The otosclerotic foci invade the annular ligament of the stapes footplate causing a diminution of the elasticity of the ligament and an increase in the acoustic impedance of the system. (Fig.1)


Fig. 1 : T = Eardrum, CC = Cochlear canal
FO = Otosclerotic foci
CAE = Outer auditory passage
The goal of the surgery is to restore good physiological vibration of the fluids in the cochlear canal. Both impedance transfer and acoustic impedance of the annular ligament should be restored at the lower tip of the piston.

The acoustic impedance transfer of the ossicular chain :

Acoustic impedance transfer is the ratio between the effective vibrating surface of the tympanic membrane and the effective vibrating surface of the stapes footplate. By virtue of a pedal-action, it is the posterior half of the footplate which by its movement activates the fluids of the inner ear.
 David LIM described a joint that develops after the age of 15, only in the posterior part of the annular ligament of the stapes footplate (Fig.2).

Fig. 2 :
Joint developed in the posterior part of the footplate
The posterior part of the footplate transmission is thus more physiological (Fig.3).


   
   Fig. 3 : Total stapedectomy gives poor high tones and too small a fenestra gives poor low tones, whereas a transmission corresponding approximately to the posterior part of the footplate will create an acoustic vibration creating a better distribution of the fluids along the cochlear canal.  
Too small fenestra in the footplate, such as a 0.4 mm one, will produce too much energy near the lower tip of the piston, and not enough farther on in the cochlear canal; moving water in a container with the tip of a finger has a different effect from moving it with the palm of the hand. The high pitched frequencies localized in the first basal turn of the cochlea are good with a small fenestra but the mid and low frequencies located in the apical remote part of the cochlea are poor if too small a fenestra has been used.

The acoustic impedance of the annular ligament of the stapes footplate:

The acoustic impedance is how a material will resist to its vibration, when a sound vibration is applied to this material.
 As otosclerosis impairs only the annular ligament of the stapes footplate, the acoustic impedance of the new annular ligament around the lower tip of the piston will depend upon the modification or not of the resistance, mass and elasticity of the drum, malleus and incus, and the quality of the resistance (dissipation force) and elasticity of the new annular ligament.
 The otosclerotic foci cause the annular ligament of the stapes footplate to lose its elasticity. It is a bony remodelling of the annular ligament.
The otosclerotic foci induce also a loss of energy due to a phenomenon of acoustic bridge.
The acoustic impedance of the ossicular chain are as in the following formula:

Z = Acoustic impedance of the annular ligament,
R = Resistance (dissipation force)*,
M = Mass*, S = Rigidity (elasticity is the contrary)*,
2f. = Acceleration (f = frequency)


 

*R = Dissipation force:

This is the resistance or dissipation force, the force applied by the annular ligament to resist excessive vibration or motion. It protects the inner ear (membrane and hair of Corti cells) against acoustic trauma and barotrauma. If this dissipation force around the lower tip of the piston is not rebuilt, the threshold at which an acoustic trauma or barotrauma will be able to damage the inner ear, hair cells and membranes is lowered.
 If the resistance or dissipation force is not strong enough, an acoustic or barotrauma may rupture the Reissner membrane (Fig.4), leading to no Na+ K+ exchange and thus a drop in the cochlear potentials. The hearing level becomes extremely poor.
Fig.4 : The Reissner membrane separates the scala vestibuli from the scala media.
Likewise, the attachments between the outer Corti hair cells and the membrana tectoria may rupture, leading to poor speech discrimination and tinnitus. As the proteolytic enzymes released by the otosclerotic foci break down the proteins (actine and myosine) of which the membranes and hair of the Corti cells are made, it is therefore especially important to rebuild good resistance (dissipation force) around the lower tip of the piston.
As the annular ligament of the stapes footplate has a certain volume of elastic fibers, we should interpose a living seal that will create around the lower tip of the piston the same volume of elastic fibers (Fig.5).


Fig.5 : The new annular ligament, showed in C, has a volume and a structure comparable to the annular ligament of the stapes footplate, showed in B.
Consequently, the level at which an acoustic trauma or barotrauma will damage the inner ear (membrane and hair of Corti cells) will be about the same, as the same protection will be provided.
The patient will get the same protection against acoustic trauma or barotrauma. Scuba diving, rough plane landings are not to be feared more than on a normal mobile footplate of an otospongiotic patient. A professional pilot or scubadiver may be able to keep his license if a videotape recording proves that this dissipation force has been actually rebuilt through an annular ligament around the lower tip of the piston.

 

*M = Mass:

The mass is multiplied by the acceleration. So it will play a role for high pitched frequencies only.
As the ossicular chain is suspended thanks to the suspensor ligament and the annular ligament of the stapes footplate, the normal ossicular chain has no mass applied to the inner ear fluids. A tiny bony spiculae helps also, making the process of the incus act as a record player arm (Fig.6). Consequently, the long process of the incus should not be traumatized (during the disarticulation for instance); if it is, the long process of the incus suspension will be altered.

  Fig.6 :
The stapes functions much like as a pick-up needle, whereas the long process of the incus plays the part of the record-player arm.
An increased mass may damage the inner ear cells and membranes ranging on the high pitched frequencies. Thus, the piston should not be heavier than the crura of the stapes. A 0.4 mm diameter Teflon piston complies with this requirement.
During tympanoplasty, placing bone having too great a mass on the head of the stapes will lead to tinnitus and poor hearing level for the high tones.

 

*S = Rigidity:

Its contrary is the elasticity.
 As the elasticity decreases (or the rigidity increases), the more the material will resist vibration.
 In this formula, elasticity is divided by frequency. Thus, it will play a role for the mid and low frequencies; as a matter of fact up to 3 kHz. The main speech frequencies are 1 and 2 kHz.
The small fenestra technique, that does not interpose a vein graft (which contains elastic fibers), will not give as good low and mid tones as a technique which will rebuild an elastic new annular ligament around the lower tip of the piston. Perichondrium (too thick and hard) and fascia both have no elastic fibers in their tissue.
The stapes footplate annular ligament has many elastic fibers in the media layer as demonstrated by D.LIM.
In the technique coupling the use of a piston and vein graft, after two years, the number of elastic fibers seen under microscope in the vein graft will be greater than those found in a vein sample taken from the wrist of the patient on the same day.
According to A.LOPEZ and L.JUBERTHIE, the elastic fibers are rebuilt from the fibroblasts (Fig. 7a et 7b).

  Fig.7a :
Old piece of vein graft- 15 years - (man, 63 years old). Histologic section. Elastic fibers colored with hydrochloric orcein.
  Fig.7b :
Old piece of vein graft - 9 years - (woman, 42 years old). Isolated elastic fiber. Ultrastructural aspect. Coloration with uranyle acetate - lead citrate.