Multifidus Muscle Changes After Biportal Endoscopic Spinal Surgery: Magnetic Resonance Imaging Evaluation

OBJECTIVE: We used magnetic resonance imaging (MRI) to assess the radiological status of the multifidus muscles (MFMs) after biportal endoscopic spinal surgery (BESS) and evaluated the extent of MFM injury and atrophy. METHODS: A total of 88 patients who had met the inclusion and exclusion criteria were enrolled in the present study. T2-weighted signal intensity MRI was performed 3 times: preoperatively, immediately postoperatively, and at the final follow-up examination. We measured the cross-sectional area of the MFM on both sides (ipsilaterally and contralaterally) and recorded the operative times. The association between the interval from surgery to the final follow-up MRI and changes in the MFMs and between the operative time and changes in the MFMs were analyzed. For the group comparisons, the patients were divided into 3 groups according to the follow-up interval. Group 1 was followed up within 2 weeks, group 2 within 2-4 weeks, and group 3 after 4 weeks. The MFM changes were recorded. RESULTS: The operative time correlated significantly with the percentage of change in the T2-weighted signal intensity ratio (SIR) for both sides (P < 0.01). At the final follow-up examination, the SIR of the ipsilateral side had decreased in group 3 (P = 0.002). The percentage of change in the SIR was smallest in group 3 (P = 0.004). CONCLUSIONS: The MFM change on MRI after BESS became significant on both sides as the operative time increased. However, the change showed a tendency to reverse within several months, and no substantial change in the MFM cross-sectional area was found. We have concluded that MFM changes after BESS might correlate with an increased operative time but will resolve over time.