulsed during friction movement, thereby resulting in the occurrence of adhesive wear. Abrasive wear occurred because the materials undergoing a long-term friction appeared to have fatigue cracks and form exfoliative plaques, some metal abrasive grains formed and were pressed again into the wear surface resulting in furrow formation caused by the secondary friction. By comparing the images, we found that the wear degree of different metal specimens ranked as follows: cobalt-chromium alloys < nickel-chromium alloy < gold-palladium alloy. Based on the hardness analysis, we also found that the higher the hardness, the milder the wear.

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引言

Dental abrasion, a normal physiological process, refers to the loss of dental hard tissue resulting from the friction between the tooth surface and the food during the chewing movement^[1]. However, in life, there are many reasons that can cause the excessive wear on the natural teeth, such as poor chewing habits and poor wear performance of repair materials. The excessive wear of the natural teeth can cause injuries to the dental hard and soft tissues, maxillofacial muscles, and joints^[2-5], resulting in a series of oral diseases, such as dentin hypersensitivity, pulpitis, maxillofacial muscle dysfunction, and temporomandibular joint disorders^[6-7]. Therefore, the wear performance of the oral materials needs to be studied in depth^[8]. In addition, an ideal repair material should have a wear performance lower than but similar to the natural tooth^[9-10], based on which, the opposite natural tooth can have a similar wear loss caused by the natural tooth.

Zirconia ceramics, a commonly used esthetic restorative material, is characterized as excellent biocompatibility, high strength, color closer to the natural tooth, and has been widely used for all-ceramic crowns, pure zirconia crowns, inlays, and implant restoration. Compared with the alumina ceramics which is also a commonly used all-ceramic crown material, the zirconia ceramics has better mechanical properties and its fracture toughness is three times higher than that of the alumina. Vult von Steyern^[11] believed that the mechanical properties of zirconia crowns were obviously superior to those of the alumina. Quinn et al ^[12]studied the physicochemical properties and microstructure of zironia and they confirmed that ziconia had excellent mechanical properties. But the main deficiency of ceramic materials is likely to cause the excessive wear of the opposite teeth during a short time. The wear performance of the ceramic materials is considered to be influenced by their physical and chemical properties as well as the fine morphology on the porcelain surface, and the latter one is particularly prominent.

Metal materials are used earlier in the prosthodontics, which are characterized as high strength, high toughness, excellent processing performance in the laboratory, relatively low cost, and have been widely used in dental crowns, inlays repair. Metal materials are also commonly used oral restorative materials in clinic. Different metal materials have their own wear performance^[13-17]. In this study, we chose zirconia and the other four kinds of oral restorative materials commonly used in clinic. Then, we observed the similarity of zirconia to the natural tooth in the wear resistance and the differences of zironic from the other four kinds of oral materials in the wear performance, based on which, we can select appropriate restorative materials for full crown restoration that have a similar wear performance to the opposite natural tooth so as to better protect the opposite natural tooth and temporomandibular joint.

材料方法

Design

A comparative observation based on experimental data.

Setting and time

The experiment was completed in the Tribology Laboratory of Qingdao Technology University from November 2012 to January 2013.

Materials

Zirconia (LAVA); gold-palladium alloy (TALLADIUM, USA); cobalt-chromium alloy, nickel-chromium alloy (Stellite, USA); vita ceramics (vita); self-curing acrylic resin (Nissin Dental Materials Co., Ltd.); steatite (Taishan Porcelain Company, Zibo); MMV-1 vertical universal friction and wear tester (Jinan Test Group Company); microhardness tester (FM-700 type, Shimadzu Corporation, Japan); scanning electron microscope (JEOL-JSM-840); porcelain furnace (3M Company, USA); high-frequency centrifugal casting machine (JG-50Ag, Tianjin); precious metal casting machine (HERACAS); blasting machine (Zibo Detong Company); ultrasonic cleaning machine (KQ3200DB, Kunshan Ultrasonic Instrument Co., Ltd.); electronic balance (AUM220, Shimadzu Corporation, Japan); dental grinding machine (Department of Prosthodontics, Stomatological Hospital of Qingdao City).

Artificial saliva formulation of ISO/TR10271: NaCl 0.4 g, KCl 0.4 g, CaCl₂ • 2H₂O 0.795 g, Na₂HPO₄ • 2H₂O 0.78 g, Na₂S • 2H₂O 0.005 g, urea 1 g, distilled water 1 000 g.

Human enamel: unerupted third molars completely removed from patients aged 18-25 years, with no surface damage, no caries, enamel hypoplasia, which were provided by the Stomatology Hospital of Qingdao City.

Methods

Specimen separation

Zirconia, gold-palladium alloy, nickel-chromium alloy, cobalt-chromium alloy, and vita ceramics were used as experimental group, and there were five specimens for each material. Human tooth enamel served as a control group, including five specimens. Specimens in the experimental group were 4.0 mm×4.0 mm× 12.0 mm in size; while in the control group, axial tooth surfaces of the natural tooth were selected to prepare 4.0 mm×4.0 mm×2.0 mm enamel specimens, and then the root was embedded using self-curing resin to prepare 4.0 mm×4.0 mm×12.0 mm specimens that was easy to be installed in the upper fixture of MMW-1 vertical universal friction and wear tester. Gold-palladium alloy, nickel-chromium alloy, and cobalt-chromium alloy were treated with sandblasting and polished using a rubber fiber wheel. Zirconia and vita ceramics were polished using tapetum and silicone, respectively. The steatite was processed into 30 disc-shaped ceramic chips, 25.0 mm in diameter, 2.0 mm in thickness.

After successful preparation, the specimens were treated with ultrasonic cleaning for 10 minutes followed by alternative washing with ethanol and acetone, and rinsed with distilled water for 10 minutes. Then, the specimens were dried and weighed using the electronic balance (accurate to 0.000 1). All the specimens were sealed and stored in a dry and clean container against wearing and contamination.

Test specimens

The zirconia ceramics, vita ceramics, gold-palladium alloy, nickel-chromium alloy, cobalt-chromium alloy and fresh natural enamel were installed in the upper fixture of MMW-1 vertical universal friction and wear machine, respectively, and steatite porcelain was installed in the lower fixture. The artificial saliva was added to immerse the specimens evenly. The testing machine was set to 80 r/min^[18], under the loading force of 42 N^[19], and the testing time was 5 hours and 12 minutes, totally 25 000 rpm. The temperature of the artificial saliva was set to 36 ℃. Steatite specimens should be replaced by the end of each test.

Density measurement and data

First, the density of each specimen was measured by dehydration method. Only one specimen, 2.0 mm in diameter and 10.0 mm in length, was prepared by each kind of oral restorative material. A pin tube of 1.0 mL was used and injected 0.5 mL water. Then, the rinsed, dried and weighed specimens made of different materials were placed into the pin tube for volume measurement (V). Specimen density could be calculated according to the following formula: ρ=m/v. Each specimen was measured three times to obtain the average density. The mass loss of the specimens after abrasion was determined using the balance, and then the volume loss was calculated as follows: the volume loss=the mass loss/the density.

Hardness test

One specimen made of each material was randomly selected, and the hardness of the specimen at five loci was determined using the microhardness tester. One specimen was randomly selected from the control group was processed as described above.

Scanning electron microscope observation

One specimen made of each material in the experimental group was randomly selected and immersed in anhydrous ethanol for ultrasonic cleaning 15 minutes, followed by cold drying, and then the specimen was placed onto a cover slip and coated with gold powder to observe the morphology of wear surface under the scanning electron microscope.

Main outcome measures

Hardness value of specimens and the volume loss of specimens and grinding materials were observed; correlation between the volume loss and hardness of specimens was analyzed; morphology of wear surface was observed.

Statistical analysis

SPSS 17.0 statistical software was used for statistical analysis. Experimental data were expressed as mean±SD. Test of normality and homogeneity of variance test were performed. Intergroup differences were compared using a least significant difference test. Correlation analysis between the volume loss and Vickers hardness value of the specimens was used. A value of P < 0.05 was considered statistically significant.

讨论

Design basis for the type of experiment

Currently, the wear test is divided into in vitro and in vivo wear tests. In vivo wear tests are mostly classified as clinical observation, which are characterized as time-consuming, many factors, non-uniform data, and big error; therefore, the in vivo test is not recommended. An abrasion testing machine is used in the in vitro test to simulate oral chewing movements after the parameters are set. The advantages of the in vitro test include unified data, short time, less interference, and experimental data have certain significance for the clinical work^[20-23]. Foreign literature has proved that the in vitro wear test is feasible^[24]. Chewing movement is complex, and the in vitro wear test is difficult to completely simulate all the chewing movement. Therefore, the majority of foreign scholars believe that the wear tester can be used to simulate a kind of abrasion form abrasion^[25]. In the present study, the MMV-1 vertical universal friction and wear tester was applied to simulate chewing movement and environment.

Comparative analysis of wear performance of zirconia and other ceramic materials

There exists a common problem for all the ceramic materials, that is, it is easy to cause excessive wear on natural teeth, which is caused by brittle fracture of the ceramic materials during the wearing^[26]. When a loading movement exits between two surfaces during frictional contact, microcracks and even fractures of the materials occur, and pits and fissures appear on the surface of materials, thereby resulting in excessive wear on the opposite natural tooth. In addition, the pores, crystal size, surface fissures of the ceramic materials can also impact on the wear^[27].

Our experimental results show that the volume loss of the grinding material caused by zirconia was higher than that by vita ceramics (P < 0.05). Because of the presence of enamel layer on the surface of vita ceramics, there are less microcracks and fissures during the wear under force which can result in less volume loss of the grinding material. It is indicated that high-quality fine polishing is of great significance for reducing natural enamel wear prior to permanent restorations following occlusal adjustment for pure zirconia all-ceramic crowns.

The volume loss of the grinding material caused by vita ceramics was lower than that caused by zirconia (P < 0.05), and the hardness of vita ceramics was also close to the natural tooth. Therefore, normal ceramic materials with relatively low wear ability and zirconia all-ceramic crowns modified with veneering porcelain are recommended for patients who have excessive wear on the opposite nature tooth and enamel hypoplasia or those who cannot accept or are inappropriate to metal crowns.

Comparative analysis of wear performance of zirconia and metal materials

The volume loss of the grinding material caused by zirconia was higher than that caused by the other three metal materials (P < 0.05). We found that in the experiment there were a large amount of metal abrasive grains on the surface of the steatite to form a metal film which served as the lubricating film placed between the two materials, relieving the abrasion between the metal material and the grinding material. However, the hardness of ceramic abrasive grains was too high to form a film that can produce a protective role. In addition, brittle fractures appeared and formed pits on the surface of ceramic materials, thus producing a large amount of abrasion.

In precious metals, most scholars believe that the gold alloy has an better anti-wear properties than ceramic materials^[28]. The gold alloy has a wear performance closest to the natural enamel and can produce minimal volume loss on the opposite natural enamel as compared with ceramic materials and non-precious metals^[29-32]. The gold-palladium alloy is a kind of low precious metal alloy, which is suitably instead of the gold alloy based on its physicochemical properties and processing performance. Out experimental data show that the gold-palladium alloy has a volume loss closest to the natural teeth, and the volume loss of the grinding material caused by the gold-palladium alloy is less than that of the rest groups, which is further proved that the gold-palladium alloy can be a good substitute for the gold alloy in the aspect of wear performance. Most scholars believe that the non-precious metal materials can result in abnormal wear on the opposite natural teeth^[13]. In the present study, the volume loss of the nickel-chromium alloy and cobalt-chromium alloy, which are two kinds of non-precious metals, was less than that of the gold-palladium alloy (P < 0.05), while the volume loss of the grinding materials caused by the nickel-chromium alloy and cobalt-chromium alloy was larger than that by the gold-palladium alloy (P < 0.05). Therefore, the nickel-chromium alloy and cobalt-chromium alloy are more likely to produce excessive wear on the opposite nature teeth compared with the gold-palladium alloy.

Among the three metal specimens used in the study, the cobalt-chromium alloy with the highest hardness caused the maximal volume loss of the grinding material, the gold-palladium alloy with the lowest hardness caused the minimal volume loss of the grinding material, and the nickel-chromium alloy ranked in the middle in terms of hardness and volume loss of the grinding material. The correlation analysis results showed that the hardness of the three kinds of metal materials was positively correlated with the volume loss of the grinding materials, confirming that the there is a positive correlation between n the hardness and wear performance of the metal materials.

Comparative analysis of wear resistance of zirconia and metal materials

Our findings have demonstrated that the higher the hardness of the metal materials, the lower the volume loss of the metal materials. Zirconia has the highest hardness among the ceramic materials, but it also has the highest volume loss, which is inconsistent with the metal materials. The atoms of the metal materials are interconnected based on “metallic keys”, and higher bond energy means better plastic deformation, elastic deformation resistance, and wear performance. Metal materials are less prone to breakage than the non-metal materials with similar hardness in case of the compressive deformation. Ceramic materials are easy to be exfoliated because of their crystal structure during the wear process and are more likely to cause their own volume loss as compared with the metal materials. Therefore, the wear process is complicated, which can be influenced by many factors. Hardness or other factors cannot be used alone to represent the wear resistance, and other indicators should be considered comprehensively.

Conclusion and prospects

Zirconia restorations, currently used in the dental restoration, are mainly pure zirconia all-ceramic crowns and zirconia all-ceramic crowns modified with veneering porcelain. Because the zirconia all-ceramic crowns modified with veneering porcelain are easy to collapse, pure zirconia all-ceramic crowns are recommended for posterior restorations that have less aesthetic requirements, which results in a problem about the difference in the wear performance between the pure ziconia and opposite natural teeth. It is thJe starting point for our experimental study. Since only rubber fiber wheels, the more commonly used in clinical work, were adopted for polishing, studies addressing the effects of different surface treatment and polishing methods on wear performance of zirconia are required. Currently, Zekang Company launched a high-permeability zirconia ceramic blocks which have been improved in aesthetic properties and surface polishing treatment than previously reported. Along with the in-depth study of wear performance of zirconia corresponding to the natural teeth and continuous progress in material sciences, pure zirconia all-ceramic crowns for dental restorations will have broad prospects.

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      氧化锆造成的对磨滑石瓷体积损失量大于3组金属材料所造成的滑石瓷体积损失量(P<0.05)，原因是实验中发现在金属材料对磨滑石瓷表面有大量金属磨粒附着形成金属薄膜，这层薄膜可作为两种材料间的润滑膜，减缓了金属材料及滑石瓷之间的磨耗，而陶瓷材料颗粒硬度较高，磨削下的颗粒无法在表面形成薄膜，不能产生介质保护效应，加之瓷层表面出现脆性断裂进而形成尖窝点隙，造成了对磨滑石瓷的大量磨耗。

　   在贵金属中，目前大多数学者认为金合金抗磨耗的性能好于陶瓷材料，磨耗性能最接近天然牙釉质，其对天然牙釉质的磨耗小于陶瓷材料及非贵金属，实验中使用的金钯合金属于低贵金属合金，目前认为其在理化性能及技工加工性能方面均适宜代替贵金属金合金。实验数据显示金钯合金自身体积损失量与天然牙最接近，对磨滑石瓷体积损失量小于其余各组材料所造成的损失量，其磨耗性能优于各组材料，进一步证明了金钯合金在磨耗性能方面可代替金合金的理论。大多学者认为非贵金属材料可造成对颌天然牙的非正常磨耗。实验选用的镍铬合金、钴铬合金属于非贵金属，其自身磨耗量小于金钯合金（P<0.05），而造成滑石瓷的体积损失量大于金钯合金(P<0.05)，相对金钯合金更容易造成对颌天然牙的过度磨耗。

      实验数据示金属试件中钴铬合金硬度值最高导致的滑石瓷体积损失最多，金钯合金硬度值最小导致的滑石瓷体积损失量最少，镍铬合金硬度值和滑石瓷体积损失量居中。经相关性分析得出，3种金属材料的硬度值与导致的滑石瓷体积损失呈正相关，证实了金属材料硬度与磨耗性能呈正相关的理论。