Multifidus morphology in persons scheduled for single-level lumbar microdiscectomy: qualitative and quantitative assessment with anatomical correlates.

Objective: The purpose of this study was 2-fold: (1) to analyze the degree and the location of lumbar multifidus asymmetry among patients scheduled for L4-L5 microdiscectomy, and (2) to determine the amount of asymmetry recognizable by a radiologist on magnetic resonance imaging.

Design: Bilateral multifidi cross-sectional areas were measured on magnetic resonance axial images of 20 patients scheduled to undergo L4-L5 microdiscectomy. The sides were compared and the location and amount of asymmetry (%)determined. The axial slices were assigned to two different categories based on their anatomical proximity to the L4-L5 disc (Disc(Ref)) and to the fascicles of the lumbar multifidus attaching to the spinous process of L5 (Muscle(Ref)). The maximal percent difference in cross-sectional area was compared between these categories. A radiologist rated the degree of asymmetry visually. These ratings were compared slice by slice to the computed percent difference in the cross-sectional area.

Results: Mean differences in multifidus cross-sectional area were 11.5 +/- 5.6% in the Disc(Ref) and 15.8 +/- 9.5% in the Muscle(Ref) categories and were statistically significant (P = 0.03). The radiologist recognized asymmetry when the quantitative difference was at least 12.6%. This level of asymmetry was identified in the Muscle(Ref) category in 45% of cases and in Disc(Ref) in 30% of cases.

Conclusions: Nearly half of the cases scheduled for lumbar microdiscetomy exhibited asymmetry of at least 12.6% in the lumbar multifidus innervated by the L5 nerve root, which is located below the level of the injured disc. A trained radiologist can readily identify this multifidus asymmetry.