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Objectives and questions: Lateral lumbar interbody fusion (LLIF) is based on a less-invasive access corridor through the retroperitoneum and psoas muscle, though concerns persist over postoperative weakness and neuropathy on the surgical side. This study investigates if the trans-psoas LLIF approach is associated with long-term changes in psoas morphology, hip flexor weakness, and lower extremity dysesthesia.

Material and methods: The authors retrospectively reviewed all LLIF cases at a single institution from January 2016 to June 2024. Inclusion required the availability of preoperative and postoperative magnetic resonance imaging. Psoas cross-sectional area was measured at the L4 upper endplate and the level of surgery. Clinical outcomes were analyzed through standardized Oxford motor examinations, standardized sensory examinations, and patient-reported symptoms as documented and reviewed by the attending surgeon.

Results: Eight hundred sixty-five patients underwent LLIF, of which 108 patients met inclusion criteria for imaging and indication (48 male, 60 female). The mean follow-up for included patients was 2.8 years (range 3.8 months-7.3 years). We observed a statistically significant mean decrease of 6.3% in psoas size on the surgical side (P < 0.001, Cohen's d = 0.19) and no significant change on the non-surgical side (P = 0.20, d = 0.07), both reflecting inconsequential effect sizes. Average hip flexor strength decreased by ≥ 1 grade in 8 (14.5%) patients on the surgical side and 5 patients (9.1%) on the nonsurgical side, with no significant difference between groups (P = 0.85).

Discussion and conclusions: Our study found that LLIF was associated with minor long-term decreases in psoas size and quality on the surgical side, small mean bilateral strength losses, and no side-specific sensory deficits.