Project Summary/Abstract Conductive hearing loss in military personnel is commonly caused by blast-related middle ear injuries, which can include tympanic membrane rupture and dislocation of the middle-ear ossicles. The current proposal seeks to evaluate methods and devices for restoring hearing not only for active-duty military personnel and Veterans who suffer from a blast-induced conductive hearing loss, but also for others who experience conductive hearing loss for various reasons (e.g., ruptured eardrum, birth defects, stiffening of middle ear bones). While these injuries are clinically treatable, the effects of middle-ear pathology on hearing are not well quantified, and clinical outcomes often fail to completely restore normal hearing for those suffering from a conductive hearing loss. The current proposal seeks to precisely measure the complex vibrational patterns along the ossicular chain and related pressures at the ends of the middle ear system; the umbo attached to the tympanic membrane and stapes attached to the cochlea, under both normal and particularly under various pathological conditions (damaged/repaired/reconstructed using prosthesis). These studies will use Laser Doppler Vibrometry, unique micro-pressure sensors and direct measures of otoacoustic emissions (OAEs). Aim 1 will evaluate the effects of systematically varied tympanic membrane perforations on middle ear sound transmission in an animal model. The role of the tympanic membrane in sound transmission will be explored and its influence on the motion of the ossicles will be characterized under varied perforation and naturally healed conditions. Measurement of distortion product otoacoustic emissions (DPOAEs) following stimulation with paired tones will be used to quantify the effect of varying degrees of damage to the tympanic membrane. In Aim 2 we will measure sound transmission in an animal model with simulated ossicular chain pathologies. In addition, the effect of an abnormal ossicular chain on the production and acoustic characteristics of DPOAEs will be systematically explored in order to develop quantitative tools for diagnosing hearing loss and for use in the design and implementation of improved methods of treatment. Aim 3 evaluates middle ear sound transmission in human cadaver temporal bone models of clinically repaired human ears. Using direct pressure and velocity measurements, the functional differences between normal middle ear structures and two currently employed methods of repairing disarticulation of the middle ear ossicular chain, incus/stapes fixation or partial/total ossicular chain replacement using prostheses, will be quantified in detail. The proposed research will provide important information that will serve to improve our ability to specifically diagnose the causes of conductive hearing loss, direct the development of improved methods of treatment, and assist in the rational design of advanced prosthetics that more effectively restore hearing.