**1. Introduction**

*'something old, something new, something borrowed, something blue…' 18th Century English Rhyme*

'Something old' – it may have evolved over geological time, it may distort with congenital, surgical, traumatic or other influences, but fundamentally anatomy has been around for a long time. This was wittily expressed by Harrop-Griffiths and Denny [1] in relation to the introduction of ultrasound to regional anaesthesia when they suggested that there are 'no new blocks, just old anatomy'. These authors would undoubtedly agree though that how we look at anatomy can change, and indeed should change as study and scholarship advances our understanding.

'Something new' – the ability of ultrasound to visualise tissue planes provides a new, or more accurately, a newly appreciated target. The explosion of named techniques using ultrasound guidance in the last several years bears testimony to this development in regional anaesthesia practice.

'Something borrowed' - The 'tissue plane' is a concept borrowed from surgical practice [2–4] and it forms the foundation of modern surgical dissection technique. An alternative term for the same idea is the 'plane of dissection'. Surgeons also talk about 'creating' a plane of dissection or 'getting into the correct (tissue) plane' [4]. The key idea which surgeons are emphasising is that of dissecting between

structures, and it matters little whether this is with a sharp (e.g., scalpel) or blunt (e.g., finger) instrument. In regional anaesthesia practice, the dissection occurs solely with a blunt (injected fluid) instrument and is termed 'hydrodissection'.

'Something blue' – it has been routine for many years in cadaveric anatomic studies in the regional anaesthesia literature to use methylene blue as a marker to track nerve block placement. These have in effect been studying spread of fluid along tissue plane/s. Once again, we find that this is not so much a new as an under-appreciated phenomenon. A note of caution however – early post-mortem changes in collagen change the structure of connective tissues [5, 6] and therefore the cadaveric model has an inherent flaw when it comes to assessing the dynamics of the tissue plane.

This chapter is divided into the following 10 sections: The tissue plane defined, History of the tissue plane in regional anaesthesia, The modern era of the tissue plane in regional anaesthesia, Tissue plane dynamics and some misunderstandings, Dye studies on tissue plane dynamics, High-definition ultrasound studies and the tissue plane, Tissue plane blocks versus compartment blocks, Systematic reviews, Research opportunities, and Conclusion.

## **2. The tissue plane defined**

A tissue plane is defined as a potential space separating structures such as organs, muscles, nerves and blood vessels [7]. It frequently but not exclusively contains fine loose areolar tissue which is easily divided by both sharp and blunt dissection [2, 8]. The finer quality of these connective tissues contrasts to the much denser connective tissue that forms fascial boundaries, e.g. the prevertebral fascia. This is an important distinction as local anaesthetic will not diffuse across a fascial boundary.

Tissue planes are located throughout the body [2]. Examples include; providing a conduit for nerves, blood vessels and lymphatics from one body region to another e.g., within the femoral canal, [8]; dividing elements of a structure into its component parts, e.g., the brachial plexus [8, 9]; or where structures can be easily separated, e.g., between the fascicles of rectus abdominis muscle and the posterior rectus sheath [10].

The surgeon divides these tissues with scalpel, scissors, probe or finger and in so doing creates the plane of dissection in order to excise tissue or access an anatomic area. In surgical practice there is also a plane of dissection which creates less bleeding, the so-called 'avascular plane' [2]. By contrast the regional anesthesiologist injects a bolus of fluid which spreads along and through the tissue plane/s, not so much dividing as separating the tissues and then diffusing into the nerve/s to create a conduction block.

It is important to keep reminding ourselves that the tissue plane is a concept, not actual anatomy. It points towards tissues which are easily dissected/divided vs. tissues which are not. In regional anaesthesia the tissue planes of interest contain nerves. In this respect it is required to know which nerves may be blocked, where and what they innervate, how and where to access the tissue plane safely and how the tissue plane/plane of dissection will spread the injected solution.

#### **3. History of the tissue plane in regional anaesthesia**

While the tissue plane concept is fundamental to surgical dissection technique [2–4], the lack of emphasis on its importance in regional anaesthesia is perhaps

#### *The Tissue Plane DOI: http://dx.doi.org/10.5772/intechopen.99533*

ironic given the number of publications in the literature which have in fact related to it prior to the introduction of ultrasound [11–34]. Indeed, the terms 'loss of resistance' and 'pop' refer to techniques accessing tissue planes, although they have not traditionally been described in that fashion.

The lack of acknowledgement of the tissue plane concept changed abruptly in 2007 with the publication of TAP (transversus abdominis plane) block [35]. In this paper the authors described the tissue plane between the fasciae of internal oblique and transversus abdominis muscles as a 'fascial plane' and then called it the 'transversus abdominis plane'. The fascial plane label stuck as did the name TAP and the search was on for others which surfaced in quick succession [36–43]. This pursuit for new targets was undoubtedly promoted by the emerging use of ultrasound and it is fair to say that subsequently there has been an explosion of interest across the spectrum of practice [44–83]. The terms 'fascial plane block' and 'interfascial plane block' have been further promoted [84] although more recently 'tissue plane' has been used [9].

The common theme of course is that all of the above involves the study of tissue planes and their dynamics of solution spread. As in surgical practice the tissue plane concept has been and is fundamental to the practice of regional anaesthesia.
