- Email: [email protected]
The reproducibility of natural head posture: A methodological study
The reproducibikity of natwal head pwture: A methodological study Michael S. CCM@~ Ph.D., 43.Ch.D#kms.), D. Orlh., D.D.P.H.QLC.S. En@), F.DiS.(R,C.S. E&n.), F.F.D.(R.C.S. Ire.),* mcl Stephen W. Y. Wei, fkD.S.@kms,), DcB;s., RtS., II#.D:S~,F.R.A.C~D.S.** Hong Kong Current clinical interest in natural head posture (NHP) derives from studies correlating NHP to craniofacial niorphology, future growth trends, and tc~respitatory~need$. It hasalE@ becanergued that NHP is the logicat reference and orienta%on position for crank&&al analysis and the publication of illustrations. The individual or group is printed as they naturally appear in life. Hence, lateral cephalometric radiographs recorded routinely in MHP would be more m$aningful ~forthe clinician. Underpinning all these potential clinical benefit& is the incompletely answeredquestion of the Clinical reproducibility of NHP. In this study a n&ural head posture, the oi%topo&ion, was defined and standardized for clinical use. The Meets of ~arqMs, ah external source of eye .reference (a-wall mirror), sex and time were evaluated in relation to the reproc&~cibi&yof recording lateral cephaiom-etric radiographs in this natural head posture. The sample comprised 217 randomly se&c&d 12-year-old Chiise children in Hong Kong. Boys @oked up more when changing from the self-balance position to the mirror eye reference position (mean change 2”, P 6 0.001). No other significant male-versus-femaie differences w&e detected. NHP reproducibility was better with a mirror (with mirror, method error = 1.9, without mirror, method error = 2.7”). No significant differences in reproducibility were detected between NHP recordings taken with and Whout-ear posts. However, without ear posts the radiographs tended to be of poor qualty. The reproducibility of same+ay repeat radiographs recorded with ear posts arid tiih a mirror (after 4 to 10 minutes and 1 to 2 hours) was 1.9”. NHP was only slightly less reproducible 3 to 6 mbn~hs later (method error = 2.4”). (AM J ORTHOD DENTOFAC ORTHOP 1988:93:2&l-8.)
m
1 he concept of natural head posture is not new. Leonardo da Vinci (14Si- 15 19) and Albrecht Diirer (147 1- 1528) used scaffoldings of horizontal and vertical lines on drawings of models positioned in “natural pose” in order to permit more accurate artistic and scientific r$plication of the human head.‘,’ In the l9th century, Von Baer and Wagner,’ and Broc@ defined the natural posture of the head as the posture of the subject when standing with his visual axis horizontal. Natural head posture (NHP) has been shown to be correlated to craniofacial morphology,5e7 future growth trends,’ and to respiratory needs.9g’” Several researchers have argued that NHP is the logical reference and orientation position for the evaluation of craniofacial mor-
From the Department of Children’s Dentistry and Orthodontics, Faculty of Dentistry, prince Philip Dentat Hospitrd, University of Hong Kong. This work was supported by Grant No; 335.251 .CC01.4c fmm the Uoiwrsity of Hoftg Kong and formed part of a thesis submitted to the University of Hong Kong for the Ph.D. degree. *senior Lecturer. **Professor and Head.
280
phology and the publication of illustrations. I1 I4 The individual or group is presented- as they appear in~life. Hence, lateral cephatalometricradiographs-recorded routinely in NHP -would be more meaningful clinically. Underpinning all these potential clinical @nefirs of routinely recording lateral cephalometric radiographs in NWP are its standardization and -reproducibility.- kf~aity questions remain unanswered. As a result of previous studies, most researchers have adopted the “orthoposition,” the standing posltlon defined by M&ave’5 as the “inte$ion position f&r w&ing.“SsWl8 Solow and Taligren5,19drew attqntion to the need to first &fine .the body and cervical postures uponwhich head posture is superimposed. They devised forward and backward head oscillationsbefore all-owing the head to settle into a “self-balance” posture. They went on to denionstrate that this self-balance ~position differed significantly in maIe subjects from the i$iP tipted wkenman external source d eye reference (a mirror) iS used. Subjects looked up more when using the mirror as reference_.5.‘9 Previous studies into the reproducibility of NHP are
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summarized in Table I. Few studies have evaluated the reproducibility of NHP during lateral cephalometric radiography. Only two studies have assessed the reproducibility in children of “orthodontic” age, Foster, Howat, and Naish,]’ and Siersbaek-Nielsen and Solow,‘* but the former examined very few subjects (n = 8). The larger study of Frankel,20 reported in more detail by Falck,” evaluated the reproducibility of NHP photographically, but the findings may not match exactly recordings made in a cephalostat. There have been no studies comparing NHP recorded with and without ear posts, despite the fact that significant differences in head position may result from proprioceptive feedback following insertion of the ear posts.‘2-‘4 Comparisons between male and female subjects are not available, and relatively few studies have evaluated NHP over time. 20.25No population standards of head and cervical column positions are available (“postural variables”). The reproducibility of NHP has not been quantified for any nonwhite populations. This study was directed at these incompletely answered questions and reports the findings from a methodological study of the reproducibility of recording lateral cephalometric radiographs in natural head posture. The variables investigated were sex, use of ear posts, an external source of eye reference (a mirror), and time. It forms part of a larger study evaluating the use of new cephalometric analyses based on natural head posture and the true horizontal.“’ From the clinical viewpoint, it may prove beneficial in the future to record in NHP all cephalometric radiographs taken in orthodontic and orthognathic surgical practices. No information would be lost, but a great deal may by gained. SUBJECTS AND METHODS The sample and allocation into test groups
The 217 children comprised a random subsample drawn from a larger research sample of 618 12-yearold Chinese children in Hong Kong. A partially stratified random sampling method based on schools had been used to select the original population sample. The children were randomly allocated to one of the first six test groups defined in Table II and had one set only of repeat lateral cephalometric radiographs recorded in NHP. The test groups were designed to each comprise a different combination of the study variables and to thereby simplify the statistical interpretation of the data. Months later, those children who returned for treatment at a suitable date were placed into the final two test groups, G and H. Allocation remained random as it was not known initially which children would return for treatment. Groups A to F comprised equal numbers of boys and girls. As the sample sizes in groups
Reproducibility
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natural head posture
281
G and H were relatively small, boys and girls were combined for analysis. The subjects returned to the waiting room between the immediate (4 to 10 minutes) repeats or took part in a complementary multidisciplinary research project between the delayed (1 to 2 hours) repeats. For group E subjects (with and without mirror), the first radiograph had to be recorded without the mirror, otherwise the subject’s head posture may have been influenced by prior knowledge that the mirror was hidden behind the cover. All radiographs were recorded in the morning with the same operators in the same air-conditioning environment. Cephalometric
technique
A General Electric 1000 x-ray unit with a Wehmer CI-2 cephalometer was modified for natural head posture recordings. I4 Modifications included moving the machine away from the wall and adding a second collimator. This allowed a lateral “ + ” beam of light to locate the position of the central x-ray beam in line with the external auditory meatus for those subjects not using the ear posts (groups A, D, and H). The true vertical was marked on each radiograph by a wire plumb line suspended in front of the aluminum soft-tissue wedge. To minimize the x-radiation exposure levels, rare earth intensifying screens were used with no grids. No individual radiographs were repeated. To ensure that each exposure was accompanied by a net benefit to the patient, all children were offered comprehensive dental treatment including orthodontic treatment. Copies of the radiographs were later used for diagnoses. A parent signed the informed consent form; the overall consent rate was 86%. “Orthoposition”
of natural head posture
All the radiographs were recorded with the subject standing in the “orthoposition” natural head posture, as originally defined by Molhave15 and as later adopted and modified by Solow and Tallgren,’ and others8,‘6.‘7 (Fig. 1). To prevent the children from swaying, it was found necessary to also define the feet position as “a comfortable distance apart and slightly diverging”14 (Fig. 2). For the self-balance position, without ear posts or mirror, the subject tilted the head forward and backward with decreasing amplitude until a comfortable position of natural balance was achieved. With the mirror, the subject was then requested to look into the reflection of their eyes in a mirror located 200 cm ahead. A black thread plumb line in front of the subject was used to “bisect” the facial reflection and to minimize lateral head rotations, especially when no ear posts were used (Fig. 3). Without ear posts this technique also served
282 Cooke and Wei
TaMe 1. The reproducibility of natural head posture (NHP) reported in previous studies in the literature (included in the table are the varying conditions under which the natural head posture was evaluated, the sample sizes, and the actual method errors [reproducibility] -obtained) .StUdy
Bjerin, 1957 Moorrees and Kean, 1958 CarIs and Leijon, 1960 Solow and Tallgren, 1971a Cavaflaro et al., 1974 Fratrkel, 1980 (Falck, 1981) Foster, 1981
Siersbazk-Nielsen and Solow, 1982
Posture
External eye reference
Head support
Time between repeats
Sitting Standing Sitting
Mirror Mirror Mirror
With posts Not stated No posts
Sitting
None
With posts
2- 10 days
Few minutes 1 week
Age range W
Sex
Sample size
M&od error (“)
M+F M+F F F M+F
22-36 22-36 18-20 18-20 14-79
35 35 66 61 1-l
1.62 1.a 2.05 ~I.54 4.60
M M M+F
22-30 22-30 Not stated. Mainly adults?
21 21 20
2.48 I .43 Mean error 1.05 2.25
With posts
2 months
Sitting
None Mirror Mirror
No posts
Approximately 3 weeks
Standing
Mirror
No posts
Few minutes
M+F
7-18
923
Standing
Wall mark at eye level
With posts
3-6 months
M+F
Mean 13.7
8
Mean error 4.1
Standing
Mirror
With posts
l-35 days
M+F
6-15
30
2.25
Standing
Table 8. Variables evaluated in the NHP reproducibility studies and their distribution within the~test groups
(each group comprised a different combination of variables) Test group
A B C D
Cephalostat and ear posts
E
No Yes Yes One film yes One film no Yes
F G H
Yes Yes No
Mirror as an external visual reference
Yes Yes Yes
Time interval between~repeats
Sample size (% bays, % @r/s. except G and H)
4-10 min 4-10 min 1-2 hr l-2 hr
30 30 30 40
One film yes One film no
4-10 min
30
NQ
4-10 min 3-6 mo 3-6 mo
30 1.5 12
Y.3
Yes Yes
to standardize the midsagittal plane to film distance. Special care was taken to ensure that the head was not moved when the ear posts were carefully inserted.
The angle between the sella-nasion line (SN) and the true vertical was used to assess the reproducibihty of the head posture (Fig. 4). To ensure that only true changes in head posture were measured and not changes arising from errors in l&lmark location, the “best-fit” superimposition technique described by Moorrees and Kean% was used. A&r accurately superimposing the
traced initial and repeat radiographs, a pin was used to transfer the precise &cations of sella and nasion from the initial the the repeat film. The SN to the vertical angle Was measured by hand y&h a large protractor to the nearest 0.5”. 23 Dahlberg’s formula,z7 method error. = x, J was applied to the difference between the inititil and repeat radiographs for each subject; The paired t test was used totit whether any si~ific4tnt.cha~~s~in.NNP had occurred under the~particuhx test conditions. Boys
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283
Table Ill. Method errors for tracing and
superimposition technique used to evaluate changes in NHP on repeat radiographs (errors were least when both radiographs had been taken with ear posts)
Comments Combined
photographs
recorded
in NHP,
with radiographs.
No “corrections” to head positions. “Corrections” were made if head position unnatural. Nineteen of the original study sample were children, 14-15 years, but the “repeats” appear to have been on the adults. Subjects looked upward more with the mirror external eye reference; average 3”; significant at 1% level. The error stated appears to be the difference between the means of time 1 and time 2 series. Not comparable to other studies? Combined photographs recorded in NHP, with radiographs. Combined self-balance position with eye focusing on wall mark positioned at eye level. Mean error is the mean of individual differences between time 1 and time 2 radiographs. All radiographs and head positioning were carried out by auxiliaries.
and girls were compared for each group. Dahlberg’s formula was also used in the analysis of method errors. Radiographs in which the mandible was postured, or in which the teeth were not in contact, were removed from the study. Plaster study models were available for all subjects. RESULTS Error of the method
The method errors found from repeat tracings and measurements are summarized in Table III. It was more accurate to superimpose on the better quality radiographs obtained when using ear posts. Without ear posts the subjects could more easily move during the exposure. The method error figures of 0.23” to 0.31” are in the identical range to those reported in the computeraided study of Solow and Tallgren.5 Errors occurring merely in reading the same lines at a later time, 0.17”, proved to be virtually half of the total method error. Reproducibility
of natural head posture
The results are summarized in Table IV. Method errors (reproducibility) ranged between 1.5” and 2.9”. For groups A, B, C, D, and F, no significant differences were observed between boys and girls, nor between the initial and the repeat series of radiographs. In group E (with and without mirror), significant
Use of ear posts Both radiographs without ear posts One radiograph with ear posts and one without Both radiographs with ear posts
Study
Method (Dahlberg,
error 1940)
A, H
0.31”
(n = 30)
D
0.30”
(n = 30)
B, C, E, F, G
0.23”
(n = 30)
differences were observed between boys and girls, and between the initial and repeat recordings. For the male series, the subjects looked up more when using the mirror as the external source of eye reference, the mean difference in head posture being 2.0”, which was significant at the P 6 0.01 level. There was no significant difference when using the mirror for the female series. For the combined (M + F) data, the change in head posture with the mirror was significant at the P = 0.001 level. Dahlberg’s method error formula is not strictly applicable if there are significant differences between the means of the two series of data. Therefore, the method error for group E data should in the strict sense, be quoted only for the female series (1.9”). The mean differences in head position shown for groups C, G, and H data imply a systematic error. In terms of the t values (standard error divided by the mean), the observed differences are not significantly different from zero. For groups G and H, the data could be interpreted as a trend to a change in NHP over time, with the subjects looking down more after 3 to 6 months. DISCUSSION
Muscle postures constitute a narrow range of positions controlled by the asynchronous firing of individual motor units within the skeletal muscles. The classic example is the “tremor” associated with attempts to remain still. Head posture is a typical case. FrHnkelzOspoke of NHP as constituting a “small range of positions.” Despite this theoretical range, NHP has been found in this study to be remarkably reproducible in 12-year-old children. The short-term reproducibility (method error) with ear posts and with mirror was found to be 1.9”. This figure is the mean of groups B and C (Table IV).
284
Cooke and Wei
Fig. 2. To prevent the children from swaying,the feet position was defined as “a comfottable dis@ncet apad and slightly diverging.” The foot of the operator is shown guiding the subject’s feet to prevent the subject frtim~looking down and-alterir$ head posture.
Fig. 1. Recording lateral cephalometric radiographs in natural head posture with the cephabstat and ear posts. The wire plumb line and suspended weight re&xd&d lhe~tiue vertical on each radiograph: the longer btack thread plumb line served to bisect the reflection of the subject’s face in the mirror and minimize lateral head rotation.
Overall, the results confirmed the findings of previous workers (Table I) that NHP is highly reproducible in the short term, but the data also provided many previously unreported findings. With the exception of group E (with and without mirror), no significant. sex differences were detected. The classical studies of Moorrees and Kean,“’ and of Solow and Tallgren5,” were each limited to one sex, female and male respectively, and no other studies have compared male-versusfemale data in the same study. The findings of group E confirmed those of Solow and Tallgn~~~‘~ that boys tended to look up more in changing from the selfbalance position to the mirror position. A 3” change was reported by Solow and Tallgren and a 2” changes was found in this study, both significant at the 0.1% level. No differences were found for the girls. A possible explanation is that girls are encouraged socially to adopt a good upright posture Andyto look straight ahead, and that they continue to adopt this posture even in the absence of an- external eye reference. Perhaps,
boys adopt a lazier head posture in the self-balance position. A more “natural” posture may occur without the use of the ear posts and cephalostat. Insertion of ear posts could itself be argued as positioning the subject “un-naturally.” Sutcher’* and Eliassonz3.*” had concluded that insertion of ear posts altered the position of the condyle within the fossa and perhaps this couid result in proprioceptive feedback altering the action of the tiuscles mz&aining head posture. in the short term, the reproducibility of NHP was the best-that is. the method error was the smallest-without the use of the cephalostat and ear post. For radiographs recorded with mirror but without ear posts, the method error was 1.5” in comparison to the recordings with ear posts, which had a method error of 1.9” (groups A and B). With-thee radiographs recorded 3 to 6 months later, however, the results were reversed -the NHP recordings being more reproducible when the ear posts were used. Withear posts the method error for the delayed, with mirror, radiographs was 2.3”. In contrast, the method error .for delayed, without- ear posts, recordings was 2.9” (groups G and H). In view of the poorer overall quality of the radiographs recorded without ear posts, the findings lose much of their initial relevance to clinical practice. The method errors of repeat films recorded with ear posts and with mirror of 1.9” in studies B and C were very small (Table IV). Analysis of the group D data failed to detect any significant differences in NHP between radiographs recorded with or without the cephalostat and ear posts. Studies adopting either of these techniques are therefore comparable. These results also support the methods of
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205
J
R2
Vert.
Horiz.
Fig. 3. Without the use of the ear posts, the location of the central x-ray beam was determined by a projected light cross (” + “). Magnification was standardized by using the black thread plumb line to bisect the reflection of the subject’s face in the mirror.
Vig and associates,28 and Showfety and associates,29 who recorded NHP outside of the cephalostat and then reproduced this position after insertion of ear posts. It should be noted that their method did not position the cervical column with respect to posture. Their method cannot therefore be used if the postural angles (variables) are relevant as, for example, in studies attempting to predict future growth from current head posture. Few previous studies had evaluated NHP reproducibility in children of an orthodontic age, that is, at the age when children would most commonly be assessed for definitive orthodontic treatment. The large study reported by Frlnkel” and Falck” recorded the NHP on photographs and then superimposed the positional changes onto lateral cephalometric radiographs. Only Foster, Howat, and Naish,” on a small sample of eight subjects, and Siersbzk-Nielsen and Solow,” on a sample of 30, have provided evidence of NHP reproducibility in orthodontic age children. The large sample of 217 lZyear-old children reported in this study, 90 of whom were in the normal clinical situation of recording radiographs in a cephalostat and with a mirror, confirms the feasibility of routinely recording NHP, reproduci-
Rl
Fig. 4. The landmarks and the angular variable, SN to the true vertical, used to assess the natural head posture and its changes on repeat recordings, are shown. The variables themselves, although not the symbols, are as originally described by Solow and Tallgren.5
bly, in children when proper care is taken and the radiographers are trained to follow a few key procedures. The findings also support the use of NHP as an improved (less variable) reference plane in cephalometric analysis and as the orientation reference plane for lateral cephalometric radiographs published in the literature. It would seem likely that in time all cephalometric radiographs in clinical practice would benefit from being recorded in NHP. Over time the reproducibility of recording NHP showed a slight tendency to diminish. For radiographs recorded with ear posts and with mirror, the radiographs repeated 4 to 10 minutes and 1 to 2 hours later had a method error of 1.9”. After 3 to 6 months, the observed method error was 2.3”. These increased method errors may represent a true deterioration over time in the ability to clinically reproduce NHP. The reduced reproducibility may also represent a true developmental change in the posture of the head during growth. Even though it is not known from these data whether the change in reproducibility represents the start of a trend, it should be noted that even after 3 to 6 months, NHP reproducibility was still clinically acceptable and still significantly less than the reported variances of conventional intracranial reference planes (ICRPs) to the vertical in NHP.5,‘2,‘4.26 The reproducibility after
288 Cooke and Wei Table IV. Summary table of the reproducibility of NHP under the varying test conditions*
p
1
Study
Design
A
variables
No ear posts; 4-10 min between films; with mirror With ear posts; 4-10 min hetween films; with mirror With ear posts; l-2 hr between films; with mirror With and without ear posts; with mirror; 1-2 hr between films With ear posts; with and without mirror; 4-10 min between films With ear posts; no mirror; 4-10 min between films With ear posts; with mirror; 3-6 mo between films No ear posts; with mirror; 3-6 mo between films
B C D
E
F G H
*The
of
actual reproducibility
between film.
the initial
is expressed
and repeat
;y$;zo;;
Se.r
n
repeat film
M + F
30
1~0.37
2.21
0.40
0.91
0.3-l
NS
1 5.3
F; 30
-7~0.20
2.70
0.49
U.-i1
0.69
NS
1.88
M-t-F
30
- 0.92
2.61
0.48
1.92
0.07
NS
1.93
M+F
40
--0.15
3.39
0.54
0.28
0.78
NS
2.31
M+F M F M + F
30 I.5 IS 30
- 1.65 -2.03 --I.27 -0.15
2.51 2.58 2.46 3.86
0.46 0.67 0.64 0.71
3.60
0.001
3.05
0.009
P < O.Qol P < 0.01
1.99 0.21
0.066 0.83
NS NS
2.10 2.28 1.91 2.69-
MS-F
IS
-t 1.27
3.16
0.81
1.55
0.14
NS
2.34
M+F
12
i 1.63
3.89
1.12
1.45
0.18
NS
2.86
series of radiographs.
error (ME A negative
=
-,
Dahlberg,
value indicates
3 to 6. months was also smaller than the changes in the angulations of the ICRPs to each other, within individuals, over time.30‘3’ The hypothesis that NHP is stable over time was supported by Tallgren and Solowz5 and by Chow, Clark, and Cooke,33 but not by Fr%nkel.*” The large sample reported by Fr%kel included children undergoing orthodontic treatment, many with functional appliantes, and the data may therefore be distorted by changes in posture brought about by functional and physiologic changes. Functional appliance treatment alters muscle form and function around the mouth. Adjoining muscle groups, for example in the neck, probably experience reciprocal changes and it seems likely that treatment-related head posture changes could result. The NHP was also recorded photographically and not radiographically in a cephalostat . Of great relevance to this study is the finding of Chow, Clark, and Cooke3’ that NHP, as measured by the angle Frankfort plane to the true vertical, was virtually identical in adult Hong Kong Chinese male subjects to that found in Hong Kong Chinese boys. Both studies used identical protocols, including the head positioning and measuring methods, For the 33 adult Chinese males with a mean age of 25 years (ranging between 1~6and 45._yea&, Chow and associates found the mean FHlvertical an-
P value
Method error of changes in head posture C’)
.yE‘
as the method
t value
of
SD
Mi
f”)
-
Sigxijicance changes in head posture
1940) for the mean sella-nasion
that the subjects
“looked
up”
to the true vertica!
more on average
angle
for the repeat
gulation to be 86.6”. The mean value for the same postural angle of the 120 Chinese boyswas 86.4”. The constancy of the NHP over the age range of 12 to 4.5 years appears to affirm that NHP does not change:over time, but it should be noted that the averaged data may have masked individual variations. As-growth also occurs over time, long-term studies require careful methodologicai control to prevent growth from distorting the findings on long-term changes in NHP. A longitudinal study is currently in progress with the aim of detecting possible changes in NHP over a IO-year period from 12 years of age. Alternative methods to the superimposition best-fit method described in this study will have to be develaped. The “fiducial” method recently described by Solow and Siersbaek-Nielsens may be suitable. A further study is also necessary to assessthe significance of changes in head posture on the location of landmarks and on the associated measurements of cephalometric variables. The reproducibility of recording lateral cephalometric radiographs in nattual head posture (NW) was evaluated in a random sample of 2 17 12-year-old Chinese children in Hong Kong, and a methodologicall
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Number 4
study of some of the factors affecting NHP was completed. The variables investigated were sex, use of ear posts, an external eye reference (a mirror), and time. NHP was found to be highly reproducible in 12year-old Chinese schoolchildren, with the most reproducible immediate recordings (4 to 10 minutes later) taken with a mirror but without ear posts. Without ear posts, however, the radiographs tended to be of poorer quality. After 3 to 6 months, NHP was more reproducible with a mirror and with ear posts. Boys looked up more when changing from the selfbalance position to the mirror eye reference position. This highly significant mean change (P S 0.001) was 2”. No significant change was noted for the girls and no other significant intersex differences in NHP reproducibility were found. NHP reproducibility was better when a mirror was used as an external source of eye reference, but no significant differences were detected between NHP recordings taken with and without ear posts. Data in the literature from these two types of studies are therefore comparable. NHP recorded with ear posts and with a mirror became slightly less reproducible over time. The method error following 4 to 10 minutes and following 1 to 2 hours was 1.9”. After 3 to 6 months, the method error was 2.3”. It is not known whether this was a trend that would have continued. Further longitudinal studies are needed to quantify NHP over time. We wish to thank the Dental Illustration Unit and the Oral Radiology Unit, University of Hong Kong, for their support. Dr. Peter K-J Yen, Reader in the Department of Children’s Dentistry and Orthodontics, was a cosupervisor for the Ph.D. thesis. REFERENCES 1. Popham AE. The drawings of Leonardo da Vinci. The Reprint Society Ltd. by arrangement with Jonathan Cape. Bungay, Suffolk, England: 1952:plates 216 and 217. 2. Aitken-Meigs J. On the mensuration of the human skull. North Am Med Chir Rev 1861;5:837-71. 3. Von Baer KE, Wagner R. Bericht uber die Zusammenkunft einiger Anthropologen im September 1861 in Gottingen zum Zwecke gemeinsamer Besprcchungen. Leipzig: Leopold Voss, 1861. 4. Broca M. Sur les projections de la tete, et sur un nouveau procede de cephalometrie. Bull Sot Anthropol 1862;3:514-44. 5. Solow B, Tallgren A. Natural head position in standing subjects. Acta Odontol Stand 1971a;29:591-607. 6. Solow B, Tallgren A. Head posture and craniofacial morphology. Am J Phys Anthropol 1976;44:417-36. 7. Solow B, Tallgren A. Dentoalveolar morphology in relation to craniocervical posture. Angle Orthod 1977;47: 15764.
8. Solow B, Siersbrek-Nielsen S. Growth changes in head posture related to craniofacial development. AM J ORTHOD 1986;89: 132-40. 9. Woodside DG, Linder-Aronson S. The channelization of upper and lower anterior face heights compared to population standard in males between ages 6 to 20 years. Eur J Orthod 1979;l: 25-40. 10. Solow B, Siersbiek-Nielsen S, Greve E. Airway adequacy, head posture, and craniofacial morphology. AM J ORTHOD 1984; 86:214-23. Il. Moorrees CFA, Tandarts MEV, Lebret LML, Glatky CB, Kent RL Jr, Reed RB. New norms for the mesh diagram analysis. AM J ORTHOD 1976;69:57-71. 12. Foster TD, Howat AP, Naish PH. Variation in cephalometric reference lines. Br J Otthod 1981;8:183-7. 13. Lundstrom A. Orientation of profile radiographs and photos intended for publication of case reports. Proc Finn Dent Sot 1981;77:105-11. 14. Cooke MS. Cephalometric analyses based on natural head posture of Chinese children in Hong Kong [Ph.D. thesis]. University of Hong Kong [submitted], 1986. 15. Molhave A. A biostatic investigation: the standing posture of man theoretically and statometrically illustrated. Copenhagen: Ejnar Munksgaard, 1958:291-300. 16. Posnick BT. Craniocervical angulation and morphologic variables in children: a cephalometric study [M. Sci. thesis]. University of North Carolina at Chapel Hill, 1978. 17. Vig PS, Showfety KJ, Phillips C. Experimental manipulation of head posture. AM J ORTHOD 1980;77:258-68. 18. Siersbek-Nielsen S, Solow B. lntra- and interexaminer variability in head posture recorded by dental auxiliaries. AM J ORTHOD 1982;82:50-7. 19. Solow B, Tallgren A. Postural changes in craniocervical relationships. Tandlaegebladet 1971b;75:1247-57. 20. Frlnkel R. The applicability of the occipital reference base in cephalometrics. AM J ORTHOD 1980;77:379-95. 21. Falck Von F. Die vorzuge der okzipitalbasis nach Frlnkel bei der femrontgenanalyse. Stomatol DDR 198 1;3 I:69570 1. 22. Sutcher HD. Effect of the intra-meatal cephalometric head holder on occlusion. 1.A.D.R. (North American Division) Programs and Abstracts: 1967;5:37. 23. Eliasson S. Light cross orientation for lateral cephalograms. Swed Dent J 1977a;1:29-38. 24. Eliasson S. Postural position of the mandible with ear-plug fixation and light-cross orientation. Swed Dent J 1977b;l: 39-43. 25. Tallgren A, Solow B. Long-term changes in hyoid position and craniocervical posture in complete denture wearers. J Dent Res 198 1;60:473. 26. Moorrees CFA, Kean MR. Natural head position, a basic consideration in the interpretation of cephalometric radiographs. Am J Phys Anthropol 1958;16:213-34. 27. Dahlberg G. Statistical methods for medical and biological students. London: Allen and Unwin, 1940. 28. Vig PS, Showfety KJ, Matteson SR. A new device to reproduce natural head position. J Dent Res 1981;6O(Special Issue A):474. 29. Showfety KJ, Vig PS, Matheson S. A simple method for taking natural-head-position cephalograms. AM J ORTHOD 1983;83:495500. 30. Riolo ML, Moyers RE, McNamara JA, Hunter WS. An atlas of
288
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craniofacial growth. Monograph 2, Craniufacial Growth Series Ann Arbor: 1974. Center for Human Growth and Development. University of Michigan. 3 I. Broadbent BH Sr, Broadbent BH Jr, Golden WH. Bolton standards of dentofacial developmental growth. St. Louis: The CV Mosby Company, 1975.
casts using face-bow records ation. J Dent 1985;13:277-X2.
as a result
of an anatomical
Reprint reytiests to. Dr. Michael S. Cooke University of Hong Kong Department of Children’s Dentistry and Orthodontics Prince Philip Dental Hospital Hospital Road Hong Kong
32. Prdhl-Anderson B, Kowalski CJ, Heydendael PHJM, eds. A mixed-longitudinal interdisciplinary study of growth and development. New York: Academic Press, 1979 33. Chow TW, Clark RKF. Cooke MS. Errors in mounting maxillary
Sterilixation update in the AIDS4wpatitiss era Robert
A. K&tad
Dallas, Texas
Most dental instruments, including orthodontic pliers, can be safe& and effectively sterilized by dry heat. For sterilization at 320” F, the American Dental Assq$ation’ recomm&ds t&t instruments be heated to that temperature and so maintained for i20 miautes. Thus if the rise time were 30 minutes, a sterilization cycle would require 150 minutes. This ADA guideiine was not developed from research with metal iristrum~nts; rather, it was borrowed from pharmaceutical industry studies with powders and oils. The purpose of cycling instruments through an oven is to destroy microbial pathogens,of which the most heat-resistant are bacterial spores. Accordingly, appropriate operational standards for ovens are those conditions of temperature and time xeeded to achieve spore~destiction. In one study* dental instruments insa closed metalcassette outfitted with a thermocouple were placed in a conventional (no blower) commercial oven preheated to 320” F. Regardlessof load weight, all biologic indicators, each carrying 106 spores of Bacillus subtifis, were destroyed within the rise time-that is, before the temperature within the closed cassetterekhed 320” F. My suggestionfor the use of conventional dry heat in orthodontk practice is the foilowing: 1. Use closed met.@cassettes. 2. Place a sporestrip* centrally in each loaded cassette. 3. Preheat sterilizer to 320” F. 4. Place loaded cassettes into oven~andremove
at 30 minutes.
5. Repeat, with cycles of 35 and 40 minutes. 6. Incubate sporestrips. Use the results ai a guide fer routine instrument recirculation. REFERENCES 1. Accepted d&al therapeutics. 40th ed. Chicago, 2. Kolstad RA, Alten EP. Dry heat sterilization: 1986;65:161 I.
Illinois: American Derrtal Associatian, 1984. guidelines
vs.
metinremqt
[Abstract].
J Dent
Res
*Sporestrips and incubation services via mail can be obtained fmm the Department of Microbiology at these locations: Indiana University Dental School, 1121 ~W. Michigan St.. Indianapol&IN 46282; University of Detroit Dental Scbwl, 2985 E. Jefferson Ave.. Detroit, Ml 48207; Ohio State University Dental School. 305 W. 12th Ave., Columbus, OK-43210.
vari
m
1 he concept of natural head posture is not new. Leonardo da Vinci (14Si- 15 19) and Albrecht Diirer (147 1- 1528) used scaffoldings of horizontal and vertical lines on drawings of models positioned in “natural pose” in order to permit more accurate artistic and scientific r$plication of the human head.‘,’ In the l9th century, Von Baer and Wagner,’ and Broc@ defined the natural posture of the head as the posture of the subject when standing with his visual axis horizontal. Natural head posture (NHP) has been shown to be correlated to craniofacial morphology,5e7 future growth trends,’ and to respiratory needs.9g’” Several researchers have argued that NHP is the logical reference and orientation position for the evaluation of craniofacial mor-
From the Department of Children’s Dentistry and Orthodontics, Faculty of Dentistry, prince Philip Dentat Hospitrd, University of Hong Kong. This work was supported by Grant No; 335.251 .CC01.4c fmm the Uoiwrsity of Hoftg Kong and formed part of a thesis submitted to the University of Hong Kong for the Ph.D. degree. *senior Lecturer. **Professor and Head.
280
phology and the publication of illustrations. I1 I4 The individual or group is presented- as they appear in~life. Hence, lateral cephatalometricradiographs-recorded routinely in NHP -would be more meaningful clinically. Underpinning all these potential clinical @nefirs of routinely recording lateral cephalometric radiographs in NWP are its standardization and -reproducibility.- kf~aity questions remain unanswered. As a result of previous studies, most researchers have adopted the “orthoposition,” the standing posltlon defined by M&ave’5 as the “inte$ion position f&r w&ing.“SsWl8 Solow and Taligren5,19drew attqntion to the need to first &fine .the body and cervical postures uponwhich head posture is superimposed. They devised forward and backward head oscillationsbefore all-owing the head to settle into a “self-balance” posture. They went on to denionstrate that this self-balance ~position differed significantly in maIe subjects from the i$iP tipted wkenman external source d eye reference (a mirror) iS used. Subjects looked up more when using the mirror as reference_.5.‘9 Previous studies into the reproducibility of NHP are
volume 93 Number 4
summarized in Table I. Few studies have evaluated the reproducibility of NHP during lateral cephalometric radiography. Only two studies have assessed the reproducibility in children of “orthodontic” age, Foster, Howat, and Naish,]’ and Siersbaek-Nielsen and Solow,‘* but the former examined very few subjects (n = 8). The larger study of Frankel,20 reported in more detail by Falck,” evaluated the reproducibility of NHP photographically, but the findings may not match exactly recordings made in a cephalostat. There have been no studies comparing NHP recorded with and without ear posts, despite the fact that significant differences in head position may result from proprioceptive feedback following insertion of the ear posts.‘2-‘4 Comparisons between male and female subjects are not available, and relatively few studies have evaluated NHP over time. 20.25No population standards of head and cervical column positions are available (“postural variables”). The reproducibility of NHP has not been quantified for any nonwhite populations. This study was directed at these incompletely answered questions and reports the findings from a methodological study of the reproducibility of recording lateral cephalometric radiographs in natural head posture. The variables investigated were sex, use of ear posts, an external source of eye reference (a mirror), and time. It forms part of a larger study evaluating the use of new cephalometric analyses based on natural head posture and the true horizontal.“’ From the clinical viewpoint, it may prove beneficial in the future to record in NHP all cephalometric radiographs taken in orthodontic and orthognathic surgical practices. No information would be lost, but a great deal may by gained. SUBJECTS AND METHODS The sample and allocation into test groups
The 217 children comprised a random subsample drawn from a larger research sample of 618 12-yearold Chinese children in Hong Kong. A partially stratified random sampling method based on schools had been used to select the original population sample. The children were randomly allocated to one of the first six test groups defined in Table II and had one set only of repeat lateral cephalometric radiographs recorded in NHP. The test groups were designed to each comprise a different combination of the study variables and to thereby simplify the statistical interpretation of the data. Months later, those children who returned for treatment at a suitable date were placed into the final two test groups, G and H. Allocation remained random as it was not known initially which children would return for treatment. Groups A to F comprised equal numbers of boys and girls. As the sample sizes in groups
Reproducibility
of
natural head posture
281
G and H were relatively small, boys and girls were combined for analysis. The subjects returned to the waiting room between the immediate (4 to 10 minutes) repeats or took part in a complementary multidisciplinary research project between the delayed (1 to 2 hours) repeats. For group E subjects (with and without mirror), the first radiograph had to be recorded without the mirror, otherwise the subject’s head posture may have been influenced by prior knowledge that the mirror was hidden behind the cover. All radiographs were recorded in the morning with the same operators in the same air-conditioning environment. Cephalometric
technique
A General Electric 1000 x-ray unit with a Wehmer CI-2 cephalometer was modified for natural head posture recordings. I4 Modifications included moving the machine away from the wall and adding a second collimator. This allowed a lateral “ + ” beam of light to locate the position of the central x-ray beam in line with the external auditory meatus for those subjects not using the ear posts (groups A, D, and H). The true vertical was marked on each radiograph by a wire plumb line suspended in front of the aluminum soft-tissue wedge. To minimize the x-radiation exposure levels, rare earth intensifying screens were used with no grids. No individual radiographs were repeated. To ensure that each exposure was accompanied by a net benefit to the patient, all children were offered comprehensive dental treatment including orthodontic treatment. Copies of the radiographs were later used for diagnoses. A parent signed the informed consent form; the overall consent rate was 86%. “Orthoposition”
of natural head posture
All the radiographs were recorded with the subject standing in the “orthoposition” natural head posture, as originally defined by Molhave15 and as later adopted and modified by Solow and Tallgren,’ and others8,‘6.‘7 (Fig. 1). To prevent the children from swaying, it was found necessary to also define the feet position as “a comfortable distance apart and slightly diverging”14 (Fig. 2). For the self-balance position, without ear posts or mirror, the subject tilted the head forward and backward with decreasing amplitude until a comfortable position of natural balance was achieved. With the mirror, the subject was then requested to look into the reflection of their eyes in a mirror located 200 cm ahead. A black thread plumb line in front of the subject was used to “bisect” the facial reflection and to minimize lateral head rotations, especially when no ear posts were used (Fig. 3). Without ear posts this technique also served
282 Cooke and Wei
TaMe 1. The reproducibility of natural head posture (NHP) reported in previous studies in the literature (included in the table are the varying conditions under which the natural head posture was evaluated, the sample sizes, and the actual method errors [reproducibility] -obtained) .StUdy
Bjerin, 1957 Moorrees and Kean, 1958 CarIs and Leijon, 1960 Solow and Tallgren, 1971a Cavaflaro et al., 1974 Fratrkel, 1980 (Falck, 1981) Foster, 1981
Siersbazk-Nielsen and Solow, 1982
Posture
External eye reference
Head support
Time between repeats
Sitting Standing Sitting
Mirror Mirror Mirror
With posts Not stated No posts
Sitting
None
With posts
2- 10 days
Few minutes 1 week
Age range W
Sex
Sample size
M&od error (“)
M+F M+F F F M+F
22-36 22-36 18-20 18-20 14-79
35 35 66 61 1-l
1.62 1.a 2.05 ~I.54 4.60
M M M+F
22-30 22-30 Not stated. Mainly adults?
21 21 20
2.48 I .43 Mean error 1.05 2.25
With posts
2 months
Sitting
None Mirror Mirror
No posts
Approximately 3 weeks
Standing
Mirror
No posts
Few minutes
M+F
7-18
923
Standing
Wall mark at eye level
With posts
3-6 months
M+F
Mean 13.7
8
Mean error 4.1
Standing
Mirror
With posts
l-35 days
M+F
6-15
30
2.25
Standing
Table 8. Variables evaluated in the NHP reproducibility studies and their distribution within the~test groups
(each group comprised a different combination of variables) Test group
A B C D
Cephalostat and ear posts
E
No Yes Yes One film yes One film no Yes
F G H
Yes Yes No
Mirror as an external visual reference
Yes Yes Yes
Time interval between~repeats
Sample size (% bays, % @r/s. except G and H)
4-10 min 4-10 min 1-2 hr l-2 hr
30 30 30 40
One film yes One film no
4-10 min
30
NQ
4-10 min 3-6 mo 3-6 mo
30 1.5 12
Y.3
Yes Yes
to standardize the midsagittal plane to film distance. Special care was taken to ensure that the head was not moved when the ear posts were carefully inserted.
The angle between the sella-nasion line (SN) and the true vertical was used to assess the reproducibihty of the head posture (Fig. 4). To ensure that only true changes in head posture were measured and not changes arising from errors in l&lmark location, the “best-fit” superimposition technique described by Moorrees and Kean% was used. A&r accurately superimposing the
traced initial and repeat radiographs, a pin was used to transfer the precise &cations of sella and nasion from the initial the the repeat film. The SN to the vertical angle Was measured by hand y&h a large protractor to the nearest 0.5”. 23 Dahlberg’s formula,z7 method error. = x, J was applied to the difference between the inititil and repeat radiographs for each subject; The paired t test was used totit whether any si~ific4tnt.cha~~s~in.NNP had occurred under the~particuhx test conditions. Boys
Volume 93 Number
Reproducibility
4
of natural head posture
283
Table Ill. Method errors for tracing and
superimposition technique used to evaluate changes in NHP on repeat radiographs (errors were least when both radiographs had been taken with ear posts)
Comments Combined
photographs
recorded
in NHP,
with radiographs.
No “corrections” to head positions. “Corrections” were made if head position unnatural. Nineteen of the original study sample were children, 14-15 years, but the “repeats” appear to have been on the adults. Subjects looked upward more with the mirror external eye reference; average 3”; significant at 1% level. The error stated appears to be the difference between the means of time 1 and time 2 series. Not comparable to other studies? Combined photographs recorded in NHP, with radiographs. Combined self-balance position with eye focusing on wall mark positioned at eye level. Mean error is the mean of individual differences between time 1 and time 2 radiographs. All radiographs and head positioning were carried out by auxiliaries.
and girls were compared for each group. Dahlberg’s formula was also used in the analysis of method errors. Radiographs in which the mandible was postured, or in which the teeth were not in contact, were removed from the study. Plaster study models were available for all subjects. RESULTS Error of the method
The method errors found from repeat tracings and measurements are summarized in Table III. It was more accurate to superimpose on the better quality radiographs obtained when using ear posts. Without ear posts the subjects could more easily move during the exposure. The method error figures of 0.23” to 0.31” are in the identical range to those reported in the computeraided study of Solow and Tallgren.5 Errors occurring merely in reading the same lines at a later time, 0.17”, proved to be virtually half of the total method error. Reproducibility
of natural head posture
The results are summarized in Table IV. Method errors (reproducibility) ranged between 1.5” and 2.9”. For groups A, B, C, D, and F, no significant differences were observed between boys and girls, nor between the initial and the repeat series of radiographs. In group E (with and without mirror), significant
Use of ear posts Both radiographs without ear posts One radiograph with ear posts and one without Both radiographs with ear posts
Study
Method (Dahlberg,
error 1940)
A, H
0.31”
(n = 30)
D
0.30”
(n = 30)
B, C, E, F, G
0.23”
(n = 30)
differences were observed between boys and girls, and between the initial and repeat recordings. For the male series, the subjects looked up more when using the mirror as the external source of eye reference, the mean difference in head posture being 2.0”, which was significant at the P 6 0.01 level. There was no significant difference when using the mirror for the female series. For the combined (M + F) data, the change in head posture with the mirror was significant at the P = 0.001 level. Dahlberg’s method error formula is not strictly applicable if there are significant differences between the means of the two series of data. Therefore, the method error for group E data should in the strict sense, be quoted only for the female series (1.9”). The mean differences in head position shown for groups C, G, and H data imply a systematic error. In terms of the t values (standard error divided by the mean), the observed differences are not significantly different from zero. For groups G and H, the data could be interpreted as a trend to a change in NHP over time, with the subjects looking down more after 3 to 6 months. DISCUSSION
Muscle postures constitute a narrow range of positions controlled by the asynchronous firing of individual motor units within the skeletal muscles. The classic example is the “tremor” associated with attempts to remain still. Head posture is a typical case. FrHnkelzOspoke of NHP as constituting a “small range of positions.” Despite this theoretical range, NHP has been found in this study to be remarkably reproducible in 12-year-old children. The short-term reproducibility (method error) with ear posts and with mirror was found to be 1.9”. This figure is the mean of groups B and C (Table IV).
284
Cooke and Wei
Fig. 2. To prevent the children from swaying,the feet position was defined as “a comfottable dis@ncet apad and slightly diverging.” The foot of the operator is shown guiding the subject’s feet to prevent the subject frtim~looking down and-alterir$ head posture.
Fig. 1. Recording lateral cephalometric radiographs in natural head posture with the cephabstat and ear posts. The wire plumb line and suspended weight re&xd&d lhe~tiue vertical on each radiograph: the longer btack thread plumb line served to bisect the reflection of the subject’s face in the mirror and minimize lateral head rotation.
Overall, the results confirmed the findings of previous workers (Table I) that NHP is highly reproducible in the short term, but the data also provided many previously unreported findings. With the exception of group E (with and without mirror), no significant. sex differences were detected. The classical studies of Moorrees and Kean,“’ and of Solow and Tallgren5,” were each limited to one sex, female and male respectively, and no other studies have compared male-versusfemale data in the same study. The findings of group E confirmed those of Solow and Tallgn~~~‘~ that boys tended to look up more in changing from the selfbalance position to the mirror position. A 3” change was reported by Solow and Tallgren and a 2” changes was found in this study, both significant at the 0.1% level. No differences were found for the girls. A possible explanation is that girls are encouraged socially to adopt a good upright posture Andyto look straight ahead, and that they continue to adopt this posture even in the absence of an- external eye reference. Perhaps,
boys adopt a lazier head posture in the self-balance position. A more “natural” posture may occur without the use of the ear posts and cephalostat. Insertion of ear posts could itself be argued as positioning the subject “un-naturally.” Sutcher’* and Eliassonz3.*” had concluded that insertion of ear posts altered the position of the condyle within the fossa and perhaps this couid result in proprioceptive feedback altering the action of the tiuscles mz&aining head posture. in the short term, the reproducibility of NHP was the best-that is. the method error was the smallest-without the use of the cephalostat and ear post. For radiographs recorded with mirror but without ear posts, the method error was 1.5” in comparison to the recordings with ear posts, which had a method error of 1.9” (groups A and B). With-thee radiographs recorded 3 to 6 months later, however, the results were reversed -the NHP recordings being more reproducible when the ear posts were used. Withear posts the method error for the delayed, with mirror, radiographs was 2.3”. In contrast, the method error .for delayed, without- ear posts, recordings was 2.9” (groups G and H). In view of the poorer overall quality of the radiographs recorded without ear posts, the findings lose much of their initial relevance to clinical practice. The method errors of repeat films recorded with ear posts and with mirror of 1.9” in studies B and C were very small (Table IV). Analysis of the group D data failed to detect any significant differences in NHP between radiographs recorded with or without the cephalostat and ear posts. Studies adopting either of these techniques are therefore comparable. These results also support the methods of
Volume 93 Number
4
Reproducibility
of natural
head posture
205
J
R2
Vert.
Horiz.
Fig. 3. Without the use of the ear posts, the location of the central x-ray beam was determined by a projected light cross (” + “). Magnification was standardized by using the black thread plumb line to bisect the reflection of the subject’s face in the mirror.
Vig and associates,28 and Showfety and associates,29 who recorded NHP outside of the cephalostat and then reproduced this position after insertion of ear posts. It should be noted that their method did not position the cervical column with respect to posture. Their method cannot therefore be used if the postural angles (variables) are relevant as, for example, in studies attempting to predict future growth from current head posture. Few previous studies had evaluated NHP reproducibility in children of an orthodontic age, that is, at the age when children would most commonly be assessed for definitive orthodontic treatment. The large study reported by Frlnkel” and Falck” recorded the NHP on photographs and then superimposed the positional changes onto lateral cephalometric radiographs. Only Foster, Howat, and Naish,” on a small sample of eight subjects, and Siersbzk-Nielsen and Solow,” on a sample of 30, have provided evidence of NHP reproducibility in orthodontic age children. The large sample of 217 lZyear-old children reported in this study, 90 of whom were in the normal clinical situation of recording radiographs in a cephalostat and with a mirror, confirms the feasibility of routinely recording NHP, reproduci-
Rl
Fig. 4. The landmarks and the angular variable, SN to the true vertical, used to assess the natural head posture and its changes on repeat recordings, are shown. The variables themselves, although not the symbols, are as originally described by Solow and Tallgren.5
bly, in children when proper care is taken and the radiographers are trained to follow a few key procedures. The findings also support the use of NHP as an improved (less variable) reference plane in cephalometric analysis and as the orientation reference plane for lateral cephalometric radiographs published in the literature. It would seem likely that in time all cephalometric radiographs in clinical practice would benefit from being recorded in NHP. Over time the reproducibility of recording NHP showed a slight tendency to diminish. For radiographs recorded with ear posts and with mirror, the radiographs repeated 4 to 10 minutes and 1 to 2 hours later had a method error of 1.9”. After 3 to 6 months, the observed method error was 2.3”. These increased method errors may represent a true deterioration over time in the ability to clinically reproduce NHP. The reduced reproducibility may also represent a true developmental change in the posture of the head during growth. Even though it is not known from these data whether the change in reproducibility represents the start of a trend, it should be noted that even after 3 to 6 months, NHP reproducibility was still clinically acceptable and still significantly less than the reported variances of conventional intracranial reference planes (ICRPs) to the vertical in NHP.5,‘2,‘4.26 The reproducibility after
288 Cooke and Wei Table IV. Summary table of the reproducibility of NHP under the varying test conditions*
p
1
Study
Design
A
variables
No ear posts; 4-10 min between films; with mirror With ear posts; 4-10 min hetween films; with mirror With ear posts; l-2 hr between films; with mirror With and without ear posts; with mirror; 1-2 hr between films With ear posts; with and without mirror; 4-10 min between films With ear posts; no mirror; 4-10 min between films With ear posts; with mirror; 3-6 mo between films No ear posts; with mirror; 3-6 mo between films
B C D
E
F G H
*The
of
actual reproducibility
between film.
the initial
is expressed
and repeat
;y$;zo;;
Se.r
n
repeat film
M + F
30
1~0.37
2.21
0.40
0.91
0.3-l
NS
1 5.3
F; 30
-7~0.20
2.70
0.49
U.-i1
0.69
NS
1.88
M-t-F
30
- 0.92
2.61
0.48
1.92
0.07
NS
1.93
M+F
40
--0.15
3.39
0.54
0.28
0.78
NS
2.31
M+F M F M + F
30 I.5 IS 30
- 1.65 -2.03 --I.27 -0.15
2.51 2.58 2.46 3.86
0.46 0.67 0.64 0.71
3.60
0.001
3.05
0.009
P < O.Qol P < 0.01
1.99 0.21
0.066 0.83
NS NS
2.10 2.28 1.91 2.69-
MS-F
IS
-t 1.27
3.16
0.81
1.55
0.14
NS
2.34
M+F
12
i 1.63
3.89
1.12
1.45
0.18
NS
2.86
series of radiographs.
error (ME A negative
=
-,
Dahlberg,
value indicates
3 to 6. months was also smaller than the changes in the angulations of the ICRPs to each other, within individuals, over time.30‘3’ The hypothesis that NHP is stable over time was supported by Tallgren and Solowz5 and by Chow, Clark, and Cooke,33 but not by Fr%nkel.*” The large sample reported by Fr%kel included children undergoing orthodontic treatment, many with functional appliantes, and the data may therefore be distorted by changes in posture brought about by functional and physiologic changes. Functional appliance treatment alters muscle form and function around the mouth. Adjoining muscle groups, for example in the neck, probably experience reciprocal changes and it seems likely that treatment-related head posture changes could result. The NHP was also recorded photographically and not radiographically in a cephalostat . Of great relevance to this study is the finding of Chow, Clark, and Cooke3’ that NHP, as measured by the angle Frankfort plane to the true vertical, was virtually identical in adult Hong Kong Chinese male subjects to that found in Hong Kong Chinese boys. Both studies used identical protocols, including the head positioning and measuring methods, For the 33 adult Chinese males with a mean age of 25 years (ranging between 1~6and 45._yea&, Chow and associates found the mean FHlvertical an-
P value
Method error of changes in head posture C’)
.yE‘
as the method
t value
of
SD
Mi
f”)
-
Sigxijicance changes in head posture
1940) for the mean sella-nasion
that the subjects
“looked
up”
to the true vertica!
more on average
angle
for the repeat
gulation to be 86.6”. The mean value for the same postural angle of the 120 Chinese boyswas 86.4”. The constancy of the NHP over the age range of 12 to 4.5 years appears to affirm that NHP does not change:over time, but it should be noted that the averaged data may have masked individual variations. As-growth also occurs over time, long-term studies require careful methodologicai control to prevent growth from distorting the findings on long-term changes in NHP. A longitudinal study is currently in progress with the aim of detecting possible changes in NHP over a IO-year period from 12 years of age. Alternative methods to the superimposition best-fit method described in this study will have to be develaped. The “fiducial” method recently described by Solow and Siersbaek-Nielsens may be suitable. A further study is also necessary to assessthe significance of changes in head posture on the location of landmarks and on the associated measurements of cephalometric variables. The reproducibility of recording lateral cephalometric radiographs in nattual head posture (NW) was evaluated in a random sample of 2 17 12-year-old Chinese children in Hong Kong, and a methodologicall
Volume 93
Reproducibility
of natural
head posture
287
Number 4
study of some of the factors affecting NHP was completed. The variables investigated were sex, use of ear posts, an external eye reference (a mirror), and time. NHP was found to be highly reproducible in 12year-old Chinese schoolchildren, with the most reproducible immediate recordings (4 to 10 minutes later) taken with a mirror but without ear posts. Without ear posts, however, the radiographs tended to be of poorer quality. After 3 to 6 months, NHP was more reproducible with a mirror and with ear posts. Boys looked up more when changing from the selfbalance position to the mirror eye reference position. This highly significant mean change (P S 0.001) was 2”. No significant change was noted for the girls and no other significant intersex differences in NHP reproducibility were found. NHP reproducibility was better when a mirror was used as an external source of eye reference, but no significant differences were detected between NHP recordings taken with and without ear posts. Data in the literature from these two types of studies are therefore comparable. NHP recorded with ear posts and with a mirror became slightly less reproducible over time. The method error following 4 to 10 minutes and following 1 to 2 hours was 1.9”. After 3 to 6 months, the method error was 2.3”. It is not known whether this was a trend that would have continued. Further longitudinal studies are needed to quantify NHP over time. We wish to thank the Dental Illustration Unit and the Oral Radiology Unit, University of Hong Kong, for their support. Dr. Peter K-J Yen, Reader in the Department of Children’s Dentistry and Orthodontics, was a cosupervisor for the Ph.D. thesis. REFERENCES 1. Popham AE. The drawings of Leonardo da Vinci. The Reprint Society Ltd. by arrangement with Jonathan Cape. Bungay, Suffolk, England: 1952:plates 216 and 217. 2. Aitken-Meigs J. On the mensuration of the human skull. North Am Med Chir Rev 1861;5:837-71. 3. Von Baer KE, Wagner R. Bericht uber die Zusammenkunft einiger Anthropologen im September 1861 in Gottingen zum Zwecke gemeinsamer Besprcchungen. Leipzig: Leopold Voss, 1861. 4. Broca M. Sur les projections de la tete, et sur un nouveau procede de cephalometrie. Bull Sot Anthropol 1862;3:514-44. 5. Solow B, Tallgren A. Natural head position in standing subjects. Acta Odontol Stand 1971a;29:591-607. 6. Solow B, Tallgren A. Head posture and craniofacial morphology. Am J Phys Anthropol 1976;44:417-36. 7. Solow B, Tallgren A. Dentoalveolar morphology in relation to craniocervical posture. Angle Orthod 1977;47: 15764.
8. Solow B, Siersbrek-Nielsen S. Growth changes in head posture related to craniofacial development. AM J ORTHOD 1986;89: 132-40. 9. Woodside DG, Linder-Aronson S. The channelization of upper and lower anterior face heights compared to population standard in males between ages 6 to 20 years. Eur J Orthod 1979;l: 25-40. 10. Solow B, Siersbiek-Nielsen S, Greve E. Airway adequacy, head posture, and craniofacial morphology. AM J ORTHOD 1984; 86:214-23. Il. Moorrees CFA, Tandarts MEV, Lebret LML, Glatky CB, Kent RL Jr, Reed RB. New norms for the mesh diagram analysis. AM J ORTHOD 1976;69:57-71. 12. Foster TD, Howat AP, Naish PH. Variation in cephalometric reference lines. Br J Otthod 1981;8:183-7. 13. Lundstrom A. Orientation of profile radiographs and photos intended for publication of case reports. Proc Finn Dent Sot 1981;77:105-11. 14. Cooke MS. Cephalometric analyses based on natural head posture of Chinese children in Hong Kong [Ph.D. thesis]. University of Hong Kong [submitted], 1986. 15. Molhave A. A biostatic investigation: the standing posture of man theoretically and statometrically illustrated. Copenhagen: Ejnar Munksgaard, 1958:291-300. 16. Posnick BT. Craniocervical angulation and morphologic variables in children: a cephalometric study [M. Sci. thesis]. University of North Carolina at Chapel Hill, 1978. 17. Vig PS, Showfety KJ, Phillips C. Experimental manipulation of head posture. AM J ORTHOD 1980;77:258-68. 18. Siersbek-Nielsen S, Solow B. lntra- and interexaminer variability in head posture recorded by dental auxiliaries. AM J ORTHOD 1982;82:50-7. 19. Solow B, Tallgren A. Postural changes in craniocervical relationships. Tandlaegebladet 1971b;75:1247-57. 20. Frlnkel R. The applicability of the occipital reference base in cephalometrics. AM J ORTHOD 1980;77:379-95. 21. Falck Von F. Die vorzuge der okzipitalbasis nach Frlnkel bei der femrontgenanalyse. Stomatol DDR 198 1;3 I:69570 1. 22. Sutcher HD. Effect of the intra-meatal cephalometric head holder on occlusion. 1.A.D.R. (North American Division) Programs and Abstracts: 1967;5:37. 23. Eliasson S. Light cross orientation for lateral cephalograms. Swed Dent J 1977a;1:29-38. 24. Eliasson S. Postural position of the mandible with ear-plug fixation and light-cross orientation. Swed Dent J 1977b;l: 39-43. 25. Tallgren A, Solow B. Long-term changes in hyoid position and craniocervical posture in complete denture wearers. J Dent Res 198 1;60:473. 26. Moorrees CFA, Kean MR. Natural head position, a basic consideration in the interpretation of cephalometric radiographs. Am J Phys Anthropol 1958;16:213-34. 27. Dahlberg G. Statistical methods for medical and biological students. London: Allen and Unwin, 1940. 28. Vig PS, Showfety KJ, Matteson SR. A new device to reproduce natural head position. J Dent Res 1981;6O(Special Issue A):474. 29. Showfety KJ, Vig PS, Matheson S. A simple method for taking natural-head-position cephalograms. AM J ORTHOD 1983;83:495500. 30. Riolo ML, Moyers RE, McNamara JA, Hunter WS. An atlas of
288
Cooke and Wei
craniofacial growth. Monograph 2, Craniufacial Growth Series Ann Arbor: 1974. Center for Human Growth and Development. University of Michigan. 3 I. Broadbent BH Sr, Broadbent BH Jr, Golden WH. Bolton standards of dentofacial developmental growth. St. Louis: The CV Mosby Company, 1975.
casts using face-bow records ation. J Dent 1985;13:277-X2.
as a result
of an anatomical
Reprint reytiests to. Dr. Michael S. Cooke University of Hong Kong Department of Children’s Dentistry and Orthodontics Prince Philip Dental Hospital Hospital Road Hong Kong
32. Prdhl-Anderson B, Kowalski CJ, Heydendael PHJM, eds. A mixed-longitudinal interdisciplinary study of growth and development. New York: Academic Press, 1979 33. Chow TW, Clark RKF. Cooke MS. Errors in mounting maxillary
Sterilixation update in the AIDS4wpatitiss era Robert
A. K&tad
Dallas, Texas
Most dental instruments, including orthodontic pliers, can be safe& and effectively sterilized by dry heat. For sterilization at 320” F, the American Dental Assq$ation’ recomm&ds t&t instruments be heated to that temperature and so maintained for i20 miautes. Thus if the rise time were 30 minutes, a sterilization cycle would require 150 minutes. This ADA guideiine was not developed from research with metal iristrum~nts; rather, it was borrowed from pharmaceutical industry studies with powders and oils. The purpose of cycling instruments through an oven is to destroy microbial pathogens,of which the most heat-resistant are bacterial spores. Accordingly, appropriate operational standards for ovens are those conditions of temperature and time xeeded to achieve spore~destiction. In one study* dental instruments insa closed metalcassette outfitted with a thermocouple were placed in a conventional (no blower) commercial oven preheated to 320” F. Regardlessof load weight, all biologic indicators, each carrying 106 spores of Bacillus subtifis, were destroyed within the rise time-that is, before the temperature within the closed cassetterekhed 320” F. My suggestionfor the use of conventional dry heat in orthodontk practice is the foilowing: 1. Use closed met.@cassettes. 2. Place a sporestrip* centrally in each loaded cassette. 3. Preheat sterilizer to 320” F. 4. Place loaded cassettes into oven~andremove
at 30 minutes.
5. Repeat, with cycles of 35 and 40 minutes. 6. Incubate sporestrips. Use the results ai a guide fer routine instrument recirculation. REFERENCES 1. Accepted d&al therapeutics. 40th ed. Chicago, 2. Kolstad RA, Alten EP. Dry heat sterilization: 1986;65:161 I.
Illinois: American Derrtal Associatian, 1984. guidelines
vs.
metinremqt
[Abstract].
J Dent
Res
*Sporestrips and incubation services via mail can be obtained fmm the Department of Microbiology at these locations: Indiana University Dental School, 1121 ~W. Michigan St.. Indianapol&IN 46282; University of Detroit Dental Scbwl, 2985 E. Jefferson Ave.. Detroit, Ml 48207; Ohio State University Dental School. 305 W. 12th Ave., Columbus, OK-43210.
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