Accuracy and Reliability of Erector Spinae Plane Injection, Retrolaminar Injection and Combined Erector Spinae and Retrolaminar Injection in Thiel Cadavers

Ultrasound guided fascial plane blocks are increasingly used in clinical practice because they have a safer profile than regional and neuraxial anaesthesia yet offer similar pain relief. In 2016, a novel ultrasound guided fascial plane block named the erector spinae plane block was introduced. The block needle targets the tip of the transverse process in a plane deep to the erector spinae muscle. However, like all fascial plane blocks, the dispersion of local anaesthesia is inaccurate and unreliable, and the mechanism of action remains unclear. Therefore, the overall objective of this thesis was to devise a new fascial plane injection technique that would provide accurate spread of injectate when performed at the third thoracic level in Thiel cadavers. This thesis hypothesised that the technique would also show similar spread characteristics when performed at upper and mid – thoracic levels. Three randomised control investigations were performed on a total of 26 Thiel cadavers including 8 erector spinae plane injections, 8 retrolaminar plane injections and 26 combined injections at the third thoracic vertebral level. Additionally, ten combined injections were performed at the sixth thoracic vertebral level. Spread characteristics, craniocaudal, lateral and to anatomical structures, were investigated using gross anatomy dissection, strain elastography and CT scans. Needle and catheter tip position were identified using X-rays. This thesis shows that the combined injection spreads more to the paravertebral space compared to erector spinae injection alone and identifies greater overall spread to predefined anatomical structures compared to retrolaminar injection alone. Similar spread characteristics, craniocaudally, laterally and to predefined anatomical structures, were observed at upper and mid - thoracic levels following the combined injection. Mechanisms of action of the erector spinae plane block, retrolaminar block and the combined injection are suggested. This thesis provides pilot data for translational clinical studies; suggests robust protocols for future cadaveric studies investigating injectate spread and introduces a novel ultrasound guided fascial plane approach which has the potential to provide analgesia after mastectomy and traumatic rib fracture. As part of the future work, three mastectomy patients have received the combined block. The cold test (ethyl chloride) was used to map the area of sensory loss. Sensory mapping showed consistent and accurate dermatomal spread over five dermatomes covering the chest wall centred on the T3 vertebral level.