Two of the causes of hearing loss are lack of function in the inner ear (cochlea) and when the sound has problems in reaching the nerve cells of the inner ear. Examples of the first include age-related hearing loss and hearing loss due to noise exposure. A patient born without external ear canals is an example of the latter for which a conventional hearing aid with a mould in the ear canal opening would not be effective. Some with this condition have normal inner ear function, as the external ear canal and the inner ear are developed at different stages during pregnancy. With normal inner anatomy, sound conducted by the skull bone improves hearing.