We read with interest the article by Danielli et al [1] comparing ultrasound (US) with nerve stimulation (NS) for posterior popliteal nerve block (PB). Because this prospective randomised trial focuses on a topic of main interest, it is of outstanding importance that the main weaknesses and limitations of this study are highlighted to avoid further confusion in this controversial topic.

First, the wording chosen by Danielli et al must be corrected. US does not offer a direct visualisation but a two dimensional image of a nerve and other structures. These images are prone to confounding artefacts. Direct visualisation is only given during surgical exposure of tissues. Furthermore, needle visualisation is not a guarantee against complications, as stated in some reviews [2], editorials [3] and case reports [4, 5].

Second, the introduction alludes to some not clinically proven advantages of US compared with NS. To avoid confusion we must highlight, that there is to date no well designed clinical trial showing US to be associated with an improvement in success rates or a clinically relevant reduction in procedural time [6-8] or complications [4, 5]. The theoretical advantages of US have still not been shown to be clinically significant compared with a NS technique.

Third, a large study has demonstrated a block success rate of 97.5% within the first 30 min including the time for the placement of popliteal catheter [9]. The one puncture success rate was of 97% in the above mentioned paper and in another similar one [10]. Compared with the very low positive outcome of 82 % in the Danielli et al study, it is difficult to understand why the authors did not reference recent larger prospective studies (500 and 1001 patients) on NS-guided PB. These studies have also shown that landmarks based on fixed distances as used in the reference literature by Singelyn et al [11] do no take into consideration the anatomical variation of the individual patient. It was shown that the distance between the knee crease and the apex of the popliteal fossa varies from 6 to 14 cm. It goes without saying, that one fixed point may not be the ideal puncture point for all patients. The point chosen by Singelyn et al [11] may introduce bias between patients since the puncture point cannot be at the same level within the popliteal fossa.

Fourth, the authors compared a multiple injection technique for the NS with a single injection technique for US and then asked patients about pain or comfort during anaesthesia. What is really a surprise is that patients preferred to receive only one needle movement? A comparison with a single injection technique [9, 10] would have been more scientific and would probably also reduced the number of needle redirections.

Fifth, Danelli et al defined block onset time as ‘the time interval between muscular landmarks palpation in the NS group or first ultrasound scan in the US group and needle removal at the end of the block in both groups’. The concept to compare landmark palpating with US scanning is not scientifically valid. NS with the introduction of the needle is the equivalent to scanning. The rest of the time is spent preparing such as fixing of the settings of the NS, entering the patient’s data in the US machine, etc. But most intriguing is that the authors used this definition (block onset time) to produce the power calculation and based this on the results of another study [12]. In the latter study one can read: ‘definition of onset of nerve block was defined as complete loss of pinprick sensation in the sciatic nerve distribution with concomitant inability to perform plantar or dorsal flexion of the foot’. This is a major flaw which compromises the validity of the results.

Finally, given the recent increase in the popularity of US guidance for peripheral nerve block, randomised trials comparing NS and US localisation methods are needed and we sincerely congratulate Danelli et al for conducting this trial. However, in order to draw any valid conclusions it is mandatory to compare US with NS techniques in a way which allows an equal comparison. Two of the differences (less needle punctures and redirections and the procedure-related pain) can be attributed to the chosen NS technique, the shorter procedure time must be attributed to subjective author’s definition. A clinical relevant advantage of US has not been demonstrated.