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

The accuracy of using computed tomography (CT) to assess interbody fusion in patients with titanium implants has been questioned in the past. Radiologists have reported difficulty assessing fusion bone quality because of metal artifact and small graft windows. A new titanium interbody implant with a large footprint and a wide graft aperture has been developed.

We conducted a study to determine the interobserver reliability of using CT to assess radiographic fusion variables with the new titanium interbody device. Patients underwent anterior lumbar interbody fusion with the same titanium interbody implant. Reconstructed CT images were obtained randomly at 6, 9, or 12 months. Two independent radiologists reviewed the scans. Interobserver reliability was calculated using the κ statistic.

Fifty-six spinal fusion levels (33 patients) were analyzed. The radiologists agreed on 345 of the 392 fusion data points reviewed (κ = .88). Agreement for solid fusion formation was 0.77.

This interbody device demonstrated minimal artifact and minimal subsidence, and trabecular bone was easily identified throughout the implant in the vast majority of cases reviewed. High interobserver agreement was noted across all radiographic variables assessed.

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References

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2. Heithoff KB, Mullin WJ, Renfrew DL, Gilbert TJ. The failure of radiographic detection of pseudarthrosis in patients with titanium lumbar interbody fusion cages. In: Proceedings of the 14th Annual Meeting of the North American Spine Society; October 20-23, 1999; Chicago, IL. Abstract 14.

3. Cizek GR, Boyd LM. Imaging pitfalls of interbody implants. Spine. 2000;25(20):2633-2636.

4. Dorchak JD, Burkus JK, Foor BD, Sanders DL. Dual paired proximity and combined BAK/proximity interbody fusion cages: radiographic results. In: Proceedings of the 15th Annual Meeting of the North American Spine Society. New Orleans, LA: North American Spine Society; 2000:83-85.

5. Santos ER, Goss DG, Morcom RK, Fraser RD. Radiologic assessment of interbody fusion using carbon fiber cages. Spine. 2003;28(10):997-1001.

6. Carreon LY, Glassman SD, Schwender JD, Subach BR, Gornet MF, Ohno S. Reliability and accuracy of fine-cut computed tomography scans to determine the status of anterior interbody fusions with metallic cages. Spine J. 2008;8(6):998-1002.

7. Fogel GR, Toohey JS, Neidre A, Brantigan JW. Fusion assessment of posterior lumbar interbody fusion using radiolucent cages: x-ray films and helical computed tomography scans compared with surgical exploration of fusion. Spine J. 2008;8(4):570-577.

8. Selby MD, Clark SR, Hall DJ, Freeman BJ. Radiologic assessment of spinal fusion. J Am Acad Orthop Surg. 2012;20(11):694-703.

9. Chafetz N, Cann CE, Morris JM, Steinbach LS, Goldberg HI, Ax L. Pseudarthrosis following lumbar fusion: detection by direct coronal CT scanning. Radiology. 1987;162(3):803-805.

10. Landis RJ, Koch GG. The measurement of observer agreement for categorical data. Biometrics. 1977;33(1):159-174.

11. Viera AJ, Garrett JM. Understanding interobserver agreement; the kappa statistic. Fam Med. 2005;37(5):360-363.

12. Burkus JK, Foley K, Haid RW, Lehuec JC. Surgical Interbody Research Group—radiographic assessment of interbody fusion devices: fusion criteria for anterior lumbar interbody surgery. Neurosurg Focus. 2001;10(4):E11.

13. Albrektsson T, Zarb G, Worthington P, Eriksson AR. The long-term efficacy of currently used dental implants: a review and proposed criteria of success. Int J Oral Maxillofac Implants. 1986;1(1):11-25.

14. De Leonardis D, Garg AK, Pecora GE. Osseointegration of rough acid-etched titanium implants: 5-year follow-up of 100 Minimatic implants. Int J Oral Maxillofac Implants. 1999;14(3):384-391.

15. Schwartz Z, Raz P, Zhao G, et al. Effect of micrometer-scale roughness on the surface of Ti6Al4V pedicle screws in vitro and in vivo. J Bone Joint Surg Am. 2008;90(11):2485-2498.

16. Olivares-Navarrete R, Gittens RA, Schneider JM, et al. Osteoblasts exhibit a more differentiated phenotype and increased bone morphogenetic protein production on titanium alloy substrates than on poly-ether-ether-ketone. Spine J. 2012;12(3):265-272.

17. Olivares-Navarrete R, Hyzy SL, Gittens RA 1st, et al. Rough titanium alloys regulate osteoblast production of angiogenic factors. Spine J. 2013;13(11):1563-1570.

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The accuracy of using computed tomography (CT) to assess lumbar interbody fusion with titanium implants has been questioned in the past.^1-4 Reports have most often focused on older technologies using paired, threaded, smooth-surface titanium devices. Some authors have reported they could not confidently assess the quality of fusions using CT because of implant artifact.^1-3

When pseudarthrosis is suspected clinically, and imaging results are inconclusive, surgical explorations may be performed with mechanical stressing of the segment to assess for motion.^2,5-7 However, surgical exploration not only has the morbidity of another surgery but may not be conclusive. Direct exploration of an interbody fusion is problematic. In some cases, there may be residual normal springing motion through posterior elements, even in the presence of a solid interbody fusion, which can be confusing.⁵ Radiologic confirmation of fusion status is therefore preferred over surgical exploration. CT is the imaging modality used most often to assess spinal fusions.^8,9

A new titanium interbody fusion implant (Endoskeleton TA; Titan Spine, Mequon, Wisconsin) preserves the endplate and has an acid-etched titanium surface for osseous integration and a wide central aperture for bone graft (Figure 1). Compared with earlier titanium implants, this design may allow for more accurate CT imaging and fusion assessment. We conducted a study to determine the interobserver reliability of using CT to evaluate bone formation and other radiographic variables with this new titanium interbody device.

Materials and Methods

After receiving institutional review board approval for this study, as well as patient consent, we obtained and analyzed CT scans of patients after they had undergone anterior lumbar interbody fusion (ALIF) at L3–S1 as part of a separate clinical outcomes study.

Each patient received an Endoskeleton TA implant. The fusion cage was packed with 2 sponges (3.0 mg per fusion level) of bone morphogenetic protein, or BMP (InFuse; Medtronic, Minneapolis, Minnesota). In addition, 1 to 3 cm³ of hydroxyapatite/β-‌tricalcium phosphate (MasterGraft, Medtronic) collagen sponge was used as graft extender to fill any remaining gaps within the cage. Pedicle screw fixation was used in all cases.

Patients were randomly assigned to have fine-cut CT scans with reconstructed images at 6, 9, or 12 months. The scans were reviewed by 2 independent radiologists who were blinded to each other’s interpretations and the clinical results. The radiographic fusion criteria are listed in Tables 1 to 3. Interobserver agreement (κ) was calculated separately for each radiographic criterion and could range from 0.00 (no agreement) to 1.00 (perfect agreement).^10,11

Results

The study involved 33 patients (17 men, 16 women) with 56 lumbar spinal fusion levels. Mean age was 46 years (range, 23-66 years). Six patients (18%) were nicotine users. Seventeen patients were scanned at 6 months, 9 at 9 months, and 7 at 12 months. There were no significant differences in results between men and women, between nicotine users and nonusers, or among patients evaluated at 6, 9, or 12 months.

The radiologists agreed on 345 of the 392 data points reviewed (κ = 0.88). Interobserver agreement results for the fusion criteria are listed in Tables 1 and 3. Interobserver agreement was 0.77 for overall fusion grade, with the radiologists noting definite fusion (grade 5) in 80% and 91% of the levels (Table 1). Other radiographic criteria are listed in Tables 2 and 3. Interobserver agreement was 0.80 for degree of artifact, 0.95 for subsidence, 0.96 for both lucency and trabecular bone, 0.77 for anterior osseous bridging, and 0.95 for cystic vertebral changes.

Discussion

Radiographic analysis of interbody fusions is an important clinical issue. Investigators have shown that CT is the radiographic method of choice for assessing fusion.^8,9 Others have reported that assessing fusion with metallic interbody implants is more difficult compared with PEEK (polyether ether ketone) or allograft bone.^3,4,5,12

Heithoff and colleagues^1,2 reported on difficulties they encountered in assessing interbody fusion with titanium implants, and their research has often been cited. The authors concluded that they could not accurately assess fusion in these cases because of artifact from the small apertures in the cages and metallic scatter. Their study was very small (8 patients, 12 surgical levels) and used paired BAK (Bagby and Kuslich) cages (Zimmer, Warsaw, Indiana).

Recently, a unique surface technology, used to manufacture osseointegrative dental implants, has been adapted for use in the spine.^13-15 Acid etching modifies the surface of titanium to create a nano-scale (micron-level) alteration. Compared with PEEK and smooth titanium, acid-etched titanium stimulates a better osteogenic environment.^16,17 As this technology is now used clinically in spinal surgery, we thought it important to revisit the issue of CT analysis for fusion assessment with the newer titanium implants.