As dentists, when we see a problem, we look to see how we can fix it from a tooth-based approach. Admittedly, some of my own past patients have been subject to this way of dental treatment.
But there are many times that one must take a step back and ask the right questions to gain a comprehensive understanding, beyond a single tooth, of the problem at hand. In this article, I will discuss the pathological discrepancies between centric relation (CR) and maximum intercuspation (MIP, also known as centric occlusion CO) and how this slide can lead to tooth failure.
Before we can discuss the teeth, we must assess the system in which the teeth work. The system is comprised of a triad of temporomandibular (TM) joint, muscle and teeth.
Let's start with centric relation. There are seven different definitions for CR in the Glossary of Prosthodontic Terms. For the sake of this article, we will consider CR as “the maxillomandibular relationship in which the condyles articulate with the thinnest avascular portion of their respective disks with the complex in the anterior superior position against the shapes of the articular eminences.”1
This position is independent of tooth position; the mandible exhibits rotary movement in this position along the transverse horizontal axis; and it is a reproducible position for clinicians. As a clinician, getting the patient into this position is a prerequisite to the analysis of the system. The finishing position in regards to this article is MIP.
Maximum intercuspation is “the complete intercuspation of the opposing teeth independent of condylar position.”1 CR and MIP only coincide in about 10 percent of the population, leaving 90 percent of the population to have a slide.1 As a clinician, you must determine if CR is a functioning position and how the patient gets from CR to MIP. Only then can you determine if the slide is pathological or physiological.
When the teeth, muscles and joints are able to function independently of one another, the system will remain stable. In a pathologic occlusion however, “the three elements of the triad are dependent on each other and the powerful muscle engrams accelerate the aging of the teeth and TM joints.”2
The best analogy that I have heard and use with my own patients is that the posterior teeth should come together like a perfectly balanced door. As the mandible closes, the TM joints (the hinges) will rotate the mandible so the lower posterior teeth (the door) will fit perfectly into the upper posterior teeth (the frame) allowing the teeth to contact harmoniously and simultaneously. This is easier said than done, as a lot of pieces have to come together constantly just at the right moment to make this perfect closure. If the door closes poorly, drags on the floor, or has to be wedged into the frame, the system will function poorly.2
Dr. Pankey notes in his book “A Philosophy of the Practice of Dentistry”: “I see patients who do not have centric relation occlusion within hollering distance.”2 The point is that very few patients have occlusions that close like a balanced door.
It is important for the first point of contact to be “one of stability, occurring simultaneously on as many teeth as possible.”2 The contacts should not occur on inclines but instead be a solid stop. Ideally, squeezing the teeth after the first point of contact should not produce any apparent slide to achieve complete closure.
Dr. Peter Dawson takes the concept a step further in his textbook “Evaluation, Diagnosis, and Treatment of Occlusal Problems” by stating: “The occlusal contours of all the posterior teeth are dictated by both condylar guidance and anterior guidance. No posterior tooth should interfere with either anterior guidance or condylar guidance. Posterior teeth may either be discluded from any lateral contact by the anterior teeth, or they must be in perfect harmonious group function with them and the condyles.”2
The system will start showing signs of stress when condylar guidance, anterior guidance and incisal guidance become dependent on one another, causing the system to age rapidly in a pathologic fashion. These signs can display as: an asymptomatic click in the TM joint, craze lines, abfractions, or soft tissue irregularities in the periodontium.2
When dependent on each other, the TM joints and the teeth will either adapt or age quickly. If the three guidance systems adapt together over time, change can be physiologic, leading to natural aging of the joints and teeth. However, if the guidance systems oppose each other strongly, this will lead to pathologic changes.
In part II of this article, I will discuss actual cases I have treated that show pathologic changes in the teeth due to a strong CR to MIP slide and how they were treated.
1Hamdan, B. Centric Relation, The Basic Reference. Published October 18th, 2014. Accessed December 11th, 2016.
2Supple, R. Anterior Guidance. Accessed December 11th, 2016.
3Christenson, G. Ask Gordon: Is there always a shift from Centric Relation (CR) to Maximum Intercuspation (MIP)? Published June 27th, 2011. Accessed December 11th, 2016.