EDUCATIONAL OBJECTIVES As a result of reading this article, physicians should be able to: 1. Understand the forces that predispose adjacent cervical segments to degeneration. 2. Understand the challenges of radiographic evaluation in the diagnosis of cervical and lumbar adjacent segment disease. 3. Describe the changes in biomechanical forces applied to adjacent segments of lumbar vertebrae with fusion. 4. Know the risk factors for adjacent segment disease in spinal fusion. Adjacent segment disease (ASD) is a broad term encompassing many complications of spinal fusion, including listhesis, instability, herniated nucleus pulposus, stenosis, hypertrophic facet arthritis, scoliosis, and vertebral compression fracture. The area of the cervical spine where most fusions occur (C3-C7) is adjacent to a highly mobile upper cervical region, and this contributes to the biomechanical stress put on the adjacent cervical segments postfusion. Studies have shown that after fusion surgery, there is increased load on adjacent segments. Definitive treatment of ASD is a topic of continuing research, but in general, treatment choices are dictated by patient age and degree of debilitation. Investigators have also studied the risk factors associated with spinal fusion that may predispose certain patients to ASD postfusion, and these data are invaluable for properly counseling patients considering spinal fusion surgery. Biomechanical studies have confirmed the added stress on adjacent segments in the cervical and lumbar spine. The diagnosis of cervical ASD is complicated given the imprecise correlation of radiographic and clinical findings. Although radiological and clinical diagnoses do not always correlate, radiographs and clinical examination dictate how a patient with prolonged pain is treated. Options for both cervical and lumbar spine ASD include fusion and/or decompression. Current studies are encouraging regarding the adoption of arthroplasty in spinal surgery, but more long-term data are required for full adoption of arthroplasty as the standard of care for prevention of ASD.
Newer navigation protocols that rely on intraoperative CT registration have improved the accuracy of imaging models and allowed surgeons to rely less on fluoroscopy. Despite concerns regarding ionizing radiation exposure to the patient, use of CT navigation systems was found to reduce radiation exposure by more than 90% compared to traditional fluoroscopic guided percutaneous surgical techniques (1). Furthermore, the surgeon's exposure to radiation is almost eliminated, as the surgeon does not need to be close to the patient during the CT image registration.The accuracy of navigation has also undergone a huge amount of progress. In addition to higher quality registration, the use of stereotactic 3D cameras allows the system to predict relative position between instruments and anatomical landmarks in real time with higher reliability (2). Amiot et al. and Yu et al. (3,4). both demonstrated that freehand pedicle screw (PS) placement had a higher rate of error and reoperation compared to navigation assisted placement. The same results were reflected in a meta-analysis of 12 studies conducted by Shin et al. (5) However, many other meta-analyses in the literature have failed to demonstrate superiority of computer assisted navigation to free hand PS instrumentation perhaps due to the heterogeneity of studies included. The accuracy of navigation has also undergone a lot of progress. In additional to higher quality registration, the use of stereotactic 3D cameras allows the system to predict relative position between instruments and anatomical landmarks in real time with higher reliability (2). Amiot et al. and Yu et al. (3,4) both demonstrated that freehand PS placement had a higher rate of error and reoperation compared to navigation assisted placement. Shin et al. completed a meta-analysis of 12 studies which also reflected the same results (5). However, many other meta-analyses in the literature have failed to demonstrate superiority of computer assisted navigation to free hand PS instrumentation perhaps due to the heterogeneity of studies included.Navigation systems rely on the use of reference trackers to keep the registration image in sync throughout the operation regardless of the positioning of the patient. Multiple modalities of anatomic tracking are in use. Pin trackers that are inserted into bony landmarks allow for accurate mapping with relatively few trackers, however
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