OKAS-3D: optoelectronic jaw movement recording system with six degrees of freedom

OKAS-3D is a further development of the single floating circle target tracker. This tracker consists of a cathode ray tube display, a lens in front of the display and a circular light-sensitive photocell placed in the image field of the lens. A small circular trace generated on the screen is focused around the photocell. The sinusoidal electrical output of the photocell contains all the relevant information regarding the position of the photocell relative to the image of the circular light trace. This gives the tracker the properties of a servo-controller with position feedback. In OKAS-3D three pairs of photocells are located on two lightweight frames attached to the upper and lower frontal teeth by individually adapted clutches. By using the formulas of rigid body mathematics, the motion of any point of the mandible can be reconstructed. The system permits a high sampling frequency of at least 300 Hz per photocell co-ordinate. The noise, linearity and accuracy are better than 0·08 mm, 0·07 mm and 0·13 mm for points in the lower incisal region, and better than 0·25 mm, 0·17 mm and 0·27 mm for points in the condylar region. Thus jaw movements can be recorded with a high spatial and temporal resolution.