Page 34 - Dental Practice South Asia 18-6

prosthodontic section

THE ABCsOF OCCLUSION

AND ARTICULATIONS

REDUCING POSITIVE ERRORS FOR LESS ADJUSTMENTS

THOMAS E. LEE

This article will focus on the ABCs (Axis, Bite, and Chewing) of
Occlusion and Articulation that can be easily implemented to create
restorations that require fewer adjustments, saving time and reduc-
ing stress. The initial step in reducing positive errors in articulation
begins with accurate impressions and bite records as any error in the
technique or material will create a high restoration.All philosophies
have the same objective of equal contacts of the occluded teeth with
no interferences in all movements. Laboratory technicians can
achieve this objective on whatever articulator they may use, yet most
restorations still need adjustments when placed in the patient’s
mouth because of inaccurate impressions and/or positive errors in
the Axis, Bite and Chewing.
An articulator is an instrument that represents the temporo-
mandibular joints (Axis) or jaws to which study casts may be
attached to simulate the static (Bite) and dynamic (Chewing) rela- FIG 1
tionship between the occlusal surfaces of the teeth during mandibu-
lar movements. Positive errors occur when the articulator under
compensates for mandibular movements, resulting in a positive fea-
ture on the occlusal surface where that feature should be smaller or
non existent.11 Positive Errors can create interferences that may
need to be adjusted in the Axis (opening and closing movements),
the Bite and Chewing (envelope of function movements) depending
on the discluding factor of the protrusive pathway, influence of
Bennett movement, and steepness of the anterior guidance.
Negative Errors occur when the articulator overcompensates for
mandibular movements, resulting in a negative feature on the
occlusal surface which allows the teeth to disclude more freely. 11

AXIS FIG 2 FIG 3
It is important to relate the teeth to the patient’s axis to simulate
more accurate axis movements in an articulator. The most common by referencing the patient’s ears which relates the study models
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error in relating study models is using a simple hinge articulator much closer to the patient’s axis to reduce positive errors. The Kois
without the use of a facebow. The axis in simple hinge articulators Dento-Facial Analyzer (DFA) is a simple instrument that incorpo-
(Figure 1, redpoint B) are always located below the patient’s axis rates a 3-Dimensional guide plane to reference how the occlusal
(Figure 1, green point A). Therefore, simple hinge articulators pro- plane relates to the face for esthetics, as well as having a functional
duce more vertical opening and closing axis movements (Figure 1, relationship of the teeth to the axis based on an average axis-incisal
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red pathway b) than the patient’s opening and closing axis move- distance of 100mm (Figure 2). This 100mm axis-incisal distance is
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ments (Figure 1, green pathway a). This positive error in axis move- supported by Monson’s Spherical Theory (4in=101.6mm) ,
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ments can create interferences in the mesial inclines of the upper Bonwill’s Equilateral Triangle , as well as other research showing
teeth and/or distal inclines of the lower teeth that will require the Kois DFA to be as functionally accurate as a facebow. 6,7 The
adjustments. 1 100mm axis-incisal distance is also engineered into the Kois
Research shows that a facebow has a statistical average to the axis Platform on the articulator which can mount study models with or

34 Dental Practice // November-December 2022 // Vol 18 No 6