Original Research

Interobserver Agreement Using Computed Tomography to Assess Radiographic Fusion Criteria With a Unique Titanium Interbody Device

Am J Orthop. 2015 February;44(2):86-89

References

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Artifact

The results of our study support the idea that the design of a titanium interbody fusion implant is important to radiographic analysis. The implant studied has a large open central aperture that appears to generate less artifact than historical controls (paired cylindrical cages) have.^1-4 Other investigators have reported fewer problems with artifact in their studies of implants incorporating larger openings for bone graft.^6,18 The radiologists in the present study found no significant problems with artifact. Less artifact is clinically important, as the remaining fusion variables can be more clearly visualized (Table 2, Figure 2).

Anterior Osseous Bridging, Subsidence, Lysis

In this study, the bony endplates were preserved. The disc and endplate cartilage was removed without reaming or drilling. Endplate reaming most likely contributes to subsidence and loss of original fixation between implant and bone interface.^1,4,12 Some authors have advocated recessing the cages deeply and then packing bone anteriorly to create a “sentinel fusion sign.”^1,2,6 Deeply seating interbody implants, instead of resting them more widely on the apophyseal ring of the vertebral endplate, may also lead to subsidence.^4,12 The issue of identifying a sentinel fusion sign is relevant only if the surgeon tries to create one. In the present study, the implant used was an impacted cage positioned on the apophyseal perimeter of the disc space, just slightly recessed, so there was no attempt to create a sentinel fusion sign, as reflected in the relatively low scores on anterior osseous bridging (48%, 52%).

Subsidence and peri-implant lysis are pathologic variables associated with motion and bone loss. Sethi and colleagues¹⁹ noted a high percentage of endplate resorption and subsidence in cases reviewed using PEEK or allograft spacers paired with BMP-2. Although BMP-2 was used in the present study, we found very low rates of subsidence (0%, 5%) and no significant peri-implant lucencies (2%, 4%) (Figure 2). Interobserver agreement for these variables was high (0.95, 0.96). We hypothesize that the combination of endplate-sparing surgical technique and implant–bone integration contributed to these results.

Trabecular Bone and Fusion Grade

The primary radiographic criterion for solid interbody fusion is trabecular bone throughout the cage, bridging the vertebral bodies. In our study, the success rates for this variable were 96% and 100%, and there was very high interobserver agreement (0.96) (Figure 3). This very high fusion rate may preclude detecting subtle differences in interobserver agreement, but to what degree, if any, is unknown. Other investigators have effectively identified trabecular bone across the interspace and throughout the cages.^6,18 The openings for bone formation were larger in the implants they used than in first-generation fusion cages but not as large as the implant openings in the present study. Larger openings may correlate with improved ability to visualize bridging bone on CT.

Radiologists and surgeons must ultimately arrive at a conclusion regarding the likelihood a fusion has occurred. Our radiologists integrated all the separate radiologic variables cited here, as well as their overall impressions of the scans, to arrive at a final grade regarding fusion quality (Figures 3, 4). Although this category provides the most interpretive latitude of all the variables examined, the results demonstrate high interobserver reliability. Agreement to exactly the same fusion grade was 0.77, and agreement to within 1 category grade was 0.95.

This study had several limitations. Surgical explorations were not clinically indicated and were not performed. There were no suspected nonunions or hardware complications, two of the most common indications for exploration. In addition, this study was conducted not to determine specific accuracy of CT (compared with surgery exploration) for fusion assessment but to assess interobserver reliability. The clinical success rates for this population were high, and no patient required revision surgery for suspected pseudarthrosis. To assess the true accuracy of CT for fusion assessment, one would have to subject patients to follow-up exploratory surgery to test fusions mechanically.

Another limitation is the lack of a single industry-accepted radiographic fusion grading system. Fusion criteria are not standardized across all studies. Our radiologists have extensive research experience and limit their practices to neuromuscular radiology with a concentration on the spine. The radiographic criteria cited here are the same criteria they use in clinical practice, when reviewing CT scans for clinicians. Last, there was no control group for direct comparison against other cages. Historical controls were cited. This does not adversely affect the conclusions of this investigation.

Conclusion

Clinicians have been reluctant to rely on CT with titanium devices because of concerns about the accuracy of image interpretations. The interbody device used in this study demonstrated minimal artifact and minimal subsidence, and trabecular bone was easily identified throughout the implant in the majority of cases reviewed. We found high interobserver agreement scores across all fusion criteria. Although surgical exploration remains the gold standard for fusion assessment, surgeons should have confidence in using CT with this titanium implant.