Figure 1: Imaging results of effect of facet joint injection with oxygen-ozone (O2–O3). Note: (A) Paramedian sagittal transverse processes view. The hyperechoic reflections of the transverse processes (TP) with their acoustic shadow produce the “trident sign.” The psoas muscle (PM) with its typical hypoechoic and striated appearance, is visible between the transverse processes. The erector spinae muscle (ESM) lies superficially to the transverse processes. (B) Paramedian sagittal articular process view. The articular processes are visible as a continuous hyperechoic line of “humps” with acoustic shadowing beneath, and the bony contour of the superior articular process (SAP) is usually more superficial than the inferior articular process (IAP). The facet joint space (arrowhead) is situated between the articular lines of the SAP and IAP. The ESM lies superficially to the articular processes. (C) Paramedian sagittal oblique view. The sacrum (arrowhead) is recognizable as a horizontal hyperechoic curvilinear structure, the L3–5 laminae (arrows) have the typical “sawtooth” appearance, and the erector spinae muscles (asterisk) are hypoechoic and lie superficial to the laminae. (D) Transverse interlaminar view. Interspinous ligament (ISL) is visible in the midline. The articular process (AP) and the TP are visible laterally on either side of the midline. (E) Ultrasound-guided facet joint injections in lateral to medial approach. The asterisk indicates the entry zone (hypoechoic space) into the lumbar facet joint between the medial aspect of the IAP and the lateral aspect of the SAP. The arrowheads indicate the needle trajectory. (F, G) Facet joint injection with O2–O3 in transverse sonogram. (F) The medial aspect of the IAP and the lateral aspect of the SAP is easily recognizable. (G) The O2–O3 mixture (arrowheads) appears as hyperechoic area in the peri-facet joint soft tissue with a well-defined shadow beneath. The O2–O3 mixture was produced by means of a Multiossigen Medical 99IR generator (Multiossigen S.p.A., Gorle, Italy). (H) Ultrasound-guided facet joint injections in caudal to cranial approach. The articular processes are visible as a continuous hyperechoic line of “humps” with acoustic shadowing beneath. The needle (arrowheads) is directed to the space (asterisk) between the articular lines of the SAP and IAP. A–D were obtained by means of a Mindray DC-60 Ultrasound (Mindray Medical S.R.L, Milano, Italy). E–H were obtained by means of a ESAOTE MyLab OMEGA Ultrasound (Esaote S.p.A, Genova, Italy).