gms | German Medical Science

For stapes surgery a great variety of prostheses are offered on the market. They differ mainly in their coupling principle. Stapes prostheses which are made of Shape Memory Alloy (SMA), simplify the coupling process significantly since they only need to be heated up above the phase transformation temperature (mostly around 50–60°C) to fix them at the long process of incus. An excessive heating of mucosa and bone, however, should be avoided. Heating the loop by means of laser, in the clinical practice the necessary laser power, pulse length and number of laser shots is discussed.

To simulate the heat flux of prosthesis, mucosa and bone at thermal input of a laser, a finite element model was developed which allowed the investigation of the heat input and its distribution. The model was validated with infrared camera measurements. Various laser settings concerning laser power, pulse length and laser shot sequence were studied and different design variants of SMA prostheses were compared.

Simulations and measurements showed for a typical SMA prosthesis considerable high mucosa temperature during the heating up process. This is the reason for mucosa evaporation which could be observed during the experiments. Thus a new SMA prostheses design (Patent DE102007008851, international Patents pending) was developed (see Figure 1 [Fig. 1]) where the heating energy is applied at specified thermo-active zones which are separated from the contact zones and thus prevent mucosa from overheating. Moreover, free zones can prevent strangulation of blood vessels and minimize risk of arrosion. Elastic spring zones reduce spring-back effect and guarantee defined contact forces.

Simulations showed that a couple of laser impulses with less power performs better than one with high laser power and for applying the same energy, a longer pulse length instead of higher laser power is beneficial. Local high temperature can be reduced by optimizing the time history of a laser shot sequence.