Pacific Coast Society of Orthodontists Bulletin Summer 2020 - 47

Let's Talk About That

It is very important to note that the maxillary
portion of the TBA for this patient will have no
labial bow. This is to avoid lingual tipping of the
maxillary incisors while waiting for the mandible
to grow forward to correct the overjet.
The construction bite to make a TBA is absolutely
critical. In the described patient's case, the wax
construction bite would be taken with the mandible advanced forward 6 mm into an edge-to-edge
A-P relationship. It is very important that the
maxillary and mandibular skeletal midlines are
lined up, and the bite should be open about 3 mm
in the incisor area.

Figure 4. (A) Twin Block design for use in the
mixed dentition stage. Note the acrylic-covered labial bow to enhance the retention of the
lower appliance. Also note that there is no labial
bow on the upper incisors. (B) Side view of the
appliance to show the clearance allowed for the
mandibular first molars to erupt to assist in the
overbite correction. The arrows indicate optional
elastic hooks that can be used for night wear of
elastics in patients who are suspected of mouth
breathing. (C) Occlusal view of upper and lower
TBA showing the midpalatal expansion screw
used to widen the maxillary arch as needed to
coordinate both arch widths as the mandible
grows forward.
and mandibular first premolars (Figure 4). For
additional retention, a labial bow will be used
from canine to canine on the mandibular anteriors. I find that encasing the labial bow in acrylic
helps to enhance the retention of the mandibular
appliance.
To minimize any proclination tendency of the
mandibular incisors, I place a 1 mm thickness of
wax relief on the lingual surfaces of the mandibular incisors on the construction model prior to
appliance fabrication. This ensures that the acrylic portion of the appliance is being supported by
the soft tissues and basal bone of the mandibular
lingual anterior area, not the teeth.

Summer 2020    PCSO Bulletin	

To correct the transverse relationship between
the maxilla and the mandible as the mandible
grows forward, a maxillary expansion screw is incorporated into the maxillary portion of the TBA.
The patient will be instructed to open this screw
one turn per week, equating to 1/4 mm of expansion per week and 1 mm of expansion per month,
as needed during the active treatment phase.
For correction of the deep overbite, it is essential
that the acrylic blocks on the upper portion of the
appliance be trimmed in such a way as to allow
free eruption of the mandibular first molars.
4. What are the benefits or advantages of using
your appliance over other approaches to Class II
treatment? One of the main reasons that I prefer
the TBA over the fixed functional appliances
is that unlike these other appliances that are
entirely tooth-borne, the TBA is at least partially
supported by soft tissue and basal bone. For this
reason, it is able to deliver more skeletal correction (i.e., more forward mandibular growth),
whereas the entirely tooth-borne appliances
result in mainly dentoalveolar changes.
Being removable might seem like a disadvantage
for the TBA. In fact, it offers some significant
advantages. Since the TBA can be removed for
eating, there are no dietary restrictions for the
patient. Because it can be removed and easily

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Pacific Coast Society of Orthodontists Bulletin Summer 2020

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