Fat infiltration of the posterior paraspinal muscles is inversely associated with the fat infiltration of the psoas muscle: a potential compensatory mechanism in the lumbar spine

BACKGROUND: The function of the paraspinal muscles and especially the psoas muscle in maintaining an upright posture is not fully understood. While usually considered solely as a hip flexor, the psoas muscle and its complex anatomy suggest that the muscle has other functions involved in stabilizing the lumbar spine. The aim of this study is to determine how the psoas muscle and the posterior paraspinal muscles (PPM; erector spinae and multifidus) interact with each other. METHODS: A retrospective review including patients undergoing posterior lumbar fusion surgery between 2014 and 2021 at a tertiary care center was conducted. Patients with a preoperative lumbar magnetic resonance imaging (MRI) scan performed within 12 months prior to surgery were considered eligible. Exclusion criteria included previous spinal surgery at any level, lumbar scoliosis with a Cobb Angle > 20° and patients with incompatible MRIs. MRI-based quantitative assessments of the cross-sectional area (CSA), the functional cross-sectional area (fCSA) and the fat area (FAT) at L4 was conducted. The degree of fat infiltration (FI) was further calculated. FI thresholds for FI(PPM) were defined according to literature and patients were divided into two groups (< or ≥ 50% FI(PPM)). RESULTS: One hundred ninetypatients (57.9% female) with a median age of 64.7 years and median BMI of 28.3 kg/m(2) met the inclusion criteria and were analyzed. Patients with a FI(PPM) ≥ 50% had a significantly lower FI in the psoas muscle in both sexes. Furthermore, a significant inverse correlation was evident between FI(PPM) and FI(Psoas) for both sexes. A significant positive correlation between FAT(PPM) and fCSA(Psoas) was also found for both sexes. No significant differences were found for both sexes in both FI(PPM) groups. CONCLUSION: As the FI(PPM) increases, the FI(Psoas) decreases. Increased FI is a surrogate marker for a decrease in muscular strength. Since the psoas and the PPM both segmentally stabilize the lumbar spine, these results may be indicative of a potential compensatory mechanism. Due to the weakened PPM, the psoas may compensate for a loss in strength in order to stabilize the spine segmentally.