Different Moro Zones of Psoas Major Affect the Clinical Outcomes after Oblique Lumbar Interbody Fusion: A Retrospective Study of 94 Patients

Abstract

Oblique lumbar interbody fusion (OLIF) has been driven to the maturity stage in recent years. However, postoperative symptoms such as thigh paresthesia resulting from intraoperative retraction of the psoas major (PM) have sometimes occurred. The aim of this study was to assess the different positions and morphology of PM muscles and their relationship with clinical outcomes after OLIF by introducing the Moro zones. Patients who underwent L4-5 OLIF at our institution between April 2019 and June 2021 were reviewed and all data were recorded. All patients were grouped by Moro zones into a Moro A cohort and a Moro I and II cohort based on the front edges of their left PM muscles. A total of 94 patients were recruited, including 57 in the Moro A group and 37 in the Moro I and II group. Postoperative thigh pain or numbness occurred in 12 (21.1%) and 2 (5.4%) patients in the Moro A group and the Moro I and II group, respectively. There was no difference in the psoas major transverse diameter (PMTD) between groups preoperatively, while longer PMTD was revealed postoperatively in the Moro A group. The operating window (OW) and psoas major sagittal diameter (PMSD) showed significant differences within and between groups. Thirteen patients had teardrop-shaped PM muscles, with 92.3% in the Moro A group showing significantly worse clinical scores at 1-week follow-up. The Moro zones of the PM affected the short-term outcomes after OLIF. Preoperative measurements and analysis of OW, PMSD and PM morphology should be performed as necessary to predict short-term outcomes.

Keywords: Moro zones; minimally invasive; oblique lumbar interbody fusion; psoas major; teardrop-shaped psoas major.

Figure 1

Measurement standards and Moro zones. (left): Disc midpoint (O); disc sagittal diameter, DSD (OA); operating window, OW (BC); psoas major transverse diameter, PMTD (DG); psoas major sagittal diameter, PMSD (EF). (right): The disc was divided into Zones A, I, II, III, IV, and P s. The psoas major muscles were divided according to the positions of their anterior borders. The left psoas major with a teardrop shape was located in Moro Zone A. The right psoas major with a normal shape was located in Moro Zone I.

Figure 2

Clinical scores at different follow-up times. The VASb (A), VASl (B), and ODI (C) scores showed worse results in the Moro A group at 1-week postoperative follow-up. *, p < 0.001.

Figure 3

The multiple-regression linear model. The preoperative OW and PMSD showed significant linear correlations with the 1-week postoperative ODI.

Figure 4

Typical case. A 55-year-old woman, presented with pain and numbness in the right lower extremity for 5 years; her condition worsened for 3 weeks. Preoperative lumbar X-ray (A,B); preoperative lumbar CT (C,D); preoperative lumbar MRI. Preoperative images showed the L4 spondylolisthesis and stenosis at L4-5 (E,G); postoperative lumbar X-ray showed L4 was reduced (H,I); postoperative lumbar CT showed the distraction of disc height (J,K); postoperative lumbar MRI showed satisfactory decompression and a huge swelling of the left psoas major (arrow) (F,N). Lumbar CT reconstruction and MRI at 1-year follow-up showed solid fusion with no cage subsidence (L). Swelling of the left psoas major reduced significantly and its area recovered (arrow) (M,O).