Orthopaedic Surgery Spine
obesity, use of contrast or radiopaque structures, can result in suboptimal navigation. The development of a 3D C-arm fluoroscopy navigation system where an isocentric C-arm (iso-C) automatically rotates around the patient represents another landmark development in the field of image-guided computer-assisted navigation. Multiplanar reconstruction is performed in the axial, coronal and sagittal views to provide high-quality images similar to those of CT-based systems.33,34 Anatomical registration is not required as a calibration target is fitted to the conventional C-arm and the optoelectronic tracking system cross-references the images obtained with the DRA attached to the patient. The main advantage is the increased accuracy of matching the pre-operative and intra-operative images. This system also provides the opportunity to apply the principles of minimally invasive surgery as there is no need for anatomical registration.
General Results of Computer-assisted Spinal Surgery Accuracy
There is improved accuracy of insertion of pedicle screws with computer-aided navigation.7,32,35–42
Studies comparing both 2D- and 3D-
based navigation have shown that in general the perforation rates of pedicle screw insertion vary between 1 and 5% in the thoracic and lumbar spines. This is in comparison with the perforation rates of 8–31% with conventional navigation techniques. In the cervical spine, there are fewer studies and the rate of misplaced screws is higher.36,43 This may be attributed to the complex anatomy in the cervical spine and difficulty in the registration process due to the changes in anatomical position. In the thoracic and lumbar spines, navigation produces relatively more lateral breaches. Few studies have also reported on the re-operation rate, which is 5–7% with the conventional technique compared with <1% with navigation-assisted pedicle screws.28,44
The predominance of lateral breaches of the pedicular cortex also increases the overall safety of the procedure using computer-assisted navigation technology by reducing the risk of neurological complications, with the proviso that significant lateral breaches may endanger vascular structures.
Clinical Outcomes
There are no studies comparing the true functional outcome of pedicle screw fixation with or without computer-aided surgery. Re- operation rates of about 7% have been reported by Amiot et al.28 with the conventional technique of screw fixation, but none with computer-aided pedicle screw fixation. A systematic review and meta-analysis of computer-aided pedicle screw fixation had shown that there was an average of 2.3% neurological complication using conventional techniques, whereas none of the patients with image- guided screw insertion had any neurological complications.45
Time
This was due to rapid registration. With a CT-based navigation system, Holly evaluated two registration techniques, i.e. paired point matching and surface matching, and concluded that in routine simple cases use of paired point matching alone does not compromise the accuracy of navigation and results in a decreased operating time compared with the addition of surface matching.46
50
The amount of time taken to insert a pedicle screw using navigation is variable and dependent on factors such as experience of the surgeon, familiarity with the navigation system and accuracy of the registration process. Rajasekaran et al. have reported the significantly shorter time required to insert a pedicle screw using the 3D isocentric fluoroscopy- based navigation.42
However, in complex cases, paired point matching supplemented with surface matching resulted in a lower mean registration error than paired matching alone and therefore was safer in preventing any breach of the pedicular cortex, even though the time taken to insert pedicle screws was longer. Schlenzka et al. showed that in normal anatomy the amount of time to insert each pedicle screw using optoelectronic tracking methods was higher; however, in complex deformity, conventional techniques were much more time-consuming. However, there was no overall difference in the mean operation time in both groups.47
Time taken to insert a pedicle screw also depends on whether single- or multilevel registration is carried out for matching pre-operative and intra-operative images.48
It has been shown that
navigation systems allowing multilevel registration significantly reduce the amount of time for pedicle screw insertion and also the total mean operative time in comparison with single-level registration.49
Radiation Exposure
There is less radiation exposure in insertion of pedicle screws using computer-assisted navigation systems compared with conventional fluoroscope-assisted techniques.14,51
This reduction is
not only in the amount of time of radiation exposure but also in the total radiation dose that is used intra-operatively. However, in the CT-based image-guided navigation system, because of the necessity for thin-sliced, high-resolution CT scans, the overall radiation exposure to the patient is significantly higher.52
Radiation exposure
to the operating theatre personnel is much lower compared with the fluoroscopy-guided system, which is particularly important in spinal surgery.53
Cost-effectiveness
There is a general agreement that CASS adds to the cost of surgery,47 although the utilisation of expensive equipment by as many surgical specialities as possible would be expected to increase cost- effectiveness.54
Pedicle screw insertion is generally a process of tactile feedback aided by image guidance. Requirements include precise knowledge of individual vertebral anatomy and the 3D orientation of each vertebra and its relation to the spinal column. With the development of imaging technology, there is a heavy reliance on the use of image guidance to achieve optimal pedicle screw position. We know that suboptimal positions will influence the biomechanics. Breach of the medial wall jeopardises the spinal canal causing iatrogenic central stenosis and an inferiorly placed screw can cause radicular symptoms. Rampersaud et al. have shown that using conventional techniques of fluoroscopy-assisted pedicle screw insertion in the thoracolumbar region results in significantly higher radiation exposure to the operating surgeon. Consequently, they have called for measures to limit the exposure to radiation during pedicle screw insertion.50
The reduced incidence of neurological injury and re-operation rate with computer-assisted techniques should offset some of this; however, there are no studies directly demonstrating this. Insufficient sample size and length of follow-up have been cited as reasons for the inability to perform a cost–benefit analysis. With the development of minimally invasive spinal surgery (MISS), a hope would be that the combination of CASS and MISS would reduce the overall morbidity of spinal surgery with less surgical dissection, reduced hospital stay, early mobilisation and early healing.
Acceptability
Although CASS has been around for about two decades, it is not as widely practised as one would expect. The main reasons listed by
EUROPEAN MUSCULOSKELETAL REVIEW
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