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The fragile buccal and
lingual cortical plates are sufficient to retain
healthy teeth under load and function due
to the shock absorbing capability of the
periodontal ligament surrounding the
natural root,
and the elastic
quality of the alveolar bone itself. An implant, without a
natural shock
absorbing ligament does not
have this advantage. The
implant-bone
interface is
much less forgiving. This implies that for
long-term success under
function, the
implant should
be surrounded
with a sufficient
volume of bone
which will assimilate these
forces. Therefore it is necessary to visualize and understand the true
bone anatomy
in both two and
three dimensions for proper fixture placement.
Utilizing CT data, the cross-sectional,
three dimensional view can be utilized to
evaluate for the best possible fixture
placement. To maximize the bone-implant interface, while creating the necessary immediate fixation and stabilization
of the implant, the author postulates the
visualization of a triangle of bone.
The wide base of the triangle demarcates the alveolar crestal bone at its
widest buccal-lingual diameter, usually
including good cortical bone while the
apex of the triangle should bisect the
residual crestal ridge [FIG.7]. The implant fixture should then bisect the designated triangle of bone [FIG. 8]. Since
the implant is now parallel to the inherent
trajectory of the bone, an angled transmucosal abutment will be required to
restore the tooth in its original position
[FIG.9].
The completed implant supported restoration should maintain the
height and width of the alveolar crest,
and return the patient to proper function
and esthetics [FIG.1O].
In a site where
the tooth has been missing for a long
period of time, the apex of the triangle
should bisect the proposed alveolar crest
for the ideal tooth position.
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Fig. 9 (above left) As the implant is placed parallel to the trajectory
of the bone, an angulated transmucosal abutment is necessary to enable
correct restoration of the tooth.
Fig. 10 (above right) The final prosthetic crown, with good
emergence profile, increased thickness of buccal cortical plate,
replaces the natural tooth in its original position.
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Augmentation/grafting procedures can be done
simultaneous to fixture placement, before, or after.
The corrected example of an immediate extraction placement can be seen
illustrated in [FIG.1 1,12]. The implant should
bisect the proposed triangle of bone to gain
the fixation and support . Although it may
appear that the emergence of the implant is at an angle to the bone1 it is actually
placed parallel to the natural bone trajectory.
The osteotomy should be prepared with full
anticipation of the final prosthetic replace
ment. This may require the fabrication of an
accurate surgical
template to help
guide the drilling
instruments. The
residual socket
may be ovoid or
dumbbell in shape
depending upon
the individual root
morphology. The
osteotomy preparation created by
cylindrical drills,
must be perfectly
cylindrical. It is
critical that the drill
be property positioned to bisect the
proposed triangle of
bone. If the
drill is drawn to the
path of the socket,
then the fixture
placement will be
compromised
and could possibly lead to perforation of the fragile
buccal plate.
When teeth are
present, the drill
should be positioned in alignment with the incisal edge
of the adjacent teeth. The body of the
implant should be positioned in alignment with the buccal or cervical aspect
of the adjacent teeth. Initial fixation of
the implant should be achieved through
the volume of bone surrounding the
apical aspect of the fixture. Often this
will result in the engagement of good
quality cortical type I or type II bone.
Often, due to the discrepancy between the cylindrical osteotomy
preparation and the residual shape
of the socket, there will be a gap between the buccal cortical plate, the implant fixture, and the lingual cortical plate.
N E X T+
P A G E |
The Triangle of Bone
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