ARTÍCULO CIENTÍFICO

 

Experimental Biomechanical Study of the Musculo-Skeletal Masticatory System. Applications to the Study of Osteosynthesis

Estudio biomecánico experimental del sistema musculo-esquelético masticatorio. Aplicaciones para el estudio de la osteosíntesis^*

 

 

 

J.L. Cebrián Carretero¹, M.T. Carrascal Morillo², G. Vincent Fraile³, F. Ortiz de Artiñano⁴

1 Servicio de Cirugía Oral y Maxilofacial.Hospital Universitario La Paz.
2 Dpto de Mecánica.
3 Laboratorio de prótesis dental.
4 Servicio Cirugía Oral y Maxilofacial. Hospital de Cuenca. España

^*Galardonado con la beca tarma SECOM 2005

Correspondence

 

 

 

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ABSTRACT

The biomechanical behavior of the masticatory system is not well known. The mathematical models that have been developed to date are limited and experimental studies have not yet solved the problem of reproducing muscular forces and stress distributions in the internal mandibular structure. A static simulator of the masticatory system was developed in the present study and threedimensional photoelasticity was used to analyze stress distribution in several physiologic and pathologic situations. The results showed that the simulator and 3D photoelasticity were useful for studying interactions and ostheosynthesis materials used in daily clinical practice.

Key words: Photoelasticity; Mandible; Biomechanics.

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RESUMEN

El comportamiento biomecánico del sistema músculoesquelético dista mucho de estar esclarecido. Los modelos matemáticos han mostrado importantes limitaciones, y los ensayos biomecánicos se han visto frustrados por la dificultad de simular las cargas musculares y la distribución de las tensiones en el espesor mandibular. En el presente trabajo se desarrolla un simulador estático del sistema músculo esquelético masticatorio que reproduce fielmente la situación fisiológica, empleándose la foto elasticidad tridimensional para el estudio de los cambios tensionales que ocurren en la estructura mandibular en diversas situaciones fisiopatológicas. Los resultados de los ensayos demuestran que la fotoelasticidad 3D aplicada en el simulador da resultados muy útiles para el análisis de la aplicación hueso-material de osteosíntesis utilizado en la práctica clínica.

Palabras clave: Fotoelasticidad; Mandíbula; Ensayos biomecánicos.

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Introduction

Speech, chewing, and swallowing are voluntary actions that take place almost unconsciously, but require the correct and coordinated function of several muscles and bones. Attempts have been made to understand mandibular function using biomechanics, the science concerned with the study of the forces that govern the motility of living beings and their external and internal effects.¹

The application of mechanical engineering knowledge to this topic has two conditionants:

a. The laws of mechanics have been formulated using models, materials, and abstract systems that are not easily applied to biological systems. The osseous material on which the muscles are inserted and that receive the motor loads clearly differ from any other material studied by mechanical engineering.²

b. The analysis of mechanical stress in bone is a problem that is so complex that it has to be approached using a combination of two methods, a numerical method of mathematical equations and an experimental method of biomechanical analysis to obtain exactly assess what is occurring.

The numerical component is the finite elements method, which has become a fundamental tool in basic science for conducting functional mechanical analyses.³ In the field of mandibular motor function, abstract numerical models have shown limitations related to the difficulty of making threedimensional studies to analyze the interaction between bone and implant.⁴

Experimental studies have been made to resolve this problem, which requires the preparation of a model or simulator of the system that is studied for application of the technique chosen.

Of the experimental techniques used to identify stresses, the photoelastic technique has been widely used for the analysis of highly complex mechanical elements. It has been used frequently to analyze the distribution of stresses in the specific case of bone.

Photoelasticity is based on an optical phenomenon called temporal birefringence. When a transparent material of suitable characteristics is subjected to mechanical loads and is examined on a photoelastic bench or with a polariscope set up for circular polarized light and cross-mounted for light extinction, band spectra called isochromes appear that correspond to the geometric sites of the points at which the difference of the principal stresses is constant, a phenomenon known as two-dimensional photoelasticity.

What occurs when we determine the state of the stresses inside any piece using the photoelastic method when the geometry of the solid does not allow it to be analyzed as a flat elastic regimen? In such cases we use the three-dimensional photoelasticity technique. Three-dimensional photoelasticity is a very precise technique that has the enormous advantage of presenting the state of the stresses present throughout the piece being study. It also can be used with scale models prepared from transparent plastics, or with plastics applied to the prototype. Translucid plastics, such as epoxy resins, are used, which have good photoelastic characteristics and a specific property that makes them suitable for use in threedimensional photoelasticity. These models, when loaded and subjected to a certain temperature for a period of time, conserve the same state of stresses when they cool to room temperature; this is the phenomenon denominated "stress freezing." The molecular explanation for the stress freezing process is that these plastics occur in two different phases: a solid phase at room temperature that is viscous at a certain temperature, and a solid phase that maintains its crystalline solid qualities at both temperatures. Therefore, when this material is warmed the loads applied cause deformation and stress in the crystalline phase, whereas the viscous phase flows freely, preserving the state of stress when the load is removed after cooling. This material is cut into thin slices without introducing new stresses and two-dimensional photoelasticity techniques are use to observe the state of the stresses inside the material.^5,6 This technique is considered ideal for studying the stresses generated in the lumbar spine, anterior cruciate ligament, and jaw.^7,10

 

Material and method

The facial skeleton was reproduced using an adult skull with the maxillae, mandibles and complete dentition. The cranial and maxillary replicate was prepared using a 1:1 scale polyurethane reproduction by obtaining a silicone cast impression of an osseous skull. The skull is the set part of the system. The mandibular prototypes were obtained from a model mandible of a healthy adult with complete permanent teeth, from which replicates were made in epoxy resin. The model was submerged in a self-polymerizing silicone bath that faithfully reproduced the mandibular size, form, and contour. This mold was filled with successive injections of the components of the epoxy resin, which polymerized. Multiple replicates of the jaw were made. The epoxy resin was chosen to its optical and mechanical properties (it has rigidity and resistance similar to bone tissue). The mandibular replicates were articulated with the maxillary bone of the skull. The masticatory musculature (Fig. 1) was replicated by attaching screws, hooks, and adhesive in the sites of muscular insertion as support for elastic bands that were stretched with a force similar to the known forces of the masticatory musculature in different positions. The masseter, lateral pterygoid, medial pterygoid, temporal, and depressor muscles were simulated.

In the specific case of the masticatory skeletal system, we studied the situation of stresses in slices of the part of the mandible with teeth (symphysis, body, and angle) after applying specific loads and preserving them by means of the stress freezing method (Figs. 2 and 3). The study positions were:

Physiologic positions, to study load distribution in healthy mandibles.

1. Position 1: Unloaded control

2. Position 2: Open mouth

3. Position 3: Closed mouth

Pathologic positions

4. Position 4: Mouth closed with elastic maxillomandibular fixation. Strict fixation and immobilization are part of the treatment of mandibular fractures, but they can be very harmful to bone.

5. Position 5: Fractures of the mandibular body repaired with different osteosynthesis materials in order to study bone-implant interactions.

Stresses were evaluated by analyzing the isochromatic lines obtained in the different study areas for each position.^5,11

 

Results

The use of the photoelastic method after stress freezing of study mo dels made it possible to evaluate stress distribution in physiologic (open and closed mouth) and pathologic (mandibular banding and osteosynthesis) circumstances. In physiologic situations the superficial or cortical paths of stress traditionally described by Seipel¹² were shown to have an important impact on the mandibular bone marrow. In pathologic circumstances, information was obtained on the harmful consequences of immobilization and on boneimplant interactions (Figs. 4 and 5).

The stress distribution in the mandibular body, which supports the greatest masticatory load, is shown in figure 4. In position 1 (4A), no isochromatics were observed, as could be expected because no stress was applied. In position 2 (4B), open mouth, stresses were minimal as there was no bite contact. In position 3 (4C), stress lines appeared in both the mandibular cortical and bone marrow that had the direction of the transmission of force due to the action of the elevator muscles. Finally, in position 4 (4D and 4E), the forces due to maxillomandibular fixation and dental wiring were added to the previous forces, which manifested as a functional overload in the region of the tooth-alveolar bone junction (arrow) and bone marrow.

Some of the particulars of maxillomandibular fixation are shown in figure 5: osteosynthesis. In figure 5A, conventional plate compression systems were applied to the bone, whereas in 5B, osteosynthesis was achieved with a screw-plate (keyhole system). As can be seen, the stress distribution and bone overload were lower in the case of screw-plate osteosynthesis, since the force is applied to the plate.

 

Discussion

When we reviewed the biomechanical test methods on which the treatment of traumatic maxillofacial bone pathology is based, we found two important limitations:

The study models did not take anatomic and functional aspects of the system into account.^13,14

1. In all of the models it was difficult to study the interphase between the implant and implant bed in the application of osteosynthesis material.

The static simulator that we present in Material and Methods is an original idea that has the advantage of using exact bone replicates and elastic materials to simulate muscular loads that are applied to the exact anatomic points of muscular insertion.

The model can be adapted for the use of any type of material as a replicate of the osseous component of the system. In our case, as the objective was to evaluate load transmission to the mandibular cortical and bone marrow after applying certain muscular forces, a transparent material with a mechanical behavior similar to bone was needed, although it did not have to have the same microscopic structure. This allowed us to study the implant-bed interphase, which is impossible with the fresh bone or treated cadaveric bone.

We believe that epoxy resin has important advantages. The behavior of epoxy resin is very similar to real bone in terms of load transfer and the interaction between implant and receptor bed, so the results obtained can be extrapolated into clinical practice.¹⁵ Three-dimensional photoelasticity offers extensive information on the directions and intensities of the principal stresses on the cortical and bone marrow of the replicates of a given bone fragment, as well as on the interphase where the interaction of prosthetic material with its receptor bed occurs.^6,7,10

The method is very sensitive, as demonstrated by the fact that in both the physiologic and pathologic positions chosen to evaluate its validity, it yielded results consistent with expectations for the muscular forces applied at each moment and the characteristics of the implant materials in their interaction with bone. For example, in figure 5B it is evident that systems that apply the force of a screw against a plate seem to be less harmful for bone than systems that compress the plate against the cortical, which are more widely used. The effect of these materials on the implant bed is clear in figure 5A.

 

Conclusions

Finally, the surgeon is offered a method that can be interpreted almost intuitively because it has a color scale associated with stress states.

The present study describes a model and reliable method for studying the biomechanical characteristics of the masticatory musculoskeletal system. In addition it offers a way to qualitatively and semiquantitatively analyze stresses acting on the system. The experimental model described here is a starting point to which anatomic and functional variables applicable to the field of mandibular osteosynthesis can be added.

In the near future, it would be useful to develop more applications of this method in order to analyze more complex loading situations and eventually reproduce real work conditions. This model allows us to analyze the behavior of different materials under cyclical load conditions. Dynamization of the system depends on adapting the mechanisms of mouth opening and closing so that they can be examined using the stress freezing process.

 

 

Correspondence:
Hospital Universitario La Paz.
Paseo de Castellana nº 216.
28040 Madrid. España

Received: 27.08.07
Accepted: 17.12.08

 

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