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الكلية كلية طب الاسنان
القسم ترميم ومعالجة الاسنان
المرحلة 4
أستاذ المادة احمد غانم مهدي الهلال
4/18/2011 7:57:52 AM
Complex restorations
Complex restorations are used to replace missing tooth structure of teeth that have fractured or are severely involved with caries or existing restorative material.
These restorations usually involve the replacement of one or more missing cusps, and often, they utilize a bonding technique. The most frequently used pin type is self-threading pin. Friction-locked and cemented pins, although still available, are rarely used.
The threads of self threaded pins engage the dentin as it is inserted, lead to retaining the pin. The elasticity (resiliency) of the dentin permits insertion of a threaded pin into a hole of smaller diameter. Although the threads of self threading pins do not engage the dentin for their entire width, the self-threading pin is the most retentive and acceptable one.
Resistance and Retention Forms:
In a tooth severely involved with caries or existing restorative material, any undermined enamel or weak tooth structure subject to fracture must be removed and restored. Usually, a weakened tooth is best restored with a properly designed indirect (usually cast) restoration that will prevent tooth fracture caused by mastication forces. However, in selected cases, preparations may be designed for amalgam that improves the resistance form of a tooth.
When conventional retention and resistance features are not adequate because of insufficient remaining tooth structure, pins, slots, and amalgam bonding techniques may be used to enhance retention form. The retention features needed depend on the amount of tooth structure remaining and the tooth being restored. As more tooth structure is lost, more auxiliary retention is required.
CONTRAINDICATIONS:
The complex amalgam restoration may be contraindicated in:
1. If the patient has significant occlusal problems.
2. If the tooth cannot be properly restored with a direct restoration because of anatomic and/or functional considerations.
3. If the area to be restored is esthetically important for the patient.
ADVANTAGES
1. Conserves Tooth Structure: The preparation for a complex amalgam restoration is usually more conservative than the preparation for an indirect restoration or a crown.
2. Appointment Time: The complex restoration can be completed in one appointment. The cast restoration requires at least two appointments.
3. Resistance and Retention Forms: Resistance and retention forms may be significantly increased by the use of pins, slots, and bonding.
4. Economics.
DISADVANTAGES
1. Dentinal Microfractures: Preparing pinholes and placing pins may create craze lines or fractures, as well as internal stresses in the dentin. Such craze lines and internal stress may have little or no clinical significance, but they may be important when minimal dentin is present.
2. Microleakage: In amalgam restorations using cavity varnish, microleakage around all types of pins has been demonstrated.
3. Decreased Strength of Amalgam: The tensile strength and horizontal strength of pin-retained amalgam restorations are significantly decreased.
4. Resistance Form: Resistance form is more difficult to develop than when preparing a tooth for a cusp capping onlay or a full crown. The complex amalgam restoration does not protect the tooth from fracture as well as an extracoronal restoration. However, amalgam restorations with cusp coverage significantly increase the fracture resistance of weakened teeth as compared to amalgam restorations without cusp coverage.
5. Penetration and Perforation: Pin retention increases the risk of penetrating into the pulp or perforating the external tooth surface.
General concepts in Tooth Preparation for Pin-Retained Amalgam Restorations
When the facial or lingual extension exceeds two thirds the distance from a primary groove toward the cusp tip (or when the facial-lingual extension of the occlusal preparation exceeds two thirds the distance between the facial and lingual cusp tips), reduction of the cusp(s) for amalgam is usually required for the development of adequate resistance form. After initial tooth preparation of a severely involved tooth, removal of any remaining infected carious dentin or removal of remaining old restorative material is usually necessary and is accomplished as described previously. A liner can be applied, if needed, and, if used, should not extend closer than 1 mm to a slot or a pin.
The depth of the pinhole varies from 1.3 to 2 mm, depending on the diameter of the pin used. However, a general guideline for pinhole depth is 2 mm.
Number of pins:
Several factors must be considered when deciding how many pins are required:
(1) The amount of missing tooth structure.
(2) The amount of dentin available to receive pins safely.
(3) The amount of retention required.
(4) The size of the pins.
As a rule, one pin per missing axial line angle or cusp should be used. Certain factors may cause the operator to alter this rule. The fewest pins possible should be used to achieve the desired retention for a given restoration. When only 2 to 3 mm of the occlusogingival height of a cusp has been removed, no pin is required because enough tooth structure remains to use conventional retention features. An excessive number of pins can fracture the tooth and significantly weaken the amalgam restoration.
Location:
Several factors aid in determining pinhole locations:
(1) Knowledge of normal pulp anatomy and external tooth contours.
(2) A current radiograph of the tooth.
(3) A periodontal probe.
(4) The patient s age.
Although the radiograph is only a two-dimensional picture of the tooth, it can give an indication of the position of the pulp chamber. The pinhole should be positioned no closer than 0.5 to 1 mm to the DEJ or no closer than 1 to 1.5 mm to the external surface of the tooth, and it also should be parallel to the adjacent external surface of the tooth.
With the drill in the contra-angle handpeice, place the drill in the gingival crevice beside the location for the pinhole, position it until it lies flat against the external surface of the tooth, and then, without changing the angulation obtained from the crevice position, move the handpiece apically and insert the drill with slight hand pressure on the dentin.
Incorrect angulation of the drill may result in pulpal exposure or external perforation. With the drill tip in its proper position and with the handpiece rotating at very low speed (300 to 500rpm), apply pressure to the drill, and prepare the pinhole in one or two movements until the depth-limiting portion of the drill is reached, and remove the drill from the pinhole. The drill should never stop rotating (from insertion to removal from the pinhole) to prevent the drill from breaking while in the pinhole.
Pin insertion:
The pin consists of a flattened head to engage the hand wrench or the appropriate handpiece chuck for threading into the pinhole. When the pin approaches the bottom of the pinhole, the head of the pin shears off, leaving a length of pin extending from the dentin. Two instruments for insertion of threaded pins are available: conventional contra-angle handpiece and hand wrenches.
General tips
1. In general, increasing the number of pins increases the retention in dentin and amalgam. However, the benefits of increasing the number of pins must be compared to the potential problems created. As the number of pins increases:
(1) The crazing of the dentin and the potential for fracture increase.
(2) The amount of available dentin between the pins decreases.
(3) The strength of the amalgam restoration decreases.
2. As the diameter of the pin increases, the retention in dentin and amalgam increases. However, as the number, depth, and diameter of pins increase, the danger of perforating into the pulp or the external tooth surface increases. A large number of long pins also can severely compromise condensation of the amalgam and the amalgam s adaptation to the pins. A pin technique should be used that permits optimal retention with minimal danger to the remaining tooth structure.
3. Horizontal pins can be used for cross-splinting to provide effective reinforcement of weak remaining cusps. However, horizontal pins are not generally recommended because of limited access for pin placement and for condensation once pins are placed.
4. Whenever three or more pinholes are placed, they should be located at different vertical levels on the tooth, if possible. This will reduce stresses resulting from pin placement in the same horizontal plane of the tooth.
5. Spacing between pins, or the interpin distance, must be considered when two or more pinholes are prepared. The optimal interpin distance depends on the size of pin to be used. The minimal interpin distance is 3 mm.
6. When the pin or the drill breaks inside the dentin, don’t try to remove it but choose another pin or drill and another location for placement, because the removal of broken pin or drill do more harm to the tooth than benefit and removes excessive tooth structure.
7. Care must be taken in placing pins for translucent anterior teeth because shadow may be shown through the restoration, compromising the esthetic of the patient.
8. Occlusal or incisal clearance is very important to prevent restoration fractures in the site of pin.
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
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