Role of Biomechanics in Periodontal Therapy

Dr. Angelo A. Caputo and Dr. Robert S. Wylie

The three-dimensional technique makes use of a special property of certain of the plastic modeling materials. When these materials are subjected to loads at specific elevated temperatures and the loads are maintained while the temperature is slowly reduced to room temperature, the stresses remain after the loads are removed. This procedure of locking in stresses is called stress freezing. With the three-dimensional technique, full geometric fidelity may be achieved (Fig. 1-16). In order to the extract three-dimensional stress data, the model must be sectioned into thin slices (Fig. 1-17). Each slice is then analyzed as a two-dimensional model. Analysis of all the sections allows the construction of the full three-dimensional stress picture. However, the necessity of slicing the model means that for each load condition and each appliance, a new model is required. In addition, the full three-dimensional model is more difficult to fabricate than the two-dimensional models. The decision to use the stress freezing technique must be made after weighing the difficulty involved in obtaining the additional stress information against the need for the more detailed and accurate information.

Figure 1-16 Figure 1-17

Quasi-three-dimensional Technique

• No restrictions on geometry, load, or stress variations
• No sectioning - examine as two-dimensional model

• Good geometric fidelity
• Multiple load situations
• Various appliances

• Incomplete three-dimensional stress picture

The quasi-three-dimensional technique was developed to capture some of the advantages of both the two- and three-dimensional techniques. This technique provides some unique advantages, with, of course, some limitations. The quasi-three-dimensional approach uses models with arbitrarily accurate geometric fidelity (Fig. 1-18). The main difference between the quasi-three-dimensional technique and the true three-dimensional technique lies in the means by which the stresses are observed and recorded. With the two- and three-dimensional techniques, models, or sections of models, are examined that do not have any significant stress variation through the model or section thickness. The quasi-three-dimensional technique does not impose the restriction of planar stress distribution. In fact, relaxation of this restriction is accepted as a limitation of the technique. In addition to the advantage of good geometric fidelity is the advantage of being able to apply multiple, complex forces to various appliances placed on the model (Fig. 1-19). Unlike the two-dimensional technique, there is no restriction that forces and stresses be planar. Further, unlike the three-dimensional technique, the model need not be destroyed to obtain the photoelastic data. The main disadvantage of the quasi-three-dimensional technique is the inability to obtain the true three-dimensional stress distribution within the model.

Figure 1-18 Figure 1-19

Combined Technique Applications

• Sequential or simultaneous use of more than one photoelastic technique

Often the most efficient means to obtain solutions to clinical problems involves the sequential or combined use of photoelastic techniques. For example, in certain complex problems, one might begin with a two-dimensional modeling procedure. Based on the results of the two-dimensional tests, a better, more well-informed formulation of a three-dimensional approach may be accomplished. Another plan of attack involves using only three-dimensional models but examining these stress-frozen models in a quasi-three-dimensional manner. After such an examination, the models may be annealed (relieved of stress) by subjecting them to the stress freezing cycle with no applied loads. In this way the effects of many load conditions and appliances may be evaluated quasi-three-dimensionally without destroying the model. Depending on the nature of the data, a decision may be made to section the model at some point. Either of these two approaches or other possible combinations will facilitate the determination of stress data on particularly complicated situations.

Photoelasticity as a predictor

Questions are raised occasionally as to the relationship between the results obtained from homogeneous, isotropic models and the biologic systems being modeled. Many investigations reported that the photoelastic technique, not necessarily surprisingly, predicted biologic responses. One of the earliest and perhaps most conclusive examples was a demonstration of a correlation between a three-dimensional, stress-frozen photoelastic model and histologic specimens in a case of canine root tipping in a cat.5 Where tension was observed in the photoelastic model, the corresponding histologic material showed the periodontal ligament fibers stretched, as illustrated in Figs. 1-20A and 1-20B. Where the model showed pressure, the histologic sections demonstrated corresponding compression of the periodontal fibers. Finally, where high fringe orders were present in the model (Fig. 1-21A), hyalinization was evidenced in the histologic material (Fig. 1-21B). The homogeneous, isotropic photoelastic model was seen to be predictive of the cellular response in a situation of root tipping.

Figure 1-20

Figure 1-20B

Figure 1-21

Figure 1-21B

Another striking example dealt with the effects of orthodontic intermaxillary Class lll mechanics on craniofacial structures.6,7 A stress freezing, quasi-three-dimensional analysis revealed that Class lll elastics affected various sutures of the craniofacial complex of the photoelastic skull. It was shown that a positive relationship existed between computerized cephalometrics and photoelastic techniques in the analysis of ten treated cases. The relationship was based on the fact that specific changes that took place during treatment were consistent with the stresses observed photoelastically.

These examples reveal the effectiveness and importance of photoelasticity to predict the effects of stress in biologic systems. These facts, together with sound modeling procedures, make the techniques of photoelasticity very useful in elucidating the biomechanical principles of clinical dentistry.

Summary

The orofacial complex presents some special difficulties with respect to predicting responses to applied forces. These difficulties include structures with varying complexity of mechanical behavior, intricate geometry, and boundary conditions that are not readily specified. Regardless of these facts, the laws of physics still hold. Application of the principles that have been briefly reviewed in this section will enable the clinician to better understand the specific situations which arise and will assist in establishing treatment modalities with improved prognoses. The principles presented here form the basis for the understanding of specialized subjects in the subsequent sections.

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References

1. Craig, R.G. Restorative Dental Materials, 7th ed. St.Louis: The C.V. Mosby Co. 1985.
2. Cochran, G.V.B. A primer of Orthopaedic Biomechanics. New York: Churchill Livingtone, 1982.
3. Fung, Y.C. Biomechanics. New York: Springer-Verlag New York, Inc., 1981.
4. Durelli, A.J., and Riley, W.F., Introduction to Photomechanics. Englewood cliffs, N.J.; Prentice-Hall, Inc., 1965.
5. Brodsky, J.F., Caputo, A.A., and Furstman, L.L. Root tipping: A photoelastic-histopathologic correlation. Am. J. Orthod. 67:1, 1975.
6. de Alba, J.A., Caputo, A.A., and Chaconas, S.J. Effects of orthodontic intermaxillary Class III mechanics on craniofacial structures: I. Photoelastic analysis. Angle Orthod. 49:21, 1979.
7. de Alba, J.A., Caputo, A.A., and Chaconas, S.J. Effects of orthodontic intermaxillary Class III mechanics on craniofacial structures: II. Computerized cephalometrics. Angle Orthod. 49:29, 1979.