Limitations of titanium dioxide and aluminum oxide as ossicular replacement materials: An evaluation of the effects of porosity on ceramic prostheses

Background: Because the performance of titanium dioxide (TiO₂) has not yet been assessed in the unique environment of the middle ear, its role as an ossicular replacement prototype in the form of a total ossicular replacement prosthesis (TORP) was tested and compared with aluminum oxide (Al₂O₃), once considered to be a suitable implant material. Methods: Ossiculoplasty was performed by implanting TORPs into the tympanic cavities of rabbits. After an implantation period of 28, 84, or 300 days, the petrous bones were extracted, whereby the biocompatibility of the prostheses was examined using light microscopy and scanning electron microscopy to determine morphologic changes in situ. Proper implant placement and functionality was tested via manual manipulation. Results: Mucosa was seen covering most of the implants by day 84. Inflammatory cells were not observed in any of the specimens examined. The macroporous TiO₂ TORPs were subjected to osseous infiltration, material dissolution, and fragmentation, whereas the microporous TiO₂ implants were subjected to an increasing frequency of fissure formations. The Al₂O₃ prostheses demonstrated signs of material dissolution by producing encapsulated aggregates during the experimental trial period. Conclusions: Neither the macroporous nor microporous oxide ceramics were able to withstand the oscillatory stress to which they were continually subjected. Although porosity allows for the rapid integration of an implant material into a biological environment, its properties are not suited to fulfill the requirements of strength and long-term stability, which are demanded of middle ear prostheses.