Occlusion, orthodontics and the traditional assesment criteria of the cast radiographic analysis (cre).
Orthodontics is a fine balance of science and art. Orthodontic treatment goals can be divided into five categories: facial aesthetics, dental aesthetics, functional occlusion, periodontal health, and stability. An “Ideal” occlusion was first classified by Angle in the early 20th century into the molar Class I, II, and III relationships. It’s interesting how Angles classification still dominates in modern orthodontic diagnosis and treatment objectives. His classification was based on the morphological relationship of the teeth and the dental arch, and a subjective opinion on how the face looked in balance when a class I occlusion was present. There was no consideration for the dynamic skeletal relationship with regard to function. Andrews elaborated on Angles’ (Angle, 1907) classification in his “six keys to normal occlusion”, which considered angulation and inclination, rotations, spacing and the occlusal plane 1. It was generally assumed that an ideal static occlusal relationship is compatible with an ideal functional occlusion, but this is not necessarily so and hasn’t been backed by hard scientific data 2. The ideal occlusion refers to an occlusion which has ideal static and functional occlusal relationship (mutually protected occlusion). Several functional occlusion types present in nature. These include a balanced occlusion, canine protected occlusion, group function occlusion, and biologic (multi-varied) occlusion. None of these occlusal schemes have been shown to dominate in nature3 but should be of considered in our diagnosis and treatment planning. Ultimately an end goal of a ‘normal’ relationship of centric occlusion to centric relation free of TMJ dysfunction. We treat individuals with natural variation, and a one size fits all approach seems counterintuitive.
A malocclusion has been described as any deviation from the physiologically acceptable contact of opposing dentition. The role of the orthodontist traditionally has been to correct this deviation primarily to improve esthetics and questionably function. This is the blend of art and science.
Orthodontics as a profession has continually looked for ways to quantify a malocclusion to compare pre-treatment and post treatment records to determine the quality of the result. Several indices are still used today such as the Occlusal index, Par index, and IOTN scale. These indices have not satisfied the American Board of Orthodontics in their examining process. An ABO committee was assembled in 1994 to establish a precise method to objectively evaluate treatment outcomes in orthodontic care from which orthodontic students are examined. A dental Cast-Radiographic Evaluation was developed from ‘field testing’ hundreds of dental casts and panoramic radiographs, while at the same time calibrating their committee. This committee felt that this was a precise way to evaluate orthodontic case outcomes. This testing was established in 1999 and continues today.
The ABO Model Grading System for scoring dental casts and panoramic radiographs contains eight criteria. These are: alignment, marginal ridges, buccolingual inclination, occlusal relationships, occlusal contacts, overjet, interproximal contacts, and root angulation(via panoramic radiograph analysis).
Let’s have a closer look at each of these ‘static’ records and reflect on its influence on a healthy functional occlusion for each individual human we treat.
Alignment. “In the anterior region, the incisal edges and lingual surfaces of the maxillary anterior teeth and the incisal edges and labial-incisal surfaces of the mandibular anterior teeth were chosen as the guide to assess anterior alignment. These are not only the functioning areas of these teeth, but they also influence esthetics if they are not arranged in proper relationship”.
Marginal ridges “are used to assess proper vertical positioning of the posterior teeth”. It is logical to have a smooth transitioning occlusal plane to avoid traumatic occlusal forces and interferences.
Buccolingual inclination “is used to assess the buccolingual angulation of the posterior teeth. In order to establish proper occlusion in maximum intercuspation and avoid balancing interferences, there should not be a significant difference between the heights of the buccal and lingual cusps of the maxillary and mandibular molars and premolars”. I don’t feel this is point is scientific.
Occlusal contacts are measured to assess the adequacy of the posterior occlusion. Again, a major objective of orthodontic treatment is to establish maximum intercuspation of opposing teeth. There is no differentiation in this measurement to appropriate forces on specific teeth. Should the posterior dentition take the majority of occlusal load? There is no record of how this ideal intercuspal position relates to central relation, which is surely extremely important.
Occlusal relationship “is used to assess the relative anteroposterior position of the maxillary and mandibular posterior teeth. The most verifiable method of scoring this criterion is to use Angle’s relationship.” This relationship shows little bearing to function or esthetics.
Overjet “is used to assess the relative transverse relationship of the posterior teeth, and the anteroposterior relationship of the anterior teeth”. This obviously can affect esthetic outcomes, but at which point does an increased overjet become unaesthetic and it is debatable whether it effects function.
Interproximal contacts “are used to determine if all spaces within the dental arch have been closed. Persistent spaces between teeth after orthodontic therapy are not only unaesthetic but can lead to food impaction”. Again, as esthetics is entirely subjective, I don’t feel this is a strong indicator of treatment success. Obviously, we don’t want to create food traps leaving small spaces and poor interproximal contacts. I have seen plenty of patients with missing teeth and large spaces who have great smiles and functioning occlusion.
Root angulation “is used to assess how well the roots of the teeth have been positioned relative to one another”. If roots are properly angulated, then sufficient bone will be present between adjacent roots to support the periodontium and soft tissue. Again, I’m not sure if science backs this point, and from what long-term cases I have seen that the roots tend to settle in a natural position influenced by function and relevant intrinsic oral forces.
In my opinion there are many flaws in this ABO CRE evaluation, in assessing outcomes of orthodontic treatment. The glaringly obvious fact that there is an individual human being from which these two diagnostic records are extracted. This individual may have multiple biological, psychological, physical, pathological or environmental variations that will influence their treatment outcomes. Indeed, a perfect ‘field tested’ occlusion does not guarantee health and a harmonious functioning craniofacial complex. There is no assessment of the how the casts of the patient relate to their smile, soft tissue balance, temporomandibular joint function or even skeletal base. There is an assumption that by achieving these aforementioned criteria all will be well. There is no assessment of, the occlusal scheme or working occlusion and/or interferences or dynamic function (smiling, mastication, speech or swallowing). There is no assessment for soft tissue function, which has a large influence on the equilibrium of oral environment. We all know the problems associated with aberrant muscle function and its related side effects on the temporomandibular complex4.
The micro aesthetics and final smile is one of the main motivating factors for orthodontic treatment so careful assessment and planning is prudent. Vertically, incisal edges and the gingival zeniths should be symmetrical and at ideal heights, and the widths of the teeth should follow the golden proportion. Recently the esthetics of the smile indicate that buccal corridors (the dark space between the dentition and the corners of the lips) should be modest or reduced, although not eliminated completely. The curvature created by the incisal edges of the anterior maxillary teeth is most esthetic when following the curvature of the lower lip upon smiling.5
In a functional occlusion, MI coincides with CR, cusps in the opposing arches are interdigitated, occlusal forces are equally spread among all the teeth, and lateral excursions have canine guidance. Group function in lateral excursions may be acceptable in cases with severe wear or differing skeletal patterns. No balancing interferences should be present. Protrusion should exhibit anterior guidance with posterior disclusion.6
The ABO CRE evaluation of ‘treatment success’ is not reflective of precision medicine which is the developing modern mantra in health sciences. There is a need to identify and respect the biological make-up of the individual and their specific treatment aspirations. Then as an orthodontist we need to try and meet these patient demands within all the constraints of their individual craniofacial complex. Treatment goals should include therapies that cause no long-lasting harm while at the same time help create beautiful smiles and an esthetic facial balance that fulfills the realistic demands of the patient. Underlying this improvement in esthetics should be a healthy functioning dentition and maxillo-mandibular complex.
1. Andrews, L. F. The six keys to normal occlusion. Amer J Orthodontics 62, 296–309 (1972).
2. Tipton, R. & Rinchuse, D. The relationship between static occlusion and functional occlusion in a dental school population. Angle Orthod 61, 57–66 (1991).
3. DiPietro, G. J. A study of occlusion as related to the Frankfort-mandibular plane angle. J Prosthet Dent 38, 452–458 (1977).
4. Okeson, J. P. Evolution of occlusion and temporomandibular disorder in orthodontics: Past, present, and future. Am J Orthod Dentofac 147, S216–S223 (2015).
5. Machado, A. 10 commandments of smile esthetics. Dent Press J Orthod 19, 136–157 (2014).
Also referenced the ABO CRE Grading system https://www.americanboardortho.com/media/1191/grading-system-casts-radiographs.pdf