Purpose of the study: Intramuscular injection of the botulinum toxin into the psoas can be proposed for permanent hip flexion due to spastic disorders. Several approaches have been described: retrograde subinguinal, anterolateral suprailiac, and posterior. Ultrasound or computed tomography can be used to guide needle position. These approaches are however limited to access to the L4 region, i.e. far from the motor points and with the risk of injury to the ureter. The purpose of this work was to determine the innervations of the psoas muscle that would be best adapted to this type of injection and thus to describe the most effective and reliable approach.

Material and methods: This anatomy study included 20 dissections to: describe vertebral insertions of the psoas major and the psoas minor and to measure their distance from the iliac crest; define the region where the ureter crosses in front of the psoas.

Results: More than 80% of the psoas muscles presented a proximal insertion on the transverse process of T12 and the body of L1; the mean length of the psoas in the adult is 27 cm above the inguinal ligament; the nerve roots collateral to the lumbar plexus are: 33% L2, 25% L3, 19% L1, 9% L4, 3% L5 and 1% T12, the remainder arising directly from the femoral nerve; the L2-L3 region is situated 4.6 cm on average above the iliac crest.

Discussion: The region facing the L2-L3 space enables access to more than 50% of the psoas nerve branches. Injection via a posterior approach situated in adults 4.6 cm above the iliac crest and identified fluoroscopically is the most reliable access. This will avoid injury to the ureter which lies lower.

Conclusion: This anatomy study described a new more effective less dangerous approach for botulinum toxin injections into the psoas muscle.