**7. High-definition ultrasound studies and the tissue plane**

There have now been several studies examining nerves of the upper and lower extremities and the soft tissues which surround and envelope these nerves [146–149]. These have given rise to the term 'paraneural sheath' of the sciatic nerve [146] and brachial plexus [149] respectively. In one recent study, the sonographic imaging was correlated with histologic specimens [149] to demonstrate what the authors described as 'fascial tissue planes' within the paraneural sheath.

It is to be noted on the images of the brachial plexus [149] that the pectoral fascia is much more visible, i.e., it exhibits greater anisotropy, than the tissues within the plexus complex. This is not artefactual – fasciae such as the pectoral or prevertebral are much sturdier connective tissue structures than the relatively fragile connective tissues surrounding the brachial plexus. The imaging also demonstrates both the tissue planes of the plexus and the phenomenon of injected fluid separating the tissue planes as it flows through the length of the plexus complex. The 'paraneural sheath' has the same anisotropy as the tissue planes within the neurovascular bundle, i.e., the same type of tissue, which is quite distinct from fascial tissue. This study demonstrates very well some characteristics of the dynamics of the tissue planes of the brachial plexus.

#### **8. Tissue plane blocks versus compartment blocks**

In some literature it has been suggested that the local anaesthetic fills a compartment to achieve its effect, particularly where the injection is made within the fascia enclosing muscles such as rectus abdominis or perhaps even the erector spinae plane block [150]. The actual phenomenon can be viewed on ultrasound in real time as the injected solution can be seen to develop a plane of dissection between the muscles and the fascia or bone. Once again, the concept of the tissue plane becomes the focus of our attention.
