Journal of Image Guided Suqpry 1:88-93 (1995) Computer-Aided Fixation of Spinal Implants Lutz-P. Nolte, Heiko Visarius, Erich Arm, Frank Langlotz, Othmar Schwarzenbach, and Lucia Zamorano M.E. Muller Institute for Biomechanics, University of Bern, Bern, Switzerland (L.-PN., H. F?, E.A., EL.); Department of Orthopaedic Surgery, University of Bern, Bern, Switzerland (03); Department of Neurobgical Surgely, Wayne State University, Detroit, Michigan (z. 2.) ABSTRACT Medical imaging provides an important basis for modern diagnosis as well as for preoperative planning of surgical procedures. However, information gained cannot be transferred directly into the operating room. Furthermore, the safety and accuracy of the surgical intervention would be improved by interactive navigation of surgical instruments. These features are provided by the system for computer-aided fixation of spinal implants described in this paper. J Image &id Surg 1:88-Y3 (1YYF). 01995 Wiey-Liss, Inc. INTRODUCTION The basic principles of stereotaxis involved in com- puter-aided fixation of spinal implants were intro- duced early this century.' However, without recent advances in three-dimensional (3-D) image recon- struction6 and computer science, this technique could not be applied effectively to clinical problems. Since the end of the 1970s, several systems for stereotactic tumor neurosurgery have been developed and clini- cally e~tablished.~.~ Although applications in the field of neurosurgery are still being developed, relatively few attempts have been made to apply these tech- niques to orthopedic surgery in general'2.'6 and to spinal surgery in partic~lar.~'*'' Spinal surgery is frequently performed using a posterior approach. The spinal anatomy allows ex- posure of the bony posterior elements only, although several surgical procedures involve the anterior ver- tebral bodies as well. The insertion of pedicle screws through the pedicle and into the vertebral body for posterior spinal fixation is one clinically relevant example. Pilot holes are prepared and screws are in- serted without any direct visual control. Image in- tensification is used, but, because of radiation expo- sure to the patient and the difficulty of use of this system, it cannot be applied during the entire screw- insertion procedure. The variability in width, height, and spatial orientation of spinal pedicles conse- quently leads to a considerable rate of misplaced screws.'O This not only limits anchoring potential but might also endanger the integrity of the surrounding anatomy, i.e., spinal cord, nerve roots, and abdomi- nal vessel^.^ A reliable and accurate system that al- lows real-time linkage of medical images and the operative field minimizes these surgical risks. MATERIALS AND METHODS Description of the System The aim of the present study is to introduce a new system for preoperativeplanning and simulation and for intraoperative guidance during spinal surgery. Therefore, stereotactic concepts based on 3-D tomographic image reconstruction have been com- bined with a space-digitizing system. As usual, all stereotactic components, i.e., sur- gical instruments(drill, pointer, probes, etc.), anatomi- cal structures (e.g., the vertebrae), and their medical images, are treated as rigid bodies. The location and orientation of a rigid body in space can be completely defined when the location of three or more Received original February 23, 1995; accepted April 3, 1995. Address correspondence/reprint requests to Lutz-P. Nolte, PhD, M.E. Miiller Institute for Biomechanics, University of Bern, P.O. Box 30,3010 Bern, Switzerland. 0 1995 Wiley-Liss, Inc. Computer Aided Surgery Downloaded from informahealthcare.com by University of Toronto on 04/29/13 For personal use only.
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Journal of Image Guided Suqpry 1:88-93 (1995)
Computer-Aided Fixation of Spinal Implants Lutz-P. Nolte, Heiko Visarius, Erich Arm, Frank Langlotz, Othmar Schwarzenbach,
and Lucia Zamorano M.E. Muller Institute for Biomechanics, University of Bern, Bern, Switzerland (L.-PN., H. F?, E.A.,
EL.); Department of Orthopaedic Surgery, University o f Bern, Bern, Switzerland (03); Department of Neurobgical Surgely, Wayne State University, Detroit, Michigan (z. 2.)
ABSTRACT Medical imaging provides an important basis for modern diagnosis as well as for preoperative planning of surgical procedures. However, information gained cannot be transferred directly into the operating room. Furthermore, the safety and accuracy of the surgical intervention would be improved by interactive navigation of surgical instruments. These features are provided by the system for computer-aided fixation of spinal implants described in this paper. J Image &id Surg 1:88-Y3 (1YYF). 01995 Wiey-Liss, Inc.
INTRODUCTION The basic principles of stereotaxis involved in com- puter-aided fixation of spinal implants were intro- duced early this century.' However, without recent advances in three-dimensional (3-D) image recon- struction6 and computer science, this technique could not be applied effectively to clinical problems. Since the end of the 1970s, several systems for stereotactic tumor neurosurgery have been developed and clini- cally e~tablished.~.~ Although applications in the field of neurosurgery are still being developed, relatively few attempts have been made to apply these tech- niques to orthopedic surgery in general'2.'6 and to spinal surgery in partic~lar.~'*''
Spinal surgery is frequently performed using a posterior approach. The spinal anatomy allows ex- posure of the bony posterior elements only, although several surgical procedures involve the anterior ver- tebral bodies as well. The insertion of pedicle screws through the pedicle and into the vertebral body for posterior spinal fixation is one clinically relevant example. Pilot holes are prepared and screws are in- serted without any direct visual control. Image in- tensification is used, but, because of radiation expo- sure to the patient and the difficulty of use of this system, it cannot be applied during the entire screw-
insertion procedure. The variability in width, height, and spatial orientation of spinal pedicles conse- quently leads to a considerable rate of misplaced screws.'O This not only limits anchoring potential but might also endanger the integrity of the surrounding anatomy, i.e., spinal cord, nerve roots, and abdomi- nal vessel^.^ A reliable and accurate system that al- lows real-time linkage of medical images and the operative field minimizes these surgical risks.
MATERIALS AND METHODS
Description of the System The aim of the present study is to introduce a new system for preoperative planning and simulation and for intraoperative guidance during spinal surgery. Therefore, stereotactic concepts based on 3-D tomographic image reconstruction have been com- bined with a space-digitizing system.
As usual, all stereotactic components, i.e., sur- gical instruments (drill, pointer, probes, etc.), anatomi- cal structures (e.g., the vertebrae), and their medical images, are treated as rigid bodies. The location and orientation of a rigid body in space can be completely defined when the location of three or more
Received original February 23, 1995; accepted April 3, 1995. Address correspondence/reprint requests to Lutz-P. Nolte, PhD, M.E. Miiller Institute for Biomechanics, University of Bern, P.O. Box 30,3010 Bern, Switzerland. 0 1995 Wiley-Liss, Inc.
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Nolte et al.: Fixation of Spinul Implants 89
noncollinear points on the body with the respect to a global space-fixed coordinate system (COS) are known. In practice the orientation and origin location of a body-fixed COS with respect to the global COS are used to describe rigid body positions.
Various technologies exist to track and/or con- trol rigid bodies in space. In medical applications, for example, articulated arms or robots as well as infrared and acoustic space digitizing systems are currently in use.9.12-16-19 During the design of our ste- reotactic system the following requirements for the space digitizer were defined: 1) Established surgical procedures should not be principally affected or sig- nificantly altered, 2) the existing surgical tool set should not be altered but only slightly modified, 3) the accuracy of the system should be at least one order of magnitude greater than the desired overall accuracy, and 4) the system should be fast enough to allow real-time instrument control and visualization.
Based on these criteria we selected an opto- electronic space digitizer (Optotrak; Northern Digi- tal, Waterloo, Ontario, Canada). The contactless sys- tem can track up to 256 pulsed infrared light-emitting diode markers (LEDs) with a maximum sampling rate of about 3,200 markers per second and is con- trolled by a personal computer. The coordinates ob- tained are transferred via local network to a Sun workstation (Sun Microsystems, Inc., Mountain View, CA), which runs the computer operating sys- tem (CAS) software. The camera can locate each LED with an accuracy of 0.15 mm within a field of view of 1.0 x 1.2 m and a distance of 2.0 m. By at- taching at least three markers onto each rigid body, using probes, location and orientation in space of the rigid body can be determined.
For the real-time display of a rigid body within a medical image, it is necessary to know the position of the rigid body in the image COS. For example, it is necessary to display the drill tip during the drill- ing of a pilot hole through a pedicle. To accomplish this, the rigid body is “transformed’ from the global COS to the image COS4. This matching of a COS in the operating room to a COS of a tomographic image is termed “skeletal registration.” In general, matching methods can be divided into four catego- ries? 1) control-point or paired-point methods (point- to-point matching), 2) moment based methods, 3) structure-based methods, and 4) similarity optimi- zation-based methods.
We chose a point-to-point method using a non- linear iterative algorithm in combination with a sur- face-matching refinement with intermediate visual checks, resulting in a four-step procedure of skeletal registration; the point-to-point step is based on four
to six characteristic anatomic landmarks, which are digitized in both systems computed tomography (CT) or magnetic resonance imaging (MRI) and on the vertebra. The resulting coordinate transformation may lack accuracy because of digitization errors in both systems and difficulty in precisely identifying anatomical locations. However, this rough solution is used as a starting vector for the iterative surface- matching refinement. Intraoperatively, any 20-40 points on the vertebral bony surface are digitized, and, preoperatively, the vertebral surface is detected from the tomographic image data and a minimum- distance analysis in the voxel world is carried out. Nonlinear matching algorithms’ are then used to pro- vide the required coordinate transformation.
Preoperative data acquisition, image recon- struction, operative planning and simulation, and real- time display of the location of surgical instruments are accomplished using the modified Neurological Surgery and Planning System (NSPS).s*lrls.ls*’o The modified program is referred to as Orthopedic Sur- gery and Planning System (OSPS). Data from tomographic images (MRI and CT) and projective images (radiography, angiography) are utilized as the basis for the planning. Multimodality abilities allow the use of all possible image sources simultaneously.
Surgical Procedure As the initial clinical application, the proposed sys- tem for computer-assisted surgery was utilized for the insertion of spinal pedicle screws. This proce- dure involves distinct preoperative and an intraoperative phases.
Preoperatively, the OSPS basic module recon- structs and displays 3-D vertebral images or mul- tiple two-dimensional (2-D) views, i.e., frontal, sag- ittal, and transverse sections from MRI or CT of the affected vertebrae. This allows planning of the sur- gical intervention. An approximate insertion axis for the pedicle screw is defined by digitizing its entry and target points. Equidistant sections perpendicu- lar to the trajectory axis are generated and displayed. The chosen trajectory is then interactively optimized and redefined. This procedure is not mandatory but was found to be particularly helpful for planning the alignment of the overall fixation construct. In this procedure, all intended pedicles are optimized with respect to one another.
For the definition of body-fixed COSs on the vertebra and the surgical instruments, custom marker probes were designed (Fig. 1). All intraoperative digi tizations are then performed with respect to the local vertebral COS defined by the dynamic reference base (DRB).
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90 Nolte et al.: Fkation of Spinal Implants
Fig. 1. Custom marker probes.
The surgical site is exposed in a standard pos- terior approach. The spinous process provides a se- cure anchoring point for the DFU3 carrying four mark- ers. A space pointer with six markers is used to digitize specific anatomic landmarks on the acces- sible posterior elements. These points are located in the CT or MRI image accordingly in order to per- form a point-to-point matching between the "real world" (vertebra) and the "virtual world" (image). Points on the dorsal aspect of the transverse pro- cesses, the facet joints, and the spinous process usu- ally provide a sufficiently accurate matching result. However, a refined surface matching digitizing any 20-40 points on the bony surfaces of the posterior elements is available and is performed in cases of inadequate accuracy of the point-to-point matching. Adequate accuracy is indicated by low RMS errors of the computed transformation as well as a small- to-negligible deviation between an anatomical land- mark as digitized by the surgeon and as displayed in the medical image on the computer screen.
For maximal control and safety, the OSPS tracking module automatically displays any point being digitized on the vertebral surface, in real time, in the tomographic image. This provides a safe method to identify inaccurate transformations, i.e., displayed and digitized points do not correspond. Once a sufficiently exact solution is found, the lo- cation of any instrumented and calibrated surgical tool can be transformed into the coordinates of the tomographic image and displayed on screen in the image. The required tool calibration is performed with a single custom calibration device for all in- struments.
The surgeon may choose from various options for real-time visualization of the pedicle hole prepa- ration. The instrument used can be displayed as a line in the above-mentioned multiple 2-D views (Fig.
2, left). Simultaneously, a circle representing the position of the instrument in a plane perpendicular to the trajectory axis may be shown together with graphical guidelines for proper adjustment of the instrument (Fig. 2, right). Both options adjust the plane of the display, in real time, to the position of the tip of the instrument. Furthermore, multiple sec- tions along the axis of intended pedicle hole prepa- ration can be calculated and displayed according to the orientation of the instrument in relation to the pedicle. This option may eliminate the need to per- form the preoperative planning (Fig. 3).
These tools guide the surgeon during pedicle hole preparation. The increase in operative time dur- ing the matching procedure is usually offset by faster hole preparation.
RESULTS AND DISCUSSION
Validation of the System For the evaluation of possible errors, a complex vali- dation study was undertaken. To justify the quality of a transformation or a digitization, the coordinates of corresponding points in each relevant COS are required. For this purpose, six precise polished tita- nium spheres were attached to a demineralized ver- tebra to provide a set of exactly defined reference points. The centers of the spheres were measured with a laser measuring device (accuracy 5 p). The re- sulting coordinates in the COS of the laser device (COSL) were termed "exact." The vertebra was scanned via CT, with slice distances of 1 and 2 mm. On-screen digitization of the sphere centers in the reconstructed CT data and digitization of the spheres using the 3-D space pointer on the vertebra resulted in three additional sets of coordinates in the COSm and COS'. The manual digitizations were performed three times by five observers to evaluate the accu- racy of digitization of a discrete point in the C0Sa and COS'. Table 1 shows the associated mean digi- tizing errors of six spheres (n = 3 observations x 5 observers x 6 spheres = 90).
Within the range of this accuracy, the means of all coordinates were adjusted to match the exact coordinates more precisely. Criteria for more pre- cise matching were all possible distances between any two points, of the sets of six, compared to the exact values. If, for example, the distance between
Table 1. Mean Digitizing Error of Six Spheres RMS digitizing error (mm; n = 90) Parameter
COS" 0.15 COSCF'"" 0.20 COSrn"" 0.22
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Nolte e t al.: Fixation of Spinal Implantr 91
Fig. 2. Real-time visualization of the pedicle hole preparation. Left: The instrument displayed as a line in the multiple 2-D views. Right: The section perpendicular to the trajectory axis, with a circle representing the position of the instrument, with graphical guidelines.
spheres 1 and 4 was 43.543 mm (dL) in COSL and 43.525 mm (dv) in COSv, dV had to be enlarged by 0.018 mm moving sphere 1' and/or sphere 4'. By using a nonlinear algorithm, all centers in COSv and COSm were shifted to minimize the differences from COSL. To preserve the validity of this proce- dure, adjustments were kept less than or approxi- mately at the average digitization error in the rel- evant COS. Table 2 summarizes the performed adjustments.
These new coordinates in COSv and COSm were used to compute the exact transformation be-
Table 2. Adjustment of Means of Coordinates Displacements (mm)
tween these two coordinate systems. A standard matching procedure was then performed three times by five observers. The error of the resulting transfor- mation is defined as the distance between transformed points of COSv into COSm using the exact transfor- mation compared to the transformation obtained by the observers (Fig. 4).
This value includes digitization errors in both COSs as well as numerical inaccuracies of the trans- formation algorithm. Table 3 gives an overview of this evaluation of the overall accuracy of the system for different CT slice distances.
To evaluate further the accuracy and reliabil- ity of the system, 20 pedicle screw hole prepara-
Table 3 . Overview of System Accuracy for Different CT Slice Distances
Transformation error (mm; n = 90) Parameter COS='mm cosmmm
RMS 0.7 1 1.74 SEM 0.07 0.4 1
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92 Nolte et al.: Fixation of Spinal Implantr
Fig. 3. Multiple cross-sections along the axis of the intended pedicle hole preparation.
tions were performed on human lumbar vertebra specimens in vitro. Based on 1 mm CT scans, the trajectories were defined as described above. Us- ing a standard 3.6 mm drill bit in an instrumented pneumatic hand drill, pilot holes for pedicle screw
p v I
Error 1-e p~~
Transformation
0 P V
Fig. 4. Transformation error.
insertion were prepared. Only the real-time display of the drill bit in the CT image was used as an aid for guidance to match the preoperatively defined trajectory. Aluminum cylinders were inserted into the holes, and the 20 pedicles were cut into 77 his- tological sections perpendicular to the cylinder axes. Each section was classified into one of the follow- ing groups: group I, cylinder centered in pedicle; group 11, cylinder touches cortex; group 111, cylin- der engages cortex; and group IV, cylinder perfo- rates cortex. Figure 5 summarizes the results of the in vitro study.
Clinical Application To date, six patients have undergone posterior fixa- tion of degenerative lumbar spinal segments. After the sixth patient, CT was introduced as a means of postoperative evaluation. A detailed analysis of these patients, focusing on the location of all individual screws, is under preparation. The introduction of the system for spinal surgery is completed. Basic con- cepts such as tool handling and marker camer de- sign have been optimized. Furthermore, software options were altered to match the demands of the in vivo situation as well as possible.
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Nolte et al.: Fixation of Spinal Implants 93
Fig. 5. Results of the in vitro study.
CONCLUSIONS The concept underlying the technique presented can be applied to a variety of surgical procedures. Image data reconstruction and manipulation modules provide powerful tools for preoperative planning and simula- tion. Intraoperatively, a four-step procedure allows ac- curate skeletal registration, the basis for accurate and safe tracking of surgical tools. Finally, the real-time dis- play of the motion of any surgical instrument combined with the corresponding tomographic image enables the surgeon to follow the preoperative plan exactly. lnitial clinical results indicate improved surgical outcome and leads to new frontiers in orthopedic surgery.
ACKNOWLEDGMENTS Supported in part by the Swiss National Science Foundation (grant no. 32-39732.93).
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