Intraforaminal Location of Thoracolumbar Radicular Arteries Providing an Anterior Radiculomedullary Artery Using Flat Panel Catheter Angiotomography

Methodologic Considerations

To assess the location of the branches providing an ARMA in a clinically relevant manner, our measurements were performed in a sagittal reconstruction of the FPCA datasets matching the fluoroscopic landmarks used during TFESI (ie, a lateral projection perpendicular to the entrance of the NF).¹⁰ The vessel measured can be considered as either a radicular artery or an ARMA because the exact point of transition from one to the other remains difficult to assess by imaging and nomenclatures show slight variations.^11,12 All the branches investigated for this study continued intradurally as contributors to the anterior spinal arterial axis.

The NF variability in size between patients and vertebral levels was accounted for by adopting the NF height as the normalization standard, considering, in particular, the relative paucity of osteophytes that form on the superior and inferior borders of the vertebral pedicles.¹³ No major differences in artery locations were noted between the raw measurement data and the normalized data (Fig 3).

Intraforaminal Location of Branches Providing an ARMA

The ISA consists of an aortic stem that divides into medial (or spinal), dorsal, and lateral branches; depending on the level considered, the lateral branch takes the name of posterior intercostal, subcostal, or lumbar artery (Fig 2A). The medial branch, when complete, enters the neural foramen and provides the retrocorporeal and prelaminar arteries, which contribute to the vertebral vascularization and the radicular artery.¹⁴ A radicular artery that supplies the spinal cord crosses the dura to continue as an anterior or posterior radiculomedullary artery or both, which, respectively, contribute to the anterior or posterior spinal arterial chains. Supply to the anterior thoracolumbar spinal cord is typically limited to a dominant thoracolumbar ARMA, the artery of Adamkiewicz, and a smaller upper thoracic ARMA, the artery of von Haller.¹⁵ Reports of paralysis resulting from vascular injuries during TFESI spurred previous studies on the location of these vessels within the NF. The investigation of Murthy et al,⁷ based on anteroposterior views only, concluded that the radicular segment most often crosses the superior aspect of the NF. Two cadaveric studies reported that branches providing an ARMA are generally located anterosuperior to the dorsal nerve root within the NF.^8,9 Our multiplanar analysis of high-resolution angiographic datasets acquired during routine clinical practice confirms and extends these observations by plotting the precise distribution of branches providing an ARMA within the NF (Fig 3).

The caliber of radicular branches that provide an ARMA measured in our study (average, 1.0 mm; range, 0.6–1.7 mm) was comparable with that in prior investigations, which have reported diameters ranging from 1.2 to 2.5 mm (average, 1.9 mm)⁸ and from 0.8 to 1.9 mm (average, 1.2 mm)⁹ (measurements obtained in anatomic specimens, including the vessel wall thickness).

Clinical Implications for Arterial Injury

Despite being associated with severe complications related to the intraforaminal location of radicular branches that provide an ARMA,³ TFESI has seen its rate of use steadily increase due to its purported superiority over alternative techniques, such as the caudal or interlaminar approaches.¹ Spinal cord ischemia and stroke occurring during TFESI may result from arterial embolization with air or particulate steroids¹ or from direct vessel injuries such as spasm, laceration, and dissection.¹⁶ The close association between branches providing an ARMA and the subpedicular notch documented in our study emphasizes the danger in making contact between the needle and the pedicle, as is typical with the subpedicular approach.¹ Multiple attempts at repositioning the needle tip, as reported in several cases of paralysis,^16,17 likely increase the risk of vessel damage.

The Role of Particulate Steroids

All 19 observations of paralysis reported so far have involved the injection of particulate steroids during TFESI.^3,4 With particle sizes ranging from 1 to 100 μm, an intra-arterial injection could result in the embolization of spinal arterioles.¹⁸ Direct arterial injection of particulate steroids has caused permanent neurologic damage in animal models, while nonparticulate steroids such as dexamethasone^19,20 and prednisolone²⁰ produced no injuries. Although it has been suggested that particulate steroids are more effective than nonparticulate steroids, no significant difference in efficacy after lumbar TFESI has been reported.^21,22 It is possible that the small number of reported complications and the widespread use of particulate steroids²¹ falsely suggest a causative relationship between particulate injectate and the risk of paralysis. Nonetheless, dexamethasone became the recommended injectate for TFESI in 2010.²¹

Posterolateral Approach

Positioning the needle in the region of the NF known as the Kambin triangle²³ has been recommended as a safer alternative to the subpedicular approach.¹ In this posterior triangle or posterolateral approach, the targeted area is defined by the posterior margin of the exiting nerve root, the endplate of the caudal vertebral level, and the articular facet of the cranial vertebral level.²³ This approach was proposed for arthroscopic discectomy,²³ which requires the transforaminal placement of large-caliber instrumentation.¹ TFESI via the posterolateral approach has shown pain reduction scores similar to the those with the subpedicular approach,²⁴ while decreasing both the amount of periprocedural pain and the risk of nerve damage.²⁵ Considering that the posterolateral approach targets the posteroinferior quadrant of the NF, where branches providing an ARMA are least likely encountered, and that it has not yet been associated with reports of paralysis suggests that it should be the preferred technique for TFESI.¹

Misconceptions Regarding Spinal Vascular Anatomy and the “Safe Triangle”

Several misconceptions have led to a slow rejection of the subpedicular approach,¹ notably the incorrect notions that the “safe triangle” helps prevent vascular injuries in addition to nerve damage and that arterial opacification is reliably detected during contrast injections performed before steroid instillation.¹ Intra-arterial needle tip placement during TFESI can, in fact, not be excluded with certainty by using intermittent fluoroscopy,^26,27 aspiration,^{27⇓⇓–30} contrast injection,¹ or even digital subtraction angiography.^1,31 In addition, none of these imaging methods would help avoid other injury mechanisms such as arterial transection or dissection. A limited understanding of spinal vascular anatomy, which is often overlooked or misrepresented,¹⁰ also plays a role.

Simon et al³² reported the presence of arteries both in the subpedicular region and the posterolateral triangle on contrast-enhanced CT scans, but the vessels observed with that technique cannot be clearly identified as branches providing an ARMA or osteomeningeal branches or even venous structures such as radiculomedullary veins or the emissary veins linking the internal and external venous plexuses at each vertebral level. Figures 4 and 5 illustrate the advantage offered by a high-resolution imaging technique to reliably distinguish such minute vascular elements on the basis of their morphology (ie, termination of the radiculomedullary veins into the epidural plexus) or opacification pattern. A similar lack of specificity is found in a report of apparent retrograde flow into a T6 spinal artery noted during a posterolateral approach,³³ which prompted Simon et al³² to suggest that the injection of any artery near the NF carries potential clinical consequences. However, the images documenting that observation only show the opacification of the external epidural venous plexus with retrograde filling of a basivertebral and intraosseous venous system. An analysis of reports of paralysis following TFESI in which the needle position was mentioned shows that spinal cord damage only occurred in association with the subpedicular approach,³ a fact weakening the role of retrograde injections and the notion that any artery in the vicinity of the NF has clinical consequences comparable with those supplying ARMAs when performing TFESI.

Study Limitations

The goal of our study was to describe the typical location of radicular branches providing an ARMA in the NF based on FPCA datasets acquired in patients principally investigated for myelopathy. This patient group may have a lower incidence of degenerative osteodiscal pathology than the population typically treated with TFESI, leading to a more stable distribution of contributors of ARMAs within the NF.