Quantification of Multifidus Atrophy and Fatty Infiltration Following a Minimally Invasive Microdiscectomy

Discussion

Traditional open approaches to the lumbar spine are known to result in atrophy of the multifidus muscle.^{10, 11} Published studies regarding multifudus muscle degeneration and fatty infiltration following MIS techniques have been limited. A study by Bresnahan et al demonstrated a significant increase in the paraspinal muscle cross sectional area on MRI in patients who underwent an MIS decompression.¹⁷ Although the imaging analysis was blinded, the results of postoperative paraspinal muscle hypertrophy were counterintuitive.

Fan et al compared the atrophy in the multifidus muscle in patients who underwent either an MIS or open posterior lumbar interbody fusion (PLIF).¹⁸ The authors demonstrated significantly lower multifidus muscle atrophy in the MIS cohort as compared to the open cohort.¹⁸ However, the indications for PLIF ranged from spondylolisthesis to large lumbar disc herniations.¹⁸ Similarly, Min et al demonstrated significantly decreased multifidus muscle degeneration following minimally invasive lumbar fusion when compared to open techniques.²⁰ However, the open patients underwent either a PLIF or a postero-lateral lumbar fusion (PLF) while all MIS patients underwent a transforaminal lumbar interbody fusion (TLIF).²⁰ As such, the purpose of this study was to rigorously quantify the multifidus muscle degeneration and fatty infiltration following a MIS lumbar mi crodiscectomy as demonstrated on MRI utilizing each patient as his or her own control.

The current study was able to quantify the anatomic changes to the multifidus muscle at an average of 6 months following a primary 1-level MIS lumbar de compression. Atrophy, as determined by the change in total CSA, was minimal and isolated to the index level. The lean CSA decreased most significantly at and below the index level, while fatty infiltration (as measured by the lean-to-total CSA ratio) increased at the adjacent levels. Additionally, a pattern of increasing atrophy and fatty infiltration caudal to the site of surgical intervention was demonstrated in the ipsilateral lean CSA and lean-to-total CSA ratio.

Although fatty degeneration, as measured by the T1 signal intensity ratio, demonstrated substantial percent change following surgery, no statistical significant differences were demonstrated between pre- and postoperative values. Recently, Zhi-Jun et al as sessed the accuracy of this measurement in a rat model.²³ The Pearson correlation coefficient demonstrated high correlation between T1 intensity ratios and histological fat scores (r = 0.804).²³ Despite these findings, the current study only demonstrated a trend in postoperative T1 intensity ratio changes. This finding may be due to the wide range of values over a proportionally smaller surgical area attributed to MIS techniques in humans as compared to open multi-level techniques in a rat model.²³

Other studies have focused on T2 intensity ratios as an accurate reflection of multifidus muscle disruption.^{19, 24} T2 signal intensity ratio measurements appear to be a better reflection of muscle edema and in flammation. These studies differ from the current study in that the analysis was performed on patients who had undergone open procedures with short-term follow up. In our analysis, the average postoperative imaging was obtained 6 months after surgery, which may have been well into the fibrotic regenerative stage of the injury response minimizing the post-surgical changes potentially observed on T2 imaging.

Fatty infiltration, as determined by the lean-to-total CSA ratio, demonstrated post-surgical changes in the current study. Given the variability in the shape and CSA of the multifidus muscle between patients, it appears intuitive that this ratio more accurately reflects true amount of fatty infiltration. Currently, there is insufficient evidence regarding the utility of the CSA ratio and its validity and reproducibility should be the subject of future research. Neverthe-less, this pattern of fatty infiltration does support previous studies regarding the complex anatomy and innervation of the multifidus muscle.^{3, 4} In addition, theses results are favorable when compared to data regarding paraspinal anatomy after traditional open posterior approaches.^{11, 19}

Despite small heterogeneous patient populations^{18, 20} and some counterintuitive results¹⁷, studies comparing changes to paraspinal musculature between open and MIS procedures have consistently favored MIS techniques. Bresnahan et al compared the paraspinal CSA in a cohort of 18 patients (9 undergoing open vs. 9 undergoing microendoscopic decompression). At an average of 16 months, the open patients demonstrated an average decrease in CSA of 5.4% while the microendoscopic decompression of steno sis (MEDS) grouped CSA ironically increased by 9.9%.¹⁷

There are clear limitations to the current study. First, the sample size was relatively small, which may have contributed to the lack of statistical significance when comparing pre- and postoperative T1 signal intensity ratios. However, consistent differences were detected across all measurements. In addition, to the best of our knowledge, the current study is the largest radiographic analysis of patients undergoing primary MIS microdiscectomies. Second, no clinical correlations were made with the radiographic findings. As such, the clinical significance of these radiographic changes cannot be ascertained. Third, although the pre- and postoperative measurements were compared with each patient operating as their own control in the paired sample T-test, the study may have benefited from a control group that included patients undergoing open decompression procedures. Additionally, the date of the pre- and postoperative imaging was not standardized and varied be tween patients. Changes demonstrated in the postoperative imaging may have been independent of the procedure and potentially due to the timing of the postoperative imaging. In addition, two surgeons performed the surgeries potentially introducing some intra-operative variability amongst differing MIS techniques. Lastly, the authors of the study matched the MIS patients to those patients who underwent an open approach lumbar microdiscectomy according to age, operative levels, and timing to imaging relative to the index surgery. However, due to the great variability of the cross-sectional area measurements in the open microdiscectomy patients, no adequate comparison could be performed between open and MIS patients.

In summary, the findings of the current study suggest primary 1-level MIS microdiscectomy results in minimal atrophy and fatty infiltration to the multifidus muscle at the index level on the ipsilateral side of the intervention. Atrophy, as measured by the lean CSA at the index level on the ipsilateral side, ranged from 2.9-18.6% with increasing atrophy demonstrated caudal to the index level. Fatty infiltration (best represented by the decrease in the lean-to-total CSA ratio) increased by 1.2% at the index level on the ipsilateral side. However, more profound fatty infiltration was demonstrated caudally toward the inferior disc level, which increased by 17.2% and 5.3% on the ipsilateral and contralateral sides, respectively.