Today, various indications exist for an obturator nerve block, and several different nerve block techniques have been described over the last century. In 2012, using 88 cadavers (176 lower limbs), Feigl et al.1 developed a new technique of an obturator nerve block This block technique was named vertical obturator nerve block, because of the perpendicular direction of the needle to the surface. The simplicity, as well as the promising results described by Feigl et al., encouraged us to test the vertical obturator nerve block for the first time under real clinical conditions.
A prospective, randomised, double-blind, parallel designed pilot trial was approved by the local ethics committee (Ethical Committee IRB00002556, N°25-387ex12/13) of the Medical University Graz, Auenbruggerplatz 2, A-8036 Graz, Austria (Chairperson Univ. Prof. DI. Dr Josef Haas) on 17 May 2013 and was prospectively registered at clinicaltrials.gov (NCT01875289, 10 June 2013, ASK). All study participants provided written informed consent before participating. The study was conducted at the University Hospital of Graz, Austria, from October 2013 to March 2014.
Of the 36 patients assessed for eligibility, 30 patients were randomised to receive either ropivacaine (ropivacaine group) or 0.9% normal sodium chloride (control group) for obturator nerve block. We had no dropouts as all 30 patients completed the study. A 2 : 1 computer-generated randomisation (ropivacaine group, n = 20/control group, n = 10) was implemented. Our institutional pharmacy produced blinded study medication: 5 ml of 0.75% ropivacaine (37.5 mg) for the ropivacaine group and 5 ml of 0.9% normal sodium chloride for the control group in identical looking syringes. Patients were scheduled for elective trauma surgery of the knee region. Inclusion criteria were American Society of Anesthesiologists physical status classification system 1 to 3 and age over 18 years. Exclusion criteria were neurologic disease, neuromuscular deficits of the lower extremities, pregnancy and signs of local infection. The vertical obturator nerve block and all measurements were performed after routine premedication and before induction of general anaesthesia. The primary outcome was adductor muscle strength, measured before and 15 min after nerve block. Like previous obturator nerve block studies, to evaluate the success of the vertical obturator nerve block we measured the adductor muscle strength by a patient's ability to squeeze a manual sphygmomanometer cuff between their thighs.2,3 In addition, patients were questioned about pain [an 11-point numeric rating scale (NRS)], and the possible regrets about participating in this study. The time from the start to the end of the nerve block was noted.
The obturator nerve block was performed according to the instructions given by Feigl et al.1 First a finger was placed lateral to the palpable pubic tubercle, then a short bevelled needle (Uniplex 25 G UP 3/80, Pajunk, Geisingen, Germany) was inserted directly laterally and adjacent to the distal part of the fingernail and advanced strictly perpendicular to the operating table. The injection depth was between 2.5 and 6 cm depending on the weight of the patient, according to Feigl et al.1 In case of bone contact (superior pubic ramus), the needle was pulled back a few millimetres, the tip of the needle was then tilted slightly distally, and readvanced till the bone was passed. In this case, the needle tip was inserted 1 cm deeper than the distance to where the bone had been encountered.
We predefined the success of the nerve block as a reduction of adductor muscle strength of more than 50% relatively to the measurement before treatment (measurement evaluation published by Sinha et al.4). Absolute difference in adductor muscle strength was analysed by a Mann–Whitney U test. A P value less than 0.05 was considered significant.
A decreased muscle strength of 20.0 mmHg was detected in the ropivacaine group, compared with an increase of 7.5 mmHg in the control group (P = 0.001; Table 1). Using the predefined cut off of a 50% decrease of adductor muscle strength a successful obturator nerve block was observed in only three patients in the ropivacaine group (3/20), and in no patient in the control group (0/10). In seven cases a bone contact was noted (7/30). No side-effects such as neurapraxia, infection, or hematomas were encountered. In all 30 patients the median NRS pain score was 3 (2, 3; lower, upper quartile). One patient described pain as NRS 6, and one as NRS 9. Every patient indicated they would be happy to have this nerve block in future. The whole nerve block lasted 130 ± 101 s.
In this study, we considered two reasons for the seemingly low success rate. First, we used quite a small volume (5 ml) of local anaesthetic compared with many other obturator nerve block studies where 10–25 ml local anaesthetic was used.4,5 Second, we tested the success of the vertical obturator nerve block by measuring the adductor muscle strength 15 min after nerve block. Owing to the institutional time restrictions before surgery, the measuring was performed very early. The onset of a motor-neuron blockade with 0.75% ropivacaine is likely to appear after 15 min.6 As a consequence, we would expect to see more successful blocks by using higher volumes of local anaesthetic and delayed testing of adductor muscle strength.
Compared with other obturator nerve block techniques, this block is encouragingly fast and easy to perform. The time to perform the vertical obturator nerve block was shorter compared with the times recorded in other studies of obturator nerve block using different techniques.3,4 Furthermore, the vertical obturator nerve block is performed more proximal than the ultrasound directed inter-adductor technique for an obturator nerve block.3–5 At this point the nerve has not yet divided into its two branches (anterior and posterior), hence a part of the sensory innervation of the hip will be blocked. Thus this block technique may be superior for reducing hip pain.
Given these advantages compared with other nerve block techniques and the promising results from the anatomical study,1 the vertical obturator nerve block should be further evaluated in clinical studies.
Acknowledgements relating to this article
Assistance with the study: none.
Financial support and sponsorship: none.
Conflicts of interest: none.
1. Feigl GC, Ulz H, Pixner T, et al. Anatomical investigation of a new vertical obturator nerve block technique. Ann Anat
2. Helayel PE, da Conceição DB, Pavei P, et al. Ultrasound-guided obturator nerve block: a preliminary report of a case series. Reg Anesth Pain Med
3. Manassero A, Bossolasco M, Ugues S, et al. Ultrasound-guided obturator nerve block: interfascial injection versus a neurostimulation-assisted technique. Reg Anesth Pain Med
4. Sinha SK, Abrams JH, Houle TT, Weller RS. Ultrasound-guided obturator nerve block: an interfascial injection approach without nerve stimulation. Reg Anesth Pain Med
5. Choquet O, Capdevila X, Bennourine K, et al. A new inguinal approach for the obturator nerve block: anatomical and randomized clinical studies. Anesthesiology
6. Cuvillon P, Nouvellon E, Ripart J, et al. A comparison of the pharmacodynamics and pharmacokinetics of bupivacaine, ropivacaine (with epinephrine) and their equal volume mixtures with lidocaine used for femoral and sciatic nerve blocks: a double-blind randomized study. Anesth Analg