How Stapes Ankylosis and Fracture Affect Middle Ear Dynamics: A Numerical Study

Numerical models of the human middle ear have been developed throughout the last 30 years, for different purposes. While several types of pathologies have been studied, stapedial disorders were seldomly explored. This papers aims to clarify how stapes fracture and some forms of stapes ankylosis, such as stapedial tendon (ST) ossification, augmented pyramidal eminence (PE) and bony bar presence, affect the sound transmission through the middle ear. In addition, the stapes dynamics is also analyzed by means of total displacement and first principal strain. For the purpose of the study, first, a three-dimensional finite element model of the human middle ear is detailed and validated under normal (healthy) conditions. The model is then modified to represent the stapedial disorders of interest. A measure is established for evaluating how the disorders reduce sound transmission through the middle ear. Results of the reduction of sound transmission showed that the different forms of stapes ankylosis affect primarily low frequencies, while the stapes fracture mostly affects high frequency sound transmission. According to the results, an augmented PE does not restrict stapes movement unless followed by some ossification of the ST. In addition, the question whether the fracture is in the anterior or posterior crus and the distance of the fractured part from the stapes footplate have a relevant role in the reduction of the sound transmission. Finally, the analysis of total displacement and first principal strain of the stapes helped to highlight some differences among the stapedial disorders.