Figure 22. Consequence of NHP recording error. This symmetric patient rotated his head 2 degrees clockwise during NHP recording. It caused the upper and lower dental midlines to look deviated when they were not. In the first method, standardized frontal and lateral facial photographs taken with the patient in the NHP serve as visual guides to manually reorient the 3D CT in the computer. Although this method is subjective, it is valuable for checking the outcome of advanced methods.
In the second method, a patient is placed in the NHP. Then, the perpendicular lights of a laser-level are projected onto the face of the patient and the level is moved until the laser’s vertical line is on the patient’s median plane and the horizontal line crosses the external auditory canals. Next, a skin marker (i.e., a pen) is used to delineate six points on the skin of the face, establishing the orientation of the reference lines directly onto the patient. Following this, radiopaque markers are tapped on the skin markings and the patient is CT scanned. After scanning, the markings are used to build an anatomical frame of reference. Unfortunately, this method has not been formally validated. A theoretical disadvantage is that it relies on skin landmarks that can be easily displaced.
The third method of reorienting a CT to the NHP uses an orientation sensor to record the NHP before CT scanning (Figure 23). The sensor is attached to the same bite-jig used for registration. Next, the patient, with the bite-jig between his teeth and the sensor in front of it, stands erect with his head in the NHP. In this posture, the pitch, roll, and yaw of the sensor are recorded. Because the sensor is orthogonal to the bite-jig frame, the orientations of the sensor and the frame are always equal. By establishing the orientation of the sensor while the patient is in the NHP, the orientation of the frame of the bite-jig for the same posture is also established. In the next step, the sensor is detached from the bite-jig and a fiducial facebow is attached orthogonally to the bite-jig, giving the bite-jig frame and the facebow the same orientation. Subsequently, the patient is CT scanned while holding the bite-jig and facebow. Thereafter, the CT, including the imaged facebow, is segmented and rendered as a 3D model. Finally, the 3D model is rotated until its facebow attains the measured NHP orientation, placing the whole 3D model in the NHP. The advantage of this method is that it has been validated in vitro and clinically.