Comparing the analgesic effects of femoral triangle block and adductor canal block following total knee arthroplasty: a systematic review and meta-analysis

Abstract

Objective

The present study compared the postoperative analgesic effects of femoral triangle block (FTB) with adductor canal block (ACB) in patients undergoing total knee arthroplasty (TKA).

Methods

Randomized controlled trials (RCTs) involving the analgesic effects of FTB and ACB post-TKA were collected by searching PubMed, Embase, Web of Science, Cochrane Library, Wanfang, CNKI, and VIP databases from inception till March 2024. The primary outcomes of the study focused on pain scores in resting and activity states, and the secondary outcomes examined the quadriceps muscle strength, postoperative adverse reaction incidence rate, and patient satisfaction scores.

Results

Six RCTs published between 2020 and 2023 were included, which involved altogether 452 patients, with 226 each in the FTB and ACB groups. No significant difference was observed in the resting and activity pain scores at 6, 12, and 24 h between the FTB and ACB groups (P > 0.05). In contrast, at 48 h, the ACB group exhibited better activity pain scores than those in the FTB group (P < 0.05). Three studies concluded that ACB could preserve quadriceps muscle strength, whereas one study concluded that FTB had an advantage in quadriceps muscle strength recovery. Postoperative patient satisfaction scores and adverse reaction incidence rates were not significantly different between the FTB and the ACB groups (P > 0.05).

Conclusion

The ACB group demonstrated a certain advantage over the FTB group in the immediate postoperative quadriceps muscle strength recovery. The FTB group showed a better analgesic effect at 48 h than the ACB group. No significant differences were observed in the postoperative adverse reaction incidence rates and patient satisfaction scores between the two groups. Nonetheless, because of the differences in the enrolled articles, further large-scale, high-quality RCTs should be conducted to verify whether ACB is more effective and safer than FTB.

Introduction

The trends of population aging indicate that an increasing number of people are undergoing total knee arthroplasty (TKA). Reducing postoperative pain and facilitating early mobilization have a positive effect on the reconstruction of the knee joint function. Therefore, efficient pain control plays an essential role in accelerating patient recovery and improving satisfaction while minimizing the risk of complications [1]. Nerve blocks that directly target the painful area provide effective analgesia and reduce the use of systemic analgesics, thereby preventing drug addiction and making a significant contribution to the early recovery phase of TKA [2].

Femoral nerve block (FNB) is an efficient and mature analgesia technology in TKA [3]. Nonetheless, FNB can cause quadriceps muscle weakness, increasing the risk of falls. Femoral triangle block (FTB) and adductor canal block (ACB) are considered efficient alternatives to FNB [4] as they affect fewer motor fibers, leading to faster functional recovery while possessing similar analgesic effects to those of FNB [3, 5]. FTB blocks the saphenous nerve, the medial femoral cutaneous nerve (MFCN), and the medial femoral nerve branches to anesthetize the anteromedial portion of the knee, thereby relieving postoperative pain with no influence on the quadriceps [6]. The main target nerve of ACB is the saphenous nerve [7], it is the only nerve consistently observed in AC. For 90% of the patients, the medial geniculate branch of the medial femoral nerve enters the fascial space near the AC entrance, from the medial vastus muscle to the adductor longus muscle out of AC [8]. The posterior branch of the obturator nerve may appear in AC and innervate the anteromedial capsule of the knee joint along with the saphenous nerve and the medial vastus muscle [9]. The medial femoral nerve is significantly associated with innervating the medial subcutaneous tissue and the knee capsule, whereas the saphenous nerve has little influence on the innervation of the knee joint. The analgesic efficacy of FTB compared with that of ACB after TKA is controversial. The present study compared the analgesic efficacy of FTB with ACB post-TKA via meta-analysis, thereby shedding more light on the selection of a more suitable nerve block modality.

Data and methods

This study conducted a systematic review of the postoperative analgesic efficacy of intraoperative FTB versus ACB in patients receiving TKA according to the PRISMA principle [10], and it has been registered on the PROSPERO platform as per the specifications (CRD42024525738).

Study eligibility criteria

The inclusion criteria are as follows: (1) Research participants are patients receiving TKA regardless of race, age, gender, and height; (2) The research type is a randomized controlled trial (RCT), regardless of the research field, and the articles are only in Chinese and English; (3) Articles comparing the intervention measures FTB and ACB nerve blocks for pain management; (4) The main outcome indicator is the pain score, determined using the numerical rating scale (NRS) and the visual analog scale (VAS), with scores in the range of 0–10,and a higher score indicating more severe symptoms. The NRS pain score adds an emoticon below the value based on the VAS pain score, and the patient can indicate the numerical value of their pain level based on the degree of pain expressed via the facial expression pattern; (5) Secondary outcomes include postoperative quadriceps muscle strength, postoperative patient satisfaction score, and postoperative adverse reaction incidence rate.

The exclusion criteria are as follows: (1) Non-randomized controlled trials; (2) Reviews or conference papers; (3) Articles for which the full text could not be obtained or data were incomplete.Any disagreements will be resolved via consultation between the two researchers or by a third party for arbitration.

Characteristics of enrolled studies

The studies included in this meta-analysis exhibited several key differences that may influence the overall results and interpretations. These differences are summarized as follows:

Study design

The included studies varied in their design, with some employing single-shot nerve blocks while others utilized continuous nerve blocks. This variation may impact the duration and intensity of analgesia provided by FTB and ACB.

ACB puncture location

The anatomical location of the adductor canal block (ACB) varied across studies. Four studies used the newly defined ACB location (midpoint level between the femoral greater trochanter and superior patella), while two studies used the traditional ACB location (midpoint level between the anterior superior iliac spine and superior patella). This anatomical variation may affect the block’s efficacy and safety profile.

Sample size and demographics

The studies included different sample sizes and patient demographics (e.g., age, gender, ASA grade). These variations could influence the generalizability of the results and the observed effects of FTB and ACB.

The methods for measuring quadriceps muscle strength and patient satisfaction scores: These variations highlight the need for standardized protocols in future research to ensure comparability and reliability of results.

Analgesic regimens

The nerve block medication regimens differed among the studies, which may contribute to the observed heterogeneity in pain scores and other outcomes.

By acknowledging these differences, we aim to provide a more transparent and comprehensive understanding of the variability in the enrolled studies. Future research should aim to standardize these factors to enhance the comparability and reliability of findings.

Search strategy

Studies comparing FTB with ACB for pain management following TKA were searched in databases, including PubMed, Embase, Web of Science, Cochrane Library, Wanfang, Chinese National Knowledge Infrastructure (CNKI), and Virus Protein domain (VIP) using the terms “femoral triangle block OR FTB” AND “adductor canal block OR ACB” AND “total knee replacement OR TKA.”(see full search strategy in Appendix).

Data extraction

Data extraction of the included studies involved baseline study data (author and publication year), study characteristics (age, sample size, American Society of Anesthesiologists grade, anesthesia method, and nerve block medication regimen), and outcome measures. We transformed other data types (median, interquartile spacing, mean ± 95% confidence interval [CI]) into mean ± SD in line with the Cochrane Handbook [11].

Literature quality evaluation

RevMan 5.4 provided by Cochrane Collaboration was applied to evaluate the enrolled study quality, including randomization, allocation concealment, selective reporting, blinding (trial participants or staff), blinding of study outcomes, complete outcome data, and other biases. Each content was evaluated from three aspects: low or high risk of bias or unclear bias. For the assessment of risk of bias, we referred to the study by De Cassai et al. [12]., which provides detailed insights into the robust evaluation of bias in clinical trials.

Publication bias assessment

To assess potential publication bias, we generated funnel plots for each outcome measure, including postoperative pain scores at rest and during activity (6, 12, 24, and 48 h), incidence of adverse reactions, and patient satisfaction scores. Due to the limited number of studies included in our meta-analysis (n = 6), we did not perform Egger’s regression test, as it is generally recommended only when there are at least 10 studies to ensure sufficient statistical power. Instead, we relied on visual inspection of the funnel plots to evaluate symmetry.

Assessment of evidence quality

This study employed GRADE Profiler 3.6 software of the Evidence Quality Grading System (GRADE system) to evaluate the evidence quality of every outcome indicator. As this study included all RCT trials, the evaluation included only five factors: risk of bias, heterogeneity, indirectness, accuracy, and other factors, with the quality ratings expressed as high, medium, low, or very low.

Statistical analysis

Statistical analyses were conducted with the RevMan 5.4 software. Continuous data were represented as mean difference (MD) and 95% CI, whereas dichotomous variables were represented as odds ratio (OR) and 95% CI. First, the heterogeneity test was conducted among the included studies, and heterogeneity was assessed based on P and I². If P > 0.10 and I² < 50%, there is minor heterogeneity in the results, so a fixed-effects model was selected for the analysis; otherwise, it is believed that the results exhibit heterogeneity. Following the exclusion of clinical heterogeneity, we attempted to identify the related factors that produce heterogeneity. Only qualitative descriptions are provided for data that cannot be meta-analyzed. P < 0.05 indicates statistical significance.

Results

Literature search results

A total of 101 related articles were retrieved; 35 duplicate articles were removed; 10 dissertations, reviews, and evaluations were removed; and 41 full-text papers with inconsistent research content and outcome indicators and inaccessible full-text were removed. Altogether, six studies were enrolled for the final analysis [13,14,15,16]There were four Chinese and two English documents (Fig. 1). Of these six studies, a single nerve block was performed in three studies [13, 14, 17] and [15, 16, 18] continuous nerve block was performed in three studies. According to the newly defined ACB block location, four studies [13, 16,17,18] had puncture sites for new ACB (midpoint level of the femoral greater trochanter and superior patella) and two for [14, 15] traditional ACB (midpoint level of the connection between the anterior superior iliac spine and superior patella). Table 1 presents the features of the enrolled studies. Figure 2 displays the risk of bias evaluation according to the Cochrane Systematic Evaluation of Interventions.

Fig. 1

Flowchart of study selection

Table 1 Baseline features of enrolled studies

Fig. 2

Risk of bias summary for the enrolled studies

Meta-analysis results

Publication bias assessment

Postoperative Pain Scores at Rest and During Activity (6, 12, 24, and 48 h):

The funnel plots for pain scores at rest and during activity at all time points (6, 12, 24, and 48 h) exhibit some degree of asymmetry. This asymmetry may be attributed to the limited number of studies included in our meta-analysis (n = 6). With fewer than 10 studies, the statistical power to detect publication bias is reduced, and funnel plots are more prone to asymmetry due to random variation rather than true publication bias. Additionally, heterogeneity in study design, patient populations, and measurement methods may contribute to the observed asymmetry(Figure S1-S8).

Incidence of adverse reactions

The funnel plot for adverse reactions also shows asymmetry. This could be influenced by the small number of studies reporting adverse events and potential underreporting of negative or null results in the literature. However, the asymmetry may not necessarily indicate publication bias but rather the limited number of studies and potential heterogeneity in reporting adverse events(Figure S9).

Patient satisfaction scores

The funnel plot for patient satisfaction scores is similarly asymmetric. This outcome had high heterogeneity (I² = 86%), which may explain the asymmetry. Differences in measurement tools and timing of assessment across studies likely contributed to this heterogeneity(Figure S10).

Pain scores

Two studies [14, 16] reported resting and activity pain scores 6 and 12 h postoperatively, with significant heterogeneity (6 h: P = 0.001, I² = 90%; P < 0.001, I² = 98%; 12 h: P < 0.001, I² = 99%; P < 0.001, I² = 97%). As few studies were enrolled, subgroup or sensitivity analysis could not be used; thus, we used the random-effects model. The results showed that no statistically significant difference was observed in the resting and activity pain scores between the two groups 6 and 12 h postoperatively (6 h: MD = 0.16, 95% CI − 0.99~1.31, P = 0.78; MD = 0.22, 95% CI − 0.61~1.06, P = 0.60; 12 h: MD = − 0.52, 95% CI − 2.63~1.60, P = 0.63; MD = − 0.08, 95% CI − 1.29~1.14, P = 0.90; Table 2; Fig. 3).

Table 2 GRADE evidence quality of every outcome indicator

Fig. 3

Forest plot comparing the resting and activity pain scores 6, 12, 24, and 48 h postoperatively in the two groups.Pain score with rest at 6 h

Four studies [14, 16,17,18] reported resting and activity pain scores 24 h postoperatively with significant heterogeneity (P < 0.001, I² = 96%; P < 0.001, I² = 96%). Subgroup analyses were conducted using different block methods and divided into single and continuous block groups. The subgroup analyses revealed that during the single nerve block, the analgesic effect of FTB at rest and activity was superior to that of ACB 24 h postoperatively (MD = − 0.84, 95% CI − 1.10~−0.58, P < 0.001; MD = − 0.81, 95% CI − 1.03~−0.59, P < 0.001). During continuous nerve block, ACB had better analgesic efficacy compared with that of FTB (MD = 0.74, 95% CI 0.50~0.99, P < 0.001; MD = 0.78, 95% CI 0.53~1.03, P < 0.001; Tables 2 and 3; Fig. 3).

Table 3 Subgroup analysis stratified by block type (continuous catheterization or single injection)

Four articles reported resting and activity pain scores at 48 h postoperatively with significant heterogeneity (P < 0.001, I² = 96%; P < 0.001, I² = 96%). Following subgroup analyses, heterogeneity was still high; therefore, sensitivity analysis was conducted. It was observed that the study by Luo 2023 significantly impacted heterogeneity. After removing this study, heterogeneity testing was conducted and no heterogeneity was observed among the studies (P = 0.24, I² = 29%; P = 0.61, I² = 0%). The fixed effects model was used to merge the effect sizes, and the results showed that there was no statistically significant difference in the resting pain scores between the two groups 48 h postoperatively (Z = 0, 39, P = 0.69). The analgesic effect of FTB was better than that of ACB in the activity state 48 h postoperatively (MD = − 0.27, 95% CI − 0.45–0.10, P = 0.002; Table 2; Fig. 3).

One study indicated that [19] the accurate ACB puncture location is the midpoint of connection from the femoral greater trochanter to the upper edge of the patella; four of the six studies included in this meta-analysis [13, 16,17,18] were for the newly defined ACB, the remaining two studies were ACB in the femoral triangle [14] and a distal ACB [15]. Therefore, depending on the ACB location, subgroup analysis was performed in two groups: FTB vs. new ACB and FTB vs. traditional ACB. No significant differences were observed in the resting and activity pain scores at 24 and 48 h in three studies (24 h: MD = 0.20, CI − 0.66 ~ 1.06, P = 0.65; MD = 0.12, CI − 0.84 ~ 1.08, P = 0.81; 48 h: MD = 0.05, CI − 0.57 ~ 0.67, P = 0.87; MD = − 0.03, CI − 0.83 ~ 1.06, P = 0.77). No statistically significant difference was observed in the postoperative adverse reaction incidence rates and satisfaction scores between the FTB and new ACB groups (P = 0.87; P = 0.38); moreover, no statistically significant difference was observed in the postoperative adverse reaction incidence rates between the FTB and traditional ACB groups (P = 0.83, Table 4).

Table 4 Subgroup analysis stratified by block location (FTB vs. new ACB and FTB vs. traditional ACB)

Adverse reaction incidence

Five studies [13,14,15, 17, 18] reported the incidence of postoperative adverse reactions. Heterogeneity was not detected across diverse studies (P = 0.75, I² = 0%), so a fixed-effects model was adopted, which revealed the absence of a significant difference in adverse reaction incidence rate between the two groups (OR = 1, 95% CI 0.6–1.67, P = 1.00; Fig. 4).In the included studies, the evaluated adverse reactions mainly included the following types: Nausea and vomiting、Urinary retention, Dizziness and headache.Among these, nausea and vomiting was the most commonly reported adverse reaction, with an incidence rate ranging 15% in the FTB group and 17% in the ACB group. Compared to the 4% incidence rate of urinary retention in the ACB group, the incidence rate in the FTB group was 2.5%. The incidence rates of dizziness and headache were 9.3% in the FTB group and 12.4% in the ACB group. However, no statistically significant differences were observed in the incidence rates of these adverse reactions between the two groups.

Fig. 4

Forest plot showing postoperative adverse reactions of the two groups

Satisfaction score

A total of four studies [15,16,17,18] reported postoperative satisfaction scores, except for one [15] for which the value could not be obtained. Three articles were finally included in the study, and heterogeneity was observed across them (P = 0.0008, I² = 86%) using the random-effects model, which revealed the absence of statistical difference in the postoperative satisfaction scores of both the groups (MD = − 0.68, 95% CI − 1.56 ~ 0.20, P = 0.13; Fig. 5).

Fig. 5

Forest plot showing the postoperative satisfaction scores of the two groups

Quadriceps muscle strength

Five studies [13, 14, 16,17,18] reported quadriceps femoris muscle strength postoperatively in patients. Considering the different methods used in different studies to measure the quadriceps femoris muscle strength, relevant data are insufficient and heterogeneous, and meta-analysis cannot be conducted; thus, the results are presented as text descriptions. The results of the three studies [13, 16, 18] believed that, compared with the FTB group, the postoperative quadriceps femoris muscle strength increased for the ACB group; in contrast, one study [14] believed that, compared with the ACB group, the postoperative quadriceps femoris muscle strength increased for the FTB group. One study [17] believed no obvious difference existed between the two groups (Table 5).

Table 5 Postoperative quadriceps femoris muscle strength of the FTB and ACB groups

Discussion

The present meta-analysis compared the safety and efficacy of ACB and FTB for analgesia post-TKA. Our results revealed that FTB had a better analgesic effect than ACB at 48 h with activity; however, ACB was superior in the preservation of quadriceps muscle strength. The postoperative adverse reaction incidence rates and patient satisfaction scores were not significantly different between the two groups.

ACB and FTB are emerging nerve block techniques targeting TKA. The cadaver study by Ishiguro et al. [20] proved that FTB can block several saphenous nerve branches and the vastus medialis muscle. Many vastus medialis nerve muscular branches pass through the deep part of the medial femoral muscle and exert a key effect on controlling the knee joint [21]. Our results showed that overall, there was no statistically significant difference in pain scores during rest and activity 6, 12, and 24 h postoperatively, but the FTB group had lower activity pain scores than the ACB group 48 h postoperatively (P < 0.05). According to the different blocking methods, further subgroup analysis was conducted, and the results showed that the analgesic effect of the FTB group was better than that of the ACB group in both resting and activity states 24 and 48 h after a single block (P < 0.05). In contrast, the analgesic effect of the ACB group was better than that of the FTB group after continuous block (P < 0.05).

This may be because a single nerve block blocks the medial femoral nerve in a short period of time as the medial femoral nerve is located in an independent fascial sheath [22], which limits the blocking effect of the local anesthetic injection into the adductor canal on the medial femoral nerve. However, the triangular block of the femur can effectively block the medial femoral nerve, so the analgesic effect of a single FTB is better than that of a single ACB. In contrast, following the implementation of the continuous nerve block, the postoperative analgesic effect for the ACB group was superior to that of the FTB group. A possible explanation is that ACB can simultaneously block the saphenous nerve, the vastus medialis muscle branch, MFCN, and the posterior obturator nerve branch; in contrast, FTB cannot achieve this effect [19, 23, 24]. Therefore, the continuous blockade can provide effective local analgesia and obtain an ideal analgesic effect [25].

However, controversy still exists regarding the precise puncture position of ACB. AC is a myoaponeurotic space that originates proximally from the femoral triangle apex and ends distally in the adductor tendon hiatus, which has been fully verified in cadaveric studies [20, 26, 27]. Currently, the most commonly used ACB puncture location for post-TKA analgesia is the midpoint of the thigh, which refers to the midpoint of the line that connects the anterior superior iliac spine with the superior edge of the patella. Bendtsen et al. [19] examined this and believed that the anterior superior iliac spine to the superior edge of the patella, the midpoint of the connecting line, and the short axis plane of the thigh formed the femoral triangle. The accurate midpoint of the thigh for ACB puncture should be considered as the midpoint of the line that connects the lower edge of the greater trochanter of the femur and the upper edge of the patella. Therefore, the presumed ACB puncture location reported in previous literature [28,29,30]is located within the femoral triangle. Of the six articles enrolled in the present study, four [13, 16,17,18]were true ACBs, and the remaining two studies were ACBs in the femoral triangle [14]and distal ACBs [15]. Thus, a subgroup analysis was conducted, which suggested no statistically significant difference in the resting and activity pain scores at 24 and 48 h postoperatively, causing ambiguity with the above conclusion. The possible reason could be the inclusion of a small number of studies; therefore, future research will focus on the actual ACB puncture site to provide more effective evidence for clinical practice.

Due to the high heterogeneity of postoperative patient satisfaction scores, subgroup analysis results based on different block methods and block positions showed that high heterogeneity was persistent. The possible reason is first, of the two studies [16, 18], one study evaluated patient satisfaction 72 h postoperatively, and one study [17] evaluated patient satisfaction 48 h postoperatively; second, fewer studies were included, resulting in higher heterogeneity. Postoperative adverse reactions and patient satisfaction are also important indicators for evaluating the analgesic effect. No statistically significant difference in postoperative satisfaction scores and adverse reaction incidence rates was observed in both groups, further illustrating the analgesic effect of FTB and ACB after TKA. As few studies have been conducted on the application of ACB and FTB in TKA and the methods of measuring quadriceps muscle strength differ among the included literature, a uniform quantification was not possible; therefore, the results are presented in the form of a text description. Further studies are needed to verify the application of FTB and ACB and the role of ACB in quadriceps muscle strength in the future. In this study, the postoperative quadriceps muscle strength of the ACB group was enhanced relative to the FTB group. This may be because the FTB block location is closer to the proximal femoral nerve than the ACB block, and most local anesthetics spread proximally within the femoral triangle. Femoral nerve paralysis is caused around the femoral nerve, leading to severe quadriceps muscle weakness [31, 32].

The observed differences between femoral triangle block (FTB) and adductor canal block (ACB) in postoperative analgesia and quadriceps muscle strength preservation can be attributed to both anatomical variations and the pharmacokinetics of local anesthetics. FTB targets multiple nerve branches, including the saphenous nerve, medial femoral cutaneous nerve, and medial femoral nerve branches, which can provide more comprehensive analgesia but may also lead to greater quadriceps muscle weakness [31, 32]. In contrast, ACB primarily targets the saphenous nerve within the adductor canal, resulting in less impact on quadriceps function and better preservation of muscle strength. This anatomical distinction likely contributes to the superior quadriceps strength preservation observed in the ACB group. Additionally, the pharmacokinetics of local anesthetics play a role in the differing analgesic effects. Continuous ACB can provide sustained analgesia by continuously blocking the saphenous nerve and its branches, while FTB may offer more immediate but shorter-lasting effects. These pharmacokinetic differences could explain the varying analgesic efficacies observed in our subgroup analyses.

While this meta-analysis focused on short-term outcomes (up to 48 h), long-term functional recovery and chronic pain management are equally critical for TKA patients. Early postoperative pain control and functional recovery are closely associated with long-term quality of life. Chronic pain is a common complication following TKA. Although our study focused on acute pain management within 48 h, effective early analgesia may play a crucial role in preventing chronic pain. By reducing the intensity and duration of early postoperative pain, both FTB and ACB could potentially mitigate the risk of central sensitization and chronic pain development. Future research should evaluate the long-term effects of FTB and ACB on chronic pain incidence, particularly in high-risk patients, such as those with preexisting chronic pain or psychological factors.

This study had some limitations. First, limited research has been conducted on the application of ACB and FTB in TKA, and thus, only six articles were included. Therefore, further evidence is required to prove the optimal nerve block method for TKA. Second, this study did not use the unified nerve block medication regimen, which requires further exploration in future studies. Third, the findings were focused on inpatients and long-term follow-up was not performed. Future research is warranted to evaluate the long-term results of ACB and FTB following TKA.

Conclusion

The FTB and ACB groups achieved comparable analgesic efficacy 6 to 24 h postoperatively. In contrast, the analgesic effect of activity at 48 h postoperatively was better in the FTB group than in the ACB group. The subgroup analysis performed using different block methods revealed that the FTB group achieved superior analgesic efficacy after a single block compared with the ACB group after a continuous block, although the analgesic effect of the ACB group outperformed that of the FTB group. The ACB group seemed to have some advantages in the rapid postoperative quadriceps muscle strength recovery; however, the postoperative adverse reaction incidence rates and patient satisfaction scores were not significantly different in both groups. Nonetheless, because of the differences in our enrolled articles, further large-scale, high-quality RCTs should be performed to demonstrate whether the safety and efficacy of ACB are better than FTB following TKA.