Imaging: Lumbosacral Fusion
Lumbosacral Fusion: Cages, Dowels, Pedicle Screws: Part 6
The assessment of a successful interbody arthrodesis is difficult and controversial. 15,30,64 The rates of "successful" fusion are approximately 20% higher when fusion is based solely on assessment of motion on plain film radiographs, as compared to methods which also require other criteria, such as the complete absence of peri-implant lucencies. 39, 64 Many studies also differ in the amount of motion permissible in a "successful" fusion, ranging from 1 to 5 degrees. 59, 60, 64, 66, 83, 101 Some suggest that the best indication of fusion with threaded interbody implants is the presence of a "sentinel sign": radiographically evident bridging trabecular bone anterior to the interbody device. 64, 66 In order to improve clinical results and assist in fusion determination, the concept of "ream long, fuse short" has been proposed. The threaded cylindrical interbody device is placed at the far posterior portion of the reamed and tapped channel, allowing room in the interspace anterior to the device for the packing of cancellous bone graft. When utilizing this method of cylindrical interbody device placement, care must be taken to ream to a sufficient depth prior to cage placement. Otherwise, attempts to seat the cage more posteriorly can result in "stripping" of the threads, a factor that has been associated with failure and implant migration. 60 In the case of threaded cortical bone dowels, this can exceed the insertional torque threshold of the implant-driver interface and cause device fracture. On the other hand, the periphery of the endplate is stronger than it is centrally, thus for biomechanical purposes the cage is best placed in the anterior aspect of the interspace.
Traditional methods of radiographically detecting pseudarthrosis are plain x-ray and CT. The standard criteria for pseudarthrosis are lucency around the cages, motion at the fused segment, and lack of viable bone extending through the cage into the vertebral marrow on reconstructed thin section, high resolution CT. High-resolution CT scans, however, have been unreliable in fusion status assessment. 15, 43, 66, 98 One recent report reviewed patients with postoperative persistent low back pain and surgically confirmed interbody pseudarthrosis. 43 The standard criteria failed to accurately predict pseudarthrosis proven at surgery. Six out of 7 patients who underwent pre-revision CT scanning had no identifiable peri-implant lucencies and 7 out of 8 patients who had pre-revision plain films showed no evidence of loosening. Due to the metal artifact at the periphery of titanium interbody cages, fusion assessment has been found to be more difficult (both on CT scans and plain films) than with threaded cortical bone dowels. 15, 98 It is clear that CT and plain films do not reliably detect pseudarthrosis preoperatively. CT as well as plain films may be falsely reassuring in evaluation of patients with titanium intervertebral lumbar spine fusion cages.
A very interesting cadaveric study on the imaging pitfalls of interbody spinal implants was reported by Cizek and Boyd. 15 They compared titanium threaded cylindrical cages, threaded bone dowels, and carbon fiber trapezoidal cages. They found the presence of bone seen extending through the cage from endplate to endplate on x-ray and CT was confused with fusion. And one may see lucencies on plain film or CT but not the other. Bone within the dowel and titanium cage devices could not be discerned by plain film. And most importantly, CT artifact was not present with the bone cage but was seen with titanium and carbon fiber. The artifact was more prominent anteriorly and posteriorly as well as the implant/endplate interface, which may obscure thin lucencies. In conclusion, current imaging techniques are suboptimal in evaluating spinal interbody implants. Errors may arise in interpreting the presence of bridging bone, lucencies, and obstruction by metallic artifact. Bridging bone may be misinterpreted as fusion, and radiolucency was better seen in threaded bone dowels compared to titanium cages.