Introduction
The fluoroscope is very common piece of equip- ment in the operating theatre and is very familiar to most trauma surgeons. Fluoroscopic control is commonly employed in orthopaedic trauma sur- gery. By providing two-dimensional images, it helps the surgeon to locate the fracture fragments, the patient's anatomy, the implants and the instru- ments during operation. During the operative treatment of fractures, fluoroscopic control is es- sential for the closed reduction of fractures and many of the minimally invasive procedures of modern orthopaedic trauma surgery. However, the disadvantages of its use are many. These in- clude the radiation exposure to all personnel, in- cluding the patient, in the operating room, the images provided are of quality nature only and non-interactive, most machines can only provide real-time images in one plane and maximally two planes, frequent repositioning of the C-arm is needed during surgery and it has always to be in the operation field.
Computer-aided orthopaedic surgery (CAOS) has been applied to many different types of orthopaedic surgery in recent years [5]. In ortho- paedic trauma, due to the diversities of fracture patterns, which continue to change until the pa- tient is anaesthetised and the fractures are re- duced and stabilised on the operating table, it is therefore essential to have the fluoroscopic image- based system to achieve accurate automatic regis- tration of the fluoroscopic images, which can be used to navigate the surgical instruments, the C- arm as well as the bone fragments during surgery.
Furthermore, the C-arm is a very common piece of equipment in almost all operating rooms and familiar to most orthopaedic surgeons; fluoro- scopic images are also readily interpreted by orthopaedic surgeons. Combining fluoroscopy and the navigation system will not impose additional major difficulty during surgery.
While navigating the real-time fluoroscopic images taken intraoperatively, fluoro-navigation
will thus help to minimise exposure to X-rays for surgeons, operating room personnel and patients, provide multiplanar views for monitoring and ac- curate positioning of implants, provide real-time interactive quantitative data of the images, expand the application of minimally invasive surgery and remove the C-arm from the operation field.
Operation Principles
With the spatial co-ordinates of a standard X-ray fluoroscope and the skeleton, on which the surgi- cal procedures are going to be carried out, regis- tered into the system, X-ray fluoroscopic images obtained intraoperatively are transferred to the navigation system with automatic scale and dis- tortion corrections. The graphical user interphase then allows the surgeon to navigate with stereo- tactic tools on the registered biplanar, tri-planar and multi-planar images. As these images are al- most the same as those obtained from the stan- dard C-arm, the interpretation of the anatomical features for navigation is simple and straightfor- ward for most surgeons. Surgical procedures can thus be carried out with the virtual fluoroscope (Fig. 6.2.1).
Technical Notes
The fluoroscopy-based navigation system allows the tracking of a surgical instrument and the superimposing of its contour onto fluoroscopy images [2]. This enables the surgeon to know pre- cisely the position of the surgical instrument at any time of the procedure without the need to take additional X-ray shots. When an image is taken it is automatically transferred to the naviga- tion system and the position of the image intensi- fier of the C-arm is localized. Distortion of the fluoro-images is common and is caused by the flexibility of the C-arm, the magnetic field of the earth as well as the interference of the other
New Technology:
Image-Guided Navigation in Orthopaedic Trauma Surgeries
K.-S. Leung
equipment in the operating room. The distortion is thus corrected by the use of the metal markers attached to the C-arm (commonly known as the phantom) (Fig. 6.2.2). With the distortion cor- rected, the image is referenced to the patient's bone, which is localized by a further tracker (Fig.
6.2.3). This allows the removal of the C-arm from the operation field while keeping the registration of the image and visualizing the actual position of the surgical instrument.
For tracking a tool, two basic technologies are available: infra-red based and electromagnetic
based. Infra-red technology is very robust and boasts a high localization accuracy, but with problems of line of sight. Line-of-sight problems are not present in the case of electromagnetic sys- tems, which also have distortion problems affect- ing the accuracy. A new generation of multiple camera-based navigation systems using infra-red technologies will combine the advantages of both technologies.
The first generation of tracking devices was characterized by cable-based active trackers. The second-generation trackers were based on passive
Fig. 6.2.1.Virtual fluoro- scopy on the computer monitor
Fig. 6.2.2.Phantom attached on the C-arm tracker to cor- rect image distortion (P). The C-arm tracker is located with
the light-emitting diodes (L) Fig. 6.2.3.A patient tracker is attached to the anterior iliac crest where the image is referred for navigation
infra-red reflecting objects, which had the prob- lem of limited number of tractable objects and visibility when the reflecting surfaces were not clean. The third generation of trackers is based on wireless battery-powered active technology and overcomes the limitations of the first two generations. In addition, these trackers are equipped with on-board electronics communicat- ing with the navigation system, which allows automatic instrument detection and software re- mote control.
Operating Procedures
The first step of the operation is to set up the sys- tem. The patient tracker is anchored to the part of the bone that is going to be operated, and is activated (Fig. 6.2.3). The C-arm tracker is also activated and the position of the devices is orien- tated to the optimal position for operation (Fig.
6.2.4). In some systems, the accuracy and reliabil- ity of the system is validated by either mapping the image of the tracker or confirmed by the po- sition of the tool tracker and the patient tracker.
The calibration of the surgical tool is thus car- ried out by attaching a tool tracker on the instru- ment that the surgeon is going to use. The cali- bration can either be done by tip calibration, which means that only the tip of the instrument
(Fig. 6.2.5) will be monitored or by tip and axis calibration, where both the tip and the axis are monitored during navigation (Fig. 6.2.6). The cali- bration can be a factory default one, or can be done during surgery with a calibration station (Fig. 6.2.7). The latter provides versatility for the application of different surgical instruments.
However, for the calibration of axis of the tool, the instrument has to be straight, while for tip ca- libration, the tool can be of any geometry. With a specifically designed axis adaptor for common in- struments in orthopaedic trauma, e.g., screw driv- er, awl, power drill, the calibration will be much easier with a hand-held calibration device where both the axis and the tip calibration can be done (Fig. 6.2.8). This facilitates surgery and minimises the duration of the procedure.
The next step is to acquire the fluoroscopic im- age for navigation. With the C-arm positioned (Fig. 6.2.9) to take the images of the skeleton where surgery will carried out, the image taken will be automatically transferred to the system with distortion correction. The images can thus be stored or assigned to different layouts for navi- gation (Fig. 6.2.10).
In the navigation, the position of the instru- ment is depicted by the numeric data on the monitor. The virtual tip and the axis thus enable the measurement of the paths (Fig. 6.2.11). This helps to plan the screws that are going to be in-
Fig. 6.2.4.C-arm tracker is activated
Fig. 6.2.5.Point calibrated instrument is monitored during navigation
Fig. 6.2.6.The axis and the tip can be monitored if the instrument is calibrated with axis and tip
serted with the diameter and the length shown on the monitor and superimposed on the images of the bone. The C-arm guidance mode helps to vi- sualise the position of the C-arm in case further images are needed for navigation and thus de- creases the number of shots for positioning (Fig.
6.2.12). More intervention can be carried out by modification of the procedures and instruments.
The Applications
Fluoro-navigation is particularly important for orthopaedic trauma as the fracture fragments are mobile and the orientations are not fixed before surgery. It is only possible to navigate the images obtained during the operation after fracture re- duction or manipulation is completed [3].
With the fluoro-navigation system, many proce- dures that require intraoperative fluoroscopic con- trol can now be done without continuous moni- toring with the fluoroscope. These procedures in- clude the following.
Fixation of Femoral Neck Fractures
with Percutaneous Cannulated Screws (Fig. 6.2.13) With the fracture closely reduced and stabilised with traction on a traction table, both the frontal and lateral images of the proximal femur are taken and registered in the system. The insertion of screw guide wires can be navigated with the patient tracker anchored on the anterior iliac crest and the power drill as the navigated tool.
Two or three screws can thus be planned and in- serted with a small incision (Fig. 6.2.14).
Fig. 6.2.7. A curved awl with the tool tracker attached is being calibrated in the calibration station
Fig. 6.2.8.A hand-held calibration device (C) for calibrating an instrument
Fig. 6.2.9.Positioning of the C-arm to take images. The yel- low dot indicates the centre of the C-arm beam
Intramedullary Locked Nails for Long Bone Fractures Entry to the medullary canal can be navigated with the proximal femoral frontal and lateral images. The awl is the navigated tool and the pa- tient tracker can be inserted onto the anterior iliac crest when a traction table is used or onto the lateral part of the greater trochanter when a traction table is not used in the nailing proce- dure.
Insertion of distal locking screws (Fig. 6.2.15) is the commonest procedure applied in the navi- gation during intramedullary nailing [9, 10]. With the current technology [12], the distal locking screw insertion still requires perfect frontal and lateral fluoroscopic images. With the patient tracker anchored to the proximal nail holding de- vice (Fig. 6.2.16), the power drill with the corre- sponding drill bit can be calibrated and thus navi- gated for screw insertion without further fluoro- scopic control. Both the length and the diameter
Fig. 6.2.10.Different layouts of the images for navigation
Fig. 6.2.11.Position of the instrument can be shown by the tip and its axis
Fig. 6.2.12.Planning of the implants before insertion
of the screws to be inserted can thus be planned.
The insertion of the drill bit can thus be navi- gated (Fig. 6.2.17).
Intramedullary Fixation
of Trochanteric Fractures (Gamma Nails) [7]
(Fig. 6.2.18)
Entry to the medullary canal is similar to that in the intramedullary nails. Lag screw position and planning are perhaps the best illustrations of the advantages of fluoro-navigation. With the same set of images of the proximal femur (provided there is minimal movement of the fracture frag- ment after femoral component insertion), the po- sition of the lag screw can be planed on both the frontal and sagittal planes simultaneously without the need of further fluoroscopic control. After in- sertion of the guide wire with navigation assis-
Fig. 6.2.13.Percutaneous cannulated screws fixation of femoral neck fracture un- der navigation
Fig. 6.2.14.Surgical incision for percutaneous screw fixation of femoral neck fracture
Fig. 6.2.15.Distal locking in intramedullary locked nail with the fluoro-navigation
tance, the lag screw can thus be inserted accu- rately. In addition, distal locking screws can be inserted, similar to those in the intramedullary locked nails.
Percutaneous Fixation
of Sacro-iliac Fracture Dislocations (Fig. 6.2.19) This is indicated in unstable fracture dislocation of the sacro-iliac joints where the stability can be restored by trans-iliac sacral screw fixation [1, 11]. With fluoro-navigation, the insertion of the screws can be done with a small incision. As the first sacral body is the bone in which the screws are to be inserted, the patient tracker should be inserted onto the anterior iliac crest of the unin- jured side. If an anterior external fixator is in- serted during the resuscitation phase, the patient tracker can also be anchored onto the frame. At least three images of the sacro-iliac complex are required for proper navigation: inlet view, outlet view and the sagittal view of the sacrum. Either the drill sleeve or the power drill can be the tool that is to be navigated. We recommend navigation of the drill sleeve as it provides a more rigid posi- tion and accurate trajectory for the guide wire in- sertion. With the images registered in the system, the entry point of the guide can be monitored on the sagittal image of the sacrum. The trajectory of the screws can thus be planned with the inlet and outlet views so that the first screw is to be in- serted into the anterior third of the S-1 body and the second screw goes inferior and into the centre of the S-1 body. Both screws go through the sacral pedicle mass superior to the S-1 foramen, which is best monitored with the outlet view.
Fig. 6.2.16.Patient tracker is attached to the proximal nail- holding jig to ensure accurate navigation of the distal locked screws
Fig. 6.2.17.Navigation of a power drill for distal locking
Fig. 6.2.18.Gamma nailing under fluoro-navigation
Percutaneous Fixation of Iliac Wing Fractures [8]
(Fig. 6.2.20a)
Fixation of iliac wing fractures is indicated when the fracture causes instability of the pelvis. The images that are needed in the navigation are ob- turator oblique view and iliac oblique view, as- sisted with outlet-iliac oblique view and inlet-iliac oblique view. With the patient tracker on the iliac crest of the uninjured side or on the external fixa- tor, a drill sleeve is calibrated to provide steady and rigid guidance. The screw is inserted in ret- rograde manner through a 1.5-cm incision, just below the AIIS, targeting towards the PIIS, verti- cally and 5±108 towards medially. The screw tra- jectory is along the roof of the acetabulum from AIIS to PIIS (Fig. 6.2.20 b).
Percutaneous Fixation of Acetabulum Fractures [8]
(Fig. 6.2.21)
This is possible in fractures that can be reduced closely by traction. These fractures may include
the high transverse fracture [13], central fracture dislocation, particularly in old patients with a de- gree of osteopenia. A traction table is preferred and the reduction of the fracture under traction is confirmed by fluoroscopy in standard oblique views. With the patient in a lateral decubitus posi- tion, the patient tracker is anchored on the ipsi- lateral ASIS, with fluoroscopic images according to the column where the screws are going to be inserted.
For the anterior column screw, inlet-iliac oblique view, ensure the pin does not go into the pelvis; for the outlet-obturator oblique view, en- sure the pin does not go into the acetabulum; as- sisted with standard inlet and outlet views. Again, a calibrated drill sleeve providing rigid and stea- dying positioning is navigated. The screw can be inserted antegrade (Fig. 6.2.21 a) via the mid- point between the iliac tubercle and the greater trochanter or retrograde through an incision in the pubic symphysis. The retrograde screw is best inserted with the patient in a supine position.
For the posterior column screw, navigation can be done with the standard iliac oblique and ob-
Fig. 6.2.19.Percutaneous screw fixation of sacro-iliac fracture dislocation under fluoro-navigation
Fig. 6.2.20.Percutaneous fixation of iliac fracture:aplanning,bscrew positions
Fig. 6.2.21.Percutaneous fixation of acetabular fractures:ascrew planning,bscrew positions
turator oblique views, assisted with outlet-obtura- tor oblique view (Fig. 6.2.20b). The entry point is through an incision on the centre of the ischial
tuberosity. The knee should be flexed and the hip externally rotated to relax the sciatic nerve. The trajectory of the screw should be along the medial side of the posterior column and directed proxi- mally in the posterior column to the centre of the dome of the acetabulum on the iliac oblique view.
This technique is also applicable in open sur- gery where screw insertion can be done in a pre- cise manner (Fig. 6.2.22), e.g. posterior column screw through ilio-inguinal approach, anterior column screw through posterior approach.
Insertion of Ilizarov Tension Wires for Complex Articular and Peri-articular Fractures (Fig. 6.2.23) This can be done after the fracture is reduced by longitudinal traction. With the images of the frac- ture taken in frontal, lateral and two oblique views, the tension wires can be inserted by navi- gating the calibrated drill sleeve or power drill.
This has the advantages of better planning and positioning of the wires. Additional procedures of screw insertion can also be done with the same technique (Fig. 6.2.24).
Fig. 6.2.22. Posterior column screw insertion during an open reduction surgery
Fig. 6.2.23.Insertion of ten- sion wires under fluoro-na- vigation of a pilon fracture
Other Applications
Many percutaneous fixation procedures that need fluoroscopic control can also be done with the same principle.
In all these applications, navigating the instru- ments with great accuracy and real-time quantita- tive display greatly facilitates the surgical proce- dures without frequent use of the fluoroscope [4].
The navigation of the C-arm also helps to de- crease the radiation exposure and guide the posi- tion of the C-arm whenever an image needs to be taken.
The interactive and quantitative data from the navigation system help in implant planning: the position of the implant with the axis defined by two to four planes, the length and the diameter can be clearly depicted with a high degree of ac- curacy. Early clinical experience has confirmed that the advantages and the extended applications of this technique will benefit many of our pa- tients. The simultaneous monitoring of the trajec- tory of the instruments in two to four views facili- tates the accurate positions of implants and in- struments without the need of frequent exposure
to X-ray. The adaptation of specific trauma in- struments further facilitates the practice of mini- mally invasive surgery. At this stage, the accuracy of the system is very acceptable and the system is also stable for most operative procedures, as men- tioned above. We have been using the system for the past one and a half years and the success rate is above 96% in all the procedures. It has become standard operative equipment in many common orthopaedic trauma surgeries in our department.
We anticipate that as the experience accumulates and with improvements in both the hardware and software, the applications will increase.
The Future
Fluoro-navigation, like all applications in compu- ter-aided surgery, is a developing technology. It is therefore expected that further improvements in the hardware and software of the system for quicker image acquisition and accurate registra- tion will be possible in the near future. With im- provement of the image quality, more precise po- sitioning and more interactivity will be possible
Fig. 6.2.24.Insertion of can- nulated screws for fixation of pilon fractures under na- vigation
between the system and the surgeons. The devel- opment of dedicated surgical instruments for orthopaedic trauma surgery in line with the further improvement of the navigation system will be another major direction of research.
With the establishment of image libraries for instruments, implants and skeleton, it will be pos- sible to minimise further the need for standard X-ray. This is applicable in procedures where in- struments and images of the implants are navi- gated. One good example is the distal locking of the intramedullary locked nails, where the image of the distal nail is navigated and not the bone.
As the dimensions of the nails are standardised for each system, the image library of the implants can thus be used in navigation without further ac- quiring the fluoroscopic images, provided there is minimal deformity of the implant after insertion.
Thus the distal locking procedure can be done without the use of the C-arm.
The combination of intraoperative three-di- mensional fluoroscopy and the navigation will be another technique for further development [6].
The registration of these three-dimensional fluoroscopic images into the navigation system al-
lows the application of the CT-based navigation system in orthopaedic trauma surgery with fluoro-navigation. The application is particularly useful in minimally invasive surgery for intra-ar- ticular and juxta-articular fractures where the fractures can be reduced by closed technique and the fixation by percutaneous screw fixation. With this technique, depressed osteochondral fragments can also be approached and reduction done with a navigated instrument. However, at this stage, fluoroscopic assistance is still needed during the procedure where fracture fragment movement is expected. With expected further improvement in the software and the image acquisition, the com- bination of fluoroscopy and three-dimensional fluoro-navigation will improve percutaneous fixa- tion of these articular fractures (Fig. 6.2.25). An- other direction of development is to explore the possibility of navigating on each individual frac- ture fragment. This will extend the technique even more to real-time fracture reduction, which at the present moment still needs fluoroscopic control.
The development of virtual fluoroscopy in fluoro-navigation has progressed rapidly in the past few years. The application of virtual fluoro-
Fig. 6.2.25.Fluoro-naviga- tion with intraoperative three-dimensional fluoro- images
scopy in surgical training needs further explora- tion. With the image libraries of bone and im- plants, surgical training, evaluation and assess- ment can also be possible with the virtual envi- ronment. A laboratory for virtual fluoroscopy training has been set up to help the surgeon in preoperative planning, practice of surgical tech- niques and assessment of surgical competence.
This represents one more step towards surgical training in a complete virtual environment.
Conclusion
The development of fluoro-navigation in ortho- paedic trauma represents one of the applications of computer-aided surgery. Its development is ex- citing and challenging. While many of these de- velopments are from the engineers, many applica- tions in surgery were considered impossible in the past. As a result of the innovative thinking and perhaps endless imagination of those who are devoted to perfection in patient care, these ap- plications have become possible and their effects have been proven. There is a great need for closer collaboration between engineers and clinicians in future developments.
Acknowledgements
I would like to thank the CAOS team of the De- partment of Orthopaedics and Traumatology of the Chinese University of Hong Kong for their as- sistance in the surgeries and the preparation of this manuscript.
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