Coiling of echogenic perineural catheters with integral stylet: A proof-of-concept randomized control trial in a sciatic nerve block simulator and a pilot study in orthopaedic-trauma patients.

Study design and settings

Two independent studies were conducted: 1) A simulation-based protocol in a Blue Phantom US simulator of sciatic nerve block. 2) A pilot study in 25 patients undergoing sciatic CPNB. All procedures were performed under US real-time visualization (5-9 MHz linear transducer; LOGIQ e; GE Healthcare, USA).

Simulation-based CPNBs methods and randomization

Two methods of perineural catheter advancement using the Blue Phantom simulator were evaluated. A 20G single orifice, echogenic straight perineural catheter, polyurethane based (communication with the manufacturer) with a stainless-steel helical coil-reinforced body and a steel integral stylet (SonoLong/Sono, PAJUNK, Germany) was used.

Method A: The catheter was advanced beyond the needle tip (under real-time US imaging) with its integral stylet extending along the entire length of the catheter ( Figure 1A).

Figure 1. A: The perineural catheter removed from catheter’s container, with the integral stylet extending along its entire length (from the beginning to the tip); B: The catheter’s integral stylet is retracted by 6 cm (arrow) so that the catheter’s distal portion features a more flexible end; C: Blue Phantom’s sciatic nerve in long axis view; D: the needle insertion angle (40 degrees) to the Blue Phantom posterior thigh surface as measured by goniometer; E: Ultrasound image of the Tuohy needle tip (arrow) as directed underneath the sciatic nerve of the simulator.

Inset: perineural catheter container. SC.N=Sciatic Nerve.

Method B: The catheter was advanced beyond the needle tip after the catheter’s integral stylet was retracted by 6 cm so that the catheter’s distal portion provided a more flexible end ( Figure 1B).

Randomization was done using a randomization generator (www.randomization.com) by means of sequentially numbered sealed opaque envelopes, including information about the sequence of method allocation.

The simulator was provided with a sciatic nerve insert (upper third of the posterior thigh) containing one nerve structure (CAE training Blue Phantom). The sciatic nerve-simulator surface distance featured different location depths (2-4 cm) ( Figure 1C).

A Tuohy needle (18G × 7.5 mm, PAJUNK, Germany) was inserted at the simulator’s posterior thigh (TS) from a lateral to medial direction (needle in-plane/nerve in-short-axis technique). The needle insertion angles (coronal plane) were always kept at 40 degrees ( Figure 1D). In both methods, the needle was aimed behind the sciatic nerve with the curved end facing the nerve ( Figure 1E). The perineural catheter was threaded through the Tuohy needle until its tip reached the needle orifice. The catheter was slowly advanced exactly 6 cm (under real-time US guidance) beyond the curved tip of the Tuohy needle, until the catheter’s distal end-portion coiled exactly under the sciatic nerve ( Figure 2A). To better visualize a catheter’s trajectory, a slight heel-toe maneuver was implemented to bring the US beam perpendicular to catheter’s distal portion. Procedural time was limited to 15 seconds (total allowed performance time for each coiling maneuver). Each attempt was stored digitally⁵ and analyzed off-line (frame by frame) by two co-investigators (EP, MR) who were blinded to group allocation. The two investigators decided independently whether the catheter coiled or not. The result was considered positive (coiled catheter) only after unanimous agreement between the two investigators. If the catheter distal end did not coil or if the catheter bypassed the nerve and then coiled, the catheter was withdrawn, the needle tip reoriented (rotation of needle shaft/tip at 90 degrees), and the catheter was readvanced. In total, three attempts were allowed (overall maximum procedural time: 45 seconds). The number of attempts to coil a catheter, as well as the proportion of cases where a catheter was successfully looped on first attempt, were measured. The patency of coiled catheters was tested after normal saline (3 ml) infusion through the catheter. Contact of saline spread with the nerve was also recorded ( Figure 2B). All images were taken under the same imaging settings (gain: 90 db, focus position at the level of the nerve, standard time gain compensation).

Figure 2. A: The coiled perineural catheter underneath the sciatic nerve (arrowheads) of the Blue Phantom simulator; B: Saline spread contacts the sciatic nerve (arrowheads) of the Blue Phantom simulator; C: The injection of LA through the perineural catheter (arrows) contacts the pantient’s sciatic nerve (arrowheads) prior to surgery; and D: 36 hours postoperatively.

C=Perineural Catheter, SC.N=Sciatic Nerve, *=LA spread.

Statistical analysis

Sample size calculation was based on a previous pilot simulation study including 10 CPNBs for each method, using a Blue Phantom nerve block simulator that contained two nerve structures with dimensions of 17cm × 13cm × 6cm (L × H × W), (CAE-Healthcare). Catheters were coiled (after maximum of three attempts) in 3/10 (30%) and in 8/10 (80%) cases, with method A and B respectively. Using an α error of 0.025 and a power of (1− β) at 80% to detect a 50% difference between the two methods, a sample size of 18 CPNBs in each group was required. To allow for increased data variability, 40 CPNBs were performed in a sciatic nerve block simulator. The D’Agostino-Pearson test assessed normality. The results were expressed as percentage (%) or median (highest value-lowest value). Proportions were compared using Fisher’s exact test; p values <0.05 were considered statistically significant. Interobserver agreement was assessed by using the inter-rater agreement statistic Kappa (MedCalc Software, Mariakerke, Belgium).

Pilot study in orthopaedic-trauma patients

To assess procedural effectiveness of the preferred coiling technique (method B), a pilot study was performed. The pilot study was conducted according to the principles expressed in the Declaration of Helsinki and was approved by the “Attikon” Hospital Ethics Committee (Approval No 342, Approval Date 09/05/2024). Informed written consent was always obtained from the patients or their surrogates.

Straight perineural catheters with coiled distal ends behind the sciatic nerve were examined in 25 patients (age: 28-55 years; male/female: 14/11; BMI: 26 (19-31) kg/m²) who had sustained tibia fractures (recruitment period: 10/05/2024-15/07/2024). Exclusion criteria: patients with infection in the subgluteal region, allergies to local anaesthetics, patients’ refusal to consent for the nerve block, neurological disorders, pregnancy.

Patients were placed in the lateral position (with the injured leg uppermost) and a US-guided sciatic nerve block was performed under aseptic conditions (TS, EP). The sciatic nerve was visualized at the upper third of the posterior thigh (short axis view). A Tuohy needle (18 G × 10 mm, SonoLong/NanoLine, PAJUNK, Germany) was inserted in a lateral to medial direction. The needle was gently advanced into the subparaneural compartment (area between the paraneural sheath and epineurium). An initial 3 ml normal saline injection distinctly elevated the various layers of the paraneural tissues and dynamically shifted the sciatic nerve upwards, thus confirming proper needle tip placement in the paraneural compartment.

Ten ml of ropivacaine (0.1%) were then injected and the LA spread diffused in this space between the multiple layers of the paraneural tissues without nerve expansion (thus excluding intraneural injection). A perineural catheter (with retracted integral stylet) (SonoLong / Sono, PAJUNK, Germany) was threaded through the needle tip and coiled behind the sciatic nerve in the paraneural space. Specifically, confirmation of correct catheter tip placement in the peraneural space was defined as ropivacaine injectate (3 ml, 0.1%) spread was distributed around and in contact with the sciatic nerve ( Figure 2C). If the distribution of the LA could not be visualized on the first infusion, 2 additional injections were performed until the distribution was clearly seen. If the LA spread could not be visually confirmed, or if, when confirmed, LA was not distributed around and in contact with the sciatic nerve, the case was excluded.

Subcutaneous tunneling (4 cm long) and placement of a transparent adhesive dressing were used to secure the catheter. All sciatic nerve blocks were followed by an adductor canal block (10 ml of ropivacaine 0.5%) and general anaesthesia. A continuous infusion 8-12 ml of ropivacaine 0,1% (ROPIVACAINE/KABI INJ.SOL 2 mg/ml, 28823.01.08, Fresenius Kabi Hellas A.E.) was commenced and an oral combination of paracetamol 325 mg with oxycodone 5 mg (DEPALGOS F.C. TAB 325+5 mg, 91337.01.01, MOLTENI SPA, Italy) was provided on an as-needed basis after surgery.

All CPNBs were assessed and data were collected in the post-anaesthetic care unit, and every 12 hours thereafter until 36 hours postoperatively from members of the acute pain service (APS) team not participating in the study. Observation of fluid under the dressing indicated catheter’s leakage. Unplanned external displacement and dislodgement of the catheter were defined: less than and equal to 6 cm (displacement) and more than 6 cm (dislodgement) movement of the catheter from the initial recorded depth at the point of insertion or from the distal end of the subcutaneous tunnel. The final catheter tip position was evaluated (under US imaging) 36 hours postoperatively and was identified by injecting 3 ml of ropivacaine (0.1%) through the catheter ( Figure 2D). If LA spread was not confirmed after 3 injections or if, when confirmed, it did not come in contact with and around the sciatic nerve, the catheter’s tip was considered dislocated.

Ethics and consent

The pilot study was conducted according to the principles expressed in the Declaration of Helsinki and was approved by the “Attikon” Hospital Ethics Committee (Approval No 342, Approval Date 09/05/2024). In accordance with the “Attikon” Hospital Ethics Committee requirements, informed written consent was always obtained from the patients or their surrogates.