Nature has amazingly design the masticatory system in such a manner to meet the functional demands in a very physiologic manner. Our occlusal, cervical, airway and postural system has been pre-determined genetically to function optimally relative to earth’s gravitational forces. An angled or slanted occlusal plane is natures design when the head is properly supported by balanced muscles.
Because of gravity, the muscles of the structural system as well as the masticatory system allows the human being to function with the head in properly balanced manner to avoid strains and fatigue to the overall postural system. A person with a forward neck posture will accommodate to maintain the flow of oxygen into his/her body. This forward neck and head posture tendency is indicative of an upward head tilt with a resulting flatter occlusal plane. The upward head tilt contributes to TMD and accommodative pathologic issues (e.g., shoulder pain, neck pain, temporal headaches). This upward head tilt is the bodies way to accommodate due a mal-aligned bite which tries to defy natures gravitational vertical forces, resulting in a flatter more leveled occlusal plane which does not represent what nature originally intended in its design.
The SN (Sella-Nasion) Plane is noted in the dental literature as a standard objective levelling reference. What you see in the lateral ceph below is one of one of my patients, male age 47 who presented with former TMD cervical neck problems. Symptoms were resolved with an orthotic. The previous upward head tilt responded positively to the orthotic therapy allowing nature to correct it’s head posture including and an improved occlusal plane. Based on this corrected and more normalized occlusal plane orientation as seen in the below lateral cephalogram I want to transfer this occlusal plane orientation to a flat analysing table on my articulator to fabricate the upper and lower restorations for FM rehab aesthetically and anatomically.
It seems to me that if SN plane is level, symptoms are gone, and patient’s head is now balanced. Recording this position is the most logical to communicate with the lab to mount the case.
To record this maxillary occlusal plane orientation I use the Fox plane as indicated in the blog articles and other publications I have written. It takes the guess work away from the laboratory as long as you implement the process properly like anything. Errors can occur during the following steps:
1. During the bite registration stage. If one use computerized mandibular scanning (jaw tracking – e.g. scan 4/5 with TENS) and does not have adequate training to accurately interpret and record this kind of neuromuscular bite registration error can occur before the case is even started.
2. If the EMG interpretation is not correct and one implements erroneous EMG diagnostic protocols (unable to decipher the differences between fatigued EMG patterns vs. normalized EMG patterns or does not monitor the cervical group EMGs believing that these EMG recordings are the same as SCM EMG recordings one will be greatly mistakened.
3. Subjective interpretation in these areas of diagnostics will also lead to failed treatment results.
I believe in using good artistic and scientific clinical judgments to record head levelness like any diagnostic protocol, using the Modified Fox Plane technique as I have formerly indicated is a simple, inexpensive and effective tool every dentist in North America can use. (By the way, every dental student has been issued a Fox Plane in dental school as a basic learning tool). Note the angle of the occlusal plane.
Clayton A. Chan, DDS, MICCMO
Occlusion Connections, 17th April 2010