impact electrosurgical heat on optical force feedback sensors

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J. Vander Sloten, P. Verdonck, M. Nyssen, J. Haueisen (Eds.): ECIFMBE 2008, IFMBE Proceedings 22, pp. 914–917, 2008

www.springerlink.com © Springer-Verlag Berlin Heidelberg 2009

The impact of electrosurgical heat on optical force feedback sensors

J.A.C. Heijmans1, M.P.H. Vleugels2,3, E. Tabak 1, T.v.d. Dool1, M.P. Oderwald1

1 TNO Science and Industry/Advanced Precision and Production Equipment, Delft, Netherlands2 EFI BV, Maastricht, Netherlands

3 Rivierenland Hospital, Netherlands

Abstract — Electrosurgery enables cutting and coagulation

(desiccation) of tissue for Minimally Invasive Surgery.

Measurements performed by TNO with an infrared camera

showed that the forceps of an endoscopic instrument can be

over 300°C in temperature. During electro-surgery the

surgeon relies on the power control and endoscopic images to

perform the procedure successfully.

Manipulation of tissue with the present forceps does not

give accurate tissue information due to the presence of friction

in the transmission mechanism and in turn force, control of the

operating surgeon is poor. The latest developed instruments

incorporates sensors and actuators that enable better control

of force application on the tissue and give a better feeling ofthe tissue to the surgeon. TNO works for EFI BV on the

development of a surgical instrument that senses and controls

the gripping force even during electrosurgery.

Electric sensors and actuators experience Electro Magnetic

Interference during the use of electro-surgery, making it

impossible to control the force. The high temperature that

arises at the forceps influences and possibly destroys the

sensors when positioned nearby the heat source. Based on the

experience with optical fiber sensors TNO has developed an

instrument that is immune to EMI and withstands

temperatures up to 200°C. This optical sensor is based on a

Fiber Bragg Grating (FBG). The FBG read out system, named

the interrogator, transfers the optical fiber signal from the

mechanical local strain. By this way the force exerted on thetissue and its resistance can be measured. However this sensor

system is also sensitive to temperature changes. To control

accurately he gripping force, the measurement must be

independent of temperature. Therefore the thermal load at the

forceps were measured and analyzed. The results are used for

the instrument design and location of sensors.

Keywords — Electrosurgery, coagulation, Minimally

Invasive Surgery, Fiber Bragg Grating, optical force sensor

I. I NTRODUCTION

Minimally Invasive Surgery is performed with instrumen-tation that enables surgery through small incisions. This typeof surgery enables faster recovery of the patient and reduces

the change on postoperative complications. The surgeon,however, makes sacrifices on its perception and ergonomics.

With the introduction of minimally invasive surgical procedures, the development of a new range of surgical

instruments has started. The majority of these instruments

are based on existing instruments, used in conventionalsurgical procedures. These evaluated into devices which are

difficult in handling and which lost force control. Moreoverinformation of the force created by the tissue resistance onthe tip of the instrument is absent. These disadvantagesadded to the poor visualization of the area of operation,decrease the usability performance of the instruments andhampers the introduction of minimal invasive surgery into

areas with higher demands on accuracy. By introducingnovel technologies from the field of robotics into the

instruments, the mechanical disadvantages of the currentinstruments can be tackled. Innovation in medicaltechnology is rather difficult due to the harsh environment

in which the instrument must operate. However, thoroughunderstanding of the medical problem and the issues relatedto human interfacing is just as important [1]. Thecooperation of medical company EFI BV with TNO Science

and Industry covers both the medical and technical field.The concept for this next generation of surgical

instrumentation comes from EFI BV, (Endoscopic Force-reflecting Instrument). The control issues related to hapticfeedback, the mechatronical and fiber optic (FO) issues areall dealt with by TNO.

II. SYSTEM OVERVIEW

A. Instrument overview

Figure 1 shows the design of an endoscopic instrumentfor surgery with haptic feedback. It consists of forceps forgripping tissue, a shaft with a rod inside for moving the

Fig. 1 Conceptual design of the endoscopic instrument


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forceps, and a handle that moves the rod relative to theshaft. This instrument is connected through a cable with an

external controller (not shown).A regular handle is shaped like a simple pair of scissors.This handle has been replaced by a type that houses twoactuators, one for actuating the forceps and one foractuating the trigger. In the present regular instruments forMinimally Invasive Surgery (MIS) the surgeon has little

feeling of the tissue which he is gripping due to friction inthe handle, the rod inside the shaft, and hinges in theforceps. The newly developed forceps has fiber optic FBGsensors that accurately measure the gripping force. Thisforce is relayed to the controller which in turn sends signalsto the actuators in the handle such that the desired force isexerted on the tissue and an exact scaled force is exerted on

the trigger at the same time which enables the surgeon tofeel the tissue characteristics. Also the force canautomatically be limited to safe values that prevent tissuedamage. The measuring range of the clamping force is up to

20N with a resolution of 10mN.A further advantage of a “wired” MIS instrument is that itallows for independent scaling of force and displacement.

This enables highly accurate operation of delicate tissue asis the case in an extreme degree for neuro- and eye surgery.One of the challenges for the endoscopic instrument is theneed to perform electrosurgery. This requires the instrumentto accurately measure the force while the forceps is chargedwith electric signals with frequencies in between 350kHzand 3.3MHz. Besides the EMC the instrument must be re-

usable and therefore allow demounting, cleaning andsterilizing by autoclavation.

B.

Fiber optic sensor, FBG

Fiber optic (FO) sensors are a relatively new type ofsensors and their application fields rapidly grow. Medical

technology is one of these applications. Here fiber optics isvery attractive due to their insensitivity to electromagneticfields, their small dimensions and their intrinsic electricalsafety [2].

The FBG sensor used for this application is used for forcemeasurement at the distal end of a surgical instrument. The

functionality is quite identical to an electrical strain gaugewith the difference that no electrical signal comes from thesensor. A grating situated in the fiber reflects a small band

of wavelength from the light that is sent in the fiber. Achange in the grating results in a change of the reflectedwavelength. This is in general a length difference of the

sensor (strain) or a temperature difference. The sensitivityof an FBG sensor expressed in wavelength shift for these

parameters is typically 10pm per Kelvin, 1.2pm per µstrain

(1 µstrain = 1×10-6 [-]).

Multiple sensors can be used inside one fiber, known asmultiplexing. The length of the grating is typically 1 to

10mm in length. The diameter of the sensor is generally in between 10m up to 100m that corresponds with the fibercore and cladding respectively.The signal can be monitored with an optical instrumentwhich is named an interrogator. This instrument analysesthe signal with interferometry or spectroscopy. To create a

realistic haptic feedback system, an interrogator is used witha bandwidth of 19kHz. This relatively high bandwidth iscommercially available nowadays [3].

III. EXPERIMENT

As the FO FBG sensor is sensitive to temperaturechanges an experiment has been performed to determine thethermal effects of electrosurgery. In order to obtain therequired force measurement accuracy the temperature must be known within 2°C. Even without electrosurgery thetemperatures vary from 20°C to 37°C, clearly indicating the

need for temperature compensation. Therefore a secondFBG is used that only measures the temperature at theforceps. However, the temperature gradient in the forcepsshould be sufficiently homogenous. Since the aim is that theinstrument can be used at all times, even duringelectrosurgery, experiments have been carried out in orderto measure thermal effects during electrosurgery. In figure 2a schematic representation is given of the experimental

setup.The Valleylab Force 30 (1) is used for both electricalcutting and desiccation. A handheld device (2) is used toinitiate the electrosurgery. The device is electrically

Fig. 2 Schematic representation of the test setup

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916 J.A.C. Heijmans, M.P.H. Vleugels, E. Tabak, T.v.d. Dool, M.P. Oderwald

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connected (3) to the endoscopic instrument (4).Thermocouples (not depicted) are glued to the forceps of

the instrument that are monitored with a multimeter (5). A piece of chicken meat (6), which is connected to the returnelectrode (7), is used to simulate human tissue. Whenelectrosurgery is performed heat is generated at the contactarea of the forceps with the tissue.

Figure 3 depicts the actual setup. The temperature

distribution is measured using an infrared camera (7), whichis placed just above the tissue. At first, the thermocouples(8) are calibrated offline. Secondly, the results from thecamera are calibrated using the thermocouples.

Fig. 3 The actual test setup in which the temperature distribution ismeasured using an infrared camera (7). A detail of the forceps and

thermocouples (8) are depicted at the corner, far right

In order for the experiments to be representative, themeasurement procedure is formulated in consultation with a

surgeon (M.Vleugels2,3

). Both (monopolair) cutting anddesiccation is performed. Cutting is done by applying a cutof approximately 3 to 4 centimeters. Next, a waiting periodis introduced of 3 seconds and than another cut is made. Inthe measurements four cuts are made. The power used forthe desiccation and cutting is depicted in figure 2.

A second experiment is carried out in which a high loadsituation has been examined. In this case the power isincreased by a factor of 2 and is applied for 20 instead of 3seconds continuously. It must be noted that the heat load isdepending on the power, the contact area between tissue andforceps and the amount of moisture in the tissue.

A third experiment is carried out with a FBG sensorfixed to the forceps. When applying cutting and desiccation

to this sample, the FBG signal is read out simultaneously.The purpose is to show the thermal effects experimentallyand to verify that the sensor and interrogator are indeedinsensitive to electromagnetic fields.

IV. R ESULTS

Measurements show that cutting results in slightly highertemperature than desiccation. Figure 4 depicts a typicaltemperature distribution during cutting. Maximumtemperatures of 160°C are measured at the tip of the

forceps.

Fig. 4 Temperature distribution in the forceps and tissueduring cutting with representative settings

The result of the second experiment (increased power) isdepicted in figure 5. The maximum temperature at the tip isapproximately 300°C.

Fig. 5 Temperature distribution in the forceps andtissue during cutting with increased heat load

The result of the third experiment is shown in figure 6,showing the response of the FBG sensor to the heat that

arises from electrosurgery. After nine seconds the forcepscuts in the tissue for three seconds. This is repeated for four

times. The FBG signal is clearly affected and therefore a

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0 10 20 30 40 50 601549.3

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simultaneous temperature measurement must be taken todecouple the clamping force from the rise in temperature.

Note that the signal from the FBG sensor is not affected bythe EMC but is only sensitive to temperature changes.

Fig. 6 Optical signal due to heating when performing electrosurgery

Finally, a study to different sensor fixations has been

performed. Results from this study showed the impact of thegeometry and materials to the reproducibility of the sensor

signal in time and after autoclavation. This has led to thedesign of a stable fixation with a reproducibility error ofless than 0.9% after 10 cycles of cleaning and autoclavation.

V. CONCLUSIONS

The realized demonstrator has shown the achievability offorce sensing during electrosurgery with a fiber opticsensor. Measuring the clamping force in the forceps with anoptical strain sensor requires that the temperature must be

taken into account. Tests have shown that duringelectrosurgery temperatures can rise up to 160°C and up to300°C for high load situations. This heat load at the forcepscauses a significant deviation in the strain read out signal of

the FBG sensor. Therefore the temperature must bemeasured separately and subtracted from the forcemeasurement. Correct fixation of the FBG sensor isessential to make the characterization reproducible. Thecooperation between medical company EFI BV andresearch institute TNO has been vital for the successful

development of this high tech surgical instrument.

VI. FUTURE DEVELOPMENTS

The FBG sensors, including the adhesive that is used tofix them, need to be able to withstand the high mechanical,

chemical, and thermal loads due to coagulation, cleaningand sterilization. Tests will therefore be conducted with

washing (100 cycles) and sterilization (1000 cycles) to seeif any degradation occurs.

A first prototype of this MIS instrument is being realized.This instrument will be used for experimental surgery oncadavers to verify the advantages of such an instrument in practice. With the experience and knowledge acquired, a

MIS product based on this technology will be realized.

R EFERENCES

1. Wieringa F, Poley M, Dumay A. et al. (2007). Avoiding pitfalls in

the road from idea to certified product (and the harsh clinicalenvironment thereafter) when innovating medical devices. In 7th

Belgian Day on Biomedical Engineering, Brussels, Belgium, 2007

2. Heijmans J, Cheng L, Wieringa F, Optical fiber sensors for medicalapplications, IFMBE Proc., 4th European Congress on Med. &

Biomed. Eng., Antwerp, Belgium, 2008

3. Cheng L, Groote Schaarsberg J, Osnabrugge van, J, et al. (2001) Novel Fiber Bragg Grating sensor system for high -speed structure

monitoring, 3rd Int. Workshop on Struct. Health Mon., Stanford,2001.

Author: J.A.C.HeijmansInstitute: TNO Science and Industry

Street: Stieltjesweg 1

City: DelftCountry: Netherlands

Email: [email protected]

impact electrosurgical heat on optical force feedback sensors

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