Arm, E.

Jost, B.

Nolte, L.-P

Rohrer, U.

Scheer, C.

Schwarzenbach, O.

Visarius, H.

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Interactive Image Guided Surgical Navigation -A First Step Towards Minimally Invasive Orthopaedic SurgeryC. Scheer, E. Arm, O. Schwarzenbach*, B. Jost, L.-P. Nolte, H. Visarius, U. RohrerM.E. Müller Institute for Biomechanics and *Department of Orthopaedic Surgery, University of Bern, 3010 Bern, SwitzerlandINTRODUCTIONIn the field of minimally invasive surgery an majorobstacle is the limited access to the anatomy of interest.Interactive image guided surgical navigation (IIGSN) hasrecently proven to be an effective means for improvingsurgical accuracy and safety in conventional openprocedures [1]. The goal of the present work was to applyIIGSN to "keyhole"-type Orthopaedic surgeries byintroducing novel registration and referencing techniques.CLINICAL ASPECTSThe current stage in open surgery stabilisation includespedicle screw insertion in the thoracolumbar and cervicalspine and during scoliosis surgery.Applied minimally invasive techniques likelaparo/thoracoscopic surgery (trans-, retroperitoneal,transthoracic) have the following short comings:time-consummgseveral specialists are involvedexpensive equipmenthigh-risk learning curveA*. ..·.m^wmFigure J: Scheme of subcutaneous spinal siabilisationOur research focus is towards minimally invasive spinestabilisation that offers potential benefits, such äs:• no muscle damage• decrease of intraoperative radiation exposure• time saving• smaller amount of anaesthesia• precise pedicle screw insertionAs a first application the percutanous delivery of so-calledSchanz-screws for external spinal stabilisation waschosen. A registration probe has been developed, whichwas carrying 6 CT-compatible markers.External äs well äs subcutaneous internal fixation issupported by this System.REALIZED SYSTEMThe challenge was the visualisation of operated Segments(see Figure 2) and the development of a fiducial markercarrier (see Figure 3) for the dedicated Implantation site.^'^^^^^^^c^^i^fa^Surgical objcctAny hone fragmenlVirtual object:Any mediccdiniage. . .T\· · · · ' · · · · - -Figure 2: Involved co-ordinate spacesBased on the concept of rigid body mechanics applied bythe motion analysis System OPTOTRAK® 3020 (NorthernDigital, Waterloo, Canada) several rigid bodies have tobe tracked in space (see Figure 2). Each one is equippedwith infrared LEDs tracked by the OPTOTRAK®. Atleast three LEDs are needed to establish a local co-ordinate System (COS).Differently from framebased stereotactic approaches [2],we are using a so-called dynamic reference base(COSDRB). All tooFs co-ordinates are later on expressedin this local COS. Right before the surgery, this base isrigidly attached to a newly developed clamp.Tests on cadaver spines with different fiducial markercarriers led to the clamp's final small design.The clamp consists of radiolucent POM (see Figure 3)and titanium spheres. These spheres are embedded atdefined positions. The clamp is attached to the spinousprocess percutaneously through a small incision underlocal anaesthesia prior to image acquisition (CT or MRIscan). It resides in this place, until the following surgeryhas been performed.The transformation between the "real" world and thepatient's digital equivalent (image) is called matching.470 Biomedizinische Technik · Band 41 · Ergänzungsband 1 · 1996Figure 3: Special clamp carryingßducial markers andiheir counterparts on the spinous process, visible in anMRI scanPreoperatively, the titanium spheres are registered in theCT image. In the OR the surgeon points towards six smallindents on the clamp that are in a known position relativeto the centres of the spheres. By knowledge of thesepaired points in COS07 and COSDRB the Workstation (Sunj Microsystems, Eschenbach, Switzerland) is able tocalculale the transformation MDRB~*CT (see Figure 2).l' The surgeon is now able to verify the matching accuracyby real-time visualisation of a pointer in the CT image.Also the fluoroscope is used to assure this.Figure 4: Graphical user Interface with interactive real-time tool visualisation in a CT scanIn the next Step the insertion of a Kirschner wire isinteractively guided (see Figure 4) by a marker carrierequipped drill guide (see Figure 5). This wire is used forthe drilling process with a hollow drill.Later, the use of fluoroscopy validated these actions andproved the accuracy of this Intervention.A preoperative planning of the screw paths (trajectories)which enables defmed guidance, äs well äs real-timedisplay of tools without planning, is possible.Custom made Software allows the interactive imageguided percutanous delivery of the Schanz-screws bydisplaying 2D views of saggital, lateral, and horizontalcuts through the CT images.Figure 6: OR scene including drill guide and dynamicreference baseAfter being validated on several cadavers, the newapproach was applied during three surgeries, one openstabilisation and two external fixators.DISCUSSIONThe proposed ncw way of referencing has already provenits practical application in three clinical cases.Together with dedicated 2D views of the anatomy to beoperated on, our System enables real-time guidance of apneumatic drill during percutanous Schanz-screw deliverywith minimum radiation exposure when compared toconventional fluoroscope guided insertions.Nevertheless, the preoperatively embedded clamp couldbe driven to smaller sizes, which would come closer tothe goal of more "minimally" invasive surgery.In future approaches the referencing/matching procedurescould be solved by intraoperative contour matching withfluoroscopy or ultrasound. Another promising Option liesin the combination of endoscopic/microscopic surgerywith existing concepts of Computer assisted orthopaedicsurgery (CAOS).REFERENCES[1] L.-P. Nolte, L. Zamorano, H. Visarius, U. Berlemann,F. Langlotz, E. Arm, O. Schwarzenbach: Clinicalevaluation of a System for precision enhancement inspine surgery. Clin. Biomech. 10:293-303 (1995)[2] L. Zamorano, C. Chavantes, M. Dujovny, G. Malik, J.Ausman: Stereotactic endoscopic interventions incystic and intraventricular brain lesions. ActaNeurochirurgica, Suppl. 54:69-76 (1992)Biomedizinische Technik · Band 41 · Ergänzungsband 1 · 1996 471

Interactive Image Guided Surgical Navigation - A First Step Towards Minimally Invasive Orthopaedic Surgery

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