自酸蚀与全酸蚀粘接剂对牙釉质和牙本质的剪切粘接强度比较

目的 比较自酸蚀与伞酸蚀粘接剂对牙釉质和牙本质剪切粘接强度,以期为临床提供参考.方法 选择因牙周病拔除的上颌中切牙20颗,使用两种自酸蚀粘接剂[A(ClearfilTM Protect Bond)、B(AdporTM PromptTM)]和两种全酸蚀粘接剂[C(SwissTEC SL Bond)、D(Single Bond)]按照厂家推荐步骤对牙釉质和牙本质进行粘接,并测试牙釉质和牙本质粘接样本的剪切粘接强度.结果 4种粘接剂对牙釉质和牙本质粘接强度分别为:粘接剂A(25.33±2.84)、(26.07±5.56)MPa;粘接剂B(17.08±5.13)、(17.93±4.70)MPa;粘接剂C(33.14±6.05)、(41.92±6.25)MPa;粘接剂D(22.51±6.25)、(21.45±7.34)MPa.粘接剂C对牙釉质和牙本质剪切粘接强度显著高于其他3种粘接剂(P<0.05);粘接剂B的剪切粘接强度显著低于其他3种粘接剂(P<0.05).结论 本研究所选用的自酸蚀两步法粘接剂的剪切粘接强度与部分伞酸蚀粘接剂相当,高于自酸蚀一步法粘接剂.     

不同树脂粘固剂对瓷与牙本质粘接强度的比较研究

目的 评价不同种类树脂粘固剂与瓷-牙本质粘接强度的差异,以期为临床提供参考.方法选取无龋坏青年人前磨牙制备牙本质粘接面,铸造直径为3 mm、高为3 mm的圆柱状瓷块(IPS e.max Press)80个,分为A、B、C、D、E 5组.分别选用树脂粘固剂A(Variolink Ⅱ)、B(MultilinkAutomix)、C(Multilink Sprint)、D(Rely X Unicem)和E(BisCem)将瓷块粘固于牙本质粘接面上,每组制成16个试样.37℃水储24 h后每组8个试样直接测试剪切粘接强度,另8个试样进行5000次冷热循环后测试粘接强度,扫描电镜观察粘接面形态.对同种粘固剂冷热循环前后的粘接强度进行双样本t检验.结果冷热循环前A组粘接强度[(22.3±3.9)MPa]最大,B组[(18.1±3.5)MPa]次之,再次为D组[(14.1±2.3)MPa]和E组[(11.7±4.2)MPa],C组[(11.3±3.6)MPa]最小.冷热循环后A组粘接强度[(17.8±2.3)MPa]仍最大,B组[(14.4±3.5)MPa]和D组[(13.2±2.5)MPa]次之,再次为E组[(8.9±3.2)MPa],C组[(7.0±2.4)MPa]最小.与冷热循环前相比,冷热循环后A、B、C 3组粘接强度均下降,差异有统计学意义(P<0.05);D组和E组粘接强度下降不明显(P>0.05).结论以全酸蚀粘接技术为基础的树脂粘固剂的粘接强度大于以自酸蚀粘接技术为基础的树脂粘固剂和自粘接型树脂粘固剂.     

两种脱敏剂对牙本质粘接剂粘接强度的影响

目的 探讨两种脱敏剂对牙本质粘接剂粘接强度和微观形态的影响,以期对临床治疗有所指导.方法 将30颗完整且无龋的人第三磨牙去除(牙合)面釉质,采用随机数字表法随机分为3组:C组(对照组)、U组(UltraEZTM处理)和M组(MI Paste处理);每组10颗牙齿.C组不做处理,U组和M组分别使用相应脱敏剂对暴露的牙本质表面进行处理.3组样本牙再各随机分为两个小组,每小组5颗牙,分别用粘接剂A(Single Bond 2)和B(Xeno Ⅲ)粘接,用复合树脂修复达4～5 mm.用慢速锯将牙齿切成粘接面积为0.9 mm×0.9 mm的长方体试件,用微拉伸测试仪测试微拉伸强度,作为粘接强度,对其进行单因素方差分析.每组随机挑选3个试件对粘接界面进行扫描电镜观察.结果 对于粘接剂A,U组粘接强度[(14.58±2.31)MPa]与C组[(15.82±2.18)MPa]的差异无统计学意义(P>0.05),而M组粘接强度[(9.90±0.79)MPa]降低.对于粘接剂B,U组粘接强度[(10.55±1.06)MPa]与C组[(10.73±1.07)MPa]的差异无统计学意义(P>0.05),而M组粘接强度[(8.74±0.87)MPa]降低.扫描电镜见各组混合层均较致密,M组两种粘接剂的树脂突较短且稀少.结论 两种脱敏剂对粘接剂粘接强度的影响不同,U组脱敏剂对全酸蚀和自酸蚀粘接剂的粘接强度无明显影响,而M组脱敏剂可降低两种粘接剂的粘接强度.     

四种树脂粘接剂与牙釉质和牙本质粘接性能的比较

目的 评价4种树脂粘接剂与牙釉质和牙本质的粘接强度以及粘接界面微观形态,以期为临床应用提供依据.方法 选取24颗新鲜拔除的无龋坏人磨牙,每颗牙制取牙釉质和牙本质试件各2片,均分为4组,每组牙釉质和牙本质试件各12片.选用4种树脂粘接剂(A组:PanaviaTMF;B组:RelyXTM ARC;C组:RelyXTM Unicem;D组:ClearfilTM SA Cement)进行粘接处理后,在扫描电镜(SEM)下观察粘接界面,并测试粘接强度.结果 B组牙釉质粘接强度[(29.38±6.28)MPa]高于D组[(27.25±7.03)MPa],但两者差异无统计学意义(P＞0.05),两者均显著高于A组[(22.92±7.13)MPa]和C组[(10.92±3.41)MPa,P＜0.05].B组牙本质粘接强度[(27.28±6.79)MPa]最高,随后依次为A组[(14.23±6.39)MPa]、D组[(10.09±3.26)MPa]和C组[(6.43±1.60)MPa],各组间差异均有统计学意义(P＜0.05).B组牙釉质粘接界面可见树脂突,A、B组牙本质粘接界面可见树脂突.结论 四种树脂粘接剂表现出不同的牙釉质、牙本质粘接性能.

Abstract：

Objective To evaluate the dentin and enamel bonding strength and resin-dentin interfaces of four resin cements. Methods Twenty-four sound freshly extracted molars were sectioned with low-speed saw under running water. Two mesial-distal enamel disc and two buccal-lingual dentin discs were sectioned from each molar. Four resin cements( A group: PanaviaTM F; B group: RelyXTM ARC; C group:RelyXTM Unicem; D group: ClearfilTM SA Cement) were applied to the surface of these enamel and dentin discs according to manufacturers' guidance. The bonded specimen were prepared for shear bond strength test and sectioned occluso-gingivally into two slabs for scanning electron microscopy(SEM) observation. Results The bonding strength between resin and enamel in B group[ (29. 38 ±6. 28) MPa] was higher than that in D group[ (27.25 ± 7.03) MPa], and both of them were significantly higher than those in A [ ( 22.92 ±7. 13) MPa] and C group [ ( 10.92 ± 3.41 ) MPa] ( P ＜ 0.05 ). The highest dentin bonding strength was shown in B group [ ( 27.28 ± 6. 79 ) MPa ], followed by A [ ( 14. 23 ± 6. 39 ) MPa ], D [ ( 10.09 ±3.26) MPa] and C group[ (6. 43 ± 1.60) MPa, P ＜ 0.05 ]. Conclusions The four resin cements have different enamel or dentin bonding properties.     

三种自酸蚀粘接剂使用方法对牙本质粘接的影响

目的 观察不同的酸蚀处理时间和使用方式对3种自酸蚀粘接剂牙本质粘接强度的影响. 方法 将3种自酸蚀粘接剂[粘接剂1(Adper Prompt,3M-Espe,U S A)、粘接剂2(XenoⅢ,Dentsply,Konstanz,Germany)、粘接剂3(Clearfil SE Bond,Kuraray,Japan)]按照酸蚀处理时间和使用方式分组,每种粘接剂均分为标准组(厂家推荐操作组)、缩短处理时间组(缩短组)、延长处理时间组(延长组)及改变使用方式组,每组均选择15个粘接样本,分别测定牙本质微拉伸粘接强度,扫描电镜下观察粘接界面. 结果 缩短、标准及延长酸蚀时间时,粘接强度分别为粘接剂1:(16.30±2.59)、(23.13±2.56)、(22.28±2.83)MPa;粘接剂2:(15.17±6.07)、(34.50±3.64)、(24.87±7.01)MPa;粘接剂3:(29.92±3.32)、(42.21±6.28)、(41.07±3.93)MPa(P<0.01).粘接剂1、2、3改变使用方式组的粘接强度分别为(12.53±3.73)、(23.98±3.86)、(48.37±4.95)MPa,分别与同种粘接剂的标准组相比差异有统计学意义(P<0.01). 结论 缩短酸蚀处理时间可显著降低自酸蚀粘接剂的粘接效果;延长酸蚀处理时间并未提高自酸蚀粘接剂的粘接效果;涂擦使用时,不同粘接剂的牙本质粘接效果不同;使用粘接剂应严格参照厂家推荐的操作步骤.     

五种牙本质粘接剂对乳牙牙本质粘接性能的比较

目的 检测5种牙本质粘接剂对乳牙牙本质的粘接性能,为牙本质粘接剂的临床使用提供依据.方法 按随机数表法将75颗因滞留拔除的乳磨牙分为5组,每组15颗.各标本磨除表面牙釉质,暴露的牙本质面分别根据不同粘接系统的说明进行酸蚀粘接,树脂充填.使用A(FL-Bond Ⅱ)、B(Clearfil Protect Bond)、C(Clearfil SE Bond)、D(AdperTM Easy One)、E(Single Bond 2)粘接系统进行粘接(分别为A、B、C、D、E组),用微拉伸测试仪检测微拉伸强度.扫描电镜观察样本的断裂类型.结果 各组的微拉伸强度分别为A组:(28.3±2.2)MPa,B组:(32.4±2.5)MPa,C组:(38.3±2.8) MPa,D组:(32.9±3.4)MPa,E组:(23.2±1.9) MPa,C组与A、E组差异有统计学意义(P＜0.01),C组与B组、D组差异无统计学意义(P＞0.05),A组与E组差异无统计学意义(P＞0.05).扫描电镜观察显示各组的断裂类型均以混合断裂为主,各组间无明显差异.结论 粘接剂B的粘接强度与粘接剂C和D相同,优于粘接剂E,可能更适用于乳牙的粘接.     

漂白对人牙釉质与树脂粘接强度的影响

目的 研究漂白对人牙釉质深层与树脂粘接强度的影响,探讨漂白后瓷贴面修复的粘接时机.方法 96个人恒牙试件均分为6个组,第1组不漂白,直接磨除釉质0.6～0.8 mm,第2～6组分别在35%过氧化氢漂白剂冷光辅助漂白后即刻、1、4、7、14 d磨除釉质0.6～0.8 mm,分别用粘接剂A(One-step plus)和B(Single bond)粘接.测量拉伸强度,作为粘接强度.结果 每个时间点两种粘接剂粘接强度的差异无统计学意义(P>0.01).A和B两种粘接剂中,第2、3、4组粘接强度[(17.79±3.53)和(19.41±3.37)MPa、(17.50±4.29)和(19.66±3.48)MPa、(18.39±3.58)和(19.53±3.21)MPa]比第1组[(25.94±4.88)和(28.02±4.36)MPa]降低约30%;第5组粘接强度[(22.46±3.98)和(23.74±5.04)MPa]与第1组的差异无统计学意义(P>0.01);第6组粘接强度[(26.03±4.47)和(27.88±4.78)MPa]与第1组相差不大.结论 漂白后人牙釉质深层与树脂粘接剂的粘接强度明显下降,最好在漂白两周后进行瓷贴面粘接.     

后牙纵折模型中两种材料粘接强度的比较

目的建立纵折后牙粘接模型,研究Super-Bond C＆B粘接剂和IBond自酸蚀粘接剂对纵折后牙的粘接效果。方法选择新鲜拔除的完整人磨牙40颗,制成后牙纵折模型,随机分为2组,每组20颗,分别用Super-Bond C＆B粘接剂和IBond自酸蚀粘接剂粘接断裂面,将粘好的折裂牙试样在37℃生理盐水中放置24h后,检测其剪切强度。结果 IBond粘接剂的剪切强度是（10.65±1.49）MPa,Super-Bond C＆B粘接剂的剪切强度是（19.48±2.57）MPa,差异有统计学意义（t=13.29,P〈0.05）。立体显微镜观察显示,所有试件破坏均表现为牙本质与粘接剂界面之间的断裂,未见有牙本质的内聚破坏发生。结论 Super-Bond C＆B粘接剂的牙本质粘接强度高于IBond粘接剂。     

唾液污染对运用自酸蚀粘接系统托槽/牙面剪切强度的影响

目的 :评价唾液污染对运用自酸蚀粘接系统托槽 /牙面剪切强度的影响。方法 :根据牙面不同污染情况分为 4组 :A组 :牙面清洁干燥 ;B组 :牙面在涂布酸性封闭剂之前受唾液污染 ;C组 :牙面在涂布封闭剂之后受唾液污染 ;D组 :牙面在封闭剂涂布前后均受唾液污染。所有样本均用光固化粘接剂 (TransbondXT 3M )粘接托槽。粘接 0 .5h后比较 4组的剪切强度及粘接剂残留指数 (ARI)。结果 :4组剪切强度存在显著性差异 (F =11.89,P <0 .0 5 )。D组剪切强度 (4 .62 86± 1.6883MPa)明显低于A组 (8.75 48± 1.915 3MPa) ,B组 (7.895 1± 1.9692MPa)和C组 (6.90 10± 1.5 162MPa)。B组和C组虽有所下降 ,但仍在临床可接受范围之内。 4组粘接剂残留指数比较无显著性差异。结论 :即使在轻微污染的环境中 (牙面在封闭剂涂布之前或之后受唾液污染 ) ,自酸蚀封闭剂也能进行有效的粘接。     

不同表面处理的纤维桩微拉伸粘接强度研究

目的:通过微拉伸粘接强度测试法测试在纤维桩表面经过6种不同的处理后与核树脂的粘接强度,探索何种表面处理方法能显著提高纤维桩与核树脂的粘接强度,为临床应用提供参考。方法:将30根石英纤维桩随机分成6组,每组5根。A组纤维桩表面涂布粘接剂,B组涂布硅烷处理剂,吹干后涂粘接剂,C组5%氢氟酸酸蚀30sec,流水冲洗吹干后处理同A组,D组5%氢氟酸酸蚀30sec,流水冲洗吹干后处理同B组,E组24%双氧水处理,后操作同A组,F组24%双氧水处理后操作同B组。在桩周分层固化核树脂,用低速锯沿纤维桩外周平行片切,再垂直粘接面片切成横截面约0.9mm×0.9mm的长方柱状试件,每组15个,用微拉伸粘接强度测试法测试纤维桩与核树脂的微拉伸粘接强度,体视显微镜观察断裂类型。结果:A组微拉伸粘接强度8.78±2.20MPa,B组9.35±1.92MPa,C组15.50±2.87MPa,D组22.98±3.24MPa,E组16.64±2.70MPa,F组24.88±3.9 0MPa。用氢氟酸酸蚀的C组和D组和用双氧水处理的E组和F组的微拉伸粘接强度明显高于A组B组(P<0.05)。用氢氟酸酸蚀后再硅烷化处理的D组比C组、用双氧水处...     

Physicochemical and mechanical characterization of a fiber-reinforced composite used as frameworks of implant-supported prostheses

ObjectivesTo characterize the physicochemical and mechanical properties of a milled fiber-reinforced composite (FRC) for implant-supported fixed dental prostheses (FDPs).MethodsFor FRC characterization, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction, Fourier-transformed infrared spectrometry, simultaneous thermogravimetric analysis and differential scanning calorimetry were performed. For fatigue testing, 3-unit FRC frameworks were fabricated with conventional (9 mm² connector area) and modified designs (12 mm² connector area and 2.5 mm-height lingual extension). A hybrid resin composite was veneered onto the frameworks. FDPs were subjected to step-stress accelerated-life fatigue testing until fracture or suspension. Use level probability Weibull curves at 300 N were plotted and the reliability for 100,000 cycles at 300, 600 and 800 N was calculated. Fractographic analysis was performed by stereomicroscope and SEM.ResultsThe FRC consisted of an epoxy resin (∼25%) matrix reinforced with inorganic particles and glass fibers (∼75%). Multi-layer continuous regular-geometry fibers were densely arranged in a parallel and bidirectional fashion in the resin matrix. Fatigue analysis demonstrated high probability of survival (99%) for FDPs at 300 N, irrespective of framework design. Conventional FDPs showed a progressive decrease in the reliability at 600 (84%) and 800 N (19%), whereas modified FDPs reliability significantly reduced only at 800 N (75%). The chief failure modes for FRC FDPs were cohesive fracture of the veneering composite on lower loads and adhesive fracture of the veneering composite at higher loads.SignificanceMilled epoxy resin matrix reinforced with glass fibers composite resulted in high probability of survival in the implant-supported prosthesis scenario.     

牙菌斑液对牙釉质矿物的饱和程度与龋活动性的关系

为了解菌斑液对牙齿矿物饱和度随糖漱口的变化，以及与龋活动性的关系，作者选择２２例年龄相同的大学生，按龋齿情况，将其分为３组。第１组无龋（无龋组），第２组ＤＭＦＳ大于１０但无活动性龋（普通龋组），第３组ＤＭＦＳ大于１０且有活动性龋（活动性龋组），测定各受试者在静止时、用蔗糖液漱口后３ｍｉｎ和７ｍｉｎ时的菌斑液中有机酸、ｐＨ值、钙离子活度（有效离子浓度）和无机成分的含量，并计算对于釉质矿物的饱和度。结果表明，糖漱口之后菌斑液乳酸浓度升高、ｐＨ值降低、总钙和离子钙浓度增加。活动性龋组的菌斑液在糖漱口后ｐＨ值和饱和度降低的幅度明显大于无龋组和普通龋组。结论：牙菌斑液对牙齿矿物的饱和度是指示个体接受致龋食物后牙齿脱矿倾向的敏感指标。     

Dental specialty,career preferences and their influencing factors among final year dental students in Saudi Arabia

Objective

The purpose of this study was to investigate evolving trends in dental post graduate specialty preferences and career aspirations among final year dental students in Saudi Arabia.

饰瓷对磨牙e.max双层全瓷冠断裂类型的影响

目的：通过抗折破坏实验研究饰瓷对磨牙e.max双层全瓷冠断裂强度和断裂类型的影响,为减少临床双层全瓷修复体崩瓷提供实验依据。方法：制作单层全瓷冠（IPS e.max Press）和双层全瓷冠（IPS e.max Press/Ce-ram）共40个（每组20个）,粘接于树脂代型后进行抗折破坏实验并记录断裂强度值;体视显微镜和扫描电子显微镜下观察断裂类型;能谱分析饰瓷-核瓷界面化学元素构成。结果：单层全瓷冠断裂强度值[（2665.4±759.2）N]与双层全瓷冠[（1431.1±404.3）N]间差异有统计学意义（P=0.000）。饰瓷内部及饰瓷-核瓷界面缺陷催生裂纹造成饰瓷崩裂是双层全瓷冠断裂的主要模式。结论：饰瓷对磨牙e.max双层全瓷冠断裂强度和断裂类型均有影响;饰瓷内部及饰瓷-核瓷界面缺陷是全瓷冠断裂的主要原因,提高饰瓷断裂强度及减少界面缺陷可减少因崩瓷造成的修复体失败。     

Comparative study on the effect of ultrasonic instruments on the root surface in vivo

The present study was designed to investigate the effectiveness of different ultrasonic instruments on the root surface. Fourteen patients with 35 single root teeth designated for extraction were recruited to the present study. Teeth were assigned to four experimental groups: group 1, piezoelectric ultrasonic device; group 2, magnetostrictive ultrasonic device; group 3, hand instrumentation; and group 4, untreated teeth (control). After instrumentation, the teeth were extracted and the presence of residual deposits (roughness and root surfaces characteristics) were analyzed. The results showed that residual deposits were similar in all tested groups: piezoelectric, 8.7%; magnetostrictive, 9.7%; hand instrumentation, 11.1% and control, 76.4%. There were statistically significant differences between control and all the experimental groups (p < 0.0001). With respect to roughness parameters evaluation, R_a and R_z of the roots treated with the different instruments showed a similar pattern (p > 0.05), but for R_t and R_y, a significant difference was observed (p < 0.05) among hand instrumentation and ultrasonic devices. SEM analysis revealed a similar root surface pattern for the ultrasonic devices, but curettes showed many instrumental scratches, deep gouges, and a relatively large amount of dentin was removed. Within the limits of the study, although the instruments produced similar results, root surfaces instrumentated with curettes were rougher and had more root surface tissue removed than with the ultrasonic device.     

The Influence of Precondensation Mercury Content and Mulling on the Transverse Strength of Amalgams Condensed After a Delay

The purpose of this study was to investigate the influence of changing the precondensation mercury content (initial mercury content) on the final strength of amalgams when the condensation was delayed. Another purpose was to study the effect on the transverse strength of mulling the amalgam mix after the delay. The material consisted of ten different brands of alloy. Three different alloy-mercury ratios were used with each alloy brand; Mix I about 50% Hg, Mix II about 54% Hg and Mix III about 59% Hg. Rectangular amalgam test pieces, measuring 2 × 2 × 12 mm, were condensed after a 5 minutes delay by hand using a load of about 17 kg/cm². The transverse strength test was performed after one week using three point loading. The five minutes delay of the condensation reduced the strength of the amalgams by 1 to 42 per cent depending on the brand of alloy and precondensation mercury content. The three preamalgamated amalgams were affected less by the delay than the other amalgams. Increasing the precondensation mercury content reduced the effect of the delay on the final strength. The «mulling» of the amalgam mix also decreased the effect of the delay. It was concluded that a moderate excess of initial mercury gives the dentist a longer condensing time thus allowing him to perform the condensing procedure with care.     

BLB种植体的临床应用及近期疗效观察

目的：评估BLB种植体的近期临床应用效果。方法：2003年2月-2005年12月，在北京通州区潞河医院口腔科用BLB种植体进行延期种植，共47枚，采用一期埋入、二期暴露的潜入式方法，全部为金属烤瓷冠修复，临床随诊观察半年至3年。结果：随诊期间无一例种植体脱落，4例因中央螺丝松动致修复体松动，2例发生种植体周围炎；种植体周围牙槽骨水平吸收小于0．2mm／年。结论：BLB种植体适合于常见牙列缺损的延期种植修复，近期疗效可靠     

桩核材料对牙本质应力分布的影响

目的 研究6种不同材料行桩核修复前后牙本质的应力分布情况,为临床桩材料的选择提供理论依据。方法 采用螺旋CT扫描数据建立上颌中切牙烤瓷桩核冠的三维有限元模型,在此模型上对用铸造Ni-Cr合金、铸造钛合金、铸造金合金、玻璃纤维树脂、聚乙烯纤维树脂及普通复合树脂6种材料行桩核修复前后的牙本质应力分布情况进行数值分析。结果 在6种材料中,与桩修复前的烤瓷熔附金属冠修复牙相比较,临床最常用的铸造Ni-Cr合金桩修复使桩尖周围牙本质的最大主应力和Von Mises应力分别升高了152％和162％;聚乙烯纤维树脂桩修复后,牙本质中的应力分布情况无明显改变。其他材料行桩修复后牙本质中应力分布的差异较大。桩植入后对牙本质应力分布模式的影响与桩的弹性模量密切相关。结论 与牙本质弹性模量接近的聚乙烯纤维树脂更适合用于桩修复。     

A Titration Method for the Determination of Acid and Alkali Production by Micro-Organisms

The purpose of this investigation was to study the influence of changing the precondensation mercury content (initial mercury content) on the early and final transverse strength of different amalgams. The material consisted of five conventional lathe cut alloys, two of which were zinc free, of one dispersion strengthened and one spherical alloy and of three lathe cut preamalgamated alloys. The amalgam was mixed with mercury using three different alloy-mercury ratios. Thus the precondensation mercury content of Mix I was about 50 per cent, of Mix II about 54 and of Mix HI about 59 per cent for all amalgams except the spherical brand. Rectangular amalgam test pieces, measuring 2 × 2 × 12 mm, were condensed by hand using a load of about 17 kg/cm². The transverse strength test was performed either after one hour or one week using three point loading. The results show that for some of the amalgams an increase of the precondensation mercury content resulted in a slight reduction of the early strength. Furthermore the results show that the early strength was lower for the preamalgamated amalgams than for the other amalgams. Increasing the precondensation mercury content did not significantly effect the final strength of six amalgam brands but increased the strength of three brands and reduced the strength of one brand. It was concluded that it is safer to use a moderate excess of initial mercury in clinical amalgam work than to try to reduce the initial mercury content as much as possible.