For patients undergoing total knee arthroplasty (TKA), evidence supports the use of continuous peripheral nerve block (CPNB) techniques with femoral or saphenous perineural local anesthetic infusions as part of a perioperative multimodal analgesic regimen [1,2,3]. Although CPNB offers clear analgesic advantages with selectivity for the operative extremity by targeting specific nerve distributions, there is rising concern over the quadriceps weakness produced by femoral nerve blocks and the potential risk of falls [4,5]. More recently, the adductor canal catheter (ACC) technique [6] has emerged as an alternative to the femoral nerve catheter (FNC) for TKA patients, because it better preserves quadriceps motor function while still providing effective pain relief [7,8], and may facilitate improved early ambulation after surgery [9].

While previous studies have assessed quadriceps strength in terms of maximum voluntary isometric contraction [10,11,12], this method requires the use of a dynamometer and is not routinely part of clinical bedside assessment. Clinical assessments commonly rely on physical examination and subjective patient-reported outcomes (e.g., numeric ratings of pain). Although the use of numeric rating scales (NRS) of patient-reported numbness has been reported previously [13,14], this measure has not yet been correlated with physical function and also remains to be evaluated as a predictor of postoperative ambulation. A patient who self-reports numbness of the quadriceps muscle on the operative extremity after TKA may be more reluctant to participate in physical therapy due to fear of falling. Given the expected increase in TKA prevalence over the next 10 to 15 years, due to an aging population and rising prevalence of osteoarthritis [15], there is a need to critically evaluate assessment techniques and incorporate patient-reported outcomes as appropriate.

We designed this study to test the hypothesis that TKA patients with an ACC will exhibit lower self-report numbness scores on an NRS compared to patients with an FNC. We also explored the potential correlation between patient-reported numbness and ambulation distance on postoperative day (POD) 1.

After receiving Institutional Review Board approval with waiver of informed consent (Stanford University School of Medicine, Stanford, CA, USA), we conducted this retrospective cohort study pooling data from two previously-published randomized clinical trials involving TKA patients [13,14]. These two sub-studies were part of a larger prospective project registered under NCT01459523 (http://clinicaltrials.gov); one compared proximal and distal ACC [14], and the other compared long-axis and short-axis ultrasound imaging for in-plane FNC insertion [13].

The data from all 94 previous clinical trial subjects were included in this study (46 FNC and 48 ACC). The FNC group was comparable to the adductor canal group in key baseline criteria (Table 1) although we did note a higher proportion of women in the femoral group relative to the ACC group (4/42 vs. 0/48, P = 0.04).

ACC patients reported lower numbness scores relative to the FNC group (median [10^th–90^th percentiles]): 0 (0–5) vs. 4 (0–10), P < 0.01. The ACC group also demonstrated an approximately seven-fold increase in ambulation on POD 1 relative to the FNC group (median [10^th–90^th percentiles]): 45.7 (6.2–91.4) vs 6.1 (0–42.7), P < 0.01. No differences were noted between the two groups on our other, secondary outcomes (Table 2). While linear models showed a statistically significant association between total daily ambulation distance and numbness on POD 1 (Beta = –2.6; 95% CI: –4.5, –0.8, P < 0.01), the R-squared was 0.1.

For TKA patients receiving perineural local anesthetic infusions as part of a multimodal analgesic regimen, ACC results in less self-reported numbness and facilitates improved early ambulation when compared to FNC with no differences in pain control. Few other studies involving perineural catheters have focused on numbness as a procedure-specific outcome after surgery [18,19,20]. While self-reported measures such as the Western Ontario McMaster Osteoarthritis Index exist and are commonly used for long-term postoperative evaluation, these measures tend to be global, incorporating multiple dimensions such as overall well-being and independence [21,22] and have not been correlated with actual physical function. To better assess the role of regional analgesia, researchers have advocated for more clinically relevant patient-reported outcome measurement tools applicable to TKA patients [23].

Perineural catheter location is an important factor determining patient-reported numbness beyond type of local anesthetic, dosing strategy, and duration of local anesthetic infusion [24]. The 4-point median difference in numbness between the ACC and FNC groups is notable given that half of the ACC patients reported no numbness, and ACC has been previously shown to better preserve quadriceps muscle strength [11,12]. Patient-reported outcomes related to TKA have focused primarily on pain intensity [23], but the present study provides evidence that self-reported numbness using NRS may be easily incorporated into the bedside assessment to provide additional information. This is a clinically important outcome because the patients' subjective feeling of numbness may decrease their willingness to participate in physical therapy; physiologically, loss of proprioception and motor weakness contribute to gait and balance problems that can affect ambulation [25].

However, in our secondary analysis, although there was a significant association between total ambulation distance and numbness, 90% of the variance remained unexplained; therefore, self-reported numbness cannot be used to predict ambulation achievement. The remaining unexplained variance emphasizes the need for further research into the factors that potentially contribute to postoperative ambulation after TKA. These results are consistent with prior reports suggesting that patient-reported outcomes correlate poorly with performance-based outcomes [23,26]. Furthermore, we found no group differences in satisfaction, pain experience, or opioid consumption. Since patients' own perceptions regarding their functional ability and physical status may differ greatly from their actual performance outcomes when measured, our data reinforce the need for both performance-based and self-reported outcomes when assessing the progress of TKA patients' postoperative rehabilitation [27].

Our study had several limitations that should be considered when interpreting the results. First, we used a retrospective study design such that the reported associations may not be true causal relationships; our study also pooled data from two separate clinical trials that were not powered to test for the present study's primary outcome, even though both studies measured it. We believe that the fact that we combined data from two studies may explain the difference in number of female patients per group, because randomization was not applied across the entire sample. However, a potential advantage of our study is that, although it was retrospective, it used prospectively collected data during the immediate postoperative period when the effects of the ACC and FNC techniques on numbness would be most likely to be found using the same standardized study methodology and case report forms. Although selection bias remains possible, all patients were treated on the same clinical pathway for TKA, in which only the placement location of CPNB varied. Second, the generalizability of the study results is limited to practices employing similar techniques and equipment to those used in this study, and may not be generalizable to other institutions or non-veteran populations.

In summary, although perineural local anesthetic infusion in the adductor canal results in improved early ambulation distance and lower patient-reported numbness after TKA compared to femoral perineural infusion, the correlation between these two variables was weak. Future studies evaluating the comparative effectiveness of regional analgesic techniques in TKA patients should include both patient-reported and performance-based outcome measurements.