When maxillary and mandibular casts are hand-held in maximum intercuspation, an anterior/posterior, back-and-forth rocking of models is usually observed. Most dentists dismiss this phenomenon to imperfections of casts (ie, flaws attributed to bubbles or distortion of impressions). Ash and Ramfjord feel that impressions are taken with teeth in the passive state, and the occlusal contacts may not be reflected the same as they would be with teeth in the full intercuspal position.1
A very significant and often overlooked feature of the dental anatomy is the curve of Spee. The curve of Spee refers to the anterior/posterior curvature of the occlusal surfaces, beginning with the cusp tip of the lower cuspid and following the buccal cusp tips of the premolars and molars.2 This definition goes back to Dr. Spee's article dated 1928.3
Although some casts may be distorted and often bubbles do exist, this researcher feels that the seesaw, back-and-forth rocking observed in hand-held casts is due to the fact that there is a unique anterior/posterior curve for the maxilla. This curve is of a different value than the curve of Spee for the mandible.
Dr. Spee offers a concept of curvature for the mandibular dental morphology, and it is assumed by the dental community that the maxilla shares common values with the mandible for its anterior/posterior curve. However, the radius of curvature for the maxilla is of a lesser value than the radius of curvature for the mandible. The maxilla possesses its own curve along the canine tip and follows through the buccal cusp tips of the premolars and molars. This is an important anatomical feature of the occlusion and warrants its own name for sake of accurate communication and study. This maxillary curve does not usually share the same value as the mandible, so congruency is not usually found clinically.
The author wishes to suggest that there are 2 anterior/posterior curves for the average dentition. This is normal, healthy, and stabile for the TMJ, occlusion, and muscles of mastication. In only about 10% of cases is there little or no rocking of hand-held casts. This is when individuals share a common centric relation (CR) and centric occlusion (CO) closure. This 10% value parallels observations of shared CR and CO by Ash and Ramfjord.1
Notice yourself facing forward, close lower and upper teeth to the point of first contact; you will note that premolars and anterior teeth touch first. This can also be verified with several patients. In a relaxed TMJ state, the molars are out of occlusion. Other times during the day, and totally unnoticed by individuals, the TMJ is in a working, or loaded, state. This comes about sometimes after a challenging meal or during random times during the day, and is perhaps related to clenching. When the TMJ is in this loaded relationship, the molars are in contact and the premolars are out of occlusion. Lower anterior incisal edges may still contact the lingual surfaces of the maxillary incisors, but at a different contact point. The loaded TMJ anterior contact point is now shifted more toward the incisal on the maxillary incisors.
The swing between loaded and relaxed occlusal contacts does not decisively vary from full anterior to full posterior (and it would if the maxillary curve was the same value as the mandibular curve of Spee). Since the maxillary curve is of a shorter radius of curvature, there are many variations of contacts of occlusion as the mandible rolls from one extreme TMJ state to another, accompanying the transitional positions of the condylar head.
When the TMJ is in the state of loaded positioning with the molars in occlusal contact, the individual's observation of the change of occlusion is particularly evasive. The mandibular postural rest seems more pronounced or facilitated, and complete closure is avoided. The varying degrees of loading undergone by the TMJ, as well as the shifting of first occlusal contacts when closing, go undetected.
When the TMJ is in a loaded state, the occlusal forces are centered on the posterior occlusion. This is logical, since loaded working areas are now closer to the hinge of condylar rotation. This loaded positioning of the mandible is very near, or specifically at, centric relation. To closely simulate the loaded positioning of the TMJ, stretch your head back as far as possible. You'll notice that the molars will contact and that the premolars are out of occlusion. When the TMJ is in a loaded position, forces are directed to the posterior. This loaded position is similar to Okeson's theory of a 45 head tilt as if for drinking.4 He describes this mandibular positioning as near centric relation. Ash and Ramfjord1, however, state that this position is seldom reached in normal mandibular chewing movements, a theory this author holds suspect.
In the author's office, casts are verified for accuracy using a Model Checker (Zeisky Dental Supply Model Checker [Zeisky Dental Supply]). Since verified, accurate casts rock anteriorly and posteriorly, and casts can be held in definitive premolar occlusion, and we can also confirm the same relationships upon closure for our patients mouths as well as our own, the author would offer these concepts as proof that a separate anterior/posterior curve of dentition exists for the maxilla. This curve needs to be identified and labeled for accurate communication.
CURVE OF WILSON
The curve of Wilson is the mediolateral curve that contacts the buccal and lingual cusp tips on each side of the arch. It results from inward inclination of the lower posterior teeth, making the lingual cusps lower than the buccal cusps on the mandibular arch; the buccal cusps are higher than the lingual cusps on the maxillary arch because of the outward inclination of the upper posterior teeth.2
Closing in a resting TMJ state, notice also that the lingual inclines of the lingual maxillary premolar cusps contact the lingual inclines of the buccal cusps of mandibular opposing teeth. Bevron shares a study illustrating a specific buccolingual relation of premolars.6 He shows that the maxillary premolars have lingual inclines of lingual cusps for a certain group of individuals that occlude to the lingual inclines of the buccal cusps of the mandibular teeth. For his group, the lower buccal cusps do not seat completely into the opposing fossa. Other groups of people possess this trait to a lesser extent.
It is difficult to say if the maxilla actually possesses its own unique value for the curve of Wilson (ie, a value for the radius of curvature that is different than the value for the mandible), or if the values for curvature for Wilson are the same, and incongruency is the result of a slightly constricted mandibular arch form. That is, the curve itself could have the same value for both arches, yet the mandibular teeth may be displaced just a bit lingually, still along the same curve path.
THEORY OF MAXIMUM INTERCUSPATION
The reasons for two anterior/posterior curves for the dentition would allow various force loads to be applied to the TMJ. Premolar contacts would be protective of the TMJ, since instant, full-force closure would be avoided. It is possible that the relaxed state of the TMJ is a time for healthy recovery of the articular disc. A rebuild with influx of nutrients may occur at this time.
If 2 curves of unique values exist for the dentition, then maximum intercuspation (MI) or centric occlusion does not exist. MI gives the connotation that basically all upper and lower teeth come together at once upon closing. Casts do rock back and forth, and this is a real reflection of the existing curves of dental morphology. When we usually bite, we are in premolar or unloaded TMJ occlusion. In a loaded state, we bite in molar occlusion.
The anterior/posterior tilting of teeth is appropriate to keep the individual teeth at right angles to the forces of occlusion. Mandibular molars tilt a little forward, and bicuspids are more upright.2 So, the angulation of teeth as well as the separate curves of the dentition are helpful in maintaining the health of the TMJ. But since the individual planes of occlusion are exemplified by 2 arcs, one curve superimposed over a different size curve, simultaneous contact of all maxillary and mandibular teeth would be impossible. Only segmental contacts occur as we go from premolar unloaded contact to molar loaded contact.
The traditional concept of maximum intercuspation between upper and lower teeth would need to be altered. For example, it is believed that MI is a tooth-directed positioning of the jaw.5 At best, MI may relate to unloaded premolar occlusion and is certainly directed by the state of the TMJ. This author would hope the term MI may be further refined; the term maximum intercuspation is nonspecific.
Whatever the reason for the curves of dentition, it would be wise to maintain this complexity of morphology both restoratively and orthodontically. For reasons of TMJ health and stability, the two anterior/posterior curves of dentition exist, and this ar-rangement should be respectfully duplicated in our prosthetic endeavors. The author would want to see full dentures as well as fixed prostheses designed not with a single shared curve, but with dual curves, replicating nature.
Orthodontists often desire an increased vertical dimension of occlusion (VDO). The easiest way to accomplish this is to straighten out the curve of Spee. Yet, the VDO could still be opened while maintaining the natural curves of dentition. It is unnatural for orthodontic treatment to flatten the curve of Spee. It is also unnatural to build a single, common shared curve for both the maxilla and mandible. To flatten the occlusion, or to make a single shared curve, would make for an unstable final result.
The Bevron study6 reveals a certain buccal/lingual relationship of upper and lower teeth. If this occlusal relationship exists for most of us, albeit to varying degrees, then placing lower buccal cusps directly into the upper fossa may lead to instability. From this point of an orthodontically induced mid-fossa relationship, the mandibular teeth would tend to drift lingually for a more natural positioning. A lingual drifting of mandibular premolars would lead to lower anterior crowding, which is the first and most obvious relapse we see related to orthodontics.
Computer scanning by companies such as Invisalign, Othoclear, or Sirona could analyze bite registrations and compute data. This information could furnish us with average measured values for the curves of occlusion to establish a guide. And further, extrapolated from this data, the manufacturers could fabricate a template to implement any necessary occlusal adjustment. Ideal results for occlusal adjustments could be attained from the use of such a template. The custom, full-arch template would have breaks or openings indicating adjustment points, as we do now with copings from the lab for crown and bridge preparation refinements.
Occlusal adjustments as taught today rely on positioning a patient's lower jaw into CR. While maintaining this position with both hands, the dentist is ready to evaluate the occlusion. The process is technique sensitive and beyond the capability of the average restorative dentist. Additionally, all of this is taught today without regard to the separate anterior/posterior curve of the maxilla. A template would take away the complexities and give the general dentist a more specific guideline for occlusal adjustment. It would offer the orthodontist a post-treatment occlusal adjustment never before available to aid post-orthodontic stability.
The maxillary arch has its own anterior/posterior curve, and this curve has a unique value for the radius of curvature separate from the mandibular curve of Spee. This curve needs to be duplicated in orthodontic and restorative endeavors. Due to the complex nature of the curves of occlusion, a template should be available to the dentist for equilibrations.
1. Ramfjord SP, Ash MM. Occlusion. 3rd ed. Philadelphia, Pa: WB Saunders Co; 1983.
2. Dawson PE. Evaluation, Diagnosis and Treatment of Occlusal Problems. 2nd ed. St Louis, Mo: Mosby Books; 1989.
3. Spee FG. Prosthetic Dentistry. 4th ed. Chicago, Ill: Medico-Dental Publishing Co; 1928.
4. Okeson JP. Management of Temporomandibular Disorders and Occlusion. 3rd ed. St Louis, Mo: CV Mosby; 1993.
5. Parker MW. The significance of occlusion in restorative dentistry. Dent Clin North Am. 1993;37:341-351.
6. Beyron H. Occlusal relations and mastication in Australian Aborigines. Acta Odontol Scand. 1964;22:597-678.
Disclosure: Dr. White invented, patented, and produces Model Checker under the company name Zeisky Dental Supply.