Development of the dynamic structure (force lines) of the middle ear ossicles in human foetuses.

OBJECTIVES To study the ontogenic development of the organisation of the human middle ear ossicles structure. MATERIAL AND METHODS 46 human temporal bones of ages varying from 32 days post-conception to newborns. RESULTS The development of the structural organisation of the malleus begins at 16 weeks via two cortical fascicles situated in the neck; at 21 weeks they extend towards the head, at 23 weeks to the lateral process and at 24 weeks to the handle. In the handle, the force lines are transmitted via three cardinal fascicles, two of them via the cortical fascicle and one via the centre, which starts after 29 weeks' development and is consolidated after 31 weeks. In the incus the force lines start at 16 weeks via two cortical fascicles situated in the long process, which progressively extend in a rostro-caudal direction between 17 and 20 weeks. At 21 weeks they occupy the whole extension of the long process and at 22 weeks the fusion of both cortical fascicles begins. From 30 weeks onwards it is strengthened by the crossing of bone trabeculae from one cortical to another. Two fascicles come out of the incus body, surrounding the medullary cavity and going in the direction of the short process. In the beginning, the stapes have two cortical fascicles in their crura. The remodelling process makes the internal cortical fascicle disappear and after 31 weeks all the force lines run through the external cortical fascicle. The tympanic membrane of the stapes footplate undergoes a remodelling process and after 28 weeks bony trabeculae are deposited. In newborns (40 weeks), the ossicles' structure is cavitary and has not been completed. The fan-shaped trabecular fascicle, which starts in the articular facets of the malleus and the incus, still has to develop.