BACKGROUND CONTEXT Pseudarthrosis or nonunion of the cervical spine is the result of failed attempted fusion and is a leading cause of postoperative axial pain and radiculopathy. Reported rates of failed cervical fusion range from 4.4 to 50%, generally higher with multilevel procedures, and pseudarthrosis accounts for 45%–56% of revision surgeries. Nonunion is difficult to detect clinically and diagnosis is based solely on symptomatology (neck pain, radiculopathy) and imaging studies. The gold standard approach to define fusion status involves surgical exploration, a last resort option. Radiographic tools are commonly used first, with computed tomography (CT) considered the most reliable method. However, CT scans are expensive and expose patients to large radiation doses (hundreds of times more than standard X-ray). The status of anterior cervical fusion can also be determined using interspinous motion analysis; the goal is to detect any movement between adjacent vertebra using dynamic radiographs. This may be accomplished by measuring the distance between the tips of adjacent spinous processes on lateral flexion and extension films. However, the technique is subjective and affected by parallax, and studies have found wide interobserver differences. This makes it difficult to accurately apply the method to assess pseudarthrosis using the proposed cutoff values (1 or 2 mm). Other literature similarly suggests that current radiographic methods do not reliably provide clinical information about abnormalities of intervertebral motion. Therefore quantitative and standardized methods for defining spinal fusion or instability are needed. PURPOSE Our hypothesis is that intervertebral motion can be more accurately detected using a simple passive implantable device that responds to the pressure differential or vertebral body endplate motion within the index disc space between the flexed and extended positions of the cervical spine. Conceptually, this dynamic motion sensor could detect and potentially prevent pseudarthrosis, a leading cause of postoperative pain and radiculopathy STUDY DESIGN/SETTING This desktop and benchtop research was conducted primarily in a chemistry laboratory. Manufacturing of prototypes and other materials was performed on-site. Radiographic images were taken at a centralized animal research facility. METHODS A cervical interbody spacer with integrated fluidic pressure sensor was developed using CAD modeling software and prototyped with 3D printing. The working principal involves a fluid well and indicator channel. The spacer was placed between Sawbones vertebra analogs and loads applied to simulate dynamic spinal positions. Previous work with a similar fluidics device used a single-column compression tester to apply loads comparable to those experienced by vertebra in the cervical region. Radiographs were also taken of a device loaded with radio-dense fluid (cesium acetate) and channel diameter of 0.5mm. RESULTS Computer simulations suggested that the device would fit and function well between cervical vertebra in the flexed and extended spinal positions. Prototypes placed between cervical bone analogs under load demonstrated that the signal would be apparent clinically. Previous work showed that fluid displacement into the indicator portion was in the appropriate scale (0–6.9 mm) under applied loads in the range experienced clinically (0–110 N); this relationship was linear and repeatable. The imaging resolution of the device with a radiocontrast agent as the indicator fluid was also within the clinical range, and the signal was apparent on radiographs. CONCLUSIONS Based on these results it appears that an interbody device with fluidic sensor is potentially a viable option for assessing fusion status in the cervical spine.Prior literature gives the range of loads expected in the cervical spine (up to ≈100N). Articles describing interspinous motion analysis suggest sufficient bone growth (fusion) occurs when the distance between the tips of adjacent cervical spinous processes changes by less than 1 mm or 2 mm between flexion and extension. Our concept offers two primary advantages over existing techniques. First, the indicator channel provides a clear and distinct signal or marker that can be plainly read on radiographs, simplifying comparisons between the flexed, neutral and extended positions and enabling monitoring over time. Second, the device introduces gain proportional to the ratio of the cross-sectional areas of the fluid well and channel, providing amplification and enabling detection of smaller changes. While a priori research demonstrated that an implantable fluidic device responded appropriately under the range of applied physiologic loads, if needed additional gain could be introduced by increasing this ratio. Future work involving finite element analysis and mechanical testing is underway and ultimately a cadaveric study will be conducted to verify the loads and characterize the device in vitro.Limitations of this work include the material choice as all prototypes were created using 3D printing. The flexural modulus of the material is important, and the device may potentially be formed with polycarbonate urethane, a stiffer and tougher flexible material commonly used in cervical disc replacements.
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Oct 1, 2019
Agnes Beckmann + 11
Open Access