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radiographic technique
Head posture and cephalometric analyses: An integrated photographic ~radiographic technique Virgilio F. Ferrario, MD," Chiarella Sforza, MD, b Domenica Germanb, MD, ~ Luca L. Dalloca, DMD, = and Alessandro Miani, Jr., MD d
Milan, Italy A photographic technique for natural head postu?e (NHP) recording that can be associated to standard radiographic exposures is presented. It allows the evaluation of radiographs, according to both the standard intracranial references and NHP. On the NHP photograph, the angle between the soft tissue nasion-pogonion line and the true vertical is calculated, and this value is used to rotate the standard radiograph around the Bolton point. The technique has been applied to 40 private orthodontic patients (24 females and 16 males, aged 7 to 20 years, mean 12.9 years). The hard tissue Frankfurt plane in NHP showed a wide variation: 80% of the patients had orbitale lower than potion (mean angle - 6 ~ relative to the ground), 20% had orbitale higher than porion (mean angle 4~ The position of this plane in NHP seemed to be different in the two sexes, with more males having the Frankfurt plane going upwards than females. The soft tissue Frankfurt plane (tragus-orbitale) in NHP was directed upwards (head extended) in 53% of patients.The two Frankfurt planes were never coincident in all subjects; the tragus was always lower and more anterior than porion. On average, the angle tragus-orbitale-porion was about 6~ In young orthodontic patients NHP is therefore highly variable, gender dependent, and cannot be deduced from mean population values. Nevertheless, the evaluation of head position should be performed in each young patient before and during the treatment, to verify how the combined effects of therapy and growth act. (AMJ ORTHODDENTOFACORTHOP 1994;106:257-66.)
M a n y recent studies have advocated the use of natural head posture (NHP) in cephalometric analyses."6 The NHP in orthodontics has been introduced since the 1950s, 6-s and the relationships between head posture, craniocervical curvature, craniofacial structure, dental positions, and airway patency have been examined in detail both in normal subjects and in patients) "t3 The NHP has been found to be highly reproducible, regardless of patients' age, gender or race, of the time lag between repeated recordings, of the radiographic or photographic technique, of the experience, or cultural background of the operator.t'7"14ts The high interpersonal variability of intracranial reference planes (such as the Frankfurt plane or the sellanasion line), 2'7s'16 the lack of functional information about head posture on conventional head films, ~ and the need for a holistic approach taking the overall ap-
From the Laboratorio di Anatomia Funzionale dell'Apparato Stomatognatico, Istituto di Anatomia Umana Norrna/e, Facolth di Medicina e Chirurgia, Universit~ degli Studi di Milano, Italy. 'Professor of Human Anatomy. bAssociate Professor of Human Anatomy. r private practice, Arcore, Milano, Italy. dpostgmduate student. Copyright 9 1994 by the American Association of Orthodontists. 0889-5406194153.00 + 0 811143639
pearance of the patient into consideration, 5"17all gave birth to new cephalometric analyses where classical intracranial references and standard positions were replaced by new lines and head postures. The proposals of Cooke and Wei z and Michiels and Toume '6 are based on the true horizontal (or vertical), and observe the head in the NHP. Bass s uses the esthetic horizontal line, which is determined by the orthodontist on visual inspection of the subject and which probably does not differ significantly from the NHP on average: These new analyses present the patients as they appeal- in life, and the modifications of facial appearance depending on head position are in some way taken into account. As stated by Claman et al.,'8 the subject's av;,areness of deviations in facial structure could modify the craniocervical posture in an attempt to mask, e.g., a too protrusive or retrusive chin. 6"t~ Moreover, Cooke and Wei 2 showed that some patients can be misjudged using standard references instead of NHP. Several standardized techniques have been proposed to obtain NHP radiographs. 1,3-5.7-8,16All these techniques require modified cepbalostats allowing for nonstandard head positioning, and could be of difficuR practical applicability. In this study, we have developed a photographic technique t'or NHP recording, which can be associated 257
258 Ferrario et aL
with standard radiographic exposures. We also present a computerized method that allows an easy and fast superimposition of the two recordings and makes radiographs of value, according to both the standard intracranial references and NHP.
MATERIALS AND METHODS Sample T h e proposed technique has been applied to 40 orthodontic patients (24 females and 16 males, aged 7 to 20 years, mean 12.9 years). The patients were randomly selected in an 8-week interval from all pretreatment subjects admitted. All the patients were northern Italian (white).
Photographic technique
American Journal of Orthodontics and Dentofacial Orthopedics September 1994
Cephalometric measurements A set of standardized landmarks was then traced on all the head films "'~9 by the same operator as shown in Fig. 1. Bilateral structures were tracted by bisecting right and left images. The points were digitized by means of a semiautomatic image analyzer (MM 1201, Summagraphics Co., Fairfield, Conn.) interfaced with an AT computer (Olivetti M380, Olivetti, Ivrea, Italy). By using a program developed for this purpose, the x-y coordinates of these points were used to generate several cephalometric measurements (angles and distances). The computer provided a list of the measurements and a plot of the digitized landmarks. Also the position of the soft tissue Frankfurt plane was determined. All the films were oriented with the hard tissue Frankfurt plane (porion-orbitale) parallel to the ground, as suggested in standard measurements.
A modification of the technique described by Ferrario et al. ~5was used, and one right-side picture was taken for each subject in centric occlusion. Subjects stood 150 cm from the camera, which was mounted on a tripod and levelled, with the optical axis of the lens horizontal and the film plane vertical. The subjects stood with bare feet, looking straight into a 20 • 100 cm mirror mounted at eye level on the wall so that they could see the reflected image of their eyes. The mirror-subject distance was 120 cm. They were then asked to assume and maintain a "natural and normal" erect posture of head and shoulders with both arms hanging free beside the trunk. This should correspond to Broca's "natural head position."!8 On each photograph, a reference line placed perpendicular to the ground by using a small spirit level (true vertical) was drawn. This allowed careful control of position of the subject relative to reference planes. All the photographs were taken with the same camera: Yashica Dental-Eye II, lens 100 mm f/4, using Ilford HPS 135 (ISO 400/27 ~ films. They were developed and printed by the same operator, employing consistent and rigorously controlled procedures. The subjects were photographed in the same room where the radiographic exposure was subsequently taken. The procedure required about 5 minutes for each subject.
On all photographs, the soft tissue Nasion (N') and the soft tissue pogonion (Pg') were traced, and the angle between the N'-Pg' line and the true vertical was calculated (Fig. 2, A). The same angle was calculated on the cephalometric films, and the difference between the two measurements was used to compute the position of the soft and hard tissue Frankfurt planes, and of the sella-nasion plane in NHP. These new values were compared with the values previously observed in the standard cephalometric orientation. Lastly, to better appreciate the position of skeletal and soft tissue landmarks in NHP, the value of angle N'-Pg' line/true vertical for each patient was fed to a computer program which provided a rotation of all the landmarks until the cephalometric N'-Pg' line coincided with the photographic one. Rotation was performed around the Bolton point (highest and most posterior point of the occipital condyles~9). This point roughly represents the fulcrum for small head flexion/extension movements, which can be assumed to be performed in the atlantooccipital joints. The new position of points relative to the coordinate axes was then plotted (Fig. 2, B).
Radiographic technique
Statistical calculations
All head films (lateral cephalometric radiographs) were taken with the Orthoceph 10E (Siemens AG, Germany), whose vertical adjustability allows the recording of standing subjects. The x-ray source has a focus of 0.6 mm. The focusmedian plane distance was 180 cm, and the film-median plane distance was 10 cm, with a final enlargement of 10%. For all subjects 18 • 24 cm films were used. A 5-ram radiopaque disk was fixed on the right tragus of each subject to allow the identificationof the soft tissue Frankfurt plane (tragus-orbitale) on the radiograms. The patient was radiographed in standing posture, the head being approximately oriented with the soft tissue Frankfurt plane parallel to the ground, with earrods in ccntric occlusion. This position does not necessarily correspond to the NHP, and was not chosen by the patient, but it is the standard position imposed by the machine.
Mean angle values were calculated with dedicated statistics for circular variables as detailed by Batschelet :~ by using the rectangular components of each angle. The same method allowed the calculation of relevant dispersion measures (the standard angular deviations). Standard angular deviation has the same meaning as standard deviation in linear statistics.
Photographic. measurements
Error of method To assess reproducibilityof NPH, five subjects (two males and three females) were photographed once, then a second time after a 2-week interval. Moreover, 10 randomly selected radiograms and corresponding NPH photographs were retraced and redigitized by a second investigator. The combined error of subject's positions, landmark location and digitization was estimated to be about 3% of the mean values of the measurements.
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P
N-N'
Nasion
f/
AR
Articulate
S
Sella
/
BO
Bolton
PO
Potion
OC
First molar occlusal point
OR
Orbitale
UI
M a x i l l a r y incisal edge
ANS
Anterior nasal spine
LI
Mandibular Incisal edge
PNS
Posterior nasal spine
UIA
Maxillary incisal apex
A-A'
Subspinal point
LIA
M~ndibular incisal apex
B-B'
Supramental point
TR
Tragus (Radiopaque marker)
PG-PG' Pogonion
P
Pronasale
GN
UL
Upper lip
Gnathion
ME
Menton
I.L
Lower
GO
Gonion
51"
Stomion
lip
Fig. 1. Hard and soft tissue points (a prime signifies the soft tissue equivalent of a hard tissue landmark) digitized on lateral head film.
RESULTS The angle between the soft tissue N'-Pg' line and the true vertical was calculated on all photographs taken in NHP. Values ranged between - 1 2 ~ and 6 ~ with a mean absolute value of 4.86 ~ (standard angular deviation 0.56~ Positive values mean that the subject's chin was more prominent than the soft tissue nasion. The hard tissue Frankfurt plane, set parallel to the ground in the standard radiographic films, was lastly rotated according to the NHP photographs. Its position showed a wide variation in our subjects: in 32 patients it went downward (orbitale lower than porion), with a
mean angle of - 5 . 7 3 ~ relative to the ground (standard angular deviation 0.64~ Eight patients showed a plane going upward (orbitale higher than porion) with a mean angle of 3.7 ~ (standard angular deviation 0.87~ In four subjects the slope of the Frankfurt plane in NHP was in the • 1~ range. The position of this plane in NHP seemed to be somehow different in the two sexes, with more males having the Frankfurt plane going upward than females. Porion was lower than orbitale in a quarter of the male patients (four subjects), with a mean angle of 3.13 ~ standard angular deviation 1.27 ~ but only in one eighth
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B
,tandard position
I
Fig. 2. A, Right side lateral photograph taken in NHP. Soft tissue nasion N' and soft tissue pogonion PG' are individualized, and the angle c~ between the N'-PG' line and the true vertical (tv) is computed. B, Cephalometric tracing of the same subject. Superimposition of the standard plot (continuous line). with the hard tissue Frankfurt plane (porion-orbitale) set parallel to the ground, and the plot in NHP (broken line). Rotation (angle ,,/ = (~ + 13 - 90 ~ was performed around the Bolton point. 13is the angle between the N' - PG' line and the hard tissue Frankfurt plane.
of the female patients (four subjects), mean angle 4.28 ~ standard angular deviation 1.29 ~ The other subjects, Whose Frankfurt plane went downward (orbitale lower than porion), had mean angles of - 4 . 0 3 ~ (males, standard angular deviation 1.34 ~) and - 6 . 2 9 ~ (females, standard angular deviation 0.59~ The soft tissue Frankfurt plane (tragus-orbitale) in NHP was directed upward (head extended) in 11 of 16 males (mean angle 5.82 ~, standard angular deviation
0.78 ~ and in 10 of 24 females (mean angle 7.04 ~ standard angular deviation 1.89~ The opposite orientation (plane going downward, i.e., head flexed) was found in five males (mean angle - 4 . 1 9 ~, standard angular deviation 0.97~ and 13 females (mean angle - 2 . 3 ~, standard angular deviation 0.49~ In one girl aged 8 years, it was horizontal, and in other six females it ranged in the + 1~ interval (Fig. 3). The two Frankfurt planes were never coincident in
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N. s u b j e c t s 6
Tr-Or plane in NHP
I
-50
I
I
I
i
Flexion
I
Go
0 o
Extenolon
1
i!'!-i.!i,!ii'10 ~
15 ~
Fig. 3. Soft tissue Frankfurt plane (tragus-orbitale) relative to horizontal in NHP. Sample split by sex.
all subjects; the tragus was always lower and more anterior than the porion. On average, the angle tragusorbitale-porion was about 6 ~ The inclination of the sella-nasion plane relative to the ground was evaluated both in the standard cephalometric position (hard tissue Frankfurt plane parallel to the ground) and in NHP. In the standard position, it was oriented upward in all subjects (sella lower than nasion), with mean angles of 7.96 ~ (standard angular deviation 0.83 ~) in males, and 9.86 ~ (standard angular deviation 0.71 ~ in females. In NHP, it became oriented downward (sella higher than nasion) in four males and three females, with mean angles of - 2 . 6 2 ~ (standard angular deviation 0.24 ~ and - 5 . 1 6 ~ (standard angular deviation 3~ respectively. In the other 21 female patients the mean angle was 6.29 ~ (standard angular deviation 1.13~ whereas in the final 12 male patients it was 7.3 ~ (standard angular deviation 1.05~ In 30 of 40 subjects, this plane was closer to the horizontal in NHP than in the "standard" position, but with a wide range of measurements (from 100, sella lower than nasion, to - 3 ~ sella higher than nasion). In the other I0 patients, the S-N plane was closer to the horizontal in the standard position (from 14~ to I~ Eight had a hard tissue Frankfurt plane going upward in NHP: of course, this extended head position increases the slope of the S-N plane.
The possible correlations between the intracranial measurements and the position of the soft tissue Frankfurt plane in NHP were analyzed. No clear relationships with the slope of the sella-nasion plane or with the skeletal class as evaluated by the ANB angle resulted. The Angle dental class (first molar relationship) was perhaps somehow correlated with the head position: 67% of Class 1I subjects had extended head, and 33% had flexed head. This ratio was inverted in Class I subjects: 64% of them had flexed head and only 36% had extended head. DISCUSSION This article described a photographic technique for the recording of NHP that was used to orient standard radiographic head films. Evaluation of the patients as they appear in life (i.e., according to NHP) currently seems of high clinical interest in orthodontics, where both esthetic and functional factors are to be considered. Several authors2"5"8'~6 proposed cephalometric analyses based on NHP determinations, which are probably of higher functional significance than standard intracranial references. Orthodontists should consider head posture, not only in the patient's first evaluation and treatment planning but also during the entire treatment period. Ortho-
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dontic treatment usually takes considerable time. An easy, inexpensive and harmless evaluation of the head posture modifications during this time is of great interest. Of course, the modifications that are due to the treatment are to be distinguished from those that are due to the patient's physiologic growth. 2,4 A growth study might be performed with a photographic technique like the one presently described, and could supplement the data banks already provided by many growth centers. In 1980, Fr/inkel 2' proposed a radiographic plus photographic technique for the recording of NHP that is similar to ours. Surprisingly, he did not consider NHP modifications during treatment, but proposed to r e f e r all longitudinal radiograms to the first NHP recording, thus missing important information. A careful determination of NHP seems mandatory whenever esthetic evaluations are to be performed. For example, in all classic cephalometric analyses, the positions of the upper and lower incisor teeth are determined with reference to intracranial planes. In the aforementioned new analyses, the position of these teeth is calculated with reference to the true horizontal 2 or to the esthetic horizontal, 5 and this position is dependent on modification of the NHP. Also the relation of the mandible to the maxilla, assessed by the horizontal A-B distance '6 depends on NHP when the true horizontal is taken as reference. All the authors proposing the NHP and the true horizontal (or vertical) as references ',35'8''6 reported a standardized technique for patient positioning in the cephalometer. Except for the technique of Michiels and Tourne, '6 all these techniques radiograph the patient in the orthoposition, with different systems for head adjustment. Michiels and Toume '6 proposed a sitting position, using a mirror as visual target. Their results can not be compared with those obtained in orthoposition because head posture has recently been shown to be significantly different in the standing versus sitting posture, with mean differences of 5 ~ in women and 8 ~ in men.'5 The technique of Bass 5 seems to be of questionable use if the orthodontic visit and the radiographic exposure are not performed the same day, because the patients are asked not to wash their cheeks until the head film has been taken. Showfety et al. 8"22 used disiance as visual target, but this can be a problem with children who are easily distracted or in many environmental situations. The protocols of Siersbaek-Nielsen and Solow I and Cooke and Wei 3 are perhaps easy to perform even in private
American Journal of Orthodontics and Dentofacial Orthopedics September 1994
practice, but it might be difficult to have a flexible radiographic equipment where the NHP can be correctly reproduced. Moreover, the same authors pointed out that, although the use of earrods can modify the selfbalance position of the head, films taken without the earrods are of poor quality. As a matter of fact, a poor quality radiographic film could destroy all the efforts performed in the recording of NHP, requiting a new exposure or a dubious interpretation. On the contrary, a standard radiographic picture could be integrated by a photographic analysis '5 recording the NHP. The combination of the two techniques (which has already been proposed by Bjerin in 1957') could be a useful tool in diagnostics. Chiu and Clark 14 showed that the use of a mirror does not influence NHP in Young adults, since the two positions are not significantly different. On the contrary, other authors have reported the adult head being 3 ~ higher in the mirror position than in the self balance position. '.~ When a group of children is considered, the differences are gender specific: boys extend the head more when looking at a mirror (about 2~ whereas girls do not modify head posture? In our study, we used a mirror as a visual target because of the young age of most of our patients. In our clinical practice, we have verified that children are easily distracted from their posture if a visual feedback is not given. Of course we took great care in assessing that NHP was not modified by the mirror, accurately adjusting its position to the patient's height. All the patients who entered this study were easily photographed in their NHP. No photographs had to be retaken. A prerogative of this technique is that the standard cephalometric analyses can still be used, together with their normal references and clinical guidelines. Radiographic exposures are directly comparable to classical cephalograms. 6 When the photographic NHP was transferred to the head film, the cephalometric landmarks were rotated around the Bolton point; as a matter of fact, this may not be the fulcrum for wide head flexion/extension movements, where the contribution of the cervical column can not be forgotten. Nevertheless, NHP seldom implies such wide movement; and this approximation may be satisfactory for all practical purposes. The soft and hard tissue Frankfurt planes were not coincident in our sample, as already reported in adult subjects, where vertical differences between potion and tragus ranged between 7 and 16 mm. 23 Forty-five percent of our patients had their head flexed when photographed in NHP, as indicated by the
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American Journal of Orthodontics and Dentofacial Orthopedics Volume 106, No. 3
soft tissue Frankfurt plane (tragus-orbitale) going downward (Fig. 3). This result partly contrasts with our previous findings in a group of healthy young adults. ~s In these subjects, the angle o f the tragus-orbitale plane measured in the standing position appeared to be directed a few degrees more upward, relative to the horizontal, both in the 57 men (mean 13.07 ~, standard angular deviation 0.70 ~) and in the 51 women (mean 13.24 ~ standard angular deviation 0.93~ corresponding to an extended position of the head. Only one woman in this group had a flexed head, with an angle of - I ~ We used the same photographic technique and the same measurement protocol in both investigations, but there are some differences between these two groups of subJects. The present group is composed of private orthodontic patients, whose ages rang e between 7 and 20 years. The first data were collected on healthy adults aged 20 to 27 years (mean 22 years), who were free from craniomandibular disorders and had sound denthi0ns. Presently, it is not possible to precisely evaluate how and to what extent malocclusion could influence head posture, but correlations had already been found. 6"t~ Head posture might also be influenced by age: In the first report, the subjects were selected in a narrow age range, and the inclination of the soft tissue Frankfurt plane had a limited variation in the sample (low standard angular deviation). The head position of the older subjects (20 years or more) was more extended than the younger ones. In young orthodontic patients, NHP is therefore highly variable, gender dependent, and not correlated with skeletal or dental characteristics. Conversely, these correlations are well established for groups of normal adults. 9,t0 All these findings show that the evaluation o f head position should be performed for each young patient, and can not be deduced from mean population values. Moreover, this evaluation should be performed not only before but also during the treatment, to verify how the combined effects o f therapy and growth act. CONCLUSIONS
The presented photographic/radiographic technique seems to be easier and faster than most o f the published techniques, both regarding the photographic protocol and the superimposition of the data obtained with the two procedures, which can be performed manually or with a computer. When the superimposition is performed by a computer, the procedure takes only a few seconds. This technique therefore could be a
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helpful tool both in clinical practice and research studies. The present sample is small, and was used to verify whether the radiographic and photographic technique could be useful in the evaluation of unselected patients. Correlations for NHP, intracranial planes, and dental variables should be analyzed in a wider group. Data are presently being collected on all patients in the private office where the study began. Furthermore, a protocol for the evaluation of radiographic frontal cephalograms in NHP is currently under investigation. REFERENCES
1. Siersbaek-Nielsen S, Solow B. Intra- and interexaminer variability in head posture recorded by dental auxiliaries. Ar~iJ OR"moo 1982;82:50-7. 2. Cooke MS, Wet SHY. A summary five-factor cephalometric analysis based on natural head posture and the true horizontal. AM J ORTHODDENTOFACORTHOP1988;93:213-23. 3. Cooke MS, Wet SHY. The reproducibility of natural head posture: a methodological study. AM J ORTHODDENTOFACORTHOP 1988;93:280-8. 4. Cooke MS. Five-year reproducibility of natural head posture: a longitudinal
study.
AM J
ORTHOD DENTOFAC ORTHOP
1990;97:489-94. 5. Bass NM. The aesthetic analysis of the face. Eur J Onhod 1991;13:343-50. 6. Lundstrrm A, Forsberg C-M, Westergren H, Lundstrrm F. A comparison between estimated and registered natural head posture. Eur J Orthod 1991;13:59-64. 7. SolowB, Tallgren A. Natural head position in standing subjects. Acta Odont Scand 1971;29:591-607. 8. Showfety KJ, Vig PS, Matteson S, Phillips C. Associations between the postural orientation of sella-nasion and skeletodental morphology. Angle Orthod 1987;57:99-112. 9. SolowB, Tallgren A. Head posture and craniofacial morphology. Am J Phys Anthrop 1976;44:417-36. I0. Solow B, Tallgren A. Dentoalveolar morphologyin relation to craniocervical posture. Angle Orthod 1977;47:157-64. 11. Mareotte MR. Head posture and dentofacial proportions. Angle Ortbod 1981;51:208-13. 12. Behlfelt K, Linder-AronsonS, Neander P. Posture of ihe head, the hyoid bone, and the tongue in children with and without enlarged tonsils. Eur J Orthod 1990;12:458-67. 13. Shiau Y-Y, Chat H-M. Body posture and hand strength of patients with temporomandibular disorder. J Craniomandib Pract 1990;8:244-5I. 14. ChiuCSW' Clark RKF"Repr~ ~ natural head positi~ J Dent 1991;19:130-1. 15. Ferrario VF, Sforza C, Miani A Jr, Tartaglia G. Craniofacial morphometry by photographic evaluations. AM J ORTItODDENrorAc ORTHOP1993;103:327-37. 16. Michiels LYF, Tourue LPM. Nasion true vertical: a proposed method for testing the clinical validity of cephalometric measurements applied to a new cephalometric reference line. Int J Adult Orthod Orthognath Surg 1990;5:43-52. 17. Ferrario VF, Sforza C, Miani A Jr, D'Addona A, Todisco M. Cephalometrics and facial shape: new thresholds by an overall
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18.
19. 20. 21. 22.
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American Journal of Orthodontics and Dentofacial Orthopedics September 1994
approach to classic standards. Int J Adult Orthod Orthognath Surg 1991;6:261-9. Claman L, Patton D, Rashid R. Standardized portrait photography for dental patients. AM J ORTHOD DENTOFAC ORTIIOP 1990;98:197-205. Salzmann JA. Ortodonzia pratica. Padova: Piccin, 1975:478515. Batschelet E. Circular statistics in biology. London: Academic Press, 1981. Fr~nkel R. The applicability of the occipital reference base in cephalometrics. AM J ORTHOD 1980;77:379-95. Showfety KJ, Vig PS, Matteson S. A simple method for taking
natural-head-position cephalograms. AM J ORTHOD1983;83:495500. 23. Pitchford JH. A reevaluation of the axis-orbital plane and the use of orbitale in a facebow transfer record. J Prosthet Dent 1991;66:349-55. Reprint requests to:
Prof. Virgilio F. Ferrario Istituto di Anatomia Umana Normale via Mangiagalli 31 1-20133 Milano Italy
AAO MEETING CALENDAR
1995mSan Francisco, Calif., May 13 to 18, Moscone Convention Center
(International Orthodontic Congress) 1996--Denver, Colo., May 11 to 15, Colorado Convention Center 1997--Philadelphia, Pa., May 3 to 7, Philadelphia Convention Center 1998--Dallas, Texas, May 16 to 20, Dallas Convention Center 1999--San Diego, Calif., May 15 to 19, San Diego Convention Center 2000mChicago, II1., April 29 to May 3, McCormick Place Convention Center
Milan, Italy A photographic technique for natural head postu?e (NHP) recording that can be associated to standard radiographic exposures is presented. It allows the evaluation of radiographs, according to both the standard intracranial references and NHP. On the NHP photograph, the angle between the soft tissue nasion-pogonion line and the true vertical is calculated, and this value is used to rotate the standard radiograph around the Bolton point. The technique has been applied to 40 private orthodontic patients (24 females and 16 males, aged 7 to 20 years, mean 12.9 years). The hard tissue Frankfurt plane in NHP showed a wide variation: 80% of the patients had orbitale lower than potion (mean angle - 6 ~ relative to the ground), 20% had orbitale higher than porion (mean angle 4~ The position of this plane in NHP seemed to be different in the two sexes, with more males having the Frankfurt plane going upwards than females. The soft tissue Frankfurt plane (tragus-orbitale) in NHP was directed upwards (head extended) in 53% of patients.The two Frankfurt planes were never coincident in all subjects; the tragus was always lower and more anterior than porion. On average, the angle tragus-orbitale-porion was about 6~ In young orthodontic patients NHP is therefore highly variable, gender dependent, and cannot be deduced from mean population values. Nevertheless, the evaluation of head position should be performed in each young patient before and during the treatment, to verify how the combined effects of therapy and growth act. (AMJ ORTHODDENTOFACORTHOP 1994;106:257-66.)
M a n y recent studies have advocated the use of natural head posture (NHP) in cephalometric analyses."6 The NHP in orthodontics has been introduced since the 1950s, 6-s and the relationships between head posture, craniocervical curvature, craniofacial structure, dental positions, and airway patency have been examined in detail both in normal subjects and in patients) "t3 The NHP has been found to be highly reproducible, regardless of patients' age, gender or race, of the time lag between repeated recordings, of the radiographic or photographic technique, of the experience, or cultural background of the operator.t'7"14ts The high interpersonal variability of intracranial reference planes (such as the Frankfurt plane or the sellanasion line), 2'7s'16 the lack of functional information about head posture on conventional head films, ~ and the need for a holistic approach taking the overall ap-
From the Laboratorio di Anatomia Funzionale dell'Apparato Stomatognatico, Istituto di Anatomia Umana Norrna/e, Facolth di Medicina e Chirurgia, Universit~ degli Studi di Milano, Italy. 'Professor of Human Anatomy. bAssociate Professor of Human Anatomy. r private practice, Arcore, Milano, Italy. dpostgmduate student. Copyright 9 1994 by the American Association of Orthodontists. 0889-5406194153.00 + 0 811143639
pearance of the patient into consideration, 5"17all gave birth to new cephalometric analyses where classical intracranial references and standard positions were replaced by new lines and head postures. The proposals of Cooke and Wei z and Michiels and Toume '6 are based on the true horizontal (or vertical), and observe the head in the NHP. Bass s uses the esthetic horizontal line, which is determined by the orthodontist on visual inspection of the subject and which probably does not differ significantly from the NHP on average: These new analyses present the patients as they appeal- in life, and the modifications of facial appearance depending on head position are in some way taken into account. As stated by Claman et al.,'8 the subject's av;,areness of deviations in facial structure could modify the craniocervical posture in an attempt to mask, e.g., a too protrusive or retrusive chin. 6"t~ Moreover, Cooke and Wei 2 showed that some patients can be misjudged using standard references instead of NHP. Several standardized techniques have been proposed to obtain NHP radiographs. 1,3-5.7-8,16All these techniques require modified cepbalostats allowing for nonstandard head positioning, and could be of difficuR practical applicability. In this study, we have developed a photographic technique t'or NHP recording, which can be associated 257
258 Ferrario et aL
with standard radiographic exposures. We also present a computerized method that allows an easy and fast superimposition of the two recordings and makes radiographs of value, according to both the standard intracranial references and NHP.
MATERIALS AND METHODS Sample T h e proposed technique has been applied to 40 orthodontic patients (24 females and 16 males, aged 7 to 20 years, mean 12.9 years). The patients were randomly selected in an 8-week interval from all pretreatment subjects admitted. All the patients were northern Italian (white).
Photographic technique
American Journal of Orthodontics and Dentofacial Orthopedics September 1994
Cephalometric measurements A set of standardized landmarks was then traced on all the head films "'~9 by the same operator as shown in Fig. 1. Bilateral structures were tracted by bisecting right and left images. The points were digitized by means of a semiautomatic image analyzer (MM 1201, Summagraphics Co., Fairfield, Conn.) interfaced with an AT computer (Olivetti M380, Olivetti, Ivrea, Italy). By using a program developed for this purpose, the x-y coordinates of these points were used to generate several cephalometric measurements (angles and distances). The computer provided a list of the measurements and a plot of the digitized landmarks. Also the position of the soft tissue Frankfurt plane was determined. All the films were oriented with the hard tissue Frankfurt plane (porion-orbitale) parallel to the ground, as suggested in standard measurements.
A modification of the technique described by Ferrario et al. ~5was used, and one right-side picture was taken for each subject in centric occlusion. Subjects stood 150 cm from the camera, which was mounted on a tripod and levelled, with the optical axis of the lens horizontal and the film plane vertical. The subjects stood with bare feet, looking straight into a 20 • 100 cm mirror mounted at eye level on the wall so that they could see the reflected image of their eyes. The mirror-subject distance was 120 cm. They were then asked to assume and maintain a "natural and normal" erect posture of head and shoulders with both arms hanging free beside the trunk. This should correspond to Broca's "natural head position."!8 On each photograph, a reference line placed perpendicular to the ground by using a small spirit level (true vertical) was drawn. This allowed careful control of position of the subject relative to reference planes. All the photographs were taken with the same camera: Yashica Dental-Eye II, lens 100 mm f/4, using Ilford HPS 135 (ISO 400/27 ~ films. They were developed and printed by the same operator, employing consistent and rigorously controlled procedures. The subjects were photographed in the same room where the radiographic exposure was subsequently taken. The procedure required about 5 minutes for each subject.
On all photographs, the soft tissue Nasion (N') and the soft tissue pogonion (Pg') were traced, and the angle between the N'-Pg' line and the true vertical was calculated (Fig. 2, A). The same angle was calculated on the cephalometric films, and the difference between the two measurements was used to compute the position of the soft and hard tissue Frankfurt planes, and of the sella-nasion plane in NHP. These new values were compared with the values previously observed in the standard cephalometric orientation. Lastly, to better appreciate the position of skeletal and soft tissue landmarks in NHP, the value of angle N'-Pg' line/true vertical for each patient was fed to a computer program which provided a rotation of all the landmarks until the cephalometric N'-Pg' line coincided with the photographic one. Rotation was performed around the Bolton point (highest and most posterior point of the occipital condyles~9). This point roughly represents the fulcrum for small head flexion/extension movements, which can be assumed to be performed in the atlantooccipital joints. The new position of points relative to the coordinate axes was then plotted (Fig. 2, B).
Radiographic technique
Statistical calculations
All head films (lateral cephalometric radiographs) were taken with the Orthoceph 10E (Siemens AG, Germany), whose vertical adjustability allows the recording of standing subjects. The x-ray source has a focus of 0.6 mm. The focusmedian plane distance was 180 cm, and the film-median plane distance was 10 cm, with a final enlargement of 10%. For all subjects 18 • 24 cm films were used. A 5-ram radiopaque disk was fixed on the right tragus of each subject to allow the identificationof the soft tissue Frankfurt plane (tragus-orbitale) on the radiograms. The patient was radiographed in standing posture, the head being approximately oriented with the soft tissue Frankfurt plane parallel to the ground, with earrods in ccntric occlusion. This position does not necessarily correspond to the NHP, and was not chosen by the patient, but it is the standard position imposed by the machine.
Mean angle values were calculated with dedicated statistics for circular variables as detailed by Batschelet :~ by using the rectangular components of each angle. The same method allowed the calculation of relevant dispersion measures (the standard angular deviations). Standard angular deviation has the same meaning as standard deviation in linear statistics.
Photographic. measurements
Error of method To assess reproducibilityof NPH, five subjects (two males and three females) were photographed once, then a second time after a 2-week interval. Moreover, 10 randomly selected radiograms and corresponding NPH photographs were retraced and redigitized by a second investigator. The combined error of subject's positions, landmark location and digitization was estimated to be about 3% of the mean values of the measurements.
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P
N-N'
Nasion
f/
AR
Articulate
S
Sella
/
BO
Bolton
PO
Potion
OC
First molar occlusal point
OR
Orbitale
UI
M a x i l l a r y incisal edge
ANS
Anterior nasal spine
LI
Mandibular Incisal edge
PNS
Posterior nasal spine
UIA
Maxillary incisal apex
A-A'
Subspinal point
LIA
M~ndibular incisal apex
B-B'
Supramental point
TR
Tragus (Radiopaque marker)
PG-PG' Pogonion
P
Pronasale
GN
UL
Upper lip
Gnathion
ME
Menton
I.L
Lower
GO
Gonion
51"
Stomion
lip
Fig. 1. Hard and soft tissue points (a prime signifies the soft tissue equivalent of a hard tissue landmark) digitized on lateral head film.
RESULTS The angle between the soft tissue N'-Pg' line and the true vertical was calculated on all photographs taken in NHP. Values ranged between - 1 2 ~ and 6 ~ with a mean absolute value of 4.86 ~ (standard angular deviation 0.56~ Positive values mean that the subject's chin was more prominent than the soft tissue nasion. The hard tissue Frankfurt plane, set parallel to the ground in the standard radiographic films, was lastly rotated according to the NHP photographs. Its position showed a wide variation in our subjects: in 32 patients it went downward (orbitale lower than porion), with a
mean angle of - 5 . 7 3 ~ relative to the ground (standard angular deviation 0.64~ Eight patients showed a plane going upward (orbitale higher than porion) with a mean angle of 3.7 ~ (standard angular deviation 0.87~ In four subjects the slope of the Frankfurt plane in NHP was in the • 1~ range. The position of this plane in NHP seemed to be somehow different in the two sexes, with more males having the Frankfurt plane going upward than females. Porion was lower than orbitale in a quarter of the male patients (four subjects), with a mean angle of 3.13 ~ standard angular deviation 1.27 ~ but only in one eighth
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American Journal of Orthodontics and Dentofacial Orthopedics September 1994
B
,tandard position
I
Fig. 2. A, Right side lateral photograph taken in NHP. Soft tissue nasion N' and soft tissue pogonion PG' are individualized, and the angle c~ between the N'-PG' line and the true vertical (tv) is computed. B, Cephalometric tracing of the same subject. Superimposition of the standard plot (continuous line). with the hard tissue Frankfurt plane (porion-orbitale) set parallel to the ground, and the plot in NHP (broken line). Rotation (angle ,,/ = (~ + 13 - 90 ~ was performed around the Bolton point. 13is the angle between the N' - PG' line and the hard tissue Frankfurt plane.
of the female patients (four subjects), mean angle 4.28 ~ standard angular deviation 1.29 ~ The other subjects, Whose Frankfurt plane went downward (orbitale lower than porion), had mean angles of - 4 . 0 3 ~ (males, standard angular deviation 1.34 ~) and - 6 . 2 9 ~ (females, standard angular deviation 0.59~ The soft tissue Frankfurt plane (tragus-orbitale) in NHP was directed upward (head extended) in 11 of 16 males (mean angle 5.82 ~, standard angular deviation
0.78 ~ and in 10 of 24 females (mean angle 7.04 ~ standard angular deviation 1.89~ The opposite orientation (plane going downward, i.e., head flexed) was found in five males (mean angle - 4 . 1 9 ~, standard angular deviation 0.97~ and 13 females (mean angle - 2 . 3 ~, standard angular deviation 0.49~ In one girl aged 8 years, it was horizontal, and in other six females it ranged in the + 1~ interval (Fig. 3). The two Frankfurt planes were never coincident in
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N. s u b j e c t s 6
Tr-Or plane in NHP
I
-50
I
I
I
i
Flexion
I
Go
0 o
Extenolon
1
i!'!-i.!i,!ii'10 ~
15 ~
Fig. 3. Soft tissue Frankfurt plane (tragus-orbitale) relative to horizontal in NHP. Sample split by sex.
all subjects; the tragus was always lower and more anterior than the porion. On average, the angle tragusorbitale-porion was about 6 ~ The inclination of the sella-nasion plane relative to the ground was evaluated both in the standard cephalometric position (hard tissue Frankfurt plane parallel to the ground) and in NHP. In the standard position, it was oriented upward in all subjects (sella lower than nasion), with mean angles of 7.96 ~ (standard angular deviation 0.83 ~) in males, and 9.86 ~ (standard angular deviation 0.71 ~ in females. In NHP, it became oriented downward (sella higher than nasion) in four males and three females, with mean angles of - 2 . 6 2 ~ (standard angular deviation 0.24 ~ and - 5 . 1 6 ~ (standard angular deviation 3~ respectively. In the other 21 female patients the mean angle was 6.29 ~ (standard angular deviation 1.13~ whereas in the final 12 male patients it was 7.3 ~ (standard angular deviation 1.05~ In 30 of 40 subjects, this plane was closer to the horizontal in NHP than in the "standard" position, but with a wide range of measurements (from 100, sella lower than nasion, to - 3 ~ sella higher than nasion). In the other I0 patients, the S-N plane was closer to the horizontal in the standard position (from 14~ to I~ Eight had a hard tissue Frankfurt plane going upward in NHP: of course, this extended head position increases the slope of the S-N plane.
The possible correlations between the intracranial measurements and the position of the soft tissue Frankfurt plane in NHP were analyzed. No clear relationships with the slope of the sella-nasion plane or with the skeletal class as evaluated by the ANB angle resulted. The Angle dental class (first molar relationship) was perhaps somehow correlated with the head position: 67% of Class 1I subjects had extended head, and 33% had flexed head. This ratio was inverted in Class I subjects: 64% of them had flexed head and only 36% had extended head. DISCUSSION This article described a photographic technique for the recording of NHP that was used to orient standard radiographic head films. Evaluation of the patients as they appear in life (i.e., according to NHP) currently seems of high clinical interest in orthodontics, where both esthetic and functional factors are to be considered. Several authors2"5"8'~6 proposed cephalometric analyses based on NHP determinations, which are probably of higher functional significance than standard intracranial references. Orthodontists should consider head posture, not only in the patient's first evaluation and treatment planning but also during the entire treatment period. Ortho-
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Ferrarioel al.
dontic treatment usually takes considerable time. An easy, inexpensive and harmless evaluation of the head posture modifications during this time is of great interest. Of course, the modifications that are due to the treatment are to be distinguished from those that are due to the patient's physiologic growth. 2,4 A growth study might be performed with a photographic technique like the one presently described, and could supplement the data banks already provided by many growth centers. In 1980, Fr/inkel 2' proposed a radiographic plus photographic technique for the recording of NHP that is similar to ours. Surprisingly, he did not consider NHP modifications during treatment, but proposed to r e f e r all longitudinal radiograms to the first NHP recording, thus missing important information. A careful determination of NHP seems mandatory whenever esthetic evaluations are to be performed. For example, in all classic cephalometric analyses, the positions of the upper and lower incisor teeth are determined with reference to intracranial planes. In the aforementioned new analyses, the position of these teeth is calculated with reference to the true horizontal 2 or to the esthetic horizontal, 5 and this position is dependent on modification of the NHP. Also the relation of the mandible to the maxilla, assessed by the horizontal A-B distance '6 depends on NHP when the true horizontal is taken as reference. All the authors proposing the NHP and the true horizontal (or vertical) as references ',35'8''6 reported a standardized technique for patient positioning in the cephalometer. Except for the technique of Michiels and Tourne, '6 all these techniques radiograph the patient in the orthoposition, with different systems for head adjustment. Michiels and Toume '6 proposed a sitting position, using a mirror as visual target. Their results can not be compared with those obtained in orthoposition because head posture has recently been shown to be significantly different in the standing versus sitting posture, with mean differences of 5 ~ in women and 8 ~ in men.'5 The technique of Bass 5 seems to be of questionable use if the orthodontic visit and the radiographic exposure are not performed the same day, because the patients are asked not to wash their cheeks until the head film has been taken. Showfety et al. 8"22 used disiance as visual target, but this can be a problem with children who are easily distracted or in many environmental situations. The protocols of Siersbaek-Nielsen and Solow I and Cooke and Wei 3 are perhaps easy to perform even in private
American Journal of Orthodontics and Dentofacial Orthopedics September 1994
practice, but it might be difficult to have a flexible radiographic equipment where the NHP can be correctly reproduced. Moreover, the same authors pointed out that, although the use of earrods can modify the selfbalance position of the head, films taken without the earrods are of poor quality. As a matter of fact, a poor quality radiographic film could destroy all the efforts performed in the recording of NHP, requiting a new exposure or a dubious interpretation. On the contrary, a standard radiographic picture could be integrated by a photographic analysis '5 recording the NHP. The combination of the two techniques (which has already been proposed by Bjerin in 1957') could be a useful tool in diagnostics. Chiu and Clark 14 showed that the use of a mirror does not influence NHP in Young adults, since the two positions are not significantly different. On the contrary, other authors have reported the adult head being 3 ~ higher in the mirror position than in the self balance position. '.~ When a group of children is considered, the differences are gender specific: boys extend the head more when looking at a mirror (about 2~ whereas girls do not modify head posture? In our study, we used a mirror as a visual target because of the young age of most of our patients. In our clinical practice, we have verified that children are easily distracted from their posture if a visual feedback is not given. Of course we took great care in assessing that NHP was not modified by the mirror, accurately adjusting its position to the patient's height. All the patients who entered this study were easily photographed in their NHP. No photographs had to be retaken. A prerogative of this technique is that the standard cephalometric analyses can still be used, together with their normal references and clinical guidelines. Radiographic exposures are directly comparable to classical cephalograms. 6 When the photographic NHP was transferred to the head film, the cephalometric landmarks were rotated around the Bolton point; as a matter of fact, this may not be the fulcrum for wide head flexion/extension movements, where the contribution of the cervical column can not be forgotten. Nevertheless, NHP seldom implies such wide movement; and this approximation may be satisfactory for all practical purposes. The soft and hard tissue Frankfurt planes were not coincident in our sample, as already reported in adult subjects, where vertical differences between potion and tragus ranged between 7 and 16 mm. 23 Forty-five percent of our patients had their head flexed when photographed in NHP, as indicated by the
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American Journal of Orthodontics and Dentofacial Orthopedics Volume 106, No. 3
soft tissue Frankfurt plane (tragus-orbitale) going downward (Fig. 3). This result partly contrasts with our previous findings in a group of healthy young adults. ~s In these subjects, the angle o f the tragus-orbitale plane measured in the standing position appeared to be directed a few degrees more upward, relative to the horizontal, both in the 57 men (mean 13.07 ~, standard angular deviation 0.70 ~) and in the 51 women (mean 13.24 ~ standard angular deviation 0.93~ corresponding to an extended position of the head. Only one woman in this group had a flexed head, with an angle of - I ~ We used the same photographic technique and the same measurement protocol in both investigations, but there are some differences between these two groups of subJects. The present group is composed of private orthodontic patients, whose ages rang e between 7 and 20 years. The first data were collected on healthy adults aged 20 to 27 years (mean 22 years), who were free from craniomandibular disorders and had sound denthi0ns. Presently, it is not possible to precisely evaluate how and to what extent malocclusion could influence head posture, but correlations had already been found. 6"t~ Head posture might also be influenced by age: In the first report, the subjects were selected in a narrow age range, and the inclination of the soft tissue Frankfurt plane had a limited variation in the sample (low standard angular deviation). The head position of the older subjects (20 years or more) was more extended than the younger ones. In young orthodontic patients, NHP is therefore highly variable, gender dependent, and not correlated with skeletal or dental characteristics. Conversely, these correlations are well established for groups of normal adults. 9,t0 All these findings show that the evaluation o f head position should be performed for each young patient, and can not be deduced from mean population values. Moreover, this evaluation should be performed not only before but also during the treatment, to verify how the combined effects o f therapy and growth act. CONCLUSIONS
The presented photographic/radiographic technique seems to be easier and faster than most o f the published techniques, both regarding the photographic protocol and the superimposition of the data obtained with the two procedures, which can be performed manually or with a computer. When the superimposition is performed by a computer, the procedure takes only a few seconds. This technique therefore could be a
263
helpful tool both in clinical practice and research studies. The present sample is small, and was used to verify whether the radiographic and photographic technique could be useful in the evaluation of unselected patients. Correlations for NHP, intracranial planes, and dental variables should be analyzed in a wider group. Data are presently being collected on all patients in the private office where the study began. Furthermore, a protocol for the evaluation of radiographic frontal cephalograms in NHP is currently under investigation. REFERENCES
1. Siersbaek-Nielsen S, Solow B. Intra- and interexaminer variability in head posture recorded by dental auxiliaries. Ar~iJ OR"moo 1982;82:50-7. 2. Cooke MS, Wet SHY. A summary five-factor cephalometric analysis based on natural head posture and the true horizontal. AM J ORTHODDENTOFACORTHOP1988;93:213-23. 3. Cooke MS, Wet SHY. The reproducibility of natural head posture: a methodological study. AM J ORTHODDENTOFACORTHOP 1988;93:280-8. 4. Cooke MS. Five-year reproducibility of natural head posture: a longitudinal
study.
AM J
ORTHOD DENTOFAC ORTHOP
1990;97:489-94. 5. Bass NM. The aesthetic analysis of the face. Eur J Onhod 1991;13:343-50. 6. Lundstrrm A, Forsberg C-M, Westergren H, Lundstrrm F. A comparison between estimated and registered natural head posture. Eur J Orthod 1991;13:59-64. 7. SolowB, Tallgren A. Natural head position in standing subjects. Acta Odont Scand 1971;29:591-607. 8. Showfety KJ, Vig PS, Matteson S, Phillips C. Associations between the postural orientation of sella-nasion and skeletodental morphology. Angle Orthod 1987;57:99-112. 9. SolowB, Tallgren A. Head posture and craniofacial morphology. Am J Phys Anthrop 1976;44:417-36. I0. Solow B, Tallgren A. Dentoalveolar morphologyin relation to craniocervical posture. Angle Orthod 1977;47:157-64. 11. Mareotte MR. Head posture and dentofacial proportions. Angle Ortbod 1981;51:208-13. 12. Behlfelt K, Linder-AronsonS, Neander P. Posture of ihe head, the hyoid bone, and the tongue in children with and without enlarged tonsils. Eur J Orthod 1990;12:458-67. 13. Shiau Y-Y, Chat H-M. Body posture and hand strength of patients with temporomandibular disorder. J Craniomandib Pract 1990;8:244-5I. 14. ChiuCSW' Clark RKF"Repr~ ~ natural head positi~ J Dent 1991;19:130-1. 15. Ferrario VF, Sforza C, Miani A Jr, Tartaglia G. Craniofacial morphometry by photographic evaluations. AM J ORTItODDENrorAc ORTHOP1993;103:327-37. 16. Michiels LYF, Tourue LPM. Nasion true vertical: a proposed method for testing the clinical validity of cephalometric measurements applied to a new cephalometric reference line. Int J Adult Orthod Orthognath Surg 1990;5:43-52. 17. Ferrario VF, Sforza C, Miani A Jr, D'Addona A, Todisco M. Cephalometrics and facial shape: new thresholds by an overall
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American Journal of Orthodontics and Dentofacial Orthopedics September 1994
approach to classic standards. Int J Adult Orthod Orthognath Surg 1991;6:261-9. Claman L, Patton D, Rashid R. Standardized portrait photography for dental patients. AM J ORTHOD DENTOFAC ORTIIOP 1990;98:197-205. Salzmann JA. Ortodonzia pratica. Padova: Piccin, 1975:478515. Batschelet E. Circular statistics in biology. London: Academic Press, 1981. Fr~nkel R. The applicability of the occipital reference base in cephalometrics. AM J ORTHOD 1980;77:379-95. Showfety KJ, Vig PS, Matteson S. A simple method for taking
natural-head-position cephalograms. AM J ORTHOD1983;83:495500. 23. Pitchford JH. A reevaluation of the axis-orbital plane and the use of orbitale in a facebow transfer record. J Prosthet Dent 1991;66:349-55. Reprint requests to:
Prof. Virgilio F. Ferrario Istituto di Anatomia Umana Normale via Mangiagalli 31 1-20133 Milano Italy
AAO MEETING CALENDAR
1995mSan Francisco, Calif., May 13 to 18, Moscone Convention Center
(International Orthodontic Congress) 1996--Denver, Colo., May 11 to 15, Colorado Convention Center 1997--Philadelphia, Pa., May 3 to 7, Philadelphia Convention Center 1998--Dallas, Texas, May 16 to 20, Dallas Convention Center 1999--San Diego, Calif., May 15 to 19, San Diego Convention Center 2000mChicago, II1., April 29 to May 3, McCormick Place Convention Center