The registration process of virtual reality, based either on fluoro-images, MRI scans or computed tomography images, to physical reality is the cornerstone of any navigation system. In the past different registration algorithms have been developed. These algorithms differ in terms of the type of image data used by the navigation system, i.e. either preoperatively acquired CT images or intraoperatively acquired fluoroscopy images, and the way virtual and physical reality is matched, i.e. registered. Generally, registration procedures based on active intraoperative registration and automated registration procedures can be differentiated. Intraoperative registration relies on the identification of anatomical landmarks, surface contours or preoperatively implanted fiducial markers. In classical pair-point registration at least three points not located on a straight line are to be identified in the surgical situs and in the data set. To provide an optimal spatial resolution these points need to have the largest possible distance from each other and need to be located on different levels. The surface registration algorithm is based on a three dimensional model which is calculated from preoperative CT data. Intraoperatively several points of the bony surface are identified with a tracked pointer. The computer then matches these clouds of points to a 3D reconstruction of the CT data. Both registration algorithms can be used simultaneously as well to increase the accuracy of the matching process. Holly et al. [36
] systematically analyzed whether the combination of both registration algorithms yields synergistic effects and found that paired point matching in conjunction with surface matching results in a lower mean registration error than paired point matching alone. Holly et al. conclude that paired point registration alone might be sufficient for routine cases provided that easily discernable landmarks are meticulously chosen. Fluoroscopy based registration is an alternative approach to CT based registration. Fluoroscopic images, either 2D or 3D, are acquired intraoperatively. The registration procedure is automated, since the fluoroscope is equipped with traceable markers Additionally a reference base is attached to the patient (usually to a posterior process in the direct vicinity of the vertebral body to be operated). The susceptibility of reference arrays to accidental manipulation during the operation has been an issue of concern, as any change in position of a reference base during the operation will inadvertently result in a navigational error. This problem has been investigated experimentally by Citak et al. [19
]. They used a femur model and the reference array was fixed with a single Schanz screw, thus the results cannot be directly transferred to the spine. However, relevant loosening occurred after only 2–3 cycles within a minimum force of 2 Nm applied. Thus care must be taken to avoid any accidental displacement of the reference arrays. In 2D-fluoroscopy based navigation conventional fluoroscopic images in two planes are acquired. The major disadvantage of 2D-fluoroscopy based navigation is the limited quality of fluoroscopic images in the thoracic spine as well as in obese and/or osteoporotic patients [40
]. 3D-fluoroscopy based navigation uses a set of intraoperatively acquired rotational images. Usually a 270° scan with 256 images is done. The automated registration process with reference arrays fixed to the patient and the fluoroscope is identical to 2D-fluoroscopy based navigation.
From the methodological point of view there is no clear evidence as to which registration technique is superior. In a experimental comparison of CT based and 3D fluoroscopy based registration techniques, Geerling et al. [29
] found no statistical difference between both modalities, however there was a tendency toward a higher accuracy if the 3D fluoroscopy based registration algorithm was applied. Intraoperative 3D fluoroscopy obviates the need for additional preoperative CT imaging, additionally the image data are acquired in the final position of the patient, e.g. prone, such that less artifacts due to different positions during imaging and intraoperatively are to be expected.
However, 3D fluoroscopy is also susceptible to systematic errors that need to be taken into account. Quinones-Hinojosa et al. [77
] performed serial measurements of accuracy at different distances from the DRB and at different time points during the operation in lumbar surgery. They found an increasing inaccuracy with increasing distance between the DRB and the vertebral body actually being operated on, and increasing duration of the surgery. An inaccuracy of more than 3 mm in 9% of the cases at a distance of three levels below the level of DRB fixation and an inaccuracy of more than 3 mm in 17% of the cases 60 min after beginning of the procedure was found (although the inaccuracy had no tendency to further increase with a longer duration of the surgical procedure).
Remains the question how much accuracy is needed for biomechanically reliable pedicle screw implantation. Although there is an abundant number of classification systems for pedicle screw placement [49
] there is no systematic investigation concerning the biomechanical effect of different degrees of pedicle screw misplacement. Additionally, the automated nature of 3D fluoroscopy based registration offers the advantage of being less time consuming and thus easier to integrate into the intra operative workflow, as no iterative interaction with navigation system is necessary. With respect to the trend toward a reduction of the invasiveness of surgical procedures 3D fluoroscopy is the only registration technique that supports minimally invasive approaches as the DRB can be clamped to a spinous process via a small incision of 1 cm length.
Generally it can be expected that 3D-fluoroscopy based image acquisition and registration will substitute CT based registration algorithms because of the less demanding logistics and the reduced radiation exposure [56
] of the patient. Another problem, at least if multi-segmental instrumentation is considered, is the limited scan volume of 3D fluoroscopes might be overcome with the advent of flat-panel technology [97
].The technical development of registration procedures has reached a point where further groundbreaking developments are not to be expected. The work of Holly et al. [36
] clearly demonstrates that the overall accuracy of a navigated procedure is only partially influenced by the registration algorithm, as they found a significant difference between the registration error and the actual navigation error in their experimental study comparing different registration techniques. Most interestingly the navigational error, being a result of the interaction between the technology and the human factor, remained the same independent of the accuracy of the registration algorithm.