Cutting a flap into the cornea

It is important while preparing for the LASIK operation for VICTUS® to work extremely precisely because the procedure presents great challenges for the technology. The laser cuts a small piece out of the cornea of the eye, referred to as a flap. The flap is then folded over so the cornea can be treated through the opening. After the operation the flap is folded shut again. It functions essentially as a natural patch.

The cornea of the human eye measures only 0.5 millimeters in thickness. The laser cuts the upper fifth of the cornea to a depth defined by the surgeon with µm prevision, then follows the curvature of the cornea with the incision.  Great accuracy is essential in this area – the eye surgeon must always be certain of the exact position.

Strain gauges provide reliability

A sensor system that functions reliably is indispensable for this application. It is used for the purpose of connecting the laser device with the eye. This is done with a "patient interface" consisting of two parts: a kind of contact lens that is drawn onto the laser, and a mounting clip that is drawn onto the eye. The pressure with which the cornea is pressed against the laser through the patient interface is controlled exactly.

Strain gauges (SG) in the device indicate whether the pressure is constantly maintained. The PMX data acquisition system from measuring specialist HBM acquires the signals of the sensors and digitalizes them, thereby providing the eye surgeon who is performing the procedure with the certainty needed to work continuously in the correct tolerance range.

The PMX system ensures that the values needed by TECHNOLAS Perfect Vision GmbH are generated reliably. Temperature stability is especially important in this regard.

Extremely reliable measurement system guaranteed

Engineers from TECHNOLAS Perfect Vision were looking specifically for a manufacturer whose core business is measuring sensors, and for a system that comes from a manufacturing environment. The PMX from HBM stands for both precision and reliability – thanks in part to the interference-proof HBM carrier frequency technology (CF) and automatic sensor detection via TEDS. The sample rate is 19.2 kilohertz per channel – for 16 measurement channels and 32 internal computing channels. Thanks to the high measuring bandwidths, signals can be processed extremely rapidly.

In the VICTUS® , a three-fold redundant SG system records the signals and transmits them to the PMX. From there they are directed via a bus system to a computer – the "brain" of the system – for analysis.

Data is transmitted by a longer analog route within the VICTUS®. Normally that is not ideal, since every attempt is usually made to keep spatial distances short when analyzing analog signals. The digitization should also be positioned directly on the sensors, but that was not possible for space reasons. However, this was no problem for the PMXÖ The amplifier returns reliable values despite the longer distance.

"Made in Germany" – an important criterion

The safety functions provided by the amplifier are also important. Technolas experts developed a safety concept together with HBM that incorporates redundancies, watchdogs and other features to monitor how the components function.

HBM experts were informed how to implement parameterization so that the required results could be generated. The engineers of TECHNOLAS Perfect Vision concluded that the PMX is a highly flexible system that can be adapted to individual needs in many ways and everything worked out very well.

Work extends over a very wide pressure range. Good resolution is important for the entire range – if possible with low lateral forces.

The engineers receive reliable data from the PMX system, and the provider's country of origin was also a factor in choosing the right system. Technolas Perfect Vision wanted a tested system they could rely on, and the "Made in Germany" label was an important criterion for that requirement.