The effect of ventilation tubes on stresses and vibration motion in the tympanic membrane: a finite element analysis.

Finite element analysis is used to determine the mechanical behaviour of structures. The deformation of a structure caused by a force can be calculated, and the stresses and strains within the component can be found. In brief, a geometric representation-a 'drawing'-of the structure is loaded into a computer, and a component is divided into 'elements' which usually have the shape of 'bricks'. A common analogy is a 'lego' brick assembly. The deformation of an element can be determined using engineering formulae, and the deformation of the whole structure can be determined when the elements are re-asssembled. There have been very few analyses of the biomechanical behaviour of ventilation tubes, or grommets, even though clinical studies have indicated that tube design and material determines extrusion rates and may influence tympanosclerosis. This paper reports a comprehensive biomechanical analysis of the effect of a grommet on the tympanic membrane. Analytical and computer simulation techniques (finite element analysis) are used to determine the changes in the vibratory motion and stresses in the membrane when a Reuter-Bobbin ventilation tube is inserted. It is found that the presence of a ventilation tube significantly affects the motion of the membrane in the neighbourhood of the implant. When the effect of implant material is investigated, it is found that the amplitude of motion of a heavier metal implant is less than a lighter polymeric implant. If it is true that higher motion predisposes towards early extrusion, then the lighter implant (polyethylene) is predicted to have a higher extrusion rate. Regarding the maximum stresses in the tympanic membrane, they form a crescent-shaped region in the anterior and posterior quadrants in the regions where tympoansclerosis is observed. The magnitude and pattern of the stress is predicted not to depend significantly on the presence of the tube. This suggests that tympanosclerosis is not determined by the implant per se and therefore that no tube design feature can be expected to prevent it.