Methodology
Value of bitewing radiographs in detection of occlusal caries NIGEL M . KING AND LINDA SHAW
Department of Children's Dentistry, Institute of Detital Surgery, Eastman Dental Hospital, University of London, London, England Kitig, N. M, & Shaw, L,: Value of bitewing radiographs in detection of occlusal caries. Community Dent, Oral Epidemiol, 1979: 7: 218-221, Abstract - A systetn of diagnosing occlusal caries from bitewing radiographs was developed in an attempt to overcome the problems of clinical diagnosis found in other studies. Standardised bitewing radiographs for 1172 Berkshire schoolchildren aged 11-13 years who were participating in a toothpaste trial, were assessed for occlusal caries under unifomi tnagtiification and illumination. The radiographic scores were then eompared with the clinical records for these subjects. The radiographic technique proved to be acceptably reproducible at 82.6 %. However, only 33,2 % of the lesions present were detected on the radiographs. It is concluded that bitewing radiographs for the detection of occlusal caries are of little value in epidemiologieal studies. Key words: earies, diagnosis; caries, epidemiology; caries, occlusal; radiographs, L. Shaw, Departtnent of Children's Dentistry, Institute of Dental Surgery, Eastman Dental Hospital, 256 Gray's Inn Road, London, WCIX 8LD, England. Accepted for publication 18 February 1979.
Caries iticidenee data can only be of signifieatice if the diagtiostic criteria employed are accurate atid easily reproducible. Lesions must be diagnosed as close to inception as possible, consistetit with the examiner being able to make the diagtiosis with the minimum of error (15). The FDI (15) further stated that: "Several studies have demonstrated that when the same group of patietits was examined by several exatniners, the differences between the number of decayed teeth diagnosed by differetit examiners was nearly as great or greater than the differetices amotigst patietits." Careful ealibration of examiners must be carried out (12), and adequate precautiotis must be observed to enable realistic evaluation of data from comparable studies (13). This problem of accurate diagtiosis has been stated to be most acute oti the occlusal surface ( 1 0 ) .
-^
•_
•
_ --
•
Diagnostic criteria for approximal lesions can be more rigidly defitied atid applied oti radiographie examination than for clinical examination as con-
trolled conditions are more readily aehieved. Examitier reproducibility of diagtiosis has bceti shown to be cotisistently higher for radiographic examitiations than for clitiical caries assessment (11). Tliis higher level of reproducibility led BAGKER-DIRKS, AMERONGEN & WiNKLER (2) to abatidoti clinical examitiatiotis and rely entirely on radiographic diagnosis. However, a similar high degree of consistency was tiot achieved wheti occlusal caries was diagnosed from radiographs by MILLER & HOBSON (9). WuEHRMANN & MANSON-HING (14) Suggested that: "The diligent use of a mirror and explorer will ordinarily detect occlusal caries before it becomes observable radiographieally". While GALAGAN & VERMILLION (7) expressed the view that diagtiosis of occlusal caries frotn radiographs would be a clinical rarity. However, only litnited data are available (9). It is therefore the purpose of this study (a) To formulate reproducible diagnostic criteria for the
0301-5661/79/040218-04$02.50/0 © 1979 Munksgaard, Copenhagen
Value of bitewing radiographs 219 recording of occlusal caries from bitewitig radiographs, (b) to compare examiner reproducibility of clinical and radiographic methods, atid (c) to compare the setisitivity of the two techniques in the diagnosis of occlusal caries.
Table 1, Intraexaminer reproducibility for diagnosis of primary occlusal caries from bitewing radiographs First examination Sound
Caries
121
0
Second examination
I MATERIAL AND METHODS
Sovtnd
| T h e clinical records and standardised bitewing radiographs ' w e r e available for 1172 Berkshire schoolchildren aged 1113 years who were participating in a toothpaste trial,
Caries
IRADIOGRAPHIC EXAMINATION All radiographs were taken using disposable film holders which had been specially developed for epidemiologieal studies. These were made front stiffened cardboard, with a tag of uniform length which projected extraorally. This allowed standardisation of the film-tube distance. A portable Mikasa Atomscope 20 X-ray machine, fitted with an automatic voltage compensator and timer, operating at a fixed rating of 68 Kvp and 15 tiiA was employed. The radiograpfis were processed in batehes under identical conditions, „ O n e examiner (N,M.K.) assessed all 1172 sets of films 'on a standard transilluminator (Vision Eng, Ltd,, Send, Surrey.), which provided X2 inagnificalion with a uniform light source. C L I N I C A L EXAMINATION The clinical examinations were conducted by two examiners (J,J,M, and L.S.) with the subject seated in a portable dental chair. An Anglepoise lamp fitted with a 100 W hulb provided uniform illumination. After 10 examinations the A s h No, 6 straight probes were discarded, D I A G N O S T I C CRITERIA Radiographie - A radiolucent area beneath the occlusal surface continuous with, and extending alotig, the atnelodentinal junction was recorded as a positive diagnosis of occlusal caries from the radiographs. Clinical — A clinical diagnosis of caries was made if, on applying gentle pressure, the probe stuck atid then required a definite pull to withdraw,
REPRODUCIBILITY STUDY Prior to commencing the study a period of examiner training w a s conducted to ensure consistent application of the diagnostic criteria, A random selection of 150 of the 1172 sets of radiographs was made by an administrative assistant a n d examined two days after the initial reading to assess intraexaminer reproducibility.
RESULTS
V
REPRODUCIBILITY STUDY On the 150 sets of radiographs that were reassessed, 576 first permatient tnolars were present;
4.
19
144 were utirestored atid therefore suitable for itivestigatioti as to caries status. At the first examitiatioti 125 teeth had beeti recorded as sound atid 19 as carious (Table 1). On re-reading, 121 were sound while 23 were found to be carious: 19 teeth were consistently diagtiosed as carious. The FDI itidex was r = 4/19 = 0.21, a percentage reproducibility of 82,6, MAIN STUDY In all, 4688 permanent first tnolar teeth were suitable for study, A total of 834 teeth were diagnosed as having occlusal caries on either clinical or radiographic examination (Table 2). Occlusal caries was diagnosed clinically in 804 teeth, 96.4 % of the total regarded as carious at either examination. Of these 804 teeth, 247 (29,6 % of the total) were also recorded as carious on the radiographic exatnination, and a further 30 occlusal surfaces (3,6 % of the total) were scored at the radiographic examitiation alotie. Thus, 96,4 % of occlusal caries were recorded at the clinical examination compared with 33,2 % at the radiographic exatnination, Restoratiotis of the buccal atid litigual surfaces obscured the amelo-dentinal junctioti atid preTable 2, Diagnosis of primary occlusal caries in 4688 permanent first molars No, carious teeth (total 834)
Pereent carious teeth
Diagnosed at both clinical and radiographic exatnination
247
29.6
Diagnosed at clinical examination only
557
66.8
30
3.6
Diagnosed at radiographic examitiation only
220
KING AND SHAW
vented radiographic diagnosis oti 44 occasions, while carious lesions in these surfaces resulted in a similar loss of data for six teeth. A further three teeth had large mesial lesions which obviously had ati occlusal component but did not fulfil the diagnostic criteria. Recalculation of the data to eliminate these problems raised the level of agreemetit between radiographic and clinical diagnosis from 29.6 to 31.6%.
DISCUSSION The presetit study has demonstrated that an examitier trained iti the use of well-defined criteria for the diagnosis of occlusal caries from bitewing radiographs can achieve an acceptable standard of reproducibility (82.6 %). This level of reproducibility exceeds that reported by SHAW & MURRAY (11) of 80 % and 74 % for the detection of caries by clinical means. The former investigator, when assessing the reprodueibility of the diagnosis of approximal earies from radiographs, ealeulated a value of 89.7 %. Other published studies have also shown radiographic reproducibility to be more consistent than clinical diagnosis (3, 4). However, not only is diagtiosis of approximal caries more reproducible from bitewitig radiographs than from clinical examinatioti, but when radiographs have been employed in clinical trials a very high proportion of the caries iticrement has been diagnosed from them. A considerable loss of data has been shown to occur when bitewing radiographs are not used in conjunction with clinical methods for the detection of occlusal caries (1, 6, 8). The present itivestigation suggests that the detection of occlusal caries from bitewing radiographs, although tnore reproducible than clinical examinatiotis, is considerably less sensitive. Only 33.2 % of the surfaces were diagnosed from the radiographs. This constitutes a 66.8 % loss of data atid confirms quantitatively the views expressed by GALAGAN & VERMILLION (7), and WUEHRMANN & MANSONHING (14).
A carious fissure may easily be obscured by superimpositioti of the large bulk of buccal and lingual enamel. Restorations and carious lesions of these surfaces cati be expected to exhibit a similar effeet. However, correction of the data to allow for these problems and for large mesio-occlusal cavities
not fulfilling the diagtiostic criteria, only raised the level of consistency by 2.0 %. The FDI (15) suggested that the most desirable unit of measurement for a particular exaniitier to use was that which is associated with the least diagnostic variability. The current study has demonstrated that higher levels of reproducibility in dir agtiosis of occlusal caries from radiographs were achieved at the expense of loss of data. Ati inverse relationship between reproducibility and setisitivity has also been indicated by DOWNER & O'MULLANE (5). Reproducible diagnostic criteria have beeti formulated for the recording of occlusal caries from bitewing radiographs. Although reproducibility of this radiographic method compared favourably with clinical diagnosis, there was a considerable loss of data. It is therefore concluded that detection of occlusal caries from radiographs is of little value iti epidemiological studies. Aekttowledgments - The authors would like to thank Professor J, J. MURRAY for making the data available for analysis, and Professor G, B, WtNTER for his help and guid-
REFERENCES 1, ARNot^D, F, A,, TRENDLEY DEAN, H , & SINGLETCA,', D ,
2,
3,
4,
5,
E,: The effect on caries incidenee of a single application of a fluoride solution to the teeth of young adult tnales of a military population, / , Dent. Res. 1944: 23: 15,5-162 BACKi.:R-DtRKS, O,, AMERONGEN, J, VAN & WiNKLt^R, K, C : A reproducible method for caries evaluation, /, Dent. Res. 1951: 30: 346-359 BERMAN, D , S,: A study of the pattern of developmetit of dental caries of the permanent dentition in selected groups of ehildren. Thesis, University of London 1970 DOWNER, M , C : Aspects of the validity of diagnosis itt the epidemiology of dental caries. Thesis, Manehester University 1974 DovvNER, M, C. & O'Mut^LANE, D, M,: A comparison of the eoncurrent validity of two epidemiologic diagnostic systems for caries evaluation. Comntunity Dettt. Oral Epidemiol. 1975: 3: 20-24
6, ENGEANDER, H , R , , SHERRILL, L , T , , MIELER, B , G,, CAREOS, J , P,, MEEt.BERG, J, R, & SENNING, R , S,: In-
ereniental rates of dental caries after repeated topical sodium fluoride applicatioits in children with lifelong consumption of fluoridated water, / , Am. Dent. Assoc. 1971: 82.-354-358 • 7, GAI.AGAN, D , J , & VERMtEEiON, J,: Diagnosis of caries by radiographic interpretation, / , Dent, Res. 1956: 55:1
33-38
:
Value of bitewing radiographs 221 ' 8. KERR, D . W . & Kt.sEE, R. G.: Two-year caries control study utilizing oral hygiene and an ammoniated dentifrice. / , Am. Dent. Assoe. 1951: 42: 180-188 9. MttXER, J. & HOBSON, P.: Determination of the prer sence of caries in fissures. Br. Dent. J. 1956: 100: 1518 10. MotxER, I, J,: Clinical criteria for the diagnosis of the incipient carious lesion. Adv. Fluorine Res. Dent. Caries Prev, 1965: 4: 67-72 11, SHAW,
L , & MURR.-W, J. J.: Inter-examiner and intra-
examiner reproducibility in clinical and radiographic diagnosis, hit, Detit. J. 1975: 25: 280-288
12, St.ACK, G, L,, J.^cKSON, D,, JAMES, P , M , C , LAWTON,
F. E.: A clinical investigation into the variability of dental caries diagnosis. Br, Dettt. J. 1958: 104: 399404 13, SuTci.it't-E, P.: Caries experience and dental treatment in children, A survey, Br. Dent. J. 1966: 121: 508-512 14, WuEitRMANN, A, H, & MANSON-HING, G, L , R , : Dental radiology. 2nd ed. C. V. Mosby, Saint Louis 1969, pp. 264-265 15, FDI: Prittciple requirements for controlled clinical trials of caries preventive agents and procedures, Teclittical Report No. 1. FDI, London 1974
Value of bitewing radiographs in detection of occlusal caries NIGEL M . KING AND LINDA SHAW
Department of Children's Dentistry, Institute of Detital Surgery, Eastman Dental Hospital, University of London, London, England Kitig, N. M, & Shaw, L,: Value of bitewing radiographs in detection of occlusal caries. Community Dent, Oral Epidemiol, 1979: 7: 218-221, Abstract - A systetn of diagnosing occlusal caries from bitewing radiographs was developed in an attempt to overcome the problems of clinical diagnosis found in other studies. Standardised bitewing radiographs for 1172 Berkshire schoolchildren aged 11-13 years who were participating in a toothpaste trial, were assessed for occlusal caries under unifomi tnagtiification and illumination. The radiographic scores were then eompared with the clinical records for these subjects. The radiographic technique proved to be acceptably reproducible at 82.6 %. However, only 33,2 % of the lesions present were detected on the radiographs. It is concluded that bitewing radiographs for the detection of occlusal caries are of little value in epidemiologieal studies. Key words: earies, diagnosis; caries, epidemiology; caries, occlusal; radiographs, L. Shaw, Departtnent of Children's Dentistry, Institute of Dental Surgery, Eastman Dental Hospital, 256 Gray's Inn Road, London, WCIX 8LD, England. Accepted for publication 18 February 1979.
Caries iticidenee data can only be of signifieatice if the diagtiostic criteria employed are accurate atid easily reproducible. Lesions must be diagnosed as close to inception as possible, consistetit with the examiner being able to make the diagtiosis with the minimum of error (15). The FDI (15) further stated that: "Several studies have demonstrated that when the same group of patietits was examined by several exatniners, the differences between the number of decayed teeth diagnosed by differetit examiners was nearly as great or greater than the differetices amotigst patietits." Careful ealibration of examiners must be carried out (12), and adequate precautiotis must be observed to enable realistic evaluation of data from comparable studies (13). This problem of accurate diagtiosis has been stated to be most acute oti the occlusal surface ( 1 0 ) .
-^
•_
•
_ --
•
Diagnostic criteria for approximal lesions can be more rigidly defitied atid applied oti radiographie examination than for clinical examination as con-
trolled conditions are more readily aehieved. Examitier reproducibility of diagtiosis has bceti shown to be cotisistently higher for radiographic examitiations than for clitiical caries assessment (11). Tliis higher level of reproducibility led BAGKER-DIRKS, AMERONGEN & WiNKLER (2) to abatidoti clinical examitiatiotis and rely entirely on radiographic diagnosis. However, a similar high degree of consistency was tiot achieved wheti occlusal caries was diagnosed from radiographs by MILLER & HOBSON (9). WuEHRMANN & MANSON-HING (14) Suggested that: "The diligent use of a mirror and explorer will ordinarily detect occlusal caries before it becomes observable radiographieally". While GALAGAN & VERMILLION (7) expressed the view that diagtiosis of occlusal caries frotn radiographs would be a clinical rarity. However, only litnited data are available (9). It is therefore the purpose of this study (a) To formulate reproducible diagnostic criteria for the
0301-5661/79/040218-04$02.50/0 © 1979 Munksgaard, Copenhagen
Value of bitewing radiographs 219 recording of occlusal caries from bitewitig radiographs, (b) to compare examiner reproducibility of clinical and radiographic methods, atid (c) to compare the setisitivity of the two techniques in the diagnosis of occlusal caries.
Table 1, Intraexaminer reproducibility for diagnosis of primary occlusal caries from bitewing radiographs First examination Sound
Caries
121
0
Second examination
I MATERIAL AND METHODS
Sovtnd
| T h e clinical records and standardised bitewing radiographs ' w e r e available for 1172 Berkshire schoolchildren aged 1113 years who were participating in a toothpaste trial,
Caries
IRADIOGRAPHIC EXAMINATION All radiographs were taken using disposable film holders which had been specially developed for epidemiologieal studies. These were made front stiffened cardboard, with a tag of uniform length which projected extraorally. This allowed standardisation of the film-tube distance. A portable Mikasa Atomscope 20 X-ray machine, fitted with an automatic voltage compensator and timer, operating at a fixed rating of 68 Kvp and 15 tiiA was employed. The radiograpfis were processed in batehes under identical conditions, „ O n e examiner (N,M.K.) assessed all 1172 sets of films 'on a standard transilluminator (Vision Eng, Ltd,, Send, Surrey.), which provided X2 inagnificalion with a uniform light source. C L I N I C A L EXAMINATION The clinical examinations were conducted by two examiners (J,J,M, and L.S.) with the subject seated in a portable dental chair. An Anglepoise lamp fitted with a 100 W hulb provided uniform illumination. After 10 examinations the A s h No, 6 straight probes were discarded, D I A G N O S T I C CRITERIA Radiographie - A radiolucent area beneath the occlusal surface continuous with, and extending alotig, the atnelodentinal junction was recorded as a positive diagnosis of occlusal caries from the radiographs. Clinical — A clinical diagnosis of caries was made if, on applying gentle pressure, the probe stuck atid then required a definite pull to withdraw,
REPRODUCIBILITY STUDY Prior to commencing the study a period of examiner training w a s conducted to ensure consistent application of the diagnostic criteria, A random selection of 150 of the 1172 sets of radiographs was made by an administrative assistant a n d examined two days after the initial reading to assess intraexaminer reproducibility.
RESULTS
V
REPRODUCIBILITY STUDY On the 150 sets of radiographs that were reassessed, 576 first permatient tnolars were present;
4.
19
144 were utirestored atid therefore suitable for itivestigatioti as to caries status. At the first examitiatioti 125 teeth had beeti recorded as sound atid 19 as carious (Table 1). On re-reading, 121 were sound while 23 were found to be carious: 19 teeth were consistently diagtiosed as carious. The FDI itidex was r = 4/19 = 0.21, a percentage reproducibility of 82,6, MAIN STUDY In all, 4688 permanent first tnolar teeth were suitable for study, A total of 834 teeth were diagnosed as having occlusal caries on either clinical or radiographic examination (Table 2). Occlusal caries was diagnosed clinically in 804 teeth, 96.4 % of the total regarded as carious at either examination. Of these 804 teeth, 247 (29,6 % of the total) were also recorded as carious on the radiographic exatnination, and a further 30 occlusal surfaces (3,6 % of the total) were scored at the radiographic examitiation alotie. Thus, 96,4 % of occlusal caries were recorded at the clinical examination compared with 33,2 % at the radiographic exatnination, Restoratiotis of the buccal atid litigual surfaces obscured the amelo-dentinal junctioti atid preTable 2, Diagnosis of primary occlusal caries in 4688 permanent first molars No, carious teeth (total 834)
Pereent carious teeth
Diagnosed at both clinical and radiographic exatnination
247
29.6
Diagnosed at clinical examination only
557
66.8
30
3.6
Diagnosed at radiographic examitiation only
220
KING AND SHAW
vented radiographic diagnosis oti 44 occasions, while carious lesions in these surfaces resulted in a similar loss of data for six teeth. A further three teeth had large mesial lesions which obviously had ati occlusal component but did not fulfil the diagnostic criteria. Recalculation of the data to eliminate these problems raised the level of agreemetit between radiographic and clinical diagnosis from 29.6 to 31.6%.
DISCUSSION The presetit study has demonstrated that an examitier trained iti the use of well-defined criteria for the diagnosis of occlusal caries from bitewing radiographs can achieve an acceptable standard of reproducibility (82.6 %). This level of reproducibility exceeds that reported by SHAW & MURRAY (11) of 80 % and 74 % for the detection of caries by clinical means. The former investigator, when assessing the reprodueibility of the diagnosis of approximal earies from radiographs, ealeulated a value of 89.7 %. Other published studies have also shown radiographic reproducibility to be more consistent than clinical diagnosis (3, 4). However, not only is diagtiosis of approximal caries more reproducible from bitewitig radiographs than from clinical examinatioti, but when radiographs have been employed in clinical trials a very high proportion of the caries iticrement has been diagnosed from them. A considerable loss of data has been shown to occur when bitewing radiographs are not used in conjunction with clinical methods for the detection of occlusal caries (1, 6, 8). The present itivestigation suggests that the detection of occlusal caries from bitewing radiographs, although tnore reproducible than clinical examinatiotis, is considerably less sensitive. Only 33.2 % of the surfaces were diagnosed from the radiographs. This constitutes a 66.8 % loss of data atid confirms quantitatively the views expressed by GALAGAN & VERMILLION (7), and WUEHRMANN & MANSONHING (14).
A carious fissure may easily be obscured by superimpositioti of the large bulk of buccal and lingual enamel. Restorations and carious lesions of these surfaces cati be expected to exhibit a similar effeet. However, correction of the data to allow for these problems and for large mesio-occlusal cavities
not fulfilling the diagtiostic criteria, only raised the level of consistency by 2.0 %. The FDI (15) suggested that the most desirable unit of measurement for a particular exaniitier to use was that which is associated with the least diagnostic variability. The current study has demonstrated that higher levels of reproducibility in dir agtiosis of occlusal caries from radiographs were achieved at the expense of loss of data. Ati inverse relationship between reproducibility and setisitivity has also been indicated by DOWNER & O'MULLANE (5). Reproducible diagnostic criteria have beeti formulated for the recording of occlusal caries from bitewing radiographs. Although reproducibility of this radiographic method compared favourably with clinical diagnosis, there was a considerable loss of data. It is therefore concluded that detection of occlusal caries from radiographs is of little value iti epidemiological studies. Aekttowledgments - The authors would like to thank Professor J, J. MURRAY for making the data available for analysis, and Professor G, B, WtNTER for his help and guid-
REFERENCES 1, ARNot^D, F, A,, TRENDLEY DEAN, H , & SINGLETCA,', D ,
2,
3,
4,
5,
E,: The effect on caries incidenee of a single application of a fluoride solution to the teeth of young adult tnales of a military population, / , Dent. Res. 1944: 23: 15,5-162 BACKi.:R-DtRKS, O,, AMERONGEN, J, VAN & WiNKLt^R, K, C : A reproducible method for caries evaluation, /, Dent. Res. 1951: 30: 346-359 BERMAN, D , S,: A study of the pattern of developmetit of dental caries of the permanent dentition in selected groups of ehildren. Thesis, University of London 1970 DOWNER, M , C : Aspects of the validity of diagnosis itt the epidemiology of dental caries. Thesis, Manehester University 1974 DovvNER, M, C. & O'Mut^LANE, D, M,: A comparison of the eoncurrent validity of two epidemiologic diagnostic systems for caries evaluation. Comntunity Dettt. Oral Epidemiol. 1975: 3: 20-24
6, ENGEANDER, H , R , , SHERRILL, L , T , , MIELER, B , G,, CAREOS, J , P,, MEEt.BERG, J, R, & SENNING, R , S,: In-
ereniental rates of dental caries after repeated topical sodium fluoride applicatioits in children with lifelong consumption of fluoridated water, / , Am. Dent. Assoc. 1971: 82.-354-358 • 7, GAI.AGAN, D , J , & VERMtEEiON, J,: Diagnosis of caries by radiographic interpretation, / , Dent, Res. 1956: 55:1
33-38
:
Value of bitewing radiographs 221 ' 8. KERR, D . W . & Kt.sEE, R. G.: Two-year caries control study utilizing oral hygiene and an ammoniated dentifrice. / , Am. Dent. Assoe. 1951: 42: 180-188 9. MttXER, J. & HOBSON, P.: Determination of the prer sence of caries in fissures. Br. Dent. J. 1956: 100: 1518 10. MotxER, I, J,: Clinical criteria for the diagnosis of the incipient carious lesion. Adv. Fluorine Res. Dent. Caries Prev, 1965: 4: 67-72 11, SHAW,
L , & MURR.-W, J. J.: Inter-examiner and intra-
examiner reproducibility in clinical and radiographic diagnosis, hit, Detit. J. 1975: 25: 280-288
12, St.ACK, G, L,, J.^cKSON, D,, JAMES, P , M , C , LAWTON,
F. E.: A clinical investigation into the variability of dental caries diagnosis. Br, Dettt. J. 1958: 104: 399404 13, SuTci.it't-E, P.: Caries experience and dental treatment in children, A survey, Br. Dent. J. 1966: 121: 508-512 14, WuEitRMANN, A, H, & MANSON-HING, G, L , R , : Dental radiology. 2nd ed. C. V. Mosby, Saint Louis 1969, pp. 264-265 15, FDI: Prittciple requirements for controlled clinical trials of caries preventive agents and procedures, Teclittical Report No. 1. FDI, London 1974
