The Science of Aligning Body Parts To Improve Function – Part 1
Evaluating the Cervical Spine and Head
By Clayton A. Chan, D.D.S.
Related reading see: What Angle is the Occlusal Plane Relative to the Horizon?
The antero-posterior (AP) posture of the head in relation to the cervical spine can be determined by a vertical plumb line from head down to the dorsal portion of the thoracic spine. (Most restorative, orthodontic and TMD dentist don’t pay attention to this aspect of head posture). Garry has reported measuring the neck curvature using a plumb line to the deepest curvature of the posterior of the cervical neck region to determine the distance and degree of forward head posture.(1) Optimal distance according to Racabado is 6 cm.(3, 4)
Racabado further defined normal cervical lordosis and normal craniovertebral relationships based on cephalometric measurements. A line that connects the posterior nasal spine to the basi-occiput is called the McGregor’s plane. The odontoid plane (OP) is a line that extends from apex to the anterior inferior angle of the odontoid process. A normal measurement of the posterior-inferior angle at the intersection of McGregor’s plane and OP is 101 degrees +/- 5 degrees (96-106 degrees). A distance between the basi-occiput to the posterior arch of the atlas is 4-9 mm (less than 4 mm indicates cranio-vertebral compression).(3, 5)
For more information and details read more: Chan, C.A.: Evaluating the Cervical Spine and Head, page 15-19.
Finding the Occlusal Plane
Many clinicians abroad have determined and confirmed that the occlusal plane has a great influence on the functional articulation and esthetic aspects of stomatognathic system. Clinicians have recognized that the occlusal plane should be at a right angle to the occlusal forces for stability of the occlusion, head and neck musculature; perpendicular to the long axis of occlusal forces produced by an optimal cervical posture and an optimized mandibular closing trajectory (path). Many neuromuscular minded dentist have falsely been misled to believe that the occlusal plane is flat or parallel relative to horizontal level. This is in fact a wrong teaching that defies natures bio-physiologic and anatomical design for health.
Note: Hand held diagnostic model casts oriented into a maximum intercuspal position is not sufficient to establish natures “true” occlusal plane orientation. Determining a physiologic dimensional relationship of the upper to lower model casts (vertical, antero-posterior, frontal, pitch, yaw and roll) is crucial if one desires to minimize mandibular to maxillary jaw torque during function. Reproducing nature’s occlusal plane orientation with the model casts using the Modified Fox Plane technique per Chan assists the dentist and lab to replicate what is in the patients mouth. Read more: “Mounting the Diagnostic Case Using the Fox Plane”.
The inclination of the occlusal plane (IOP) is one of the key factors governing occlusal balance. (6) Determination of inclination of the occlusal plane is an important step before construction of full arch restorative and full denture type cases. Evaluating and assessing the bilateral occlusal plane of the maxillary arch for symmetry, balance and form is desirable to meet the demands of function and appearance. Anteroposterior IOP is typically determined with a device called the Fox occlusal plane, (7) which is commonly positioned parallel to Camper’s plane or can be used to capture the occlusal plane slope by orienting the Fox plane (modified according to Chan) parallel to the horizontal level (the floor) and patient’s head positioned at level using bite recording material.(8) Ear bows or face bows have been traditionally been used in the gnathologic arena to register the steepness and tilt of the occlusal plane using the auditory meatus, nasion and orbitale as reference points.
Note: Radiographic images of before restorative treatment, progress (stabilized mandible at optimized mandibular position) and after (finished restored occlusal plane at optimized mandibular position). All images were recorded with the patient’s head position (“unposed” purposely – not at Frankfurt plane) to determine how the mandibular occlusal positioning can effect the occlusal plane and head posture. Initial radiographic shows a flatter occlusal plane relative to horizontal level (red line), patient’s head is slightly tilted upward with neck strain. As the patient is stabilized with an improved gneuromuscular occlusal position the head and cervical/neck also changes orientation relative to horizontal level. Patient’s neck pain is resolved using a lower fixed bonded orthotic over the existing restorative dentistry. Final imaging shows improved occlusal plane (angled relative to horizontal level. Patient no longer complains of neck and shoulder pain. To read more about this case and “What Angle is the Occlusal Plane Relative to Horizon”
Flat Maxillary Cast Mountings
Maxillary cast mounting and orientation is critical in phase II diagnosis and treatment. Maxillary cast that are mounted in a manner that depicts a flat to upward slope anteriorly (e.g. classic HIP – hamular notch, incisive foramen, flat mount) often times will unknowingly build in pathologic vector of forces due to an abnormal occlusal plane orientation.
- This will lead to toothy looking smiles, especially in the second bicuspid molar region and will also result in vertically taller mandibular posterior clinical crowns.
- A curve of Spee is often compromised and minimal due to inadequate space between the desired occlusal plane and tooth preparation during laboratory waxing and crown fabrication.
- To compensate for this the clinician will need to be more aggressive in maxillary posterior occlusal prep reduction which can lead to pulpal infringement and endodontic therapy.
- Skewed crown to root ratio in both the upper and lower arches will be present making arch and crown form lacking in natures anatomic proportions.
Downward Sloping Maxillary Cast Mounting
Maxillary cast mountings that show a natural downward sloping occlusal plane incline (e.g. modified Fox plane level mount according to C. A. Chan) allows for balanced vector of muscular and occlusal forces due to a more physiologic plane orientation that matches an optimized cervical, head and craniomandibular posture.
- Softer natural smile line leads to a more pleasing esthetic smile.
- Less posterior occlusal prep reduction in either maxillary or mandibular arches is required.
- Easy to develop a curve of Spee.
- Crown to root ratio tends to be closer to natures golden proportions.
References:
- Garry, JF: Upper Airway Compromise and Musculo-skeletal Dysfunction of the Head and Neck (MSD), 1977.
- Yee, D: Weight of the human head. Department of Anatomy and Histology, University of Sydney, Australia. danny.oz.au/anthropology/notes/human-head-weight.html. December 13, 2006.
- Tilley, L and Hickman, DM: TMD-An Upper Quarter Condition. Anthology of ICCMO. Vol. V. 1995.
- Racabado, M: Physical Therapy and Dentistry: An Overview, J of Craniomandibular Practice, 1982; Vol. 1, pp46-49.
- Racabado M: Dentistry I. Racabado Institute for Carniomandibular and Vertebral Therapeutics, Atlanta: Institute of Graduate Health Sciences, 1984.
- Ogawa T, et al.: Inclination of the occlusal plane and occlusal guidance as contributing factors in mastication. J Dent. 1998 Nov;26(8):641-7.
- Trubyte (Fox) Occlusal Plane Plate: Dentsply International, Trubyte, Occlusal Fox Plane Item #92232, (800) 877-0020.
- Chan, CA: Architecting the Occlusal Plane. Aurum Ceramic Continuum. Vol. 10, Issue 2, May 2006.
© 2009 Clayton A. Chan, DDS. All Rights Reserved.
