An experimental study of hydroxyapatite prosthesis in ossicular reconstructive surgery: long-term follow-up observation in the middle ear of cat

Abstract

The author have conducted a series of experimental studies, using cats, to clarify the biocompatibility of Hydroxyapatite (HA) with the middle ear tissue in the cat. Partial ossicular replacement prosthesis of 99.5% of density was designed for the experiment. After removing the incus, the prosthesis was placed between the stapes and the malleus to reconstruct the ossicular chain.Postoperative hearing was assessed by measurment of the threshold of auditory brainstem response. Histological specimens of the middle ear after the operation were made using a cutting band saw at various postoperative periods. The maximum duration was 450 days. The specimens served for light microscopic observation. In addition, the HA prosthesis with surrounding tissue was observed by a scanning electron microscope and by a microradiography.The hearing threshold returned to certain level and remained stable within a month after the surgery, in all cats except for one, when the postoperative inflammation was well controlled. Fifteen days after the surgery, a fibrous tissue grew in layers around the ossicle in contact with HA. The HA prosthesis and the ossicles in the middle ear cavity were wholly covered with a connective tissue, 30 days after the surgery. Furthermore, the scanning electron microscopy revealed that the surface of the HA was quite promptly covered with the fibrous tissue mainly consisting of collagen fibers. This indicated the high affinity of the HA to middle ear tissue. On the face of junction between the HA and the ossicles the growth of thin layer of the connective tissue was evident and no osseous ankylosis which disturbed the mobility of the ossicular chain was recognized.Throughout the postoperative course, neither absorption, resolution nor disfigurement of the HA was found. The foreign body giant cells were hardly found under the microscopic observation. From the assessment of both hearing tests and histological findings, it can be said that the HA prosthesis is stabilized in the middle ear cavity within a month. The present experimental study confirmed the high grade of biocompatibility of HA and presented an important information for the clinical application of HA as the ossicular replacement prosthesis.