Hip flexor spasticity in patients with upper motor neuron syndrome of multiple etiologies has been managed with botulinum neurotoxin injections mainly targeting the “iliopsoas” muscle. A lumbar approach to target psoas major (PM) has been described; however, it has not been incorporated into clinical practice due to perceived risk of injury to surrounding structures. This study will investigate the feasibility and accuracy of ultrasound-guided (UG)-PM injection using a lumbar approach by assessing the intra/extramuscular injectate spread in cadaveric specimens.

In eight lightly embalmed specimens, toluidine blue dye/saline was injected into PM using a UG-posterior lumbar approach. The posterior abdominal wall was exposed, and dye spread and surrounding structures digitized and modeled in 3D. The area and vertebral level of dye spread, distance of dye to the inferior vena cava (IVC), and abdominal aorta (AA) and dye spread to adjacent organs were quantified.

The models enabled visualization of the dye spread in 3D. Mean area of dye spread was 24.4 ± 2.8 cm2; most commonly between L2 and L4 vertebral levels. Mean distance of the dye to AA was 3.2 ± 1.2 cm and to IVC was 1.8 ± 0.4 cm. Dye spread remained intramuscular in all but one specimen. No dye spread occurred to any adjacent organs.

The injection of PM using the UG-posterior lumbar approach was consistent and without spread to surrounding structures. This technique alone or in addition to the anterior approach is expected to have better clinical outcomes in the treatment of hip flexor spasticity. Further clinical studies are required.