This document discusses the difficulty in consistently locating the cephalometric landmark known as Point A or Subspinale on lateral cephalograms due to conflicting anatomical details. It proposes using an alternate reference point located by drawing a line through the long axis of the maxillary central incisor root and dropping a perpendicular line from the midpoint between the upper third and lower two thirds of the root. The length of this perpendicular line was found to be approximately 3.0 mm anterior to the root, providing a suitable estimated Point A to use when the true Point A cannot be accurately identified. Using this estimated Point A allows the nasal-anterior line to be drawn with reasonable accuracy as an alternative.

1. POINT A REVISITED
UNDER THE GUIDANCE
OF-
DR. MRIDULA TREHAN
PROFESSOR & HEAD
Rick L Jacobson, Point A revisited , Am.J.Orthod ,
1980.
PRESENTED BY-
DEEKSHA BHANOTIA
PG 1ST YEAR
Department of Orthodontics &
Dentofacial Orthopaedics

3. INTRODUCTION
O 0rthodontic interest in cephalometric radiography of hard
and soft tissues in the region of cranium and face has led to
a continued quest for exact identification of anatomic
landmarks.
O The extreme range or radiolucency between bone and soft
tissues makes it impossible to locate consistently all such
landmarks on routine radiographs.
O Various techniques, such as the rare earth phosphors in
intensifying screens,x attenuation of the radiation beam, or
even the use of a second beam, have been applied to
improve physical image quality.
O In spite of improved techniques, occasionally certain
landmarks are still difficult to locate because of conflicting
anatomic details or conceptual judgment.’ Among such
landmarks is POINT A OR SUBSPINALAE.

4. POINT.A(SUBSPINALE)
O Subspinale is a midline point whose relationship to
the anterior teeth in a lateral head film may be
influenced by head position.
O It is located somewhere between the root apex (and
even occasionally above this point) and the coronal
third of the root of the tooth (Fig. 1).
O Subspinale is defined anthropologically as “the
deepest midline point on the pre- maxilla between
anterior nasal spine and prosthion.
O “ Bjork” defined it as the “deepest point on the
contour of the alveolar projection between the
spinal point and prosthion.”
O The projection of the cheeks in living persons
frequently obscures this landmark in cephalograms.

5. O Because of the difficulty in locating point A,
alternate points have been sought.
O Van der Linden, in the determination of the
anterior outline of the apical base, suggested
the use of point L, which is located on the
anterior surface of the image of the labial
lamella at the region of the apex of the
maxillary incisors.
O Jarabak and Fizzell’ identified a point 2
mm. ahead of the root apex as a redefinition
of point A.

6. O Defined originally by Downs, although not an ideal
reference point, was nevertheless a valid one and
possibly the best there is in this area of the maxilla.
O This is in accord with the findings of the Second
Research Workshop on Roentgenographic
Cephalometrics.
O Point A marks the anterior extremity of the
maxillary base. Since almost all cephalometric
analyses use point A and/or the NA plane as a
reference point or plane from which to relate
skeletal and dental landmarks, it is most important
that this landmark be identified.

7. O In instances where point A is obfuscated
for whatever reason (discussed earlier),
another means should be sought to judge
the location of this point with acceptable
accuracy.
O The purpose of the present article is to
identify such a point and thus enable
the NA line to be drawn with
reasonable accuracy.

9. METHOD AND MATERIAL
O Thirty-three lateral cephalometric radiographs were
selected on the basis of excellence of image quality
and ability to locate point A or subspinale with a
considerable degree of accuracy
O Xeroradiographs were used in preference to
conventional lateral cephalometric head films since it
has been shown that craniofacial landmarks
(particularly point A) are more easily identified on
these positive radiographs.
O Acetate paper (0.003 inch in thickness) was placed
over the radiographs, and nasion and point A were
identified in each instance.
O These points were joined by a line NA drawn on the
acetate tracing paper.

10. O The outline of the maxillary central incisor
was traced and a line was drawn through
the long axis of the root of this tooth. On
this long axis the following points were
located-:
O X = Root apex.
O Y = Junction of upper third and lower two
thirds of root length.
O Z = Midpoint of root length.

12. O From these points, lines were dropped perpendicular to the
NA line, and points X1 ,Y1 and Z1, were thus identified
(Fig. 3).
O The lengths of the lines X-X,, Y-Y,, and Z-Z, were
measured to within 0.5 mm. and recorded.
O The means, variance, standard deviation, and standard
error of the means of the measurements were calculated in
an effort to identify which of the landmarks (X,, Y,, or Z,)
could be used as reference points from which to
approximate the NA line.
O In other words, if for any reason point A could not readily
be identified, could any of the landmarks (X, , Y1, or Z,) be
used as a guide for the determination of the NA line with
any degree of accuracy?
O Variation in SNA angle or the angle between the upper
incisor and NA was minimal.

14. FINDINGS
O The length of the line Y-Y1, which was
3.03 mm., reflected a lesser coefficient of
variance (1.1555) than either X-XI (C.V.
1.287) or Z-Z, (C. V. 1.767).
O Likewise, the standard deviation of 1.075
for the Y-Y, line is less than either the X-X,
(S.D. 1.134) or Z-Z1 (S.D. 1.329) lines
(Table I). Consequently, the Y-Y1
measurement of approximately 3.0 mm.
was the parameter of choice.

16. O Thus, if in lieu of Y1, estimated point A (An)
was substituted, a line drawn from nasion
through estimated point A (An) would be one
which would most closely approximate the
true NA line.
O Therefore,in the event of point A being
difficult to locate, an estimated NA line
could be drawn from nasion through point
AE which, in turn, could be located by
plotting a point 3 mm. ahead of a point
between the upper third and lower two
thirds of the root axis of the maxillary
central incisor.

17. DISCUSSION
O Tipping the crown of a maxillary incisor tooth
palatally has the effect of moving the crown
portion of the root palatally and the root apex
labially or anteriorly with the center of rotation
somewhere near the middle fifth (between 40
and 60 percent) of the root length.
O Christiansen and Burstone” found the center
of rotation to be located between 53 and 55
percent from apex to alveolar crest. Davidian”
computed it to be between 49 and 54 percent
from root apex to alveolar crest.

18. O A newer study located the center 0f
rotation at a point 40 percent of the root
length from the apex.
O As the point of’ force application to the
crown shifts incisally, the axis of rotation
follows , but not to the same extent .

19. O It is evident from the foregoing that a point closer
to the center of the root of a tooth is less
vulnerable to displacement than, say, a point close
to the root apex during crown- tipping procedures.
O Thus, if an alternate or substitute A point were to
be sought. a point 3 mm. ahead of a point between
the upper third and lower two thirds of the root axis
should be suitable and preferable to any located
near the root apex.
O The suitability of estimated point A (AE) is
compounded by the fact that a line drawn from
nasion through AF: more closely approximates true
NA line than any other point.

20. CONCLUSION
O Point A cannot be accurately identified in all
radiographs.
O In instances where this landmark is not clearly
discernible , an alternative means of
estimating the anterior extremity of maxillary
base is shown.
O A point plotted 3.0mm. Labial to a point
between the upper third and lower two thirds
of the long axis of the root of the maxillary
central incisor was found to be a suitable point
(estimated point A ) through which to draw a
line one which closely approximates the true
NA plane.

21. REFERENCE ARTICLES
1)A CRITICAL EVALUATION OF CEPHALOMETRIC “A” POINT
AND PROPOSAL OF A MORE SIGNIFICANT LANDMARK BY
JOEL.A KALAFA.
O The cephalometric landmark, point “A,” was investigatcd with regard
to definition, location, and usefulness in cephalometric analyses.
Lateral cephalometric films of twenty-five dried skulls and fifteen
childrelr, all having at least one maxillary central incisor in position,
were studied. A 30 gauge needle was used to outline the contour of
bone covering the root of the maxillary central incisor, and on the
dried skulls a fine strip of aluminum dry foil outlined the midline of
the maxilla below ANS.
O The contour and thickness of bone over the incisor were noted, and
the difference between the midline “ A” point as defined by Downs
and the nearest point on the needle was measured. From the
contour and position of bone covering the root of the incisor*, it was
concluded that it would be impossible to locate (by inspection of a
cephalometric film) any landmark on this hone. It was also shown
that the midline “A” point and the bone over the incisor bore no
relationship to each other.

22. Therefore, only the “A” point as defined by
Downs can actually be located by
inspection on a lateral cephalometric film.
Since it is desirable to find a landmark in
the area that, would more readily
demonstrate change due to therapy, a new
point, called “TA!’ point,, was formulated.
The location of “TA” point was based on
the average thickness of bone over the
maxillary central incisor root, the position of
“A” point. and the utilization of the palatal
plane for reference. Also, a method for
superposition of “TA” point was
demonstrated on tracings using fifteen pre
and posttreatment records.

23. THANK-YOU