Radiologic assessment of interbody fusion (IF) is controversial; thin-slice CT is the present gold standard despite artifacts due to the metal implant that hinder interpretation. The present study aimed to assess the contribution of MRI in IF assessment after instrumented posterior lumbar interbody fusion (PLIF) using tantalum intervertebral implants. The study hypothesis was that fusion following PLIF can be assessed on MRI.

A single-center retrospective study included 52 single-level PLIF procedures (42 for degenerative disc disease and 10 for isthmic spondylolisthesis) using 2 tantalum intervertebral cages without interbody bone graft. Fusion was assessed at 1 year on static and dynamic X-ray and on MRI with a dedicated protocol. Two senior osteoarticular radiologists screened frontal and sagittal MRI slices for continuous cancellous interbody bridges. Consolidation was considered acquired (grade I) in case of continuous bridges on at least 2 successive frontal or sagittal slices, possible (grade II) in case of continuous bridge on just 1 slice, or absent (grade III) in case of no or discontinuous bridge.

Forty-eight patients were included, with a mean 55 months’ follow-up (range, 25–74 months). There were no hardware failures. Intervertebral mobility on dynamic X-ray was systematically less than 5°. Fusion on MRI was grade I in 71% of cases, grade II in 8% and grade III in 21%. Interobserver agreement was 100% on X-ray and 81% on MRI, with kappa coefficient=0.56 (range, 0.35–0.77).

Tantalum implants without bone autograft enabled satisfactory standard radiographic study. MRI provided imperfect assessment of fusion, being contributive only in case of positive findings (71% in the present study); when no bridge is detected on MRI, no conclusion can be drawn. Progress in CT to reduce artifacts may improve fusion assessment, unless bone integration on both sides of the implant is considered to be sufficient for interbody continuity, without a continuous bone bridge between endplates being a requirement.

IV.