FACTORS AFFECTING FRACTURE OF TEETH DURING EXTRACTION A. J. MACGREGOR, M.Ch.D., F.D.S.R.C.S.

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1 FACTRS AFFECTING FRACTURE F TEET DURING EXTRACTIN A. J. MACGREGR, M.Ch.D., F.D.S.R.C.S. The Dental School and ospital, Leeds, r FRACTURE of teeth commonly complicates their extraction, and the rate of tooth fracture is generally considered to depend upon local anatomy and the skill of the operator. A great deal of indirect evidence, well summarised by owe (x962), has been recorded to support the belief that some teeth are more difficult to extract than others. Radiographic surveys have shown, for example, that twice as many roots are retained in the mandible than in the maxilla, and that over six times as many teeth are retained in the posterior areas compared with the anterior areas of the jaws. Direct evidence is limited to a paper by Amies & Sealey (I955) in which the results of the extraction of 24,3I 9 teeth were described. They found a fracture rate of 5"r4 per cent. and showed that, with the exception of the upper first premolar, the fracture rate of every tooth in the mandible was higher than that for its counterpart in the maxilla. The importance of anatomical considerations has therefore been adequately established. The effect of the extractor's skill upon fracture rate would also seem to be obvious. A certain period will have to elapse during training before the necessary skills are acquired, and it is in the nature of things that some practitioners will be better than others. If this is true then it could be postulated that there will be differences in the fracture rates in different practices and moreover it could be expected that undergraduates would fracture a greater proportion of teeth than experienced dentists. This paper contains data relating to 9,695 teeth extracted predominately by undergraduates. A comparison is made between their results and those of Amies and Sealey, and although there are considerable difficulties in interpretation it would seem that the part that skill plays in determining the fracture rate is not as obvious as it would appear at first sight. The literature relating to tooth extraction contains no precise information about the effects of age and sex on the incidence of tooth fracture during extraction. It seemed likely that these variables would be important and they were therefore included in this study. A note was also made of the number of teeth extracted at one session as this was felt to be an indication of the difficulty of each extraction. CLINICAL MATERIAL AND METD F ANALYSIS Records were made of all extractions of permanent teeth carried out under local anaesthesia in the Leeds Dental ospital for a period of one year. The age and sex of the patient were noted. The number of teeth extracted was recorded as one or more than one i.e. single or multiple. A note was also made of the experience of the operator, i.e. staff or year of study of student. Clinical records were transferred to ollerith cards which were computer processed. The calculations necessary for the analysis of multiple regression were carried out by means of an electronic computation. 55

2 56 BRITIS JURNAL F RAL SURGERY RESULTS Records were made for I, I99 extractions. Information concerning 5o4 extractions was incomplete in respect of one or more of the factors being studied and these cards were rejected. f the remaining 9695 teeth 862 were fractured during extraction, an incidence of 8"89 per cent. The influence of the factors was as follows. Age (Table I). Extractions were carried out on patients with an age range of I to 89 years. Data have been analysed in five-year age groups. The largest number of extractions in any five-year age group was that for 21 to 25 years which was ver seven thousand of the extractions took place between the ages of 16 and 45 years. The highest fracture rate at 11'69 per cent. occurred in the 21 to 25 years age group. There appeared to be a falling off of the fracture rate above the age of 35 years, the rate up to 35 years being I I' 4 per cent. and over that level 6'30 per cent. (Fig. I). Sex (Table II). More than twice as many teeth were extracted from males than females; 6771 compared with The fracture rate in males was 9"87 per cent. and in females 6"64 per cent. Site (Table III). There was a marked variation in the fracture rates at different sites. The majority offtactures were from the molar region. The highest fracture rate of 19"1 per cent. was for the upper first premolar. Molar teeth, excluding the upper third molar, had fracture rates varying from I I "16 to 13'69 per cent. There was a great contrast between the fracture rate of 9"23 per cent. for upper canines and 299 per cent. for lower canines. The lower incisor teeth had the lowest rate of fracture. Number (Table IV). When only one tooth was extracted for a patient the fracture rate was 11"85 per cent. for 3941 teeth. If more than one tooth was extracted at one operation, and this applied to 5754 teeth the fracture rate was 6"86 per cent. Experience of perator (Table V). Students in the fourth year, after two academic terms of experience in the oral surgery department, had a fracture rate of 9'42 per cent. After a further three terms in the fifth year their rate was 9"56 per cent. During the sixth year which begins in ctober and ends in March--the latter part being spent mainly in preparation for the final examination --the fracture rate decreased to 7'29 per cent. The fracture rate for members of staff, who were predominantly house surgeons and registrars, was 78o per cent. Further Statistical Analysis. The variables taken into consideration were almost certainly capable of interaction. The high rate of fracture found below the age of 35 may have been due to a higher proportion of molar extractions in that age group, and not in fact a direct age effect. Further study could be made by looking at the age distribution as it affected molars alone. Such an approach is still limited ~ecause having found the age relationship to be valid for molars it may be asked how this is affected by sex. Increasing the number of factors taken

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4 58 BRITIS JURNAL F RAL SURGERY TABLE II TE EFFECT F SEX UPN TE FRACTURE Teeth Teeth Per cent. Extracted Fractured Male "87 Female ' [l ~ Percentage fracture rate in five year a~le groups FIG. I TABLE IV TE EFFECT F NUMBER F TEET UPN TE FRACTURE RATE Teeth Teeth Per cent. Extracted Fractured Fractured Single "85 Multiple "86

FACTRS AFFECTING FRACTURE F TEET DURING EXTRACTIN 59 into consideration however decreases the number of cases available for study to a point where valid statistical methods are inapplicable.

5 FACTRS AFFECTING FRACTURE F TEET DURING EXTRACTIN 59 into consideration however decreases the number of cases available for study to a point where valid statistical methods are inapplicable. What was required was a method of analysis which would give estimates of the individual effects of the factors as though they were acting independently. An appropriate technique had been developed by Lidwell (1961) for the analogous problem of studying factors affecting post-operative sepsis in general surgery. This technique is an analysis of multiple regression adapted for use with non-metric variables. The mathematical basis for it is given in the original article and it has since been applied to the study of the aetiology of dry socket (MacGregor, 1966). TABLE V TE EFFECT F EXPERIENCE F PERATR UPN TE FRACTURE RATE Year of study Teeth Teeth Per cent. of operator Extracted Fractured Fractured 4 3oi " "56 6 II "29 Staff I924 ~r5o 7"80 Essentially it is supposed that the event in which one is interested, say for example tooth fracture, occurs as a result of a number of factors which may be supposed to be acting additively. If Y is the number of fractures which occurs as a result of extracting teeth then Y = blxl + b2x2 + "'" where the b's are the factors to be taken into consideration and the x's are the number of teeth affected by each of these factors, b 1 may be age group under 35. b~ the age group over 35, ba single extraction b4 upper first premolars and so on, In this study Y the total number of fractures is known and so are xl x2, etc. If the data are suitably arranged then the b's may be calculated and the resulting co-efficients are estimates of the independent power of each of these variables in causing fractures. It should be noted that P the probability of any individual tooth extraction resulting in a fracture is the sum of the co-efficients. What has been described is the additive form of analysis of multiple regression. Lidwell has also developed a multiplicative form which he considers to be biologically more appropriate, but a computer programme is not yet developed for its routine use. It can be seen that the number of variables taken into consideration in this type of analysis can be very large. The data processing and computation involved is laborious even when the latter is carried out by electronic computer and it seemed reasonable to limit the number of variables chosen to those shown in Table VI. The fit of the regression was decided by using the co-efficients to calculate the expected number of fractured teeth in each of the 48 cells of the classification and

6 60 BRITIS JURNAL F RAL SURGERY comparing these wkh the observed figures. The chi square value was 24'81, the degrees of freedom were 30 and the probabilky of fit 0"7, which indicated remarkably good agreement. The results in general agreed with those of the simpler analysis. Variates found to be significant were, under 35 years male, single extraction, upper first premolar, and molar teeth except upper third molars. The variates omitted take the value zero. Fourth and 5th year have been combined as one variate as were 6th and staff. There were no significant differences between either of their values and zero so that the effect of experience which had been indicated by the chi square test could be considered to arise from interaction with other factors. TABLE VI TE EFFECT F AGE AND TER FACTRS N TT FRACTURE Coefficients of the regression equation P = Y = bl xt + b~ x~ + - Coefficient Standard error I Under 35 Male..... Single Extraction. Upper First Premolars Molar Teeth except Upper Wisdom 4th and 5th Year 6th Year and Staff 0"0,:907 o'o3z?e o'o339r o.±362z o.o6~6" 4 0" '00264 ~-0"00610 ~0'0062I ±-06Z ±'Z Z89 ±0" "00702 ±o'oo76i Variance absorbed by regression I4"65 Residual variance 0.08 Variates excluded e.g. over 35, female, etc. take the value zero. The coefficients which exceed twice their standard error are in italics. The co-elticients may be used to predict the likelihood of any extraction resulting in fracture. It may be calculated that if a single upper premolar is extracted from a male age 3o years, then the probability of fracture is o'o339z +o'i362i +o'o3132+ooi907 = 0"2205 = P This value of 0"2205 indicates that each tooth in those circumstances has over a one in five chance of fracturing or that 22 "o5 per cent. of such teeth will fracture. The value may be compared with the fracture rate for upper first premolars which is found in Table III and is I9'I per cent. The close agreement between these figures implies that upper premolars were often extracted in unfavourable circumstances and examination of data showed this to be the case. DISCUSSIN The overall incidence of fracture in the present study was 8"89 per cent. and at first sight this rate seemed rather high when compared with fracture rate of Amies and Sealey which was 5'74 per cent. There are however differences in the extraction population at risk which makes comparison liable to error. ne of the known differences is the distribution of the extractions according to site. Amies

$If each site fracture rate found at Leeds is used to re-calculate an overall fracture rate based on Amies and Sealey's anatomical distribution of teeth, then the Leeds group would be expected to have$

7 FACTRS AFFECTING FRACTURE F TEET DURING EXTRACTIN 61 and Sealey had a larger proportion of canine and incisor teeth than the Leeds group. If each site fracture rate found at Leeds is used to re-calculate an overall fracture rate based on Amies and Sealey's anatomical distribution of teeth, then the Leeds group would be expected to have a fracture rate of 7"o5 per cent. The difference between the fracture rates is not now so pronounced. There are other indications that the pattern of operating of Amies and Sealey might tend to make their fracture rate more favourable. The larger proportion of incisor teeth suggests that more clearances were being performed and if this is so they would be likely to be carrying out more multiple extractions on an older population, both of which factors would on the Leeds evidence reduce their fracture rate. These arguments tend to weaken the hypothesis that experience has a great part to play in improving a dental surgeons' ability to extract teeth without causing their fracture. The present study provides some direct information relevant to experience. Simple analysis showed that the 6th year and staff had a lower fracture rate than 4th and 5th year students, but analysis of multiple regression indicated that this was not an independent effect. ne interpretation would be that the more senior students and hospital staff extract the easier teeth, but this is somewhat paradoxical as a deliberate attempt is made to avoid presenting the juniors with the more difficult extractions. This finding may be an indication that some important factor relevant to tooth fracture has been left out of the study but it is more likely that possible tooth fracture is not the only grounds for patient selection. The difficult patients may not necessarily have the difficult extractions. The most puzzling feature of the comparison between the surveys is the discrepancy in the site distribution of the fractures. The chief difference between the two surveys arises in the upper canine and first premolar areas: for the Leeds group the respective fracture rates are 9"23 and 191 per cent. respectively and for the Australians 2'86 and 12"72 per cent. What is more surprising however, is that although the Australians have a better fracture rate for the upper teeth with the exception of the premolars their rate is worse for all lower teeth. This Antipodean inversion is not easy to explain. As far as is known the extraction techniques are similar in principle in the two countries and it may be doubted that the genetic make up of the two populations is greatly dissimilar. It is this finding more than any other which indicates that further work is needed before it may be accepted that increased experience leads to a lowering in the fracture rate. The effects of age and sex upon the tooth fracture rate have been demonstrated for the first time. The fracture rate diminishes after the age of 35 years and is lower in females than in males at all ages. It is difficult to imagine how these results could arise from a difference in attitude to the different classes of patient in that for example operators applied more skill to females and older patients. Probably there are basic physical dissimilarities in the structure of teeth and bone which account for the results obtained. Sex differences in the root structure of teeth do not appear to have been recorded. As age advances deposits of secondary dentine and cementum may serve to strengthen the tooth against fracturing forces despite the fact that occasional bizarre forms of hypercementosis make fracture more likely. There are well known sex differences in general bone structure which may account for the differences in the fracture rates. The cortices of femurs are thicker in males than in females and this also may be true for the mandible. It is well known that in age there is loss of bony support

8 62 BRITIS JURNAL F RAL SURGERY due to periodontal disease and it may be that this becomes critical as far as extractions are concerned at about the age of 35 years. Another possible factor is mandibular osteoporosis, which also increases in age (Woodhead and Atkinson, I969) and may facilitate extractions in the older age groups. SUMMARY Analysis of the records of 9,695 extractions which resulted in 862 fractures has shown convincingly that in addition to site, age and sex have independent effects upon the fracture rate. The effect of experience is moot and this point has been discussed in some detail. ACKNWLED GEMENTS i am greatly indebted to Dr.. M. Lidwell without whose patient instruction I could not have understood the analysis of multiple regression. The computations necessary for this analysis were carried out in the Department of Statistics of Rothamsted Experimental Station and I am most grateful to Mr. J.. A. Dunwoody whose generous assistance made this possible. REFERENCES AMIES, A. B. P. & SEALEY, V. T. (I955). Aus. J. Dent. 39, 394. owe, G. L., (I962). Ann. roy. Coll. Surg. Engl. 3o, 309. LIDWELL,. M. (I96I). J. yg. (Camb.) 39, 259. MACGREGR, A. J. (1966). Aetiology of dry socket. A clinical Investigation. M.Ch.D. Thesis, University of Leeds. WDEAD, C. & ATKINSN, P. J., (I969). Prec. Br. Soc. Study Prosth. Dent. In press.

A. J. MACGREGOR, D.D.K., M.CH.D., F.D.S.R.C.S. and G. E. TOMLINSON, B.SC.

British Jotrrd of Oral Surgery 17 (1979-80) 71-76 AN APPARATUS FOR MEASURING THE FORCES OF DENTAL EXTRACTION A. J. MACGREGOR, D.D.K., M.CH.D., F.D.S.R.C.S. and G. E. TOMLINSON, B.SC. Leeds Dental Hospital