Positioning the Catheter Tip Anterior or Posterior to the Saphenous Nerve in Continuous Adductor Canal Block: A Mono-Centric Retrospective Comparative Study

Introduction

Nowadays, total knee arthroplasty (TKA) is recognized as gold standard for end-stage osteoarthritis treatment.^1,2 Multiple studies have suggested that over 60% of patients undergone TKA may experience from moderate to severe pain immediately after surgery,^3–6 which in turn may limit early mobilization and recovery of the knee function while increasing the risk of adverse effects related to being bed-bound.^7–9 Considering that the number of TKAs in progressively increasing worldwide, it becomes crucial to implement anesthesiologic strategies for the control of postoperative pain.¹⁰

Among different techniques, ultrasound guided Adductor Canal Block (ACB) has emerged as an effective procedure to relieve postoperative pain after knee surgery.^11–13 Compared with common methods, such as epidural and intravenous analgesia, ACB can provide a satisfactory control of pain and reduce opioid use. As compared to a femoral nerve block, ACB allows to spare the quadriceps motor function, helping anticipate the rehabilitation of the patient.^14–17 Additionally, the possibility to perform a continuous adductor canal block (cACB) with delivery of the local anesthetic for 24–48 hours after surgery is a further advantage of this technique to ensure optimal pain control for a longer period.^18–20 While the effectiveness of the cACB block positioned at different levels in the adductor canal has been investigated by many authors, to the best of our knowledge none of them has evaluated its efficacy in relation to the anatomical closeness of the tip of the catheter to the saphenous nerve and to the femoral artery.

With this background, we performed a retrospective study aiming to compare two different catheter tip locations in relation to the saphenous nerve for cACB technique, ie, anteriorly or posteriorly to the nerve. We hypothesized that an anterior positioning might be less effective in terms of pain control and carry a higher incidence rate of pump block due to the tension of the deep fascia and subsequent compression of the tip of the catheter.

Material and Methods

This was a Level III mono-centric retrospective comparative study performed at the Department of Surgical Sciences, Orthopedic Trauma and Emergencies of the University of Naples Federico II (Naples, Italy). The study protocol was approved by the ethical committee of the University of Naples Federico II. Patient consent was not required (as confirmed by the ethical committee) because of the retrospective nature of the study and the use of data collected in daily clinical practice. All data regarding patients were handled as confidential by the authors of the study. The design of this study followed the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) statement. All procedures performed in the study were in accordance with the ethical standards as laid down in the Declaration of Helsinki (1964) and its later amendments or comparable ethical standards.

Data about patients who underwent TKA between January 2020 and November 2021 were recovered from our departmental archive. All data, using MS Office Excel 2007 (Microsoft, Redmond, WA, USA), were registered on a pre-filled form and stored in a computerized database.

Inclusion Criteria

• Patients undergone TKA;
• Aged 50–85 years;
• Body mass index (BMI) of 18–35 kg/m²;
• American Society of Anesthesiologists (ASA) physical status classification of I to III;
• Subarachnoid technique for anesthesiology plane;
• Continuous adductor canal block performed by an anesthetist with considerable experience. We considered an anesthetist with considerable experience who had at least 5 years of experience with loco-regional anesthesia as main operator and an average of over 100 procedures per year.
• Patients whose medical charts are fully accessible.

Exclusion Criteria

• Contraindications to subarachnoid technique;
• Failure to perform the pre-operative mobilization test;
• Neurologic defects in the block site;
• Surgical procedures longer than 3 hours;
• Patients in whom the treatment protocol could not be fully applied.

Study Population

In the timeframe highlighted, 120 patients (120 knees) underwent TKA.12 cases with surgical procedures longer than 3 hours, 2 cases with neurologic defects in the block site and 4 cases of failure to perform the pre-operative mobilization test were excluded, 102 patients were included in the study.

Interventions

The cACB was performed immediately after surgery in a dedicated room, while vital signs were monitoring. Ultrasound (US) transducer (Sonosite HLF38x 13–6 MHz, Fujifilm Sonosite Europe, Amsterdam, Netherlands) was used for scanning the medial aspect of the thigh in a transverse axial plane. The femoral artery was identified underneath the sartorius muscle using the color Doppler in the proximal third of the thigh, then the transducer was moved caudally along the long axis of the thigh. At the beginning of the adductor canal, the needle entry point was marked on the skin. The femoral vein was then found in a lower position and the saphenous nerve, as a hyper-echoic structure with a round cross-section, was lateral to the artery. A needle (Vygon Value Life, Italy) 18-Gauge 30° tip with a split cannula 85-mm long was inserted through the sartorius muscle, along the same anatomical plane from lateral-to-medial orientation of the transducer. The procedure was performed with real-time ultrasound guidance toward the targeted nerve. A 20-gauge Silverstim (Vygon Value Life, Italy) echogenic and stimulating catheter was inserted 2–3 cm beyond the tip through a split cannula anteriorly or posteriorly to the nerve after the administration of 20 mL of 0.5% ropivacaine in all patients under US guidance.

 A before-and-after study was performed. A correct placement of the catheter tip was obtained by retracting the catheter slowly while injecting 5-mL of a saline solution under US-guidance. Tegaderm chlorhexidine gluconate transparent securement dressing protects catheter site and secures devices to the skin.

At the end of the procedure, postoperative analgesia was initiated through intermittent bolus PIB/PCA (programmed intermittent bolus/patient-controlled analgesia) pump with boluses of Ropivacaine 0,2% (6 mL per hour) and rescue bolus (5 mL per hours, with 60 minutes of lock-out time).

Data Extraction

Demographic patients data (age, sex, BMI), postoperative pain every 8 hours after surgery measured with Visual Analog Scale (VAS) (scale 0–10, where 0 = no pain and 10 = worst imaginable pain), ambulation ability, pump block and the rate of catheter dislodgement and leakage were detected from medical charts (Figure 1).

Figure 1 Flowchart.

Statistical Analysis

Patients were divided into two groups based on the position of the catheter tip relative to the saphenous nerve, ie, Group 1 (G1) if the tip was anterior (Figure 2) and Group 2 (G2) if the tip was posterior (Figure 3) to the nerve. The chi-square test was used to compare the groups with respect to categorical variables. A Student’s t-test was used to compare continuous variables as mean and standard deviation for the patients in both groups. Statistical significance was set with a p-value of 0.05. A post-hoc analysis was run considering the proportion of patient with VAS > 4 points as main outcome of the study. The statistical analysis was carried out using the IBM SPSS software (version 20.0, IBM Corporation, New York, USA).

Figure 2 Continuous Adductor Canal Block (cACB): catheter tip anterior position. A 20 Gauge catheter was inserted through the split cannula anteriorly to the saphenous nerve. Sartorius muscle (S); saphenous nerve (N); femoral artery (A); local anesthetic (AL).

Figure 3 Continuous Adductor Canal Block (cACB): catheter tip posterior position. A 20 Gauge catheter was inserted through the split cannula posteriorly to the saphenous nerve. Sartorius muscle (S); saphenous nerve (N); femoral artery (A).

Results

Overall, 48 patients (48 knees) were included in G1 and 54 patients (54 knees) in G2. The two groups were comparable in terms of demographics except for the BMI which was higher in G2 compared to G1 (26.5 vs 29.4; p-value <0.01) (Table 1). Ambulation ability, while analgesia was performed with continuous adductor canal catheter, occurred in all patients of both groups in the postoperative period (Table 2). Analgesia was optimal in the first 8 hours for all patients of both groups; in fact, all patients reported a VAS of zero. After the first 8 postoperative hours in Group 1, 17 patients (35.4%) had a VAS greater than 4, while in group 2 only 3 patients (5.6%) had a VAS greater than 4 (p-value <0.01) (Table 3). The episodes of pump block were significantly less in G2 than in G1 (3.7% vs 20.8%; p-value <0.01). On the other hand, the catheter displacement rate was similar in the two groups (2.1% for G1 vs 3.7% for G2; p-value >0.05). No episode of leakage was recorded. The post-hoc analysis (proportion G1: 0.36 vs proportion G2: 0.05; α = 0.05) detected that a total sample of 102 (ratio: 0.85) patients led to a power (1-β) of 0.99, which was considered satisfactory.

Table 1 Characteristics of Patients

Table 2 Outcomes

Table 3 VAS After Surgery

Discussion

In this study, we found that in cACB for TKA the position of the catheter tip relative to the saphenous nerve played a role in terms of efficacy and incidence of complication for the block. As a matter of fact, the catheter tip posterior to the nerve led to a better postoperative analgesia and a reduced risk of episodes of pump block compared to catheter tip anterior location. As far as we know, this is the first study to assess and compare the outcome of cACB based on different positions of the tip of the catheter around the saphenous nerve.

Postoperative pain management is always and no matter what the challenge. An adequate analgesia facilitates early rehabilitation and reduces postoperative incidence of complication.^21,22 Dong et al managed a meta-analysis regarding analgesic regimen after TKA, and they concluded that continuous Femoral Nerve Block (cFNB) was the most effective analgesia method.²³ However, FNB was also associated with weakness of the quadriceps muscle, which delayed ambulation and therefore slowed down the whole rehabilitation phase.^24,25 On the other side, the ACB was a technique widely applied as a postoperative analgesic treatment in patients undergoing TKA, defining a purely sensory block.^26,27 ACB is considered as a more appropriate analgesic method for patients undergoing TKA compared to FNB because it confers better ambulation ability. Moreover, it is well accepted by both anesthetist and orthopedic surgeon.^28,29 Furthermore, in a recent meta-analysis, Chen et al confirmed that cACB was the most effective method for alleviating pain and promoting ambulation ability in patients after TKA. These results suggest that postoperative analgesia for patients undergoing TKA should always have a longtime effect.³⁰ The ACB can provide reliable sensory blockade in the innervation saphenous nerve area. In addition, it has variable effects on the sensory components of the knee, in fact, extension of this block by diffusion to other nerves like the peroneal and tibial nerves or even femoral nerve has been described in the literature. An injection into the distal part of the adductor canal would spread alongside the femoral artery and vein through the adductor hiatus and into the popliteal fossa, thereby reaching the popliteal plexus and the genicular branch of the posterior obturator nerve.³¹ None of our patients had pain in the back of the knee after 20 mL of anaesthetic mixture bolus.³²

From a technical standpoint, when performing the in-plane technique, the needle tip must always be in real-time ultrasound vision.^33–35 Before positioning the perineural catheter, the injection of an anesthetic mixture bolus and the subsequent spread of liquid within the targeted fascial plane allowed to get a separation of the two fasciae of the compartment, hence the catheter positioning was easier.^36,37 Catheter displacement is common in continuous peripheral nerve blocks, and improper positioning of the distal tip of catheters may occur frequently.³⁸ We believe that the low incidence of postoperative catheter displacement in our cohort was related to the technique adopted. Regardless of its position around the saphenous nerve, the catheter was always inserted more laterally into the adductor canal so that at least 3–4 cm of it were placed between the two fasciae of the compartment which contributed to keep it more stable in its position. Furthermore, the catheter through split cannula method allowed to advance more easily the catheter 2–3 cm beyond the same cannula and to retract it in real-time ultrasound vision versus the target nerve,³⁹ which in turn reduced the risk of secondary displacement.

Before 2020, in our department cACB was always performed placing the catheter tip in a sub sartorial position, anterior to the saphenous nerve. In our experience, this often led to episodes of pump block with mandatory/automatic bolus, which we thought being secondary to an increased tension of the upper fascia of the adductor canal.⁴⁰ These cases prompted us to reflect upon the positioning of the catheter, hence we decided to perform cACB by positioning the catheter tip posterior to the saphenous nerve and a before-and-after study was performed. The study involves evaluating the effects of an intervention comparing the previous outcomes with those measured afterwards.⁴¹ In our clinical context, although bolus pumps had been used for a long time instead of continuous infusion pumps, there was still a high incidence of pumps block especially in the adductor canal. This prompted us to look for a more adequate position of the catheter tip with respect to the saphenous nerve. In fact, we thought that the sub sartorial fascia by closing obstructed the catheter tip anteriorly to the saphenous nerve and this prompted us to position it posteriorly to the nerve.

Our study has some limitations. Firstly, the retrospective design of the study, with risk of bias based on the interpretation of results. Secondly, the small number of patients included. Even if no power analysis was led at the beginning of the study, we believe that the lack of previous comparative analysis investigating the outcome after cACB taking into account the position of the catheter tip and the saphenous nerve increases the value of our results. We advocate further prospective and comparative studies to confirm or disprove our findings. Thirdly, the two groups differed by BMI, which theoretically might affect our findings as well. However, based on previous literature, we would have expected a worse outcome in G2 (with a higher BMI), therefore we deduced that this variable likely played a negligible role in the comparison of the two groups.

Conclusion

In cACB for TKA, we found that positioning the catheter tip anterior to the saphenous nerve may lead to a greater postoperative analgesia and reduce the risk of pump block compared to placing the catheter tip posterior to the nerve. We encourage further studies on this subject in order to define which variables might influence the success and complication rate of cACB in the setting of lower limb surgery.