Determining prosthesis length:

Estimate the proper length of the piston by placing a 4.5 mm Causse measuring piston in the middle ear (Fig. 10a). The proper fit allows the upper end of the piston to be 0.25 mm higher than the lower surface of the long process of the incus, while the lower end of the piston just rests on the stapes footplate in its posterior half. The prosthesis is then trimmed to length in a trimming block (Fig. 10b, c).

fig 10 a fig 10 bfig 10 c Fig. 10 a,b et c : Causse measuring piston® and trimming block. et trimming block.

Creation of stapedotomy or fenestra a) If the footplate is partially fixed: The crura are vaporized with an HGM Argon laser ® using the Gherini-Causse endo-otoprobe ® (beginning with the posterior crus) (Fig.11). In the usual situation where the anterior crus is not visible, the fiber optic palpates the anterior crus before vaporizing it using a setting of 2 watts and 0.2 seconds. Creation of the fenestra with the Argon laser: a round "rosette" is created in the stapes footplate with the Gherini-Causse endo-otoprobe ®. Settings of 1.2 watts and 0.2 seconds pulse duration are used. Each laser impact site may or may not overlap the previous impact point until the shape and size of the "rosette" is complete (Fig.12a, b).

	Fig.11 Gherini-Causse Endo-Otoprobe ®
	Fig.12 a The Gherini-Causse laser Endo-Otoprobe ® creates 200 micron spots which fragilize the stapes footplate.
	Fig.12 b A rosette of spots determines the ulterior 0.8 mm stapedotomy.

The endo-otoprobe reaches the anterior crus whereas a micromanipulator set on the microscope cannot. The fiber optic diffracts the argon beam. The power decreases immediately (200 micron only) thanks to the diffraction of the beam (Fig.13). The smoothing of the fenestra is then performed with the microdrill (Fig.14 a and b).  The Skeeter microdrill with a 0.7 mm diamond bur removes the remaining bone and bone char and in addition smoothes the edges of the fenestra.

	Fig.13 with the use of the micromanipulator and the probe, the energy density is decreased.
	Fig.14 a The Causse Skeeter Oto-tool ® abrading 0.8 mm of the footplate in the posterior half of the stapes footplate approximately from one edge of the annular ligament to the other.
	Fig.14 b The stapedotomy diameter is 0.8 mm, i.e. between the small fenestra and the stapedectomy.

It should not be necessary for the bur to penetrate very far into the vestibule to accomplish this. b) If the footplate is fixed: We omit the use of the laser. A 0.3 mm safety hole is directly drilled in the footplate with the Causse Skeeter Oto-tool ® (Xomed-Treace, USA) (Fig.15).  Then the two crura, beginning with the posterior one, are abraded. Simply breaking off the crura with a sharp pick is wrong, as this may mobilize the footplate. The safety hole is then enlarged until a 0.8 mm hole is completed Fig.16). The diamond dust abrades the bone, no pressure of the bur must be applied to the stapes footplate for fear of fracturing it.

Fig.15 Causse Skeeter Oto-tool ®

Fig.16 Abrasion of the stapes crura with diamond dust.

The fenestra stands in the posterior part of the stapes footplate in order: a/ to distance the piston from the membranous labyrinth: By centering the fenestra toward the posterior footplate, any chance of inadvertent damage to the underlying saccule is minimized. The utricule is far posteriorly located behind the inferior surface of the footplate (Fig.17). If there is an adhesion between the saccule and the footplate, it is located on the level of the annular ligament of the stapes footplate. Thus, a stapedotomy must be performed and not a stapedectomy which brings the annular ligament with the footplate.

Fig.17 Stapedotomy must be performed where there is least risk of presence of the membranous labyrinth under the stapes footplate.

b/ to avoid the spread of proteolytic enzymes into the vestibule: breaking the anterior otosclerotic foci may release proteolytic enzymes into the fluids. These enzymes may damage the membranes and Corti hair cells structure. c/ to avoid bleeding into the vestibules: breaking the anterior foci may lead to such bleeding, as the foci are highly vascularized most of the time. The blood may float on the surface of the perilymph, or penetrate into the vestibule leading to fibrous bands clogging of the cochlear canal. Revision surgery in such a situation will never be effective. d/ to rebuild an impedance transfer and an acoustic impedance of the new annular ligament as we have shown it in the biophysics background of this technique.