Redefining Occlusion

Occlusion has been the source of intense debates in our profession for the past several decades. Much of the confusion about occlusion could be resolved if we redefined it. Occlusion is usually defined as how the teeth fit together at the first molars, as outlined by Angle.¹

The shortcoming of defining occlusion solely by tooth position is that the mandible is comprised not only of the teeth on the front end, but also the condyles and disks on the back end. Defining occlusion at the tooth level inherently limits the ability to evaluate the articulation of the entire masticatory system.

The anatomic reality is that instead of having one point of articulation (the teeth), there are three points of articulation that must be evaluated to offer our patients appropriate treatment options and a reasonable prognosis. By redefining occlusion as how the lower jaw fits to the upper jaw, an occlusal analysis consists of evaluating how the mandibular teeth fit to the maxillary teeth, how the right condyle disk assembly fits against the right glenoid fossa and how the left condyle disk assembly fits against the left glenoid fossa.

When evaluating the occlusion both at the back end of the system, as well as the front of the system, it becomes evident that many of the tooth-based malocclusions that have confused our profession can be explained by changes at the temporomandibular joint level.²

Normal TMJs are comprised of the soft tissue component (the disk) covering the hard tissue (condyle).³ When joints undergo structural alteration, the initial tissue deformation typically occurs at the soft tissue level resulting in a loss of ligament attachment between the disk and the condyle. This can occur at the posterior ligament attachment, the lateral ligament attachment, the medial ligament attachment or a combination of the ligament attachments.⁴

Schellhas⁵ discussed internal derangements either retarding or arresting condylar growth ultimately resulting in either mandibular deficiency or asymmetry.

Flores-Mir⁶ wrote that TMJ disk abnormality was associated with reduced forward growth of both the mandibular and maxillary bodies, as well as reduced downward growth of the mandibular ramus.

Tallents⁷ outlined the relation to displaced disks in growing children and suggests that they are at risk to have at least altered mandibular growth with possible development of a retrognathic facial appearance and increased lower facial height.

The two common factors in these articles is that many of the tooth-based malocclusions we see in clinical practice occur as a result of structural changes in the TMJ, which occur far earlier in life than previously assumed.

Given the early onset of structural changes in the TMJs⁸ and the impact on not only the dentition but also the skeletal bases and the airway, our charge in dentistry is to recognize the structural changes in TMJs as early as possible. Since many growing patients will not report pain⁹, the key to diagnosis becomes recognizing the occlusal changes that occur in young patients with structurally altered TMJs.

The key factor in recognizing the occlusal changes is evaluating the occlusion in a fully seated condylar position. This will allow for an accurate assessment of the anterior tooth relationship and an insight into possible structural alterations of the TMJ in asymptomatic growing patients.

If the anterior teeth are uncoupled in a fully seated condylar position greater than the thickness of the disk (2-3mm), structural changes in the TMJ should be a part of the differential diagnosis during the treatment planning process.

Jim McKee, D.D.S., is a member of Spear Resident Faculty.

References

1. Angle EH. Classification of Malocclusion. Dental Cosmos;1899:41:248-264.

2. Schellhas KP, Piper MA, Omlie MR. Facial skeleton remodeling due to temporomandibular joint degeneration: An imaging study of 100 patients. Am J Roentgenol 1990;11(3):541-551.

3. Alomar X, Medrano J, Cabratosa J, Clavero JA, Lorente M, Serra I, Monill JM, Salvador A. Anatomy of the temporomandibular joint. Sems Ultrasound CT and MRI 2007;28:170-183.

4. Choukas N, Sicher H. The structure of the temporomandibular joint. Oral Surgery, Oral Med Oral Pathol. 1960;13(10):1205-1213.

5. Schellhas KP, Pollei SR, Wilkes CH. Pediatric internal derangements of the temporomandibular joint: effect on facial development. Am J Orthod Dentofac Orthop 1993;104(1):51–9.

6. Longitudinal study of temporomandibular joint disk status and craniofacial growth. Flores-Mir C, Nebbe B, Heo G, Major PW. Am J Orthod Dentofac Orthop. 2006;130(3):324–30.

7. Tallents RH, Stein S, Macher DJ, Katzberg RW, Murphy W. Predisposing and Precipitating Factors in Temporomandibular Disorders. Semin Orthod. 2012;18(1):10-29.

8. Kircos LT, Ortendahl DA, Mark AS, Arakawa M. Magnetic resonance imaging of the TMJ disk in asymptomatic volunteers. J Oral Maxillofac Surg. 1987;45:852-4.

9. Karibe H, Goddard G, Kawakami T, et al. Comparison of subjective symptoms of temporomandibular disorders in young patients by age and gender. J Craniomandib Pract. 2012;30(2):114-120.