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ix parts or sextants. A specially designed probe is used to test the condition (l, 2, 3 or 4) of the gum around the tooth selected as the index tooth for each sextant.
     If several clinical signs are present simultaneously, the most severe is selected.
     WHO has compiled data on over IOO surveys carried out in the age group 35-44 years. These data should be treated with caution, since very few of them provide a national estimate.
     Nevertheless they are of great interest because they consistently show a similar pattern frequency and severity of involvement, which challenges some generally accepted ideas about the distribution and the etiological process of periodontal disease.
     The data show that the percentage of people who have deep pockets and the mean number of sextants per person also displaying deep pockets are low to very low. This means that the severe forms of periodontal disease, those requiring complicated surgery are far from common.
     Moreover, there seems to be no difference in frequency between industrialized countries and developing countries for the severe forms of periodontal disease.
     On the other hand, the initial forms (bleeding and calculus) are much more prevalent in the developing countries.
     In the light of these data it may be stated that generalized periodontal destruction is rare in 40-year-old adults. Some people show some signs of such destruction, but only a limited part of their dentition is affected. It seems that the initial forms (bleeding and calculus) do not necessarily lead on to the advanced stages of the disease, except in certain minority groups.
   
How can these diseases be prevented?
     After a few days of careful cleaning of the teeth, the bleeding stops in the vast majority of cases and the inflammation, the cause of much discomfort, also regresses.
     There are various possible prevention policies: hygiene f8r the most common and least serious forms; development of products that act against the destructive types of periodontitis. A whole field of research to protect the risk groups is opening up.

VOCABULARY
l. gum, gingiva 牙龈
2. calculus, tartar 牙结石
3. Food debris 食物残渣
4. Dental plaque 牙菌斑
5. index, indices 指数，索引
6. mean DMFT 龋(牙)均
7. D-Decayed 龋
8. M - Missing (on account of Caries) 失
9. F-Filled 补
10. Key ages (index age groups) 指数年龄组
11. oral health situation 口腔健康状况
12. regular intervals 定期，间隔一定时间
13. monitor 监测，监督
14. caries prevalence 患龋
15. simplified 简化的
16. reliable 可靠的
17. epidemiological survey 流行病学调查
18. preventive progromme 预防项目(计划)
19. on request 根据要求
20. data bank 资料库(数据瘁)
21. compile (d) 编制
22. Sharp contrast (s) 鲜明对比
23. downward 向下的，下降的
24. upward 向上的，上升的
25. spectacular fall 引人注目的下降
26. virtually 事实上，实际上：
27. weighted for population 加权人口
28. Justify, justifies 证明(认为)……有道理，为•…．•提供依据
29. optimism 乐观(主义)
30. graph 座标图，曲线图
31. worsening (使)恶化、变坏
32. halt (ed) 停止、止步，休息
33. unprecedented 空前的，无先例的
34. supervised 在监督指导下的
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