2. • Various cranial reference planes
have been used as baselines from
which to determine the degree of
jaw dysplasia.. These reference
planes deal with cranial
architecture and so are useful in
relating the jaws to the cranium.
3. • Measurements from the
cranial base, however do not
always provide a reliable
expression of
anteroposterior jaw
relationships in the
dentoalveolar complex.
4. • The ANB angle (the difference between SNA
and SNB angles) is the most commonly used
measurement forevaluating the anteroposterior disharmony of the
jaws. The average ANB angle is 2 degrees.
Angles greater than this mean indicate
tendencies toward class II jaw disharmonies while
smaller angles ( extending to –ve readings) reflect
class III jaw discrepancies .
5. • Although the ANB angle is a
reliable indication of
anteroposterior jaw
relationship, there are many
situations in which this reading
cannot be relied on .
6. • These situations are due to craniofacial skeletal
variations such as :
1. The anteroposterior spatial relationship of the jaws
relative to the cranium (nasion). For example,
Standard
ANB=2
Long cranial
base(retroposit
ioned jaw)
Short cranial
base(jaws
positioned
forward to
7. • in prognathous faces, the ANB angle increases,
whereas in jaws that are relatively retrusive, this
angle is reduced while in both conditions, the
relationship of the jaws to each other remains
unchanged.
Long cranial
base(retropositi
oned jaw) ANB=
Short cranial
base(jaws positioned
forward to nasion)
Standard
ANB=2
8. • Binder recognized the
geometric effects at
work in the ANB angle.
He showed that for
every 5-mm anterior
displacement of nasion
horizontally, the ANB
angle changed 2.5
degrees.
Vertical positioning
of nasion results in these AN
angles:
1 = 2 degrees,
2 = 1 degree,
3 = 0 degrees.
9. • A 5-mm upward displacement of nasion altered the ANB
angle 0.5 degrees; a downward displacement of nasion
changed the ANB angle 1 degree
Horizontal
positioning of
nasion results
in these ANB
angles:
1 = 2 degrees,
2 = 8.5
degrees
3 = - 4 . 5
degrees.
10. 2. The rotational effect of the jaws relative to the
anterior cranial base. A clockwise rotation of the jaws
relative to S-N plane(in a patient facing right) would
cause the ANB angle to increase in size producing
Class II and vice versa while the relationship of the
jaws to each other is still unchanged.
Counterclockwise
rotation of the jaws
Clockwise rotation
of the jaws ANB= 8
Standard
ANB =2
11. The rotational effect of the S-N line virtually has no
anteroposterior position effect on the nasion point;
therefore, the ANB angle reading is minimally affected by
angular deviations of S-N from the horizontal
12. • The Wits appraisal was developed in an effort to reduce
these inconsistencies found within the ANB angle. The
wits appraisal is a measure of the extent to which the jaws
are related to each other. This appraisal relates point A
and point B to the occlusal plane, providing a
measurement of the anteroposterior relationship of the
jaws nearer the teeth than is provided by the ANB angle.
13. • The Wits appraisal is a linear
measurement and not an analysis
in itself.
14. • The Wits appraisal uses a functional occlusal plane,
which is the plane of maximum intercuspation of
posteriors (drawn on lateral cehalometric tracing through
the region of the overlapping cusps of the first premolars
and first molars). In the mixed dentition, a horizontal
plane can usually be drawn through the overlap of the
cusps of both deciduous molars and the permanent first
molars.
15. correct location or representation of the occlusal plane in
Wits appraisal is very important, sometimes the left and
right sides of the posterior teeth do not coincide or
superimpose correctly. The latter problem may be due to
true dentofacial asymmetry,
asymmetric location of the external auditory
meatus, and /or in correct positioning of the head
in the cephalostat.
16. To overcome this discrepancy, a plane is
drawn midway between the two posterior
segments.
• Perpendicular lines are drawn from points A
and B to this occlusal plane. The points on
the occlusal plane is named AO and BO
respectively.
17. Positive values occur when point AO is
ahead of (anterior to) point BO, and negative
values occur when point BO is anterior to
point AO
18. • The average jaw relationship according to wits reading
is -1.0mm for men and 0 mm for women. In skeletal
class II, point BO would be located well behind point
AO(positive reading), whereas in skeletal class III, the
wits reading would be negative(point BO is ahead of
point AO).
19. • . The severity of anteroposterior disharmony depends
on the amount of deviation of wits reading from the
mean.. Therefore, the Wits appraisal is intended not as
a single diagnostic criterion but as an additional
measurement, which may be included in the existing
cephalometric analysis to aid in the assessment of the
degree of anteroposterior jaw disharmony. Moreover, its
usefulness in orthognathic surgery is self-evident.
24. THESE OBSERVATIONS QUITE NATURALLY LEAD
ONE TO ASK THE QUESTION, “CAN ONE
DETERMINE WHETHER OR
NOT THE ANB ANGLE IS
RELIABLE?”
25. • Subsequent investigations have indicated that if the
mandibular plane angle (Go-Gn to S-N) reading is
considerably higher or lower than the mean of 32
degrees (plus or minus 1 SD of 5 degrees), the ANB
reading as a reliable indication of anteroposterior jaw
discrepancy is suspected.
• Simply stated, this means that ANB readings are
suspected in patients having mandibular plane
angles of greater than 37 degrees or less than 27
degrees.
26. CONCLUSION:
• The Wits appraisal is a linear measurement and not an
analysis perse. It is simply an adjunctive diagnostic aid
that may prove useful in assessing the extent of
anteroposterior skeletal dysplasia and in determining the
reliability of the ANB angle.
• Identification of landmarks and interpretation of findings of
a three dimensional object on a two-dimensional image
cannot be regarded as truly stable. No single parameter in
cephalometry should be relied on entirely and interpreted
as an absolute value, Correlation between the linear and
angular measurements is a must for interpretation.
