Nolte, L-P

Visarius, H.

Zamorano, L.

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A Pilot Study About Computer-Assisted Spine SurgeryL-P. Nolte1, H. Visarius1, L. Zamorano21 M.E. Müller Institute for Biomechanics, University of Bern, 3010 Bern, CH2Dept. of Neurological Surgery, Wayne State University, Detroit, MI, USAIntroduction. Spinal fixation relying on transpedicularscrews was introduced in the early 1960s and hasrecently been the focus of increased attention. Despitethe improved fixation many problems are still involvedin the process of screw insertipn. Successfulplacement of a pedicle screw requires the accurateIdentification of the ideal screw axis. Biomechanicalstudies suggest an optimal position for the screw tip tobe äs near to the anterior cortex of the vertebral bodyäs possible. Any major deviation causes Perforation ofthe pedicle or the anterior vertebral cortex and mayresult in neurological or vascular damage.Morphometric measurements of the critical pedicledimensions, i.e. the transverse width and angle haverevealed significant variability. Moreover, it has beenshown by three-dimensional reconstruction oftomographic images that the greatest width andoptimal angle measured from a single image slice maynot correspond to the true optimal screw axis throughthe pedicle. This is confirmed clinically äsconsiderable failure rates in screw location andcomplications have been reported. We propose anovel technique for spinal surgery that combines pre-operative tomographic imaging with principles ofStereotaxis. It provides a sound basis for surgicalplanning and Simulation äs well äs real-timevisualization of surgical tools.Principles of Image Guided Stereotactic SpineSurgery. The proposed System for Computer assistedsurgery (CAS) is conceptually based on Stereotaxis, amethod for guided localization of surgical objectswithin body parts without direct access to their interior.Despite the formulation of the basic principles earlythis Century only recent technology transfer frommedical imaging, Computer science, 'and spacedigitization allowed for an Implementation intoComputer assisted surgery. Systems for frame-basedtumor stereotaxis have been reported since the late70s.All stereotactic components, the navigator (e.g. spinesurgery tools, pointers), the surgical object (e.g.anatomic hard tissue structures such äs vertebrae),and the associated virtual object (e.g. medical images)are treated äs rigid bodies. Mathematically, thelocation of a rigid body in space can be determined bythe location of three or more non-collinear points(markers) on the body with respect to a global space-fixed coordinate system (COS). The markers aredescribed in a local body-fixed COS. Varioustechniques exist to track and control a rigid body. Inmedical applications articulated arms or robots äs welläs infrared or acoustic motion analysis Systems arecurrently in use.The following criteria were applied to our stereotactictracking System. The device should: (a) not interferewith and require only slight modifications of Standardsurgical procedures and Instrumentation, (b) keepsurgeons "in the loop", (c) provide sufficient accuracy,and (d) be fast enough to allow real-time Instrumentcontrol and visualization.Based on these criteria we selected an opto-electronicmotion analysis System, the OPTOTRAK 3020(Northern Digital, Waterloo, CAN) for intra-operativedigitization. Probes instrumented with light emittingdiodes (LED) can easily be mounted on existingsurgical Instruments and/or anatomic structures. Both,the vertebra and the camera may move during thesurgical procedure without affecting the accuracy ofthe system.Spinal Skeletal Registration. Generally, skeletalregistration is required to enable coordinatetransformation of the "real world" data of the operativesitus into the "virtual world" data of the CT image. Forthe spine it includes the registration of the bonysurface of each vertebra involved in the procedure. Inorder to obtain a sufficiently accurate transformationwithin a tolerable time a combination of paired pointand surface matching methods were used for thisstudy.Surgical System and Procedures. The system hasbeen applied in over 200 cases of neurologicalsurgery and three cases (7 vertebrae) of spinal fusion.OSPS - As a basis for the proposed surgical systemwe use the Orthopaedic Surgery Planning System(OSPS). The modular features of the OSPS Softwareenvironment have been described in recent papers.Special matching, planning, tracking, and visualization260 Biomedizinische Technik Band 39 · Ergänzungsband · 1994algorithms which meet all the outlined requirements ofImage guided Computer assisted spine surgery havebeen developed and integrated into the OSPSenvironment.Pro-operative sfeps - Tomographie scans, CT and/orMRI, are obtained of all vertebrae to be operated on.A tomographic Image visualization modulereconstructs and displays the three-dimensionalvertebral Images in multiple two-dimensional views.An approximate axis for pedicle screw insertion iscreated by defining its entry and target point by thesurgeon. Equidistant serial sections orthogonal to thechosen trajectory (surgeon's perspective) may bedisplayed to check the insertion axis along the entirepedicle.Intra-operative sfeps - Our tool set for Image guidedpilot hole preparation consists bf: (a) a space pointer(Northern Digital, Waterloo, CAN) for digitization ofpoints on the vertebral surface, (b) a Standard AOpneumatic hand drill (Synthes, Waldenburg, CH) witha 3.6mm drill bit, (c) a custom opto-electronic devicefor intra-operative calibration of the drill, and (d) acustom vertebral marker carrier.The vertebral marker carrier must be fixed to thevertebra, so that there is no relative motion betweenthe vertebra and this carrier. A suitable and accessibleanchorage point is normally provided by the spinousprocess. For the paired point skeletal registration thepre-operatively chosen anatomic landmarks arelocated and digitized with the space pointer. To checkthe quality of the obtained transformation the OSPSPosition tracking module offers the automatic real-timedisplay of tomographic sections corresponding to anypoint on the vertebra digitized with the space pointer.In case of insufficient accuracy the surface matchingcan be applied. For this procedure thirty to sixty pointson the accessible vertebral surface are digitized. Oncethe matching process is completed the surgeonprecisely locates and marks the pre-operativelydefined entry point with the space pointer.Before preparation of the pilot hole the pneumatic drillis calibrated which is done hand-hold. The calibrateddrill acts äs a tracking device initiating the real-timedisplay of the tomographic sections corresponding tothe tip of the bit. During the actual drilling phaseguiding functions support the surgeon to match thechosen trajectory: real-time display of (a) colored linesin the sagittal, transverse, and frontal imagesrepresenting the drill's bit, (b) a circle matching thescrew diameter displayed in the actual slice of thesurgeon's perspective, (c) distances between the tip ofthe drill's bit and the target point äs well äs betweenthe tip of the drill's bit and the trajectory, and (d) anglebetween the axis of the drill's bit and the trajectory.In Vitro Application. Two human lumbar spines wereremoved within 24 hours of death. CT scans wereobtained of both spines (SIEMENS Somatom Plus-SErlangen, FRG) with 2mm thick slices spaced 1mmand 2mm apart. After disarticulation each singlevertebra was fixed to a reference frame with thespinous process pointing upwards imitating the intra-operative exposure. The paired point matching usingsix characteristic anatomic landmarks on the posteriorbony surfaces and the surface matching wereperformed äs described above.The entry point of the optimal axis of pedicle screwinsertion (trajectory) was identified with the spacepointer and prepared by drilling a shallow hole with ashort 2mm hand drill. The pilot hole was preparedunder image guidance, along the chosen trajectory,with the 3.6mm diameter drill bit fitted in thepneumatic hand drill. No additional visualization ororientation aid was used. Both pedicles then had a3.5mm diameter steel cylinder inserted in the resultingholes to increase their visibiiity in the X-rays.The pedicles were then sectioned perpendicular to thescrew axes with a precision band saw (1.5.-2.5mmsection thickness). All histological sections were X-rayed and classified. To simulate a Standard pediclescrew circles with a 6mm diameter were concentricallyprojected onto the image of the metal cylinder in eachhistological section.77 histological sections were analyzed. A screwPosition centered in the pedicle and either nottouching or concentrically engaging the cortical wallwas considered äs ideal. This was achieved in 70cases. A slight deviation from the ideal axis with anengagement of the pedicular cortex up to one half ofits thickness was observed in 7 cases. No cases werefound with Perforation of the pedicular walls.Conclusion. The proposed System fulfills thepreviously defined demands: It complementsestablished surgical techniques and associated toolsets effectively keeping the surgeon "in the loopM. Itprovides sufficient in vitro accuracy and real-timevisualization of surgical tools. Since there is no needfor intra-operative image intensifying and the processof Identification of landmarks is guided the total time ofthe surgical Intervention should not be affected. Weare in the process of establishing the CAS System in aspecific operating theatre to gain further experiencenot only in transpedicular screw insertion, but also infurther orthopaedic applications, e g. periacetabularosteotomy, Implantation of acetabular prostheticcomponents, cruciate ligament surgery, etc. In ouropinion, the proposed System of CAS has the potentialto become a useful tool in daily orthopaedic surgery.Blomedlzinlsche Technik Band 30 · Ergünzungsband · 1694 261

A Pilot Study About Computer-Assisted Spine Surgery

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