Understanding Forces and Moments 2
24
Functional Curves
Spee
These curves
can be helpful
in determining
which arch is
involved and to
what degree.
Monson
Wilson
The long axis should
lie parallel to the
Internal Pterygoid
resulting in axial
loading (stability).
2
Curve of Monson
1
3
Curve of Wilson
Figure 2-30
Figure 2-31
Three important functional curves are shown in Figure 2-30. In
Figure 2-31, it can be clearly seen that excellent axial loading is
achieved in #1, as the curves of Monson and Wilson nicely
coincide. However, in #2 there is an excessive curve of Monson
while in #3 there is a reverse curve of Wilson. In the latter two
cases there is a loss of axial loading which is apparent. These
discrepancies can very easily result from vertical forces acting
through the molars tubes as shown earlier. It has been shown
that eruptive forces through molar tubes create lingual crown
moments while intrusive forces acting through molar tubes
result in buccal crown moments.
The following illustrations will show the potential buccal and
lingual displacements that may occur as a result of vertical
forces acting through the molar tubes. If the second molars have
not yet erupted and the first molars are displaced without the
orthodontist being aware of such displacement, then upon
second molar eruption it may mistakenly be assumed that
second molars are at fault. As a result, treating to the first molar
width may then result in a faulty curve of Monson or Wilson.
Understanding Forces and Moments 2
Lingual
Crown
Displacement
1
Buccal
Crown
Displacement
Original Molar Width
2
3
Change in Molar Width
2nd Molar
Width is
Normal
Figure 2-32
25
1
Original Molar Width
2
3
Change in Molar Width
2nd Molar
Width is
Normal
Figure 2-33
In Figure 2-32 #1, the eruptive
Second Molars are in
force has caused the first
molars to move lingually as
normal position.
observed in #2. In #3, the
second molars have now
erupted. It remains important
to know which of the molars
Second Molars are in normal transverse dimension. are out of position. In Figure
2-33 the same series of events
Figure 2-34
has occurred with first molars
moving buccally due to intrusive forces acting through the
molars. It can be observed that second molar eruption may
create the illusion that they have erupted too far to the lingual. In
Figure 2-34 it can be seen that casual observation could easily
lead one to believe the first molars are normal in width with
second molars being the problem.
The above movements make it important for the clinician to
include the functional curves of Monson and Wilson in
observing treatment progress. A failure to harmonize these
Understanding Forces and Moments 2
26
curves may result in functional problems involving axial loading
and leading to later instability.
This concludes the chapter regarding forces and moments. What
may have appeared to be quite elementary at this point will
prove to be highly important in applying fundamental mechanics
in everyday treatment.
Most of what is contained in this book has not been taught as
part of an orthodontic curriculum. By understanding the
contents presented there will be many opportunities to treat
patients in a unique manner regarding the applied mechanics. In
addition it will be discovered that there are many approaches
available that will lessen the need for patient cooperation
without the need for appliances that displace lower incisors
because of the undesirable reciprocal effects when treating
opposing arches with interarch appliances. You are about to
discover many ways of providing intra-arch solutions for many
malocclusions that will help to avoid placing appliances on
opposing arches which may be normal and require no change.