No falls were noted during rehabilitation in either group. In contrast, the two groups differed significantly with regard to near-falls with knee-buckling (FNB, 14/37 [37.8%]; ACB, 4/36 [11.1%]; p = 0.0068). Ambulation with parallel bars began significantly earlier in the ACB group compared with the FNB group. The mean MMT scores of the quadriceps of the patients who were able to ambulate with parallel bars on POD 1 were 2.82 (95% CI: 2.49–3.14) in the ACB group, significantly higher than the 1.97 (95% CI: 1.48–2.45) in the FNB group. When evaluated by NRS, no statistically significant difference in pain relief between the two groups was seen.
Feibel et al. and Pelt et al. reported rates of falls after TKA with FNB continuous infusion of 0.7% (8 in 1190 cases)  and 2.7% (19 in 707 cases) , respectively. Using continuous infusion, Bolarinwa et al. reported a fall rate of 0.13% (1 of 791 cases) with ACB compared to 1.3% (11 in 834 cases) with FNB, and recommended ACB as the preferred regional analgesia for the TKA procedure . In our smaller study, the fall rate was 0% in both groups.
We defined knee-buckling as a near-fall since it can lead to falls, especially in the elderly . Despite a significantly higher BMI in the ACB group, the frequency of knee-buckling was 14 patients (37.8%) in the FNB group compared to 4 (11.1%) in the ACB group (p = 0.0068). This number appears higher than that in the study by Thacher et al., which reported 17/129 patients (13.2%) in the FNB group and 3/150 patients (2.0%) in the ACB group . This difference might be due to the older patient age at the time of surgery in our cases, differences in anesthesia administration (continuous infusion following bolus in our study vs. bolus alone in the previous report ), and possible differences in rehabilitation protocols. In particular, the difference between continuous catheter infusion and a single bolus administration may be significant. All of the near-falls with knee-buckling were confirmed by physical therapists, who monitored patients closely during the rehabilitation course. All knee-buckling episodes occurred within PODs 1–3, which may outline the minimum period when this close monitoring should be performed, although Thacher et al.’s study reported that 26 hours may be the window during which patients who receive a single administration of either FNB or ACB are most vulnerable . Elkassaban et al. reported stopping the continuous infusion on the morning of POD 1 before starting physical therapy. Using the Tinelli fall risk scale [25, 49] and MMT values of the quadriceps, they found that the relative risk of fall with FNB was higher than that with ACB. In our study, patients received continuous infusion blocks, which were removed on POD 2 or 3, widening the window. This suggests that careful supervision of all standing and ambulation, especially in FNB patients, is required until 24 hours after nerve blocks are removed to minimize fall risk.
A number of studies have suggested that ACB is associated with more prompt and better advancement of ambulation ability than FNB [24, 26, 31, 37]. Shah et al. reported that the ambulation ability, the timed up-and-go (TUG), the ten-meter walk test, and the 30-second chair stand test  all had significantly better results with continuous ACB than continuous FNB anesthetic infusions (51.81s vs. 180.06s, 67.0s vs. 273.70s, 5.25 vs.1.52, p < 0.001, respectively) after removal of the catheter . In our study, the quadriceps muscle strength of most patients did not recover sufficiently for them to perform unsupported tests, including the TUG, the ten-meter walk test, and the 30-second chair test, which Shah et al. did on POD 1 and 2.
In terms of quadriceps strength, although ACB is considered to be an almost pure sensory block, Jaeger et al., in a study of healthy volunteers, using a single bolus of 30 mL of 0.1% ropivacaine, reported that the mean reduction in quadriceps strength compared with 30 mL of saline in the opposite limb was 8% with ACB and 49% with FNB . Kwofie et al., using single 15 mL bolus of 3% chloroprocaine, reported that isometric contraction of the quadriceps declined by 11% 60 min after ACB block in one leg and 95% with FNB in the contralateral leg in healthy volunteers , which suggests that ACB does have a slight motor effect and still requires close attention from nursing staff during the early postoperative period.
In our retrospective study, we evaluated the intensity of postoperative pain in addition to the safety of ACB using NRS on POD 1, 2, and 3. Some studies have suggested that ACB is inferior to FNB with regard to the need for supplementary opioids [36, 39]. However, most studies, including those of Mudumbai et al. and Thacher et al. [36, 39], have supported the idea that ACB and FNB are equally effective in terms of pain relief [24, 35–37, 39, 50, 51]. Our study showed no statistically significant differences in NRS-11 scores between the two groups, indicating that ACB provided effective pain relief equivalent to that of FNB.
We demonstrated significant differences in mobility when walking with parallel bars and in the frequency of near-falls with knee-buckling between patients receiving continuous FNB vs. ACB, with earlier ambulation and fewer near-falls with knee-buckling in the ACB patients. The differences between the two groups may be due to the lower effect of ACB on quadriceps strength.
This study was limited by its retrospective design and small number of patients.