The fate of autogenous free-fat grafts after posterior lumbar surgery: Part 1. A postoperative serial magnetic resonance imaging study

Study Design. Time-related changes in free-grafted fat were investigated by postoperative serial magnetic resonance imaging in 22 patients with degenerative spinal disease who underwent posterior lumbar decompressive surgery. Objectives. To analyze the size and quality of the grafted fat on magnetic resonance imaging after posterior lumbar surgery. Summary of Background Data. Epidural fat grafts have been used to prevent epidural and perineural fibroses. Evaluations of changes in grafted fat have been reported, as observed on computed tomography scans. However, there are no published reports of clinical serial magnetic resonance imaging observations of grafted fat after posterior lumbar surgery. Methods. Axial T1- and T2-weighted magnetic resonance imaging was performed at 3, 7, 21, and 42 days as well as 1 year after surgery. The signal intensity of the fat was classified on the T1-weighted image as Grade 1 (almost equal to subcutaneous fat tissue), Grade 2 (low-signal intensity as compared with subcutaneous fat tissue), Grade 3 (speckled intensity), or Grade 4 (signal void, suggesting the change to scar tissue. The time-related, cross-sectional area of the subarachnoidal space was measured from the T2-weighted image. Results. The size of the grafted fat was reduced to approximately 57% after 42 days, and to about 33% after more than 1 year, as compared with the condition 3 days after surgery. In time, the shape of the grafted fat was changed along the shape of the dura mater. During the early stage (within 6 weeks after surgery), the signal intensity of the grafted fat was lower than that of normal subcutaneous fat tissue (Grades 2-4: 40.9-59.1%). However, the intensity had recovered to normal status (Grade 1) by 1 year after surgery. Conclusions. The total amount of grafted fat used in posterior lumbar surgery is reduced. However, as observed on magnetic resonance imaging, it is alive and remodeled along the shape of the dura mater in relation to its shrinkage and reexpansion. This remodeling of the grafted fat is meaningful and effective in protecting the spinal nerve. This report clarifies the fate of the grafted fat from the findings of postoperative serial magnetic resonance imaging after lumbar decompressive surgery.