Middle ear implants vary in type as they can be used by many different indications, containing hearing impairment and diseases, illnesses or accidents causing middle ear damage.
It is intended to reconstruct the middle ear conductive system after the patient suffered one of the following circumstances:
The implants can be divided in two groups. The first group is represented by implants for partial ossicular chain – PORP (partial ossicular replacement prosthesis). The second group is intended to replace the entire ossicular chain – TORP (total ossicular replacement prosthesis).
Chosing the material for the implants, after first tests on animals, the bone cement PALACOS R was sellected. Concurrently, the bone cement CEMEX Isoplastic was tested, by means of which the same results were achieved. The cement Palacos R was finally chosen as the light green colour of it can help surgeons to distinguish the implant from tissues. After hardening (outside of the patient´s body), the cement is chemically permanent, stable in body liquids and mechanically firm and solid but also elastic. The implants can be shaped by milling.
Body of the implant is a rectangular or circular base with round edge, thickness of which is 0,5 to 0,8 mm. This is intended to be placed at the eardrum. The oposite part is provided with a shaft rising from the base and being extended with a cylinder part. According to the type of the implant, the shaft is either coaxial or declined to 14°. The declination of the shaft provides is convenient for anatomic environment as the base of stapes lies out of the eardrum and ear canal axle. Implants with coaxial shaft are intended for cases with steaper eardrum axle and base of stapes.
In case the stapes is functional, PORP implants are used. The base has the same shape as by TORP implants but the shaft is much more massive, hollow and shorter. Diameter of the shaft is heavy-beaded to enable creation of a circular or cylinder-shaped hole at the end, after the length is adjusted. This part of the implant adjustment is important concerning embedding the shaft in the stapes joint.
Another type of implant is PORP – incus replacement. This implant is cylinder-shaped. One end of the implant is provided with semicircular perpendicular slit. According to the type, the slit is placed either axial or aside the axle and is inteded for embedding in the malleus i.e. manubrium of malleus. This shape is also intended to be finalised by the surgeon at the operation. The opposite part is provided with a hole and a cylinder-shaped recess intended as a guide while adjusting the implant length to the respective anatomic conditions. This part is embedded in the stapes joint which results in ossicular chain restoration and restored hearing. Of all middle ear implants, this model is the smallest one.
Another PORP implant is intended to be replacement of a missing or damaged incus and damaged stapes at the same time, in case the base of stapes is well-preserved and active. The shape of the implant, joint to the manubrium of malleus is the same as by incus replacements - semicircular perpendicular slit. The opposite implant part is diminished leading to a cutting edge which enables this part to be embedded in the remaining stapes suprastructures. Implant placed this way remains fixed in the position chosen by the surgeon.