【Introduction】 Vertebral bone marrow edema (BME) reflects various pathologies including osteoporotic vertebral fracture, disc degeneration with Modic type 1 change and osteomyelitis. Among patients who underwent posterior/transforaminal lumbar interbody fusion (PLIF/TLIF), we often observed vertebral BME on postoperative magnetic resonance imaging. However, its clinical relevance remains unclear. The purpose of this study is to examine the appearance of BME on postoperative MRI in those who showed normal healing course of PLIF/TLIF and clarify its clinical relevance.

【Methods】 Using a prospectively established database, 97 consecutive patients who had undergone PLIF/TLIF for lumbar degenerative disease were retrospectively reviewed. Patterns and extent of BME (percentage to whole vertebral area) were evaluated on MRI at postoperative week 3.  Presence or absence of cage subsidence, cage back out and screw loosening were assessed on X-ray images at postoperative week 3. Back pain was also evaluated by visual analogue scale (VAS).

【Results】 Excluding surgical site infection, implant failure and early revision cases, 90 patients (30 men and 60 women, mean age 71.2 ± 9.2 years) were eligible for analysis. Fifty-eight patients had one cage inserted (Single cage group), and 32 patients had two cages placed bilaterally (Dual cage group). Sixty-eight patients underwent a single-level fusion, and 22 patients underwent a 2-level fusion. Of 180 instrumented vertebrae, BME was found in 148 vertebrae. BME patterns were categorized into the following four types: no edema type (17.7%, 32/180 vertebrae), anterior corner type (37.7%, 68/180 vertebrae), around-the-cage focal type (42.2%, 76/180 vertebrae) and diffuse type (2.2%, 4/180 vertebrae). Single cage group had 6.9% in no edema type, 46.3% in anterior corner type, 45.7% in around-the-cage focal type, and 0.9% in diffuse type. In the dual cage group, there were 37.5% in no edema type, 21.9% in anterior corner type, 36.0% in around-the-cage focal type, and 4.7% in diffuse type. Single cage group had significantly higher rate of BME than the dual cage group (93.1% versus 62.5%, p<.001 in chi-square test). Anterior corner type was significantly more frequent in the single cage group than the dual cage group (p<.001 in chi-square test). BME extent was significantly greater in the single cage group than the dual cage group (38.8% vs 20.1%, p<.001 in student`s t-test), and was significantly greater in patients with cage subsidence than those without cage subsidence (45.7% vs 29.6%, p<.001 in student`s t-test). BME extent had no significant correlation with cage back out, screw loosening and VAS of back pain.  

【Discussion】 The current results showed that BME was frequently observed even in the normal healing course of lumbar interbody fusion; the rate reached 93.1% in the single cage group and 62.5% in the dual cage group. However, diffuse type BME was rarely observed, and the incidence was 0.9% in the single cage group and 4.7% in the dual cage group. Diffuse type BME might be one of the findings related to pathological events. As well, BME extent was significantly greater in cage subsidence group, suggesting a potential parameter predicting cage subsidence.