Influence of angular positioning of the prosthesis in stapes surgeries with a NiTiBond prosthesis: Investigation in cadaveric temporal bones

In incus stapedotomy surgeries, the longitudinal direction of the piston prosthesis should ideally be perpendicular to the stapes footplate. However, in reality, some amounts of angular deviation of the prosthesis from the ideal angular position is unavoidable due to anatomical constraints and surgical conditions. This study aims to evaluate the influence of angular positioning of the prosthesis on surgical outcomes in incus stapedotomy and to provide surgical guidelines related to practical tolerance of the angular positioning. In this study, this influence was assessed with a Kurz NiTiBond prosthesis (0.4-mm diameter) and fenestra sizes of 0.5- and 0.6-mm diameter in cadaveric temporal bones (n = 7 including 2 preliminary tests). Angular position of the prosthesis relative to the footplate was modulated by rotating the stapes about the long and short axes of the footplate. At each angular position, the tympanic membrane was acoustically stimulated in the frequency range of 0.2-10 kHz, and motion of the prosthesis was measured using a Laser Doppler vibrometer (LDV). Furthermore, micro-computed tomography (micro-CT) data of the middle-ear ossicles were used for anatomical analysis of angular positioning of the prosthesis. The results showed that changes of angular position of the prosthesis relative to the stapes footplate do not cause significant changes of prosthesis motion until a certain angular position threshold, and sharply attenuate prosthesis motion when the angular position reaches the threshold. The threshold of the angular position, as the tilting angle of the prosthesis from the direction normal to the stapes footplate, was 26.9 ± 2.5° with the fenestration hole of 0.5-mm diameter and 30.6 ± 3.0° with the fenestration hole of 0.6-mm diameter (n = 5, p < 0.01 for difference between the two fenestra sizes). Analysis of the middle-ear anatomy in this study revealed that the tolerances of the angular positions of the prosthesis does not always cover possible positions of prosthesis crimping. This study suggests that if an anterior offset of the stapes head and/or the thickened footplate is suspected, efforts to locate prosthesis crimping closer to the tip of the incus and/or to make a sufficiently large fenestration hole are favorable