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Dent. Med. Probl. 2003, 40, 1, 129–134
ISSN 1644−387X
CARLOS SILVA, A. PINHÃO FERREIRA
Frankfort Plane vs. Natural Head Posture
in Cephalometric Diagnosis
Płaszczyzna frankfurcka w odniesieniu do naturalnego położenia głowy
w diagnostyce cefalometrycznej
1
2
Orthodontic Department at Fernando Pessoa University, Porto, Portugal
Orthodontic Department at State University, Porto, Portugal
Abstract
The main objective of the present article, is to draw the attention to the advantages of a normalized position of the pa−
tient’s head in the cephalostat, determined by factors of physiological nature – natural head position (NHP). The adop−
tion of this position, has been insistently suggested in literature not only to minimize the distortions caused by erroneo−
us positions of the patient’s head in the cephalostat, but also to overtake, as well, the inherent distortion concerning the
use of Frankfort Horizontal as an orientation plane, in cephalometric analysis (Dent. Med. Probl. 2003, 40, 1, 129–134).
Key words: Frankfort plane, cephalostat, natural head posture, NHP.
Streszczenie
Głównym celem pracy jest zwrócenie uwagi na zalety znormalizowanego ułożenia głowy pacjenta w cefalostacie.
Ułożenie to jest zdeterminowane przez czynniki natury fizjologicznej – naturalne położenie głowy (natural head
position – NHP). Uzyskanie tej pozycji, szeroko rekomendowane w piśmiennictwie, nie tylko minimalizuje znie−
kształcenia wywołane przez niewłaściwe ułożenie głowy pacjenta w cefalostacie. Umożliwia również zmniejsze−
nie zniekształcenia związanego nierozerwalnie z zastosowaniem płaszczyzny frankfurckiej w analizie cefalome−
trycznej (Dent. Med. Probl. 2003, 40, 1, 129–134).
Słowa kluczowe: płaszczyzna frankfurcka, naturalne położenie głowy, NHP.
The Orthodontists have to stop abusing cepha−
lometrics and start to use the cephalostat as
a true scientific instrument [1]
Coben (1979)
In 1931, Broadbent in USA and Hofrath in
Germany, launched simultaneously but indepen−
dently, the bases of modern technique for cephalo−
metric analysis in lateral craniofacial head film, by
describing the cephalostat. This device, consisting
in a dispositive linked to the X−ray apparatus, per−
mitted the immobilization of the head by means of
two auricular supports and aimed for a patronised
reproduction of head positioning in X−ray films.
Hofrath used the lateral incidence with the pa−
tient in a 2 meters distance, and Broadbent used
the lateral and postero−anterior incidence, with the
patient in a 5 feet distance.
The device described by Broadbent (Broad−
bent−Bolton cephalostat), very rapidly won univer−
sal recognition, keeping its basic conception until
our days.
Since then, cephalometric methods has been
enriched with craniometrical landmarks previous−
ly invisible to the anthropologists. In 1957, Krog−
man and Sassouni, making an exhaustive study of
the literature, quantified 44 parameters of evalua−
tion and cephalometric methods.
The world of the Orthodontists was now full
of orientation and reference planes, lineal and an−
gular measurements, as well as craniometrical
landmarks. All of this, representing an effort to fa−
cilitate communication, diagnosis, clinical sup−
port, investigation and/or teaching.
130
C. SILVA, A. P. FERREIRA
The Errors Caused by Bad
Positioning of the Head
The position of the patient’s head in the cepha−
lostat, assumes particular relevance because it can
lead to diagnostic errors, specially concerning the
sagittal individual localization of the jaws, as well
as its reciprocal relation, as demonstrated by Feu−
er in 1974 [2] and Tng, Chan, Cook and Hagg in
1993 [3].
These errors, are essentially the result of a de−
viant head’s posture in the sagittal sense (flexion
or extension), since the cephalostat’s olives limit
the rotating and inclinating movements of the he−
ad. Possible exceptions, can result from the exi−
stence of asymmetrical acustic external channels,
or from an asymmetrical cephalostat [4].
The errors resulting from the head’s rotation,
do not seem to be very significant in the distortion
caused by cephalometric measurements. Gron [5]
concluded that rotations up to 5 degrees had a des−
picable effect in such measurements, and Van
Aken [6] referred that rotations up to 4 degrees
didn’t cause distortions on the profile, bigger than
insignificant 0.2 mm.
According to Tng et al. [3], the errors are much
more sensible when the head tilts sagittaly (ventral
or dorsal), being the consequences a real “change
in the structural base relationship or a default in
perception of the cephalometric references”, spe−
cially those located on the curved surfaces.
To avoid not only the errors caused by an in−
correct position of the head in the cephalostat, but
also others resulting from the variability of refe−
rential intracranial planes, several authors have be−
en defending for a long time, that lateral head
films (profile X−rays) for diagnosis purpose should
be taken with the patient assuming a position phy−
siologically determined, designated by “Natural
Head Posture” (NHP) [2, 7–15].
Natural Head Posture
This position seems to be highly reproducible
in adults and children, men and women, Cauca−
sians and non Caucasians, and shows a variance of
only 4 degrees, according to the studies made by
Cook [8], which is lower than the variance attribu−
ted to the majority of the most used intracranial re−
ference planes [16].
The concept of NHP was introduced in 1861
by Von Baer and Wagner [17], followed by Broca
[18] in 1862, who defined NHP “as the position of
the head in a standing up individual, with his visu−
al axis oriented horizontally”.
In 1876, Schmidt [cot. in 19] referred that this
procedure could be improved if the operator was
allowed to make small subjective adjustments to
the position of the head. Along time, other defini−
tions were added to improve the accuracy of the
process; Molhave [11], for instance, defined NHP
as “the position assumed by the individual, just be−
fore walking” (intention position) and Downs [20,
21] suggested the “use of a mirror in front of the
patient, who should be looking directly into his
own reflected pupils”.
The “position of maximum comfort”, based in
the patient’s self−balance perception and “looking
to a distant horizon”, were also suggested to achie−
ve NHP [14].
Based on the observation that the individual
variations of the natural head posture, are smaller
than the inter−individuals variations of the intra−
cranial reference planes, its use has been stron−
gly recommended for cephalometric analysis pur−
pose (9, 12, 14, 22]. This NHP criterion, had
been adopted yet by Sassouni in 1955, incorpora−
ting Broca’s optical plane, in his well known
analysis.
The 50th decade of the last century, was alrea−
dy marked by the beginning of a riot against both,
the Sella−Nasion stability and Frankfort plane true
horizontality, and definitely recognizes the need to
use natural head posture (NHP), in lateral head
films for cephalometric purpose.
The first to study NHP comprehensibly was
Bjerin [23], who found an error of only 1.6 degree
to the seated position, and 1.3 degree to the stand
up position.
Lundström [10] finds an error of 2 degrees,
just like Moorrees and Kean [12] did, that could be
reduced to 1.5 degree, when the operator corrected
obvious deviations of the patient’s head.
Siersbaek−Nielsen and Solow [24] studied
NHP when performed by auxiliary dental assi−
stants, using the mirror technique, having found an
error of only 2.3 degrees.
This shows the simplicity and reproducibility
of the procedure, and puts to the evidence that this
technique doesn’t require a great deal of anatomi−
cal and technical concepts from the operator.
Some more contributions were given related to
the better ways of finding and transferring this po−
sition (NHP) to the cephalostat the most widely
followed proposed by Moorrees in 1958, and pro−
bably the most accurate, ons suggested by Show−
fety, Vig and Matteson [14]. These latter authors,
used an “air bubble” indicator system attached to
the patient’s lateral head side, determining NHP
before, they were placed in the cephalostat, and
confirming it through the level indicator, immedia−
tely before the X−ray was taken. By this process,
131
Frankfort Plane vs. Natural Head Posture in Cephalometric Diagnosis
according to their results, the error could be limi−
ted to a maximum of 2 degrees.
Moorrees and Kean’s [12] method to determi−
ne patient’s NHP in the cephalostat, is certainly
the most widely used process and so it’s worth to
resuming step by step,
– Once in the cephalostat the patient should
be looking at his own eyes reflected into a mirror
placed in front of him.
– The operator should observe the patient at
his side and confirm that the pupil is exactly in the
middle of the eye. If not so, the patient head’s po−
sition should be readjusted.
– The ear supports should than be placed in
front of the tragus, slightly touching the skin and
making a support to the head in the transversal
plan. The patient should be comfortable and rela−
xed, with his arms pendent along the body and the
feet diverging slightly in the anterior direction.
– Verify if the head is not rotated or tilted.
– Observe the patient from their frontal side,
confirm if the head posture is correct and than ad−
just the frontal support slightly touching the skin,
just in front of Nasion point, for vertical support.
– Make a final verification and take the X−ray.
– All the procedures should take one to three
minutes.
Cleall in 1966 [25], Murphy et al. in 1991
[26], Preston et al. in 1997 [16], Usumez and
Orhan in 2001 [27] and also others, used various
methods like cinefluorography, inclinometers lin−
ked to glasses spectacle rims, etc,…in order to
simplify the methods of transferring NHP to the
cephalostat and register it in lateral head radio−
graphs.
However, according to Showfety, Vig and
Matteson´s studies [14], it seems to exist a tenden−
cy of extension or flexion in natural head posture,
in relation to the cervical vertebrae, or even to the
true vertical, motivated by the anatomical structu−
re of the patient seemingly exists.
Situations like, 1 – reduced posterior facial he−
ight or increased anterior facial height, 2 – reduced
anteroposterior craniofacial dimension, 3 – increa−
sed mandibular inclination in relation to the ante−
rior cranial base, 4 – facial retrognatism, 5 – cra−
nial base height enlarged and 6 – reduced nasopha−
ryngeal space – can cause a tendency to head
extension, while the opposite situations can cause
a flexion tendency.
This fact alerts to the inadequate use of the
Frankfort plane to orientate lateral X−rays, since
the patients showing these characteristics, repre−
sent the great majority in an Orthodontist’s office.
This idea is underlined by Ferrario et al. [28],
who found a medium angulation of 13 degrees be−
tween the true horizontal and the Frankfort plane
for the stand up position, and 5 degrees for the sea−
ted position.
On the other hand, this also suggests that NHP
can vary according to individual position, which is
accepted by Preston et al. [16], who found a ten−
dency of 2 degrees for head extension, while wal−
king, when compared to NHP in an orthostatic po−
sition.
These facts, arise the question how natural he−
ad positions is to adopted, for cephalometric dia−
gnosis purpose.
Taking into consideration the impossibility of
executing an X−ray in a dynamic situation, the ge−
neralization of the static position in the cephalo−
stat, the reduced margin of error and the high de−
gree of reproducibility of NHP it seems therefore
reasonable to go back to Moorrees NHP concept,
in order to register craniofacial structures for
orthodontic diagnosis.
Executing a lateral head film according to NHP
criteria, also permits, through the inclusion of a true
vertical (plumb−line) in the register, to orientate the
X−ray in the work desk, reproducing patient’s NHP.
If the “chassis” used in the cephalostat is properly
verified and positioned in a way that its margins re−
present true verticals then, the margins of the radio−
graphy can be used as a true vertical representative,
as Gianelly and Dietz suggested [29].
Natural head posture is, in this manner, highly
recommended and most essential for cephalome−
tric diagnosis accuracy, and should be preferred to
any other intracranial plane.
Its reproducibility is high, easy to achieve and,
according to Siersbaek−Nielsen and Solow [30],
constant in each individual along time. This does
not seem to happen with common used intracranial
planes, that evidence strong inter and intra−indivi−
dual variations with age, according to Cook’s stu−
dies [8].
NHP Limitations
in the Cephalometric
Diagnosis Context
However, it cannot be forgotten that a control−
led head position can’t, by itself, avoid “systematic
errors”, such as the X−ray magnification or other
possible technical defaults (anode characteristics,
X−ray film quality, filters, etc.) or random “errors”,
related to the structures identification, measure−
ments itself or operator’s experience and skill.
Periodical controls to the X−ray apparatus,
should be made to reduce and/or avoid systematic
error and also whenever one wants to conduct
a scientific study.
132
Fig. 1. Lateral head film from a patient obtained in na−
tural head posture, putting to evidence the lack of ho−
rizontality of Frankfort plane and leading, in consequ−
ence, to an erroneous diagnosis of the location of both
maxilla and mandible, by McNamara line (red lines),
identifying a bi−protrusion situation. If a true horizon−
tal was used associated to NHP in de same patient
(green line), instead of a tilted Frankfort plane, the
diagnosis would be more accurate (retrusion situation)
and totally opposite to the one influenced by the tilted
Frankfort plane. The clinical implications in treatment
planning are obvious.
Ryc. 1. Cefalometryczne zdjęcie boczne głowy w jej
nawykowym ustawieniu, wykazujące brak równole−
głości płaszczyzny frankfurckiej do poziomu. Fakt ten
prowadzi do błędnego oszacowania położenia szczęki
i żuchwy względem linii McNamary (linie czerwone),
a w konsekwencji – do błędnego rozpoznania: progna−
cja z progenią. Gdyby zastosowano autentyczną pła−
szczyznę poziomą w nawykowym ustawieniu głowy
(linia zielona), wówczas rozpoznanie byłoby dokła−
dniejsze i całkowicie odmienne od wcześniejszego: re−
trognacja z retrogenią. Wymowa kliniczna, a tym sa−
mym wpływ opisanych różnic na plan leczenia wydają
się oczywiste
The ears support of the cephalostat (olives),
should be sagittal and vertically symmetrical, in
a way that can guarantee that the head’s med−sa−
gittal plane is parallel to the X−ray film surface,
one idea that is usually presumed but that can be
false.
This fact is of great significance, since it
occurs a systematic error of distortion, both in li−
neal and angular measurements, whenever these
variables are not related to mid−sagittal plane
structures. One example of angular projected and
distorted measurement in lateral head film, is, for
instance, the measurement of Gonial angle [31].
C. SILVA, A. P. FERREIRA
This means that the great majority of the line−
al and angular variables in a cephalometric analy−
sis represent, in fact, projected structures, therefo−
re causing distorted measurements and error. This
can be amplified by an X−ray apparatus malfunc−
tion or in bad conditions, plus the amplification
and other technical errors related to the opera−
tor’s skill.
One good example of convergence of systema−
tic and random errors that can have a great impact
on cephalometric diagnosis, are intracranial referen−
ce planes, specially Frankfort horizontal, given its
widely preferential use. It can be affected by the po−
sition of the head in the cephalostat, improperly ca−
libration of the apparatus. F.H. referential points are
neither located in mid−sagittal plane or even in the
same plane of the head these points are also difficult
to be identified in lateral head film and, at last, this
plane can also vary inter and intra−individually.
According to Houston [4, 32] the best way to
minimize random errors, it would be the one consi−
sting in multiple measurements, given the difficulty
of use of samples sufficiently representative and the
ethics limitations in irradiating patients repeatedly.
On the other hand, the definition of the syste−
matic error should be reported in every study, sin−
ce, besides use of different samples, one can be
comparing situations where repeatability condi−
tions do not exist (Fig. 1).
Discussion and Conclusion
Independently of the systematic and random
errors in cephalometric diagnosis, the patient’s po−
sition in NHP criteria in the cephalostat, being
a relatively easy procedure, never reached, unfor−
tunately, the acceptance, the divulgation and gene−
ralization probably due to the fact, that F.H.p is
they diserve.
It is possible that the main reason for this fact,
lies on the circumstance that the Frankfort hori−
zontal plane is simultaneously used as an orienta−
tion and a reference one. That natural head postu−
re (NHP) most certainly helps to go round the er−
rors associated to bad positioning of the head in
the cephalostat and also to the radiography orien−
tation in the desk table, with the resultant distor−
tions, but it cannot eliminate those distortions cau−
sed by the use of Frankfort plane as reference to
the generality of measurements, which are in a ve−
ry significant number.
However, the great majority of the described
analysis that achieved great popularity and accep−
tance within the orthodontic world, use it as the
main reference plane.
Maybe because of this fact, Spradley [33] sta−
Frankfort Plane ys. Natural Head Posture in Cephalometric Diagnosis
ted, “None of the cephalometric methods known till
now can describe the face correctly, without depen−
ding on variable intracranial anatomic references or
soft facial tissues such as nose, lips and chin”.
A new method of analysis of craniofacial
structures should be developed, taking NHP and
133
a true vertical (or horizontal) as references, in
order to replace the intracranial planes in use to−
day, in conventional cephalometric methods.
This fact has been constantly suggested throu−
ghout time in orthodontic literature.
However, such method has not yet been.
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Address for correspondence:
Carlos Silva
Orthodontic Department at Fernando Pessoa University, Porto, Portugal
e−mail: [email protected]
Received: 29.10.2002
Revised: 26.11.2002
Accepted: 3.12.2002
Praca wpłynęła do Redakcji: 29.10.2002 r.
Po recenzji: 26.11.2002 r.
Zaakceptowano do druku: 3.12.2002 r.