Transferring the Appropriate Construction Plane with the use an Earbow/Articulator Correction Platform

Michael C. Fling DDS

With today’s emphasis of incorporating “aesthetics” along with function, conveying the correct position of the maxillary cast on the articulator becomes mandatory. As the position of the maxillary central incisors dominant the smile, incisal edge length and position are obviously important. It is imperative that the clinician convey to the technician the appropriate “construction plane” so the restorations can be fabricated consistent with the patient’s “esthetic plane”. It is this “construction plane” that will then dictate the restorations incisal edge relationship to the lower lip, the anterior segment incisal relation to the “esthetic plane”, and thus the cant or angulations of the interproximal embrasures. Clinically, if the restorations incisal edges are not consistent with the patient’s esthetic plane, the embrasures will be canted and the anterior segment will appear to slope, creating an unbalanced smile. The use of a facebow or earbow has been advocated to reference the appropriate hinge axis to the incisal/occlusal surfaces of the maxillary cast. Assuming the horizontal plane from right to left external auditory meatus is parallel to the patient’s aesthetic plane and assuming this plane is parallel to the floor, the facebow will register the incisal edge relation of the maxillary teeth to the articulator without distortion, allowing the technician to reference the lab bench as the appropriate “Construction Plane”.(Fig.1)

( Earbow Plane = Aesthetic Plane = Construction Plane )

Figure 1

However, if the incisal edge position of the patient’s anterior segment is coincident to the desired “aesthetic plane” (which is parallel to the floor) but the external auditory meatus is such that one side is higher than the other, the result is a plane registered on the earbow that is not parallel to the “aesthetic plane” or the floor. In this scenario, because the articulator cannot correct for this lack of consistency between the external auditory meatus plane and the desired “aesthetic plane”, the earbow will then transfer to the articulator canting the maxillary model. Now the canted cast relates an incorrect “Construction Plane” for the technician.

Now the technician will fabricate the restorations with the incsal edges parallel to the lab bench, as there is no way for the technician to know the maxillary cast has been canted. Then, as the restorations are transferred back to the mouth, the incisal edge plane will slope and not parallel the desired “aesthetic plane”. Because the proximal embrasures are contoured 90 degrees to the inaccurate “Construction Plane”, the restorations will cant or appear to be tilted, yielding an unacceptable result. (Fig.2)

Incisal plane of teeth and preparations Maxillary cast is canted as earbow can are parallel to aesthetic plane. Earbow not correct for discrepancy between plane is not parallel to aesthetic plane. earbow and desired aesthetic plane.

Restorations are constructed with incisal Clinically, final incisal plane is not plane paralleling work bench. consistent with desired aesthetic plane. Figure 2

Note that the articulator cannot correct for the canted plane registered by the earbow, resulting in a “Construction Plane” that allows fabrication of restorations that when seated, are tilted from the desired aesthetic plane.

Many approaches have been taken to solve this dilemma. The Kois Dento-Facial

Analyzer orients the maxillary cast taking into account the horizontal, vertical and saggital plane. Relying on research that supports that the majority of the population measures 100mm from hinge axis to incisal edge of the central incisor, this mechanism doesn’t rely on a record that is dependent on the external auditory meatus for its registration. While this is a very useful tool, it does require a specific registration device and specific articulator. So what if you don’t have this articulator? Another method to solve the problem of the horizontal plane from each external auditory meatus not being coincident with the aesthetic plane, is to use a conventional earbow to make a “Corrected Earbow” registration. By pulling one side of the earbow out of the external auditory meatus, and “correcting” the earbow to parallel the floor (or aesthetic plane of the patient), the record will not be rotated when fixed on the articulator as the earbow has been “corrected” to be coincident with the “aesthetic plane”. ( Fig. 3) Thus, the incisal edges of the cast will correlate on the articulator to parallel the lab bench. Then as the technician builds the restorations, the incisal edges or “construction plane” of the cast is coincident to the aesthetic plane. Because the interproximal embrasures are fabricated 90 degrees to the lab bench top, they will then transfer clinically to be correct vertically and the incisal edge horizontal plane will not be canted. This is a result of the earbow not having to be rotated when it is transferred to the articulator.

Note that the earbow has been corrected by moving it downward on the left side to parallel the desired aesthetic plane. Now restorations are constructed so they transfer to the mouth coincident to the aesthetic plane. Figure 3

The dilemma with this scenario is how this “correction” affects the relationship of

the cast to the hinge axis. Because a corrected earbow moves one earpiece out of the external auditory meatus, as an example, the left side of the earbow is moved down to correct the discrepancy in the external auditory meatus position, this then creates an altered distance from hinge axis to cusp tips than what actually exists clinically. ( Fig.4 ) In scenarios where construction occurs to an open vertical this will affect the arch of closure, the steepness of the occlusal inclines, and even affect centric stop positions of the constructed restorations. This would also have an affect on the opposing cusp to groove relationship from a lateral rotational perspective. While this factor may be relatively inconsequential, in a case where a steep curve of spee exists, or in a case that requires a very flat angle of disclusion where an increase in steepness of posterior inclines would create lateral interferences, this could be a significant factor.

Arch of Closure

Lateral Excursive Contacts

Centric Stop Position

How can we have the best of both worlds?

Figure 4

So how can we correct the earbow without altering the true relationship of teeth to hinge axis distance? After preparations are complete, take a “Construction Plane Bite Stick” registration, embedding a stick into registration material, insuring it is parallel to the desired “aesthetic plane”. Now take an earbow, regardless of the coincidence of the plane from ear to ear to the desired aesthetic plane. Now mount the maxillary cast. ( Fig.5 ) Because the earbow has not been “corrected” to be coincident to the aesthetic plane, no alteration in distance from hinge axis to occlusal plane has been registered. This ensures an accurate arch of closure. However the “construction plane” is not parallel to the lab bench top.

Construction Plane Bitestick

Figure 5

After mounting the casts, the bite stick registration is put in place. Now the entire articulator is tilted until the “Construction Plan Bite Stick” is parallel to the desired aesthetic plane or lab bench top. The articulator’s feet are corrected to maintain this position. ( Fig.6 ) Now the restorations are fabricated as the cast construction plane is now coincident to the aesthetic plane and is parallel to the bench top , without any alterations in distance from the hinge axis to occlusal plane. Restorations are now ensured to be fabricated to optimum esthetics and function.

Feet Correction

Figure 6 Figure 7 A card with a rectangular window can be placed on the lab bench which shields the view of the canted articulator. This allows the technician to concentrate on the desired aesthetic plane without distraction of the canted articulator.( Fig.7 )

A simple wood block has been fabricated to accept the feet of a given articulator.

The base of the block has adjustable feet which are utilized to correct the inclination so that the “construction plane bitestick” can be coincide with the lab bench top.( Fig. 8 )

( Fig. 8 ) **** Please see our products page for information on how to purchase your own Fling Table

In summary, utilization of a “Construction Plane Bite Stick” and correcting the articulator’s inclination so the desired aesthetic plane and construction plane are parallel to the lab bench top ensures predictable results. By correcting the articulator’s inclination, the distance from the hinge axis to occlusal plane is not altered, allowing the technician to construct restorations that are will transfer to the mouth predictably in both form and function.