Out-of-plane (OOP) vs. in-plane (IP) needle alignment for ultrasound-guided regional anesthesia remains controversial. Rather than advocating a didactic single approach for all blocks, the reality is that both have relative strengths and weaknesses for specific block techniques. Therefore, both the in-plane and out of plane techniques have a place in modern ultrasound-guided regional anesthesia.
The out-of-plane approach was initially the most frequently described, and is popular for a number of reasons. Many anesthesiologists are already familiar with this approach for venous cannulation. Second, for perineural catheter placement using short-axis nerve imaging, out-of-plane alignment places the catheter along the long axis of the nerve/plexus, potentially promoting catheter advancement along it. Catheter alignment along the long axis of the nerve also maximies the surface area of catheter in contact with the nerve, which may be important for multi-orifice catheters where orifices are positioned up to 1.5 cm from the catheter tip. Finally, and arguably the most important, orientation of a short bevelled or Tuohy tipped needle along the long axis of a nerve or plexus virtually eliminates the intraneural needle placement risk. Intraneural needle placement has yet to be reported with this needle-to-nerve orientation – although it is probably possible with the large sciatic nerve. The main problem with the out-of-plane approach is the inability to visualise the needle tip, and therefore, a lack of needle tip precision. Out-of-plane assessment of needle tip position requires the operator generate a 3 dimensional image, which can be conceptually challenging. However, with superficial blocks, the needle can be orientated more in line with (along the long axis of) the nerve; therefore, inadvertent needle advancement beyond the ultrasound beam should not significantly compromise needle-nerve proximity. Finally, caution should be exercised during out-of-plane needle advancement to avoid puncturing important structures (e.g. vessels) as needle advancement is, to a large extent, "blind". This is more of a problem for deep blocks (significant distance to negotiate), particularly where the target is approached "obliquely" and therefore beyond the area directly in front of the ultrasound beam. A good example is out-of-plane infraclavicular block where oblique needle advancement (posteriorly, laterally and caudad) may result in inadvertent puncture of an invisible superficial vessel.
The main advantage of the in-plane approach is the potential ability to visualise the needle shaft and tip throughout needle advancement, which renders it a theoretically more precise method. However, the approach has two major limitations. First, the needle may deviate from the ultrasound beam and be lost from view. Second, even if the needle remains within the ultrasound beam, shaft and tip visualisation can be poor particularly with steep needle angles (the ultrasound machine only displays ultrasound waves reflected back to the transducer). Because in-plane approaches place the needle more perpendicular to the nerve, the risk of needle impalement may be increased. That said, the in-plane approach may be preferred for deeper blocks (e.g. infraclavicular, sciatic), as the out-of-plane approach also becomes more challenging for these deeper blocks.
Obstructions to the needle path may necessitate the use of one approach. For example, access between the extensor tendons of the foot to reach the deep peroneal nerve necessitates the out-of-plane approach. Also, at the level of the bifurcation of the sciatic nerve, access to the space between the common peroneal and tibial nerves is facilitated with out-of-plane needle alignment, which facilitates needle orientation directly posterior to anterior. With the out-of-plane approach, use of a blunt needle is essential, as a blunt needle facilitates the assessment of needle tip position by causing tissue displacement (a sharp needle will cut through the tissues and cause minimal tissue displacement).
The following needle-probe alignments are recommended:
Interscalene block – out-of-plane (consider in-plane for SS blocks)
Infraclavicular block – in-plane (consider out-of-plane for catheter placement)
Distal upper extremity blocks – in-plane
Femoral block – out-of-plane or in-plane (in-plane for SS blocks)
Sciatic block – in-plane (consider out-of-plane for catheter placement or when intentionally injecting between the origin of the tibial/common peroneal divisions)
Ankle block (with patient supine):
Deep peroneal – out-of-plane
Posterior tibial – out-of-plane
Short saphenous - in-plane