An anatomical study of the thoracic paraspinal block and the clinical implication for pain management

Abstract

The thoracic paravertebral block is a well-established for regional anesthesia of the hemi-thorax in patients undergoing breast and thoracic surgeries. However, there remains a potential risk of pneumothorax or unintentional neuraxial injection. Recently, the superficial injection technique such as the erector spinae plane block, midpoint transverse process to pleura block, and intertransverse process (ITP) block have been utilized as alternatives to conventional thoracic paravertebral block. Several approaches have been reported with anatomical evidence of paravertebral spread and clinical effectiveness. However, the concept of the thoracic paravertebral space (TPVS) was shaped by clinical needs and has not been fully elucidated as a clearly delineated space from an anatomical perspective. Also, the injectate spread pattern and the mechanism of the spinal nerve blockade with many block techniques have not been fully investigated. Therefore, we aimed to clarify the three-dimensional (3D) structures of the TPVS and adjacent tissues and to investigate the potential conduits for nerve blockade in this area using manual and serial sectional dissection, histological examination, micro-computed tomography (micro-CT) and cadaveric evaluation of the ITP block injection. Twenty-four embalmed cadavers and three non-embalmed cadavers were used in this study. Micro-CT images of the TPVS and its adjacent ligamentous tissues were acquired, and 3D images were reconstructed. Manual dissection and histologic examination of these structures meticulously complemented the images. To confirm our findings, the dye-spreading pattern after ultrasound-guided ITP block injection of 20mL dye solution at the T4-5 level of the thoracic spine was evaluated. Micro-CT images clearly showed the serial topography of the TPVS and its adjacent spaces. First, the TPVS was a very narrow space with the posterior intercostal vessels very close to the pleura. Second, the superior costotransverse ligament (SCTL) incompletely formed the posterior wall of the TPVS between the internal intercostal membrane and the vertebral body. Third, the retro-SCTL space broadly communicated with the TPVS via a medial slit, costotransverse space, intervertebral foramen, and erector spinae compartment. Fourth, the costotransverse space was intersegmentally connected to the adjacent retro-SCTL space. Fifth, single-level ITP injection with a dye solution resulted in a multilevel segmental paravertebral spread in cadaveric evaluation. A non-destructive, multi-sectional approach using 3D micro-CT more comprehensively demonstrated the real topography of the intricate TPVS than previous cadaver studies. The posterior boundary and connectivity of the TPVS provides an anatomical rationale for the notion that paravertebral spread is achieved with an injection outside this space. The retro-SCTL space, the target area for the ITP block, has potential anatomical pathways to the TPVS. The medial slit and the costotransverse space provided the anatomical conduit for the anterior and intersegmental paravertebral spread of the ITP block.