The cast round RPA clasp

The incorporation of a cast round clasp into the RPA design has been presented. When properly used, the cast round clasp eliminates the need for blockout of the facial aspect of the terminal abutment normally required with the half-round clasp. This technique includes surveying the refractory cast, which ensures optimal placement of the round clasp in relation to the height of contour.     

Proximal plate in conventional circumferential cast clasp retention

Improper guiding planes are commonly used with conventional removable partial denture cast clasps. Short guiding planes with improper proximal plates decrease retention and cause repetitive traumatic movement of abutments. This article discusses the importance of guiding planes and proximal plates for conventional tooth-supported removable partial dentures with circumferential clasp. The retention force of the clasps was analyzed geometrically. If the guiding plane is short or the prepared surface is oblique, the contact between the proximal plate and the guiding plane or oblique plane diminishes at an early stage, which results in reduction of retention. The guiding plane should be parallel to the path of insertion and must be of a certain length for adequate retention. An elongated proximal plate that is parallel to the path of insertion may compensate for retention deficiency when an adequate guiding plane cannot be prepared.     

Aulen树脂卡环与Co—Cr卡环循环疲劳及对牙面磨损的研究

目的：比较Aulen树脂卡环与Co—Cr卡环在离体牙上循环疲劳测试中的脱位力衰减趋势,观察其对离体牙的磨损。方法：制作Aulen树脂卡环和Co—Cr卡环,利用多功能生物力学测定仪进行循环疲劳测试,对比其脱位力衰减情况,扫描电镜观察实验前后离体牙的磨损情况;结果：卡环在循环疲劳测试后其脱位力均不同程度下降,Aulen树脂卡环的摩擦区域无明显划痕,Co—Cr卡环的摩擦区域形成明显的划痕,破坏了原来平滑的表面。结论：Aulen树脂卡环固位力较Co—Cr卡环小,但衰减缓慢,Aulen树脂卡环对牙面的磨损程度小于Co—Cr卡环。     

Use of an adams clasp for a cast unilateral removable partial denture

A unilateral complete dentition with tight intercuspal contacts complicates the design of the interocclusal retentive elements of a cast removable partial denture. Use of an Adams clasp, which is connected to a predesigned retentive mesh in the metal framework, is suggested as a conservative and economical treatment option in this situation. Guidelines for minimizing the risk of breakage of the Adams clasp are presented.     

Repair with a surveyed cast clasp while patient retains the partial denture

A technique is described that allows a removable partial denture with a broken clasp or a removable partial denture in which an abutment has been extracted to be restored by the reattachment of a new cast clasp component or a complete surveyed clasp assembly. The technique is applicable to most clasp designs and can include attachment to the acrylic denture base or the metal major connector. The technique is distinguished from other repair techniques by (1) providing a cast clasp replacement, (2) allowing the patient to retain the prosthesis during the repair, (3) generating a precisely formed surveyed clasp assembly by the laboratory, and (4) including an efficient transfer mechanism for the precision clasp created in the laboratory to be attached to the removable partial denture in the dental office. A two-step impression procedure is used to ensure proper relation of the partial denture to the abutments. (J Prosthet Dent 1997;77:624-9.)     

Comparison of cast Ti-Ni alloy clasp retention with conventional removable partial denture clasps

STATEMENT OF PROBLEM: The elastic property of Ti-Ni alloy may be a desirable property for cast removable partial dentures. However, little is known about the retentive properties of cast Ti-Ni alloy clasps. PURPOSE: This in vitro study investigated the retentive force of various types of clasps during repeated cycles of placement and removal to determine whether Ti-Ni alloy clasps maintain their initial retentive force under varied conditions, including different retentive undercut depths and clasp size (thickness and width). MATERIAL AND METHODS: The test models were developed using a mandibular dentiform with a horizontal plane of occlusion. Two complete metal abutment crowns were made for the left second molar, differing only in retentive undercut depths. A total of 98 clasps (n=7) were fabricated, including 14 wrought wire clasps and 28 clasps from each of the following alloys: Co-Cr alloy (Biosil), Type IV gold alloy (Cast-4), and Ti-Ni alloy (TN-10). Clasps were made to engage one of 2 retentive undercut depths (0.25 mm or 0.75 mm) and were made in 2 sizes (0.8 mm or 1.4 mm). Each clasp was radiographically examined for casting defects and porosity. The force (N) required to remove the clasps was measured using a universal testing machine with a crosshead speed of 10 mm/min. After measuring the retentive force, a masticatory simulator was used to cycle the clasps on and off the metal crown 500 times, simulating the insertion and removal of a removable partial denture clasp. The cycling sequence was repeated 10 times, and retention force was measured after each sequence. The internal surface of the clasp arms was examined for evidence of metal fatigue using scanning electron microscopy. Data were subjected to 2-way and 4-way analysis of variance, followed by Scheffe's multiple comparison test (alpha=.05). RESULTS: Co-Cr alloy and gold alloy clasps in the 0.25-mm retentive undercut groups experienced a gradual decrease in retentive force measurements (P<.001). In contrast, the Ti-Ni alloy clasps maintained a retentive force of approximately 1.8 N and 2.6 N for the 0.8 mm and 1.4 mm clasp groups, respectively. Although Ti-Ni alloy clasps' retentive force was reduced slightly after the first cycling sequence, it was negligible compared with other clasps tested. The wrought wire clasps also retained their retentive force until the final cycling sequence. A similar trend was found in the clasp groups engaging 0.75-mm retentive undercuts. CONCLUSION: Although the end-point retention for all the clasps was similar, there was less change in the retentive force of the cast Ti-Ni alloy clasps after repeated cycling sequences of simulated placement and removal.     

Influence of various polishing methods on pulp temperature

Objective

After orthodontic debonding, adhesive removal can lead to rises in pulp temperature, causing histological changes or pulp necrosis. The objective of this study was to measure increases in pulp temperature during adhesive removal using different instruments and various cooling procedures.

Materials and methods

A thermoelement was introduced into the pulp chamber of 10?human incisors. The teeth were immersed in a 36°C water bath up to the cementoenamel junction. Two carbide burs, one polishing disk and two rubber points were used for polishing. All measurements were taken over a 10?s period by a single investigator, under slight pressure and with constant motion. Three cooling procedures were examined: no cooling, air cooling and water cooling. Pulp temperatures were measured before polishing and after 10?s of polishing.

Results

Without cooling, the two rubber points revealed clinically relevant temperature increases of 6.1°C and 12.4°C. Cooling with air and with water reduced pulp temperature in conjunction with all polishing methods. Air cooling was most efficient, except in combination with the polishing disk.

Conclusion

Under these study conditions, carbide burs and polishing disks can be used safely and without risk to the pulp, even without cooling. On the other hand, rubber points cause a marked increase in pulp temperature when used without cooling.     

Accurate procedure for transferring a cast replacement clasp arm to a removable partial denture

This article describes a simple, quick and accurate procedure to replace a broken bar-type clasp with a new clasp arm. This procedure involves a resin matrix that transfers a new clasp arm from a working cast to a removable partial denture. This simple procedure enables quick and precise replacement of a broken clasp arm.     

抛光方法对钴铬合金铸造全口义齿密合度的影响

［摘要］ 目的 探讨5种抛光方法对钴铬合金铸造全口义齿密合度的影响。方法 铸造上颌钴铬合金全口基托板30个,随机分为6组,1组作为对照,5组分别用光整、电解、打磨、打磨后电解、打磨后光整等5种方法抛光。在相当于上颌第二磨牙远中部位切断义齿基托与标准模型,数字万能工具显微镜分别测量上颌全口义齿基托组织面与标准模型齿槽嵴顶点、腭中缝点及其两者的中点之间共5个部位的间隙,并用电子数显卡尺测量5个点的基托板厚度。结果 6组铸造基托板的厚度,除了光整组试样的厚度与抛光前试样厚度无统计学差别外(P>0.05),其余各组均与抛光前试样厚度有统计学差别(P<0.01)。抛光前、后5点间隙值没有组间统计学差别(P>0.05)。结论 5种抛光方法对上颌铸造全口基托的密合度无影响。     

Guidelines for the choice of circumferential wrought wire and cast clasp arms for removable partial dentures

PurposeThe aim of this study was to establish guidelines for the selection of cast and wrought-wire clasps for removable partial dentures (RPDs) that would be appropriate for clinically encountered undercuts and facial curvatures.MethodsRandomly selected discarded casts were collected and 30 premolars and 30 molars were surveyed, sectioned to a line representing the clasp and scanned using a flatbed scanner. The average clasp curvature and length for each group was determined and a three-dimensional model printed, to which wrought wire clasps of 0.9- and 1.0-mm diameter were adapted. Standard wax clasp patterns were adapted and cast in a stellite alloy. Each clasp was deformed beyond its proportional limit; and the forces exerted at that limit and at deflections of 0.25 mm, 0.5 mm, and 0.75 mm were measured, and a safety limit was calculated that would ensure elastic deformation at the required undercuts.ResultsA table was produced with guidelines for those clasps that would provide the highest retentive force within the proposed safety limit. The highest forces were provided by cast clasps in a 0.25-mm undercut. Wrought round wire of 1-mm diameter provided the next highest retentive forces, in a 0.25-mm undercut for premolar clasps arms and 0.5-mm for molar clasps.ConclusionsThe results provide valid guidelines for the use of combinations of clasp material and undercut that would exert the maximum retentive force without deformation for both short (premolar) as well as long (molar) clasps, for wrought and cast clasps.     

钴铬合金铸造三臂卡环固位臂宽度设计与基牙倒凹关系的三维有限元分析

目的用三维有限元方法分析钻铬合金铸造三臂卡环固位臂在摘取时的应力分布，探讨3种宽度固位臂可放置的最大倒凹深度，为可摘局部义齿的设计和制作提供参考。方法以下颌第二磨牙为基牙，建立基牙和3种宽度固位臂的三维有限元模型，设定固位臂宽厚比为3：1，设计卡臂尖和固位臂中部进入基牙的倒凹深度不同，在固位臂中部施加动态位移载荷（以3mm／s速度，使卡环向上产生3mm的位移），计算固位臂从基牙上摘取时的接触受力。采用的统计学方法为直线相关与回归分析和两条回归直线的比较。结果固位臂的最大应力与卡臂尖进入基牙的倒凹深度呈正相关关系，与固位臂中部1／3进入基牙的倒凹深度无明显相关性。当卡臂尖进入基牙的倒凹深度相同时，随固位臂宽度的增大，固位臂最大应力的增加有统计学意义（P〈0．01）。1．8、1．6和1．4mm^3种宽度的钻铬合金固位臂可放人基牙的最大倒凹深度分别为0．25mm、0．30mm和0．35mm。结论当宽厚比一定时，固位臂越宽，卡臂尖可进入基牙的最大倒凹深度越小。为防止固位臂永久变形，应根据卡臂尖进入基牙的倒凹深度选择不同宽度的固位臂。