This Facebow Must Be Wrong! The Model Will Not Fit.By Gary DeWood on June 8, 2015 | comments
Has this ever happened to you? The upper model is trimmed normally, you position it on the bitefork, close the upper member of the articulator, and the articulator will not close onto the mounting jig because it is hitting the upper model. That little ache in your neck starts coming back as you begin to play back what could have happened.
Why did you not stand the patient up for the facebow? Why did you hurry through it? Did you check the references? Why did you let Sally take that facebow? Why did you not check the facebow before the patient left? Why does this only happen on that one person in the schedule that nobody wants to call and ask to come back?
What did we do wrong?
Before making that assumption let's consider the possibilities. By the way, an error in taking the facebow is always one of them but it is not at the top of my list.
I love my SAM articulator, and if you have chosen to use the SAM system and you use it as we recommend, you are using it a bit differently than intended by its designers. The problem illustrated in Figure 1 can occur with any articulator if the method of taking the facebow is altered as we have altered it for our use of the SAM system.
The anterior/posterior position of the model on the articulator is determined by the placement of the earpieces in the ear canal. Once the bite fork has been adapted to the maxillary teeth this becomes a fixed distance that will not be influenced by the vertical positioning of the facebow assembly on the face. (Refer to Figures 2 and 3)
Note that the measurement between Dan's ear canal and the incisal edge of the maxillary incisors is unchanged in spite of a significant difference in the vertical positioning of the facebow on his face. The red lines are exactly the same length.
The facebow in the SAM system has a NASION attachment (Refer to Figures 4 and 5) that acts as the third reference point, positioning the model in the articulator in a vertical position. All articulator systems have one of these although different systems choose different references. The Denar earbow use infraorbitale as its third reference point with a pointer that angles off of the earbow to locate the reference on the face.
Using the third reference point sometimes results in an upward tilt of the facebow relative to the floor when the patient's head is positioned in a level position. Dan was asked to stand and look at the horizon, and his head position was noted from the side. Care was taken when the facebow was placed so that his tilt and the angle of his head did not change with the facebow placement. The very significant change is evident in Figures 2, 3, 4 and 5. The tilt created when using nasion is leveled when the jig is attached to the mounting device, causing a shift of the cast downward in the articulator.
We have chosen to use the facebow differently, and we level it with the horizon so that the mounted upper model is in the articulator as the teeth are in the patient relative to the floor and to the benchtop. Sometimes this results in a lack of adequate space to mount the model with the magnetic transfer mounting. For these situations we utilize a screw-on mounting plate that requires much less space. (Refer to Figures 6 – 9)
It's always a good idea to have a few of these screw on plates around. While they are not often necessary, they are a tool for mounting the casts in a relationship to the articulator that we believe serves to maintain a very important reference, which is the teeth as they are in the patient to the horizon.
Of course all of these relationships can be changed simply by tilting the patient's head – so be sure that the head is oriented as the person stands looking at the far off horizon. Remember that hanging anything on the front of their face will make them want to tip back to counter that weight, so look before you start and guide the patient to a natural position.
Gary DeWood, D.D.S., M.S., is Executive Vice President of Spear Education and a member of Spear Resident Faculty.