Motor branches to the quadratus lumborum arise from the ventral rami of T12 and L1-L4, entering the muscle on its anterior surface from the lumbar plexus within the substance of psoas. The QL receives a segmental, multilevel motor supply.
The nerve to quadratus lumborum is at risk during retroperitoneal lumbar approaches and LLIF procedures where the QL is traversed. Its multilevel supply means complete denervation requires injury at multiple levels. QL denervation produces loss of lateral lumbar stability and pelvic obliquity. EMG of the QL is performed in complex lumbar nerve root injury workup to assess T12 and upper lumbar plexus function.
Loss of QL motor branches from multi-level retroperitoneal approach producing lateral trunk imbalance and pelvic tilt, contributing to poor outcome after lateral lumbar surgery in patients with pre-existing QL weakness.
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