Combined use of three agents containing stannous fluoride: a prophylactic paste, a solution and a dentifrice

Effect on dental caries in children in a nonfluoride area of:

Combined use of three agents containing stannous fluoride: a prophylactic paste, a solution and a dentifrice

David Bixler,* D .D .S., Ph.D., and Joseph C. M uhler,* D .D .S., Ph.D., Indianapolis

Five groups of children, all natives of a community with 0.05 ppm of fluoride in the water supply, were given different combinations of three stannous fluoride treatments; these were stannous fluoride prophylactic paste, topically applied aqueous stannous fluoride solution and home use of a stannous fluoride denti­ frice. 'Children given all three treatments had the highest reduction in dental caries after 6 and 12 months.

Considerable literature has appeared doc­ umenting the effectiveness of fluorides in reducing tooth decay when added to com­ munal water supplies at an optimal con­ centration. In an attempt to reduce the caries prevalence even further, topical application of sodium fluoride in chil­ dren whose teeth calcified while they lived in a region with an optimal level of fluoride in the water supply was evalu­ ated in the hope that a combination of these two procedures would produce ad­ ditive effects. Such studies have demonstrated that no added benefit resulted from the so­

dium fluoride topical treatments.1,2 Stan­ nous fluoride, however, has greater utility than sodium fluoride, and topical appli­ cation of this compound in children whose permanent teeth had calcified in a community whose water contained the optimal amount of fluoride has shown benefits in addition to those from the sys­ temic ingestion of fluoride from a com­ munity water supply.3 Such findings suggested that the use of other clinical technics in addition to topical application of aqueous stannous fluoride solution could produce even further benefits. Evidence to suggest that combining dif­ ferent technics of topical stannous fluo­ ride therapy may produce cumulative effects was strengthened by a clinical study4 in which children living in a non­ fluoride region received a single topical application of stannous fluoride every six months during the three-year study and used a SnF2-Ca2P207 dentifrice at home. Comparable children received placebo treatments. T he results showed a caries reduction of approximately 60 per cent. These results suggested the use of a prophylactic paste containing SnF2 and a compatible polishing agent as a third pos­

BIXLER—MUHLER .. .VOLUME 68, JU N E 1964 • 25/793

sibility for using stannous fluoride in a “multiple therapy” approach. T he use of prophylactic pastes contain­ ing sodium fluoride is not new. As early as 1946, Bibby and others5 suggested the use of sodium fluoride in a dental pro­ phylactic paste. They evaluated a paste containing 1 per cent sodium fluoride in children 6 to 15 years old and reported a caries reduction ranging from 25 to 43 per cent depending on the number of treatments. Wellock, Roberts and Bibby,6 however, were unable to confirm these data in a repeat study using a sodium flu­ oride paste in children 10 to 12 years old. T he use of stannous fluoride in a pro­ phylactic paste is complicated by the fact that most polishing agents contain sub­ stances which inactivate the stannous ion, the fluoride ion or both. A complex magnesium-calcium silicate polishing agent was found to be highly compatible with SnF2, and this method was used as the basis for formulating a compatible clean­ ing and polishing mixture to be mixed with SnF2 and used for dental prophy­ laxis.7 We, therefore, initiated a laboratory program to study the problem and found that aqueous SnF2 mixed with the polish­ ing agent was more effective as an anti­ solubility agent than was solid stannous fluoride added directly to a nonaqueous abrasive system. T he greatest antisolu­ bility effectiveness resulted when the stannous fluoride-containing prophylactic paste, the stannous fluoride topical appli­ cation and the SnF2-Ca2P20 7 dentifrice were used in combination with each other. Such a finding in the laboratory prompted us to speculate on how clini­ cally effective a combination of these three topical technics of using stannous fluoride would be when used together in a region with an optimal level and region with a low level of fluoride in the water. Such a program was planned and con­ ducted by the division of preventive den­ tistry at Indiana University and the den­

tal division of the Indiana State Board of Health. This report concerns the clini­ cal evaluation of the three topical tech­ nics using stannous fluoride in a nonfluo­ ride, region. T he combined use of the same three technics in a fluoride region will be reported separately.8 M ETHODS

A total of 1,250 children, ranging in age from 5 to 18 years, was recruited from residents of Bloomington, Ind., whose communal water supply has a fluoride content of 0.05 fig. per milliliter. Only those children were used who had been permanent residents of Bloomington all their lives. T he children were divided into nine classes on the basis of previous caries experience. Within each class, they were assigned to one of five groups by a randomizing procedure which maintained almost equal numbers of subjects in each group. H alf of each group was examined by examiner A ; the remaining half, by examiner B. T he children in group 1 served as con­ trols and received a dental prophylaxis with the prophylactic paste without the stannous fluoride. Afterward, they re­ ceived a topical application of distilled water and were given for home use a den­ tifrice identical to the one containing stannous fluoride but without stannous fluoride (Sn 2P20 7). Children in group 2 received a prophylaxis with the stannous fluoride prophylactic paste (the SnF2 concentration in the final mixture is 8.9 per cen t), a topical application of dis­ tilled water and the nonfluoride dentifrice for home use. Group 3 was composed of children who received the stannous fluo­ ride prophylactic paste, a topical applica­ tion of distilled water and the SnF2C a 2P20 7 dentifrice for home use. Group 4 children received a prophylaxis with the stannous fluoride prophylactic paste, a topical application of 8 per cent aque­ ous stannous fluoride and the nonfluoride dentifrice. The children in group 5 re­

26/794 • THE JOURNAL OF THE AMERICAN DENTAL ASSOCIATION

Table 1 • Initial balance for all subjects as to age and DMFS, for examiners A and B No. of subjects

Group

DMFS mean

Age range

Age mean

DMFS range

Examiner A 1

126

11.50

0-51

10.82

6-17

2

113

11.43

0-81

10.90

6-17

3

120

10.95

0-58

11.46

5-17

4

113

9.95

0-57

11.91

6-17

5

118

11.90

0-62

12.21

6-17

Examiner B 1

122

7.27

0-49

10.14

5-17

2

119

7.61

0-59

10.66

6-18

3

121

7.05

0-70

10.68

6-17

4

120

6.80

0-40

10.13

6-17

5

130

8.04

0-64

10.80

6-17

Table 2 • Age distribution of subjects starting test (age in years! Examiner A Age 5 6 7 8 9 10 11 12 13 14 15 16 17 18 Total

1

2

3

_

_

7 11 18 15 13 11 13 12 12 7 4 3

5 15 17 8 16 14 11 11 6 6 2 2

1 2 11 22 12 6 19 11 15 7 3 5 6

126

113

120

Examiner B

4

5

_

_

3 10 19 15 10 15 11 8 8 5 6 3

1 9 8 14 15 9 14 15 11 13 6 3

2 9 21 22 11 18 10 6 7 3 9 2 2

113

118

122

ceived all three stannous fluoride treat­ ments; that is, the stannous fluoride pro­ phylactic paste, the stannous fluoride topical application and the SnF2-Ca2P2 0 T dentifrice. The clinical technic for the application of the prophylactic paste containing the stannous fluoride has been published else­

1

2

3

_

4

5

;1 16 17 12 6 17 13 10 3 4 5 4 1

5 17 21 17 10 9 12 13 5 7 4 1

1 4 22 17 13 12 8 15 18 2 5 0 3

20 17 18 11 9 12 16 7 9 3 2

119

121

120

130

6

where,9 as has the procedure for topical application of the 8 per cent stannous fluoride aqueous solution.9,10 No special brushing or home-care in­ structions were given to any of the pa­ tients. The dentifrices were 4 to 10 weeks old when distributed to the subjects and were used at ages ranging up to 89 weeks.

BIXLER—MUHLER . . . VOLUME 68, JU N E 1964 • 27/795

Table 3 • Group balance as to age and DMFS for all subjects complet­ ing six months of test, for examiners A and B No. of subjects

Group

DMFS mean

DMFS range

Age range

Age mean

Examiner A 1

120

11.52

0-51

11.10

6-17

2

109

¡1.46

0-81

10.78

6-17

3

115

10.93

0-58

11.21

5-17

4

108

9.81

0-45

11.09

6-17

5

113

11.90

0-62

11.90

6-17

5-17

Examiner B 1

118

6.80

0-49

10.14

2

114

7.22

0-59

10.50

6-18

3

110

7.34

0-70

10.88

6-17

4

120

6.80

0-40

10.13

5-17

5

129

8.02

0-64

10.78

6-17

Table 4 • Age distribution of subjects completing six months of test (age in years) Examiner A

Examiner B

Age

1

2

3

4

5

1

5 6 7 8 9 10 11 12 13 14 15 16 17 18

6 11 17 15 12 11 12 12 11 7 4 2

4 14 17 8 16 14 10 11 6 6 2 1

1 2 11 21 11 5 19 11 15 6 3 4 6

3 10 19 15 9 15 10 7 6 5 6 3

1 8 8 13 15 9 13 15 11 12 6 2

2 9 20 21 11 17 10 6 7 3 8 2 2

Total

120

109

115

108

113

118

T he conduct of this study was identical to that previously reported.11 Each sub­ ject was examined clinically and with the use of either a series of 5-film or 7-film, bitewing roentgenograms initially and at 6- and 12-month intervals. T he dental prophylaxis and topical treatments were repeated at the same intervals.

2

3

4

5

11 16 18 12 6 16 13 8 3 4 5 1 1

4 14 18 16 8 9 12 13 5 7 3 1

1 4 22 17 13 12 8 15 18 2 5 0 3

6 20 17 18 11 9 12 15 7 9 3 2

114

110

120

129

r e s u l t s a n d d is c u s sio n

The initial balance according to age and the caries experience of all subjects for both examiners is shown in Tables 1 and 2, whereas Tables 3-6 show balance at the end of 6 and of 12 months for subjects remaining in the study.

28/796 • THE JOURNAL OF THE AMERICAN DENTAL ASSOCIATION

Table 5 • Group balance as to age and DMFS for all subjects completing 12 months of test, for both examiners A and B No. of subjects

Group

DMFS mean

DMFS range

Age mean

Age range

Examiner A 1

113

11.64

0-51

10.68

6-17

2

99

11.82

0-47

10.54

6-17

3

109

11.33

0-58

10.73

5-17

4

104

10.42

0-45

10.82

6-17

5

108

11.64

0-62

11.48

6-17

5-17

Examiner B 1

110

6.48

0-43

9.74

2

106

7.39

0-59

10.06

6-17

3

110

7.28

0-70

10.27

6-17

4

108

6.61

0-40

10.05

5-17

5

122

7.93

0-64

10.42

6-17

Table 6 • Age distribution of subjects completing one year of test (age in years) Examiner A Age

1

2

5 6 7 8 9 10 11 12 13 14 15 16 17

—

—

6 10 16 14 12 10 12 10 10 7 4 2

4 13 13 8 14 11 9 11 4 6 4 2

1 2 10 21 10 5 18 10 14 7 3 4 4

113

99

109

Total

3

4

Examiner B 5

1

2

3

4

5

—

—

—

1 7 8 13 14 10 13 14 10 11 6 1

2 9 16 20 11 15 10 6 7 3 7 2 2

—

3 10 15 14 8 15 10 7 6 6 6 3

11 15 17 11 5 13 12 7 3 4 6 2

4 15 20 15 8 9 11 13 4 7 3 1

1 3 21 16 12 10 7 14 15 2 4 0 3

103

108

110

106

110

108

T he data in Table 1 show that the ini­ tial mean D M F S of the various groups examined by examiner A was approxi­ mately 4 D M F surfaces higher than the children examined by examiner B. This difference is thought to result from the examination technics practiced by the two examiners. In a previous report in

—

5 20 15 18 10 7 11 15 7 9 3 2 122

which the same two examiners partici­ pated,12 the same directional differences were found. Difference in age of the sub­ jects examined by the two examiners, however, may be another factor in ex­ plaining these differences. Tables 3 and 5 show the same relative differences as seen in Table 1 between examiners for those

BIXLER-MUHLER . . . VOLUME 68, JU N E 1964 • 29/797

subjects completing the 6- and 12-month periods of the study. T he data in Table 7 show the mean D M F teeth and D M F surfaces incre­ ments for both examiners in those sub­ jects who completed 6 months of study. T he findings of this study are similar regardless of whether the D M F teeth in­ dex or D M F surfaces index is used for comparative purposes. T o facilitate the discussion of the results references will be m ade only to the clinical findings using the D M F surfaces index; however, the re­ sults using both indices are shown in this and subsequent tables referring to the clinical results. Fo r examiner A, the subjects in the control group had a mean caries incre­ ment of 2.69 surfaces. Those children treated with the stannous fluoride pro­ phylactic paste (group 2) had a mean surface increment of 2.22 representing a 17.5 per cent reduction, which difference was not significant at the 0.05 level of

confidence. The D M F S increments ob­ tained for both groups 3 and 4 were simi­ lar and represented a 47.2 per cent reduc­ tion for group 3 and a 46.5 per cent reduction for group 4, both of which were highly significant as judged by their low probabilities. Group 5 children, who re­ ceived all three stannous fluoride treat­ ments, had the smallest D M F S increment representing a 97.4 per cent reduction when compared with the control group, a difference which was highly significant. T he results obtained by examiner B for the 6-month interval are also sum­ marized in T able 7. T he children in the control group had a mean caries incre­ ment of 2.14 surfaces, whereas the sub­ jects treated with the stannous fluoride prophylactic paste (group 2) had a D M F S increment of 1.28. This repre­ sented a 40.0 per cent reduction and was significant at the 0.003 level of confi­ dence. This result was in contrast to that of examiner A whose data indicated no

Table 7 • DMFT and DMFS increments in subjects completing six months of test, for Examiners A and B DMFT Group*

No. of subjects

Mean

DMFS P

Reduction (% )

Mean

P

Reduction <%)

Examiner A 2.69 (0.313)

1

120

1.16 (0.155) f

2

109

1.01 (0.141)

0.46500

12.9

2.22 (0.217)

0.21500

17.5

3

115

0.77 (0.136)

0.05700

33.6

1.42 (0.194)

0.00046

47.2

4

108

0.76 (0.131)

0.04800

34.5

1.44 (0.210)

0.00076

46.5

5

113

0.13 (0.127)

<0.00001

88.8

0.07 (0.190)

<0.00001

97.4

40.2

Examiner B 2.14 (0.219)

1

118

1.06 10.109)

2

114

0.65 (0.112)

0.00850

38.7

1.28 (0.195)

0.00330

3

no

0.73 (0.093)

0.02150

31.1

1.34 (0.177)

0.00425

37.4

4

120

0.50 (0.084)

0.00003

52.8

0.86 (0.129)

<0.00001

59.8

5

129

0.25 (0.071)

<0.00001

76.4

0.49 (0.119)

<0.00001

77.1

*1, control; 2, prophylaxis with SnF2 ; 3, prophylaxis with SnF2, plus SnF2 dentifrice; 4, prophylaxis with SnF2 , plus topical SnF2; 5, prophylaxis with SnF2 , plus topical SnF2, plus SnF2 dentifrice. fNumber in parenthesis is the standard error of the mean.

30/798 • THE JOURNAL OF THE AMERICAN DENTAL ASSOCIATION

Table 8 • DMFT and DMFS increments in subjects completing 12 months of test, for examiners A and B DMFT Group*

No. of subjects

Mean

DMFS P

Reduction (%>

Mean

P

Reduction 1%)

34.6

Examiner A 1

113

2.58 10.221) f

2

99

1.79 10.199)

0.00800

30.6

3.42 (0.315)

0.00048

109

1.77 (0.159)

0.00280 '

31.4

3.28 (0.260)

0.00008

37.3

4

103

1.39 (0.169)

<0.00001

46.1

2.74 (0.277)

<0.00001

47.6

5

108

0.71 (0.142)

<0.00001

72.5

1.33 (0.232)

<0.00001

74.6

34.2

3

'

5.23 (0.430)

Examiner B 1

110

2.32 (0.190)

2

106

1.42 (0.153)

0.00017

38.8

4.42 (0.359) 2.91 (0.316)

0.00170

3

110

1.69 (0.161)

0.01100

27.2

2.99 (0.287)

0.00170

32.4

4

108

1.39 (0.136)

0.00005

40.1

2.57 (0.270)

0.00001

41.9

5

122

1.05 (0.115)

<0.00001

54.7

1.84 (0.232)

<0.00001

58.4

*1,. control; 2, prophylaxis with SnF2 ; 3, prophylaxis with SnF2 , plus SnF2 dentifrice; 4, prophylaxis with SnF2 , plus topical SnF2 ; 5, prophylaxis with SnF2, plus topical SnF2 , plus SnF2 dentifrice. fNumber In parenthesis is the standard error of the mean.

significant reduction in terms of the D M F S index for those children receiving only the SnF2 prophylactic paste treat­ ment. T he reason for these differences is not known, but may lie in the variability of such clinical trials. At the 12-month period both examiners found a significant reduction through the use of the prophylactic paste alone. The children in group 3 had a D M FS incre­ ment of 1.34 which represented a 37.4 per cent reduction. Group 4 children had a D M F S increment of 0.86 representing a 59.8 per cent reduction and the children in group 5, who received all three forms of stannous fluoride treatments, had a D M F S increment of 0.49, which is a 77.1 per cent reduction in dental caries. All of the various combination treatments pro­ vided significant reductions for examiner B when compared with the control group. These data suggest that the use of more than one topical stannous fluoride technic results in cumulative effects.

T he 12-month results obtained by ex­ aminer A are summarized in T able 8. T he control subjects had a mean D M F S incre­ ment of 5.23 and those receiving the stan­ nous fluoride prophylactic paste 3.42, this difference being highly significant. In con­ trast to the 6-month data, in which the increments for groups 3 and 4 were almost identical, the 12-month data for the same two groups showed separations with the children in group 3 having a new surface increment of 3.28 and those of group 4, 2.74. These represent reductions of 37.3 and 47.3, respectively, both of which are highly significant. These corroborate the findings of examiner B both at 6 months and 12 months, as will be shown. As with the 6-month data, the children in group 5 showed the greatest anticariogenic effect of all the groups. T he 12-month results of examiner B also appear in T able 8. T he control group had a D M F S increment of 4.42. For all practical purposes, the percentage reduc­

BIX LER -M U H LER... VOLUME 68, JU N E 1964 • 31/799

vTable 9 • Percentage of subjects showing the indicated DMFS increments at 12 months in various groups DMFS increment (percentage of subjects) Group

0 or less

1

2

3

4-5

6-7

8-9

10-11

12 or more 917)

9(11)

12(12)

12(19)

16(18)

18(12)

10(6)

5(4)

2

15125)

15(17)

20(12)

8(12)

24(20)

4(7)

6(4)

5(1)

2(2)

3

16(18)

15(19)

16(20)

1317)

24(23)

7(6)

7(7)

2(0)

0(0)

4

24(22)

17(24)

15(18)

12(17)

18(6)

8(10)

3(3)

3(0)

0(0)

5

42(41)

25123)

10(12)

7(8)

10(10)

4(5)

1(1)

1(0)

0(0)

1

9111)*

*Examiner A values shown outside brackets; examiner B values shown inside brackets.

tions found at 12 months for examiner B are the same as were found at 6 months. The increments for each group are, of course, much greater. When the similar findings of the two examiners are combined, the following observations appear logical: 1. T he group receiving all three forms of stannous fluoride therapy (group 5) had the lowest caries increment of any of the experimental groups. This was ap­ parent as both the 6- and 12-month inter­ vals and for both dental caries indices. 2. No specific statements can be made concerning the contribution of each fluo­ ride treatment to the total effect of the combinations of fluoride treatments since not all possible combinations were tested in this study; however, there is a definite directional tendency indicating that addi­ tional stannous fluoride treatments pro­ duce additional anticariogenic benefits. Again, though, it should be noted that the results of this study do not unequivo­ cally show whether the use of all three topical treatments is better than the com­ bination of only aqueous 8 per cent stan­ nous fluoride topical applications plus the SnF2-Ca2P207 dentifrice, since this latter group was not included in this study. T he data presented in Tables 7 and 8 can be evaluated in a different way in order to define their practical significance for the general practitioner. If the num­

bers of subjects in each group are listed according to the total number of new D M F surfaces recorded during the exper­ imental period of 12 months, a frequency distribution of D M F S increments for each group may be obtained. Such a summary of the data combined for both examiners appear in T ab le 9. T he clinical results for both examiners are similar. These data show that 64 to 67 per cent of all subjects in group 5 developed one or less new le­ sion during the 12 months, whereas 11 to 14 per cent of the subjects in the control group had 10 or more new D M F surfaces. M ore outstanding is the observation that 41 to 42 per cent of the subjects in the “ three-way” stannous fluoride treatment group remained free of caries. The impli­ cation of this finding in terms of the sav­ ings in time for the general practitioner is most important. It means that he may fulfill a substantial portion of his patients’ needs with a minimum expenditure of time and effort through this preventive dental service. Certainly, communal fluoridation is the single most efficient and effective means of reducing tooth decay for popu­ lation groups; however, these data sug­ gest that, for those persons who cannot receive the benefits of communal fluori­ dation, dentistry has available a practical technic which will reduce caries experi­ ence significantly. Its value becomes more apparent if the incurred cost and the

32/800 • THE JO U R N A L OF THE A M ERICAN DENTAL ASSOCIATION

time required are compared with other forms of dental treatment. SU M M A RY

A clinical study was conducted in which five groups of children were given differ­ ent combinations of three stannous fluo­ ride treatments: (1) stannous fluoride prophylactic paste, (2) topically applied aqueous stannous fluoride solution and (3) the home use of a SnF2-Ca2P20T dentifrice. All of the children in this study were lifelong residents of a community whose water supply contained 0.05 ppm of fluoride. Children were divided into five equal groups; two examiners, each examined half of the children. T he results of this study indicate that: ( 1 ) stannous fluoride, when incorporated into a compatible prophylactic paste, re­ duces dental caries in children, and (2) the use of a combination of three differ­ ent methods of stannous fluoride treat­ ment (in a compatible dentifrice) will result in a significant reduction in human dental caries. The latter combination was superior to the other combination of methods tested in this study.

This study was supported, in part, by a grant from the Procter & Gamble Company, Cincinnati. *Department of biochemistry, Indiana University Medi­ cal Center, Indianapolis. 1. Galagan, D. J., and Vermillion, J. R. Effect of topical fluorides on teeth matured on fluoride-bearing water. Pub. Health Rep. 70:1114 Nov. 1955. 2. Downs, R. A., and Pelton, W. J. Effect of topically applied fluorides in dental caries experience on children residing in fluoride areas. J. Den. Children 18:2 3rd quart. 1951. 3. Muhler, J. C. Anticariogenic effectiveness of a single application of stannous fluoride in children re­ siding in an optimal communal fluoride area. II. Results at the end of 30 months. J.A.D.A. 61:431 Oct. I960. 4. Muhler, J. C. Practical method for reducing den­ tal caries in children not receiving the established bene­ fits of communal fluoridation. J. Den. Children 28:5 1st quart. 1961. 5. Bibby, B. G., and others. Preliminary reports on the effect on dental caries of the use of sodium fluoride in a prophylactic cleaning mixture and in a mouthwash. J . D. Res. 25:207 Aug. 1946. 6. Wellock, W . D.; Roberts, J. F., and Bibby, B. G. Effect of an acidulated fluoride mouthwash on dental caries. J . D. Res. 27:497 Aug. 1948. 7. Mericle, Mack. Studies concerning the effectiveness of a stannous fluoride prophylaxis mixture. J. D. Res. 40:714 July-Aug. 1961 Abst. 8. Gish, C. W., and Muhler, J . C. Effectiveness of a SnF2-Ca2P207 dentifrice on dental caries in children whose teeth calcified in a natural fluoride area. Unpub­ lished data. 9. Dudding, Nancy J., and Muhler, J. C. Technique of application of stannous fluoride In a compatible prophylactic paste and as a topical agent. J . Den. Children 29:219 4th quart. 1962. 10. Muhler, J . C. Topical treatment of the teeth with stannous fluoride—single application technique. J. Den. Children 25:306 4th quart. 1958. 11. Muhler, J. C. Effect of a stannous fluoride denti­ frice on caries reduction in children during a three-year study period. J.A.D.A. 64:216 Feb. 1962. 12. Bixler, D., and Muhler, J. C. Experimental clinical human caries test design and interpretation. J.A.D.A. 65:482 Oct. 1962.

Are you going to the 105th annual session in San Francisco this N ovem ber? I f so, along the O cean Beach area of San Francisco are m any interesting sights. W ith its pounding surf along O cean Beach, the Pacific O cean is fascinating, but swimming is dangerous there. N earby is Fabulous Sutro’s M useum (a unique nickelodeon p a lac e ), Playland-at-theBeach, and the C liff H ouse. T h e C liff House is a restaurant that has an excellent view of Seal Rocks, 400 feet off the shore, where sea lions, with some veteran bulls that are said to weigh as m uch as 2,500 pounds, bark and lum ber over the rocks.