<P> When gas pressure within the middle ear is less than atmospheric pressure, the eardrum can become sucked into the middle ear space . This is caused by inadequate opening of the Eustachian tube and absorption of air from the middle ear space . People with a patulous Eustachian tube may also cause negative middle ear pressure by repeatedly sniffing to try to keep their Eustachian tube closed . </P> <P> The middle layer of the pars tensa is strengthened by fibres of collagen protein . This layer may be weaker in the postero - superior quadrant (top rear quarter) or after the eardrum heals after perforation or tympanostomy tubes (grommets) so predispose to retraction in these areas . The pars flaccida is prone to retraction as it does not contain the same stiffening layer of collagen . </P> <P> All over the body, new skin cells are continually produced to replace old skin cells which dry out and slough off . Growth of new cells on the surface of the eardrum is unusual in that the new cells migrate over the surface and move out along the ear canal . Even if migration along the ear canal is blocked, new cells continue to grow so the surface of the eardrum becomes larger . This process of proliferation and migration can result in enlargement of a retraction pocket so that the eardrum expands and grows deeper into the ear . </P> <P> The majority of tympanic membrane retractions remain stable for long periods of time, or may even resolve spontaneously so that eardrum becomes normal again . Not all retractions are able to resolve even if middle ear pressure normalizes, as the retracted segment may become adherent to other structures within the middle ear . Some retractions continue to progress and grow more deeply into the ear . This can result in erosion of bone and accumulation of dead skin (keratin) within the ear . </P>

The point of maximum retraction of the tympanic membrane is the