Magnetic Resonance Imaging Evaluation of the Effects of Surgical Invasiveness on Paravertebral Muscles Following Muscle-preserving Interlaminar Decompression (MILD).

STUDY DESIGN: A retrospective study.

OBJECTIVES: To determine the extent of damage to the paravertebral muscles following muscle-preserving interlaminar decompression (MILD) using magnetic resonance imaging (MRI) to evaluate changes in the multifidus muscle (MF).

SUMMARY OF BACKGROUND DATA: Short-term surgical outcomes of MILD for lumbar spinal canal stenosis (LSCS) are satisfactory; however, the extent of damage to the paravertebral muscles following MILD remains unclear.

METHODS: Thirty-four patients (18 males/16 females; mean age: 72.6 y-old) who had LSCS treated with MILD were retrospectively investigated. A total of 61 decompressed disc levels [L2/3(5); L3/4(21); L4/5(30); L5/S(5)] and 34 non-decompressed levels (L1/2) were assessed. There was one decompressed disc level in 12 cases, two in 17 cases and three in five cases. MRIs were obtained before surgery, and at three and 12-18 months after surgery, using the same scanner. The rate of paravertebral muscle atrophy was evaluated to compare the area of the MF in the T2-weighted axial plane (intervertebral disc level) pre- and post-operatively, using OsiriX® Medical Imaging Software. Changes in muscle signal intensity were also recorded. Statistical analysis was performed using three-way ANOVA with the post-hoc Fisher PSLD test.

RESULTS: The rate of MF atrophy was 4.0% at the decompressed levels and 2.1% at the non-decompressed levels. There were no changes of signal intensity in the MF between the pre- and post-operative periods. In decompressed levels, muscle atrophy and signal intensity were significantly improved from three months to 12-18 months after surgery. The number and level of the decompressed discs did not affect the extent of muscle injury.

CONCLUSIONS: The extent of paravertebral muscle injury after MILD is satisfactory. The midline interlaminar approach used in this technique may prevent local denervation and irreversible damage to the paravertebral muscles. These results indicate that MILD is useful to treat LSCS less invasively.