STEREORADIOGRAPHY AS A PROMISING METHOD OF INTRAOPERATIVE NAVIGATION IN SPINE SURGERY

Aims. The study was aimed at the assessment of feasibility of the clinical use of stereoradiographic navigation for implantation of spine stabilising systems. Materials and methods. A stereoradiographic method is proposed for assessing the volume characteristics of structures with complex geometry, with summation of images of multiple objects. The X-ray parallax effect is used, which enables obtaining three-dimensional images after the hardware computer processing. Five patients received decompressive-stabilising operations to the extent of flavectomy, foraminotomy, transpedicular fixation and interbody stabilisation (TLIF). In the course of surgery, the conventional X-ray control as a stage of obtaining stereoscopic images, and stereoscopic navigation were used sequentially. Results. The authors demonstrated the feasibility of high-quality intraoperative visualisation of the main anatomical landmarks used in transpedicular implantation without increasing the radiation dose and surgery duration. Methodological principles and techniques that improve the quality of stereoradiographic images were provided. Conclusions. The efficacy of stereoradiography in stabilisation operations at all regions of the spine was demonstrated, including X-ray shielded regions of the inferior cervical and thoracic vertebra.


INTRODUCTION.
X-ray images are widely used in spinal surgery for intraoperative navigation and control [1].
The method is affordable, familiar, and easy to use [2]. State-of-the-art equipment can adequately visualize the anatomical "landmarks" of the thoracic and lumbar spine, i.e. the sites used for implanting stabilizing transpedicular structures. At the same time, the use of X-ray images for intraoperative control is limited by the complexity of anatomy, as well as by the shielding effects of adjacent anatomical regions, especially in the lower cervical or upper thoracic spine [3][4][5].
Difficulties may arise when taking the volumetric measurements on flat X-ray scans, which is especially challenging for novice surgeons or when the spine is deformed [5]. Various intraoperative computer navigation methods have been proposed and are in use today to improve the outcomes of spinal surgery [1; 2; 6-8]. However, despite implantation being quite accurate even in the context of altered and complex anatomy, both methods have disadvantages pertaining to the duration and difficulty of preoperative planning, to the errors of adapting the method to the patientspecific anatomy; besides, the methods require advanced and rather expensive equipment [9][10][11][12].
Stereo X-ray imaging and navigation have earlier been tested for percutaneous transhepatic cholangiography [13]. The method generates a 3D X-ray image using a standard X-ray set. The algorithm is as follows: to record the first X-ray image (the baseline); to take the second image, to rotate the C-arm by 4 to 5 degrees. The X-ray images are software-processed and shown on a special stereo display or using stereo glasses; alternatively, anaglyph images (a red one and a blue one) can be generated for viewing through anaglyph glasses [14; 15]. The automatic recording and display of a stereo X-ray image take 0.5 seconds at max.
This paper dwells upon the preliminary results of testing stereo X-ray intraoperative navigation clinically when used for spinal surgery.
Goals and Objectives. The goal hereof is to test the feasibility of using stereo X-ray intraoperative navigation in spinal surgery in a clinical setting. The objectives are: 1. To find the optimal conditions of using stereo X-ray navigation for various spinal regions.
2. To compare the visualization parameters of the main X-ray landmarks, the radiation load and the duration of the study under standard and stereo X-ray navigation.
3. To test the feasibility of using stereo X-ray navigation for transpedicular implantation. The method was tested clinically when performing surgery on five patients treated against degenerative spondylolisthesis with lumbar instability. Patients were aged 63 on average, including 3 women and 2 men, none with severe somatic pathology.

MATERIALS AND METHODS. This study was done in two stages
The study protocol followed guidelines for experimental investigation with human subjects in accordance with the Declaration of Helsinki and was approved by the ethics committee. Written informed consent was obtained from each patient (or official representative) before the study.
The patient model was selected based on the relatively large size of lumbar vertebrae coupled with the implantation technique being rather independent from the navigation system in use. Every patient underwent stabilizing decompression surgery: flavectomy, foraminotomy, transpedicular fixation, and TLIF. The surgeons used standard X-ray imaging to generate stereo images, which were further used for stereo navigation. Figure 1 shows a surgeon's workplace used for such examination. The surgeons were asked to help optimize the visualization of the anatomical landmarks when using stereo X-rays as compared to standard single-view X-rays. Expert assessment used "yes or no" questions (± in the Table) for a variety of conditions for different spinal segments, see Table. Visualization of anatomical landmarks in standard and stereo X- Using contrasts to mark the areas in front of and behind the specimen improves the perception of depth in 3D X-rays. These can be special contrasts for radiography or makeshift contrasts: wound retractors, wires, or leads, see Figure 3.  Standard intraoperative X-ray imaging involves frontal and lateral radiography; at least two images are taken: 0.8 + 1.0 mSv for the frontal view, 1.8 mSv for the lateral view. Stereo X-rays are generated from two consecutively taken images per view: 0.8x2 = 1.6 mSv for the frontal view; 1.0x2 = 2 mSv for the lateral view. Thus, the patient's exposure will be 1.8 mSv on average, which is not significantly different from the standard direct + lateral imaging. Stereo X-ray navigation enabled the surgeons to compare vertebral elements volume-wise and to implant the screws after a single radiographic examination, which resulted in lesser exposure, as less X-rays had to be taken per surgery.
Standard X-ray examination takes time to take two images, then to switch the C-arm from the frontal position to the lateral one, then switch it back; thus, a standard procedure would take 40