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and the other teeth are still receiving periodontal treatment. The Rochette design is also used for immediate insertion bridges so that the bridge can be removed when the tissues have healed and the pontic adapted to the ridge or the bridge remade.
Historically, the medium-mechanical retentive systems were developed after the Rochette and Maryland designs in an attempt to overcome the disadvantages of these described above. However, they have disadvantages of their own in being less retentive than the micro-mechanical system and yet having thicker metal retainers and a thicker cement film. One advantage, however, is that they can be made in any metal, including precious metals, whereas the etched systems can only be made in base metal alloys which are etchable.
Disadvantage of minimal- preparation bridges in general
As the metal plate is added to the surface of the tooth rather than replacing part of it, the thickness of the tooth is increased and may-for example, in a normal Class incisor relationship interfere with the occlusion unless space is created orthodontically or by grinding the opposing teeth.
The margin of the retainer inevitably produces a ledge where plaque can collect. This is a problem especially in the replacement of lower incisors. Here plaque and calculus deposits are common on the lingual surface towards the gingival margin and the presence of such a ledge can only make it more difficult for the patient to clean in this area. Another example of a design that would prevent good oral hygiene is shown in Figure 96a.
Practical Points
∙ The four basic bridge designs differ in the support provide at each end of the pontic.
∙The basic designs can be combined to give, for example, a fixed-fixed/cantilever design.
∙ Simpler, less destructive minimal- preparation bridges are useful, particularly in younger patiens.
∙ Bridges that are made to be removable by the dentist make further endodontic or periodontal treatment possible; those patients can remove make cleaning easier.
∙Inexperienced operators should not use spring cantilever designs as a soft option to preparing sound anterior teeth.
VOCABULARY
l. A bridge 桥
2. A minimal- preparation bridge 最小预备的桥
3. A removable bridge 活动桥
4. A precision attachment partial denture 精密附着局部义齿
5. A partial denture 局部义齿
6. Occlusal stability 合稳定
7. retention 固位
8. dentition 牙列
9. Periodontal splinting 牙周夹板
10. Secondary caries 继发龋
11. microleakage 微漏
12. abutment 基牙
13. pontic 桥体
14. retainer 固位体
15. span 跨度、桥距
16. saddle 桥鞍
17. pier 桥墩
18. unit 单位
19. connector 连接体
20. fixed-fixed bridge 固定—固定桥
21. fixed-moveable bridge 固定—活动桥
22. Cantilever bridge 悬臂桥
23. spring cantilever bridge 弹性悬臂桥
24. Conventional bridge 传统桥
25. Metal Bridge 金属桥
26. Metal-ceramic bridge 金属烤瓷桥
27. Ceramic bridge 烤瓷桥
28. leverage force 杠杆力