**6. Dye studies on tissue plane dynamics**

The regional anaesthesia literature already has a reasonably large number of dye studies [11–34, 44–51, 54–68, 70–83] and they encompass most of the techniques in current common usage. One could be forgiven for believing that we now have most of the answers, but this is an area where for various reasons there is still room for improvement. Our work on the axillary tunnel raised some generic questions to be answered in other areas – under what conditions do flow characteristics change, can flow characteristics be manipulated to clinical advantage and what anatomic features can be disadvantageous in terms of flow characteristics? These questions infer that the tissue plane is a dynamic environment and that this can be used to clinical advantage.

It has been unusual in this type of research work to use any model other than cadavers but for various reasons these cannot provide an accurate replication for clinical application. Connective tissue changes in quality and appearance very soon after death [5, 6]. As pointed out by Ivanusic et al. [65] it is also not possible to investigate for any block-related phenomenae that may be linked to physiological occurrences such as breathing in a dead body. Specific block-related positioning is not possible in cadaveric specimens. At best, with carefully planned and executed dissection, cadaveric models provide a sort of basic static road map of where fluid may spread and what nerve/s might be blocked.

In the above context a dye and latex study by Mayes et al. [139] investigated serratus anterior plane block (SAP) as a potential analgesic technique for rib fractures. It demonstrated a clear plane of dissection between serratus anterior and the external intercostal muscle/ribs, bathing the lateral cutaneous branches of the intercostal nerves in solution as per the description in the original publication [42]. This means that the technique will not provide analgesia for rib fractures, a conclusion framed by the authors in subtle language [139], because the intercostal nerves which innervate the ribs will not be blocked. Does trauma to the chest wall provide a pathway to these nerves? This is unlikely and not a dependable mechanism in this author's opinion. Can solution track along the lateral cutaneous branches to the intercostal nerves of origin? This again is unlikely as it requires the passage of these nerves through the muscle layers of the chest wall to be the path of least resistance as the injected fluid flows through the tissue plane. It is up to those who wish to prove any of these mechanisms to create the models and test their hypotheses.

Another technical issue at play in dye studies relates to where the tip of the catheter is located when injections are made. In a study on continuous parasacral sciatic nerve block [31], the investigators used 8 ml of dye for confirmation of location of their perineural catheters. In our work on the axillary tunnel [133], we realised that we could only locate the tip of the catheter by using a much smaller volume of injectate of 2 ml, a larger volume obscuring the location.

Adductor canal block [54] has gained popularity as an analgesic technique posttotal knee joint replacement due to less motor block than femoral nerve block [140] albeit with some accompanying loss of pain relief [140, 141]. There have been several dye studies examining this technique [142–145], with ensuing cautions about volume used, where the injection is placed to limit spread of injectate beyond the adductor canal, and consideration of the impact of tourniquets on the spread of solution. However, there has been no work directly measuring the potential volume of the canal, nor how this might change with application of a tourniquet, nor what flow restrictions if any are present, all of which would require 3-dimensional imaging. This is a more costly investigative modality but provides much more information.

The cadaveric dye studies of ESP block [64, 65] and the subsequent volunteer study of ESP block [96] have been discussed earlier in this chapter. Perhaps this illustrates the peril of having cadaveric studies as stand-alone evidence of efficacy. The volunteer study certainly seems to be one potential bridging option to clinical practice but may not always be possible due to safety considerations. Some authors do combine their cadaveric studies with a clinical case series as additional evidence, but is this practice consistent with the current ethical standards in human research? Multiple pathways are probably likely required to establish efficacy and safety across the range of techniques and indications prior to widespread adoption in clinical practice. In this respect, randomised controlled trials are only as good as the science which underpins them.
