Medial approach of ultrasound-guided costoclavicular plexus block and its effects on regional perfussion

Rev Esp Anestesiol Reanim. 2017;64(4):198---205

Revista Española de Anestesiología y Reanimación www.elsevier.es/redar

ORIGINAL ARTICLE

Medial approach of ultrasound-guided costoclavicular plexus block and its effects on regional perfussion夽 D. Nieuwveld a , V. Mojica a , A.E. Herrera a , J. Pomés b , A. Prats c , X. Sala-Blanch d,∗ a

Máster en Competencias Médicas Avanzadas, Facultad de Medicina, Universitat de Barcelona, Barcelona, Spain Sección de radiología músculo-esquelética, Centro de Diagnóstico por la Imagen (CDI), Hospital Clínic, Universitat de Barcelona, Barcelona, Spain c Facultad de Medicina, Universitat de Barcelona, Barcelona, Spain d Hospital Clínic, Universitat de Barcelona, profesor asociado de Anatomía, Facultad de Medicina, Universitat de Barcelona, Barcelona, Spain b

Received 26 July 2016; accepted 19 September 2016 Available online 17 March 2017

KEYWORDS Infraclavicular block; Costoclavicular space; Sympathetic block; Brachial artery flow; Perfusion index; Skin temperature

Abstract Introduction: Ultrasound-guided infraclavicular block in the costoclavicular space located between the clavicle and the first rib, reaches the secondary trunks when they are clustered together and lateral to the axillary artery. This block is most often performed through a lateral approach, the difficulty being finding the coracoid process an obstacle and guiding the needle towards the vessels and pleura. A medial approach, meaning from inside to outside, will avoid these structures. Traditionally the assessment of a successful block is through motor or sensitive responses but a sympathetic fibre block can also be evaluated measuring the changes in humeral artery blood flow, skin temperature and/or perfusion index. Objective: To describe the medial approach of the ultrasound-guided costoclavicular block evaluating its development by motor and sensitive response and measurement of sympathetic changes. Materials and methods: Description of the technique and administration of 20 ml of contrast in a fresh cadaver model, evaluating the distribution with CT-scan and sagittal sections of the anatomic piece. Subsequently in a clinical phase, including 11 patients, we evaluated the establishment of motor, sensitive and sympathetic blocks. We evaluated the sympathetic changes reflected by humeral artery blood flow, skin temperature and distal perfusion index. Results: In the anatomical model the block was conducted without difficulties, showing an adequate periclavicular distribution of the contrast in the CT-scan and in sagittal sections, reaching

夽 Please cite this article as: Nieuwveld D, Mojica V, Herrera AE, Pomés J, Prats A, Sala-Blanch X. Descripción del bloqueo del plexo braquial ecoguiado en espacio costoclavicular mediante abordaje medial y evaluación de la alteración en la perfusión regional secundaria. Rev Esp Anestesiol Reanim. 2017;64:198---205. ∗ Corresponding author. E-mail addresses: [email protected], [email protected] (X. Sala-Blanch).

2341-1929/© 2016 Sociedad Espa˜ nola de Anestesiolog´ıa, Reanimaci´ on y Terap´ eutica del Dolor. Published by Elsevier Espa˜ na, S.L.U. All rights reserved.

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the interscalenic space as far as the secondary trunks. Successful blocks were observed in 91% of patients after 25 min. All the parameters reflecting sympathetic block increased significantly. The humeral artery blood flow showed an increase from 108 ± 86 to 188 ± 141 ml/min (p = 0.05), skin temperature from 32.1 ± 2 to 32.8 ± 9 ◦ C (p = 0.03) and perfusion index from 4 ± 3 to 9 ± 5 (p = 0.003). Conclusions: The medial approach of the ultrasound-guided costoclavicular block is anatomically feasible, with high clinical effectiveness using 20 ml of 1.5% mepivacaine. The sympathetic block can be evaluated with all three parameters studied. © 2016 Sociedad Espa˜ nola de Anestesiolog´ıa, Reanimaci´ on y Terap´ eutica del Dolor. Published by Elsevier Espa˜ na, S.L.U. All rights reserved.

PALABRAS CLAVE Bloqueo infraclavicular; Espacio costoclavicular; Bloqueo simpático; Flujo arteria humeral; Índice de perfusión; Temperatura cutánea

Descripción del bloqueo del plexo braquial ecoguiado en espacio costoclavicular mediante abordaje medial y evaluación de la alteración en la perfusión regional secundaria Resumen Introducción: El bloqueo infraclavicular ecoguiado en el espacio costoclavicular, situado entre la clavícula y la segunda costilla, pretende acceder a los troncos secundarios del plexo braquial cuando se hallan agrupados y laterales a la arteria axilar. Habitualmente se realiza mediante abordaje lateral, con la dificultad de la interposición de la apófisis coracoides y la dirección de la aguja hacia los vasos y la pleura. Un abordaje medial, es decir de interno a externo, evita estas estructuras. Tradicionalmente evaluamos el resultado del bloqueo infraclavicular mediante la valoración sensitiva y motora; no obstante, el bloqueo de las fibras simpáticas podría evaluarse objetivamente a través de los cambios en el flujo arterial, la temperatura cutánea y/o el índice de perfusión de la extremidad. Objetivo: Describir el bloqueo costoclavicular ecoguiado con acceso medial, evaluando su desarrollo mediante la evaluación motora, sensitiva y simpática. Materiales y métodos: Descripción inicial de la técnica y punción ecoguiada con contraste en cadáver, evaluando la distribución de un volumen de 20 ml mediante tomografía computarizada (TC) y secciones sagitales de la pieza anatómica. Posteriormente, una fase clínica con inclusión de 11 pacientes a quienes se evaluó la instauración del bloqueo motor, sensitivo y simpático. Este último a través de la medición del flujo humeral, el índice de perfusión digital y la temperatura cutánea distal. Resultados: En el cadáver se realizó el acceso sin dificultades y se evidenció una adecuada distribución periclavicular de medio de contraste en la TC y en las secciones, alcanzando desde el espacio interescalénico hasta los troncos secundarios. El 91% de los pacientes presentó bloqueo quirúrgico a los 25 min. Todos los parámetros de bloqueo simpático evaluados aumentaron significativamente. El flujo arterial humeral aumentó de 108 ± 86 a 188 ± 141 ml/min (p = 0,05). La temperatura cutánea de 32,1 ± 2 a 32,8 ± 9 ◦ C (p = 0,03) y el índice de perfusión de 4 ± 3 a 9 ± 5 (p = 0,003). Conclusiones: El abordaje medial del bloqueo costoclavicular ecoguiado fue anatómicamente factible y con elevada eficacia clínica tras 20 ml de mepivacaína al 1,5%. El bloqueo simpático obtenido puede evaluarse mediante los 3 parámetros estudiados. © 2016 Sociedad Espa˜ nola de Anestesiolog´ıa, Reanimaci´ on y Terap´ eutica del Dolor. Publicado por Elsevier Espa˜ na, S.L.U. Todos los derechos reservados.

Introduction Regional anaesthesia techniques, particularly at the level of the brachial plexus, are now widely used in clinical practice. Traditionally, these blocks have been performed at the interscalene level for surgeries involving the shoulder girdle, or at the axillary level for those involving the distal upper extremity.1 However, interest in periclavicular blocks using a supra or infraclavicular approach, which are usually associated with complications due to proximity to vessels

and pleura, has been rekindled following the introduction of ultrasound-guided techniques. The infraclavicular region contains the costoclavicular space, located posterior and deep to the midpoint of the clavicle. It is bounded anteriorally by the subclavius and the clavicular head of the pectoralis major muscle, and posteriorally by the anterior chest wall at the level of the serratus muscle.2,3 The cords of the brachial plexus pass through this space, together with the subclavian vessels, which at this point are known as the axillary vessels. At this level, the cords of the brachial

200 plexus always lie lateral (latero-posterior) to the axillary artery. They are clustered together and more superficial3,4 with respect to the traditional descriptions of the coracoid infraclavicular block in the parasagittal plane, which is deep to the pectoralis minor. These anatomical features make the costoclavicular space an attractive site for ultrasound imaging, and several ultrasound-guided approaches have been described: posterior infraclavicular block,5 retroclavicular block6 and, more recently, costoclavicular block.2,3 A lateral approach to ultrasound-guided costoclavicular block has been described, in which needle is inserted in-plane from a lateral to medial direction.2 In this technique, the entry point of the needle is often limited by the coracoid process, and it is directed towards vessels and pleura. These risks could be avoided by using a medial to lateral access. Since their introduction, the effectiveness of regional nerve block techniques has been measured by sensory and motor parameters, such as the pinprick test and response elicited from certain muscle groups.7,8 As nerve cords are formed of sensory, motor and autonomic nerve fibres9 the brachial plexus block also induces a sympatholytic effect that can cause vasodilation, with a corresponding increase in local temperature and peripheral perfusion distal to the blockade site.10---12 Skin temperature measurement is a simple method of assessing changes in perfusion, and nerve block is achieved with a increases of over 1 ◦ C.13 Alterations in blood flow have traditionally been measured by Doppler effect, and today this determinations can be performed in the operating room itself.11,14 Finally, the development of non-invasive haemodynamic monitoring based on pulse oximetry has led to the introduction of the perfusion index (PI).15 The PI is the ratio of pulsatile to non-pulsatile blood flow, and is an indication of peripheral blood flow. After the onset of sympathetic blockade, PI values have been observed to increase up to 100% over baseline.12,15 Our objective was to describe the medial approach to ultrasound-guided costoclavicular block, and to evaluate the extent of analgesia by measuring motor, sensory and sympathetic parameters. The quality of sympathetic blockade was determined by measuring blood flow in the brachial artery at the level of the elbow, PI using a thumb sensor, and skin temperature on the palm of the hand.

Methods First phase (anatomical) Following approval by the clinical research ethics committee (CREC), a needle was inserted from medial to lateral in the costoclavicular space under ultrasound guidance in a fresh cryopreserved cadaver. The M-Turbo ultrasound system (Sonosite Inc., Bothell, WA, USA) with a 6---13 MHz linear probe (HFL 38X Sonosite Inc., Bothell, WA, USA) was used. The transducer was placed under and parallel to the clavicle, angled slightly cranial. The following landmarks were identified: second rib, subclavian and serratus muscles superior and inferior to the plexus, respectively; the axillary artery, and lateral to this, the triangle formed by the lateral, medial and posterior cords. An anaesthetist trained in regional anaesthesia performed the technique using a 50 mm neurostimulation needle (Stimuplex D, Braun,

D. Nieuwveld et al. Mengusen, Germany). The needle was inserted in plane from medial to lateral and advanced until the tip was visualised between the artery and the nerve cords. Twenty ml of solution (17 ml saline solution, 2 ml iodinated contrast, and 1 ml methylene blue) was administered. Following this, a computed tomography (CT) image of the anatomical piece was obtained and digitally reconstructed to visualise the pattern of distribution of the contrast medium. After the CT scan, the anatomical piece was frozen at −20 ◦ C for 48 h. Following this, 2---2.5 cm-wide sagittal slices were obtained. The slices were photographed to assess the distribution of the contrast medium in relation to the brachial plexus.

Second stage Following approval by the CREC, 11 ASA I and II patients aged between 18 and 65 years undergoing distal upper limb surgery in the major outpatient surgery unit of a teaching hospital were included in the study. The patients were given information about the study in the surgical admissions lounge and signed an informed consent form before being included. Exclusion criteria were: refusal to participate, allergy to local anaesthetics, coagulopathy or any neurological or vascular pathology that would affect the accuracy of flow measurements, such as dysautonomia, Parkinson’s disease, peripheral arterial disease and neuropathies. Study patients were administered 1 mg of intravenous midazolam and then transferred to the procedure room, which was kept at a constant temperature of 24 ◦ C. PI was monitored on the thumb of both hands using a pulse oximeter (Oxy-100 pulse oximeter, GIMA SpA, Italy) and palmar skin temperature at the level of the diaphysis of the third metacarpal using a skin thermometer (Phillips Medical Systems, Eindhoven, Holland, accurate to 0.1 ± ◦ C); brachial artery flow was measured at a point 2 cm from the elbow crease using the M-Turbo ultrasound system (Sonosite Inc., Bothell, WA, USA) pre-configured for a vascular examination and a linear HFL 38X/13---16 MHz transducer. The axial section of the artery was visualised to measure the diameter, following which the longitudinal section was identified and arterial flow measured using pulse wave Doppler. The transducer was placed to obtain an angle of incidence of less than 60◦ to measure over 80% of the lumen of the artery in the Doppler window. After baseline measurements had been recorded, anaesthesia was started. Under sterile conditions, with the patient supine and the upper limb abducted at 90◦ , the ultrasound-guided costoclavicular block was performed using the medial approach described in the anatomical stage above. We identified the axillary artery and, lateral to this, the cords of the brachial plexus (Fig. 1). The neurostimulation needle (Stimuplex Braun 50 mm) was inserted in plane medial to the transducer at an angle of approximately 60◦ and advanced towards the artery and the cords (between the medial and posterior cord, always below the lateral cord). Hydrodissection was usually used to advance the needle between the artery and the lateral cord. We recorded all patient-reported paraesthesia. For added safety, we also used neurostimulation, administering local anaesthesia when motor response disappeared at 0.3 mA or less (stimulation parameters 2 Hz and 100 ms duration). If aspiration was negative, all patients received 20 ml 1.5% mepivacaine

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Figure 1 Location of the ultrasound transducer in the costoclavicular approach (A) and ultrasound window obtained for the costoclavicular block (B1), showing the disposition of the plexus lateral to the axillary artery (B2). A. Ax.: axillary artery; FL: lateral cord; FM: medial cord; FP: posterior cord; PB: brachial plexus.

in 1 min. Following administration, the spread of the block was monitored a 5, 15 and 15 min. We scored the extent of anaesthesia in each territory studied on a scale of 0---3 (0 = complete block, 3 = no block). Motor blockade was measured according to muscle contraction, with 3 being normal contraction against resistance, 2 weak contraction against resistance, 1 movement with no resistance, and 0 incapacity to move the muscle. Muscles assessed for motor function were the biceps (C6), triceps (C7) and hand flexors (C8). Sensory blockade was scored using the pinprick method, with 3 being normal sensation, 2 reduced sensitivity with less sensation of pain, 1 sensitivity with no painful sensation, and 0 absence of sensitivity. Sensory blockade was measured on the back of the hand, between the thumb and forefinger (C6), on the tip of the third (C7) and fifth (C8) fingers. Blockade was considered successful when sensitivity at 25 min of anaesthesia administration was less than 2 (0---1) in all territories. Sympathetic, temperature and PI parameters were measured simultaneously on the blocked and contralateral limb. Post-blockade brachial artery flow was measured at baseline and at 25 min. We based sample size calculation on regional haemodynamic changes, mainly indicated in our case by brachial artery blood flow. In their pilot study, Li et al.11 showed that brachial artery flow measured in 4 patients was tripled, so a minimum of 5 patients were needed to obtain a power of 80%. We decided to recruit 11 patients. Statistical analysis was performed using BM SPSS Statistics version 20.0. Given the characteristics of the sample, nonparametric statistical tests were applied. The chi-square test or Fisher’s test were used to analyse qualitative variables. Differences in quantitative variables were compared using the Wilcoxon test. Statistical significance was set at p < 0.05.

Results The infraclavicular brachial plexus block in the costoclavicular space performed on a fresh cadaver provided adequate ultrasound visualisation of anatomical reference structures: second rib, pleura, serratus, subclavian and pectoralis major muscles. Using an in plane medial approach, we obtained adequate visualisation of the path of the needle until the tip was positioned between the axillary artery and the lateral,

medial and posterior cords. Twenty ml of anaesthetic was administered under direct view, observing good distribution around these vascular-nervous structures (Fig. 2). The digital reconstruction of the CT image showed an antero-posterior spread of anaesthetic injected at the periclavicular level, reaching the cords. Anteriorally, the anaesthetic reached the pleural dome and interscalene region (at the level of the primary trunks), and posteriorally it reaches beyond the second rib, under the pectoralis minor, at the level of the cords (Fig. 3A). The anatomical images obtained from the sagittal sections of the limb showed excellent correlation with CT images. The anaesthetic was seen to extend from the interscalene region proximally, and distally to the deep axillary space and pectoralis minor. Adequate staining of all components of the brachial plexus was observed (Fig. 3B).

Clinical study Eleven patients undergoing upper limb surgery using infraclavicular brachial plexus block were included. Six patients were classed ASA I and 5 ASA II. Six patients were women and 5 men, with a mean age of 46 ± 18 years, weight 70 ± 15 kg, height 168 ± 12 cm, with a body mass index of 24 ± 3. Nerve block was performed within 7.5 ± 3.5 min without incident in all patients. Surgery involved the right extremity in 3 patients and the left in 8. One of the patients (9%) reported paraesthesia during needle insertion. In 5 patients (45%), no motor response was observed at up to 1 mA, while the remaining 6 (55%) presented motor response to stimulation at >0.3 mA. Nerve block was effective in 91% of patients (10/11) at 25 min after needle insertion. Three patients (27%) presented sensory block between C6 and C8 at 5 min, and 8 (78%) at 15 min. At 5 min post-injection, 1 patient presented complete motor block. This increased to 7 at 15 min, and 10 at 25 min (Fig. 4). In the remaining patient, motor block was still incomplete at 25 min, and required supplemental radial nerve block above the elbow. No blockade-related complications were observed. Palmar temperature of the blocked limb increased significantly from 5 min post-blockade (32.1 ± 2 ◦ C to 32.8 ± 9 ◦ C;

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Figure 2 Ultrasound image obtained from the cadaver puncture, showing the axillary artery and adjacent nerve cords, with the needle between the plexus cords. Anaesthesia volume is shown surrounding the plexus cords. A. Ax.: axillary artery; FL: lateral cord; FM: medial cord; FP: posterior cord; PB: brachial plexus;

Figure 3 CT reconstruction of volume (A) and sagittal section of puncture site obtained from the anatomical piece (B). Arrows: extent of spread shown by CT contrast. A. Ax.: axillary artery; FL: lateral cord; FM: medial cord: FP: posterior cord; PB: brachial plexus; 1: clavicle; 2: 1st rib; 3: sternum; 4: coracoid process; 5: 2nd rib. Sensory blockade

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Figure 4 Changes in sensory and motor blockade after block costoclavicular. Mean results. Sensory block measure on hand (C6 in posterior interdigital area between thumb and second finger; C7 in distal phalanx of the third finger, and C8 in distal phalanx of fifth finger). Motor blockade (C6 in forearm flexion: biceps brachii; C7 in forearm extension: triceps brachii and C8 in flexion of fingers).

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Figure 5 Boxplot showing changes in palmar temperature and perfusion index obtained in ipsilateral and contralateral arm after costoclavicular block, from baseline to 5, 15 and 25 min after administration of local anaesthetic.

p = 0.029), with no changes in contralateral temperature (33.2 ± 0.9 ◦ C to 33.7 ± 1.6 ◦ C; p = 0.475), as shown in Fig. 5. The diameter of the brachial artery at the elbow did not significantly increase after onset of blockade (36 ± 7 mm to 38 ± 8 mm; p = 0.141), while flow was significantly increased (108 ± 86 ml/min to 188 ± 141 ml/min; p = 0.05). In the blocked limb, PI significantly increased at 5 min postblockade, from 4 ± 3 to 9 ± 5 in the thumb; (p = 0.003), and 12 ± 5 at 15 min in the index finger (p = 0.003). No significant changes were observed in the contralateral limb at 5 min (PI 5 ± 5 to 5 ± 4; p = 0.386), or 15 min, (PI 5 ± 5 to 5 ± 4; p = 0.656 post-blockade (Fig. 5).

Discussion Our series shows that the medial costoclavicular approach proposed is both anatomically feasible and highly effective (>90% onset at 25 min) even at low volumes (20 ml). It also shows that onset of sympathetic blockade can be evaluated using the three study parameters: temperature, blood flow and PI. PI is a simple, continuous monitoring parameter that we believe has considerable potential. Factors such a visualisation, puncture site, direction, needle path, and volume of local anaesthetic used will, in the hands of an expert, determine the safety of the technique. During the anatomical phase of the study, we obtained good visualisation of the cords lying lateral to the brachial

artery. As described by Karmakar et al.2 and Sala-Blanch et al.,3 the disposition of the cords in the costoclavicular space marks the difference between this and other, more distal approaches, in which they are clustered around the axillary artery.16 In 1973, in the early years of modern regional neurostimulation-guided anaesthesia, Raj et al.17 described the distal infraclavicular approach from a medial to lateral direction to avoid the pleura, which is similar to the approach described by us. This approach is made safer today by the introduction of ultrasound to visualise and avoid blood vessels during needle insertion. Patient positioning in Raj et al.17 is the same as that used in our study, and has been shown to reduce the depth of neural and vascular structures.18 However, we used a slightly more cranial needle path, which allowed the local anaesthetic to spread to both infraclavicular and supraclavicular territories (Fig. 3). When situating the needle in the centre of the neurovascular bundle it is also important to bear in mind the continuous fascia-enclosed space described by Winnie et al.19 in 1964, although with the introduction of ultrasound guidance, anaesthetists tend now to focus more on the nerve structures. This is why anaesthetic administered at this site is able to spread through the sheath from the interscalene to the infraclavicular region distal to the anatomical piece, thus reducing anaesthesia requirements from the 30 ml traditionally needed for this approach8 to the 20 ml used in our series to achieve complete blockade. The anaesthetic could also spread to the suprascapular

204 nerve, suggesting that this technique could also be useful in shoulder surgery. Further studies are needed to confirm this hypothesis. In the second stage, we observed a high success rate, with complete blockade achieved in 91% of patients at 25 min, although onset in 78% of patients occurred within 15 min. This shows that onset of blockade is as rapid as other nerve block approaches used. Nevertheless, certain strategies can improve visualisation of the deep structures targeted in this approach, such as abduction of the arm18 and the craniomedial angle of the transducer (parallel to the long axis of the clavicle) with a slight cranial tilt, which visualises the subclavian artery while excluding the cephalic vein from the ultrasound window. Monitoring the changes caused by sympathetic blockade showed a high correlation with onset of motor and sensory blockade. Other authors have shown that these changes are manifest at 10 min from administration of the block.15 In our patients, we observed significant increases in both temperature and PI 5 min after administration. Brachial artery blood flow, measured after onset of blockade (25 min after administration) to monitor local perfusion changes, almost doubled in all patients. However, it is important to note that changes also occurred in the patient in whom blockade was unsuccessful. This is to be expected in the case of PI, as the pulse oximeter was affixed to the thumb (lateral cord). Brachial artery flow also increased, as all cords can influence upper extremity arteriolar tone. The advantage of combining skin temperature and PI is that both parameters are non-invasive, objective and observerindependent, they do not require patient cooperation, and can be measured continuously. This makes PI an attractive technique for simple, reproducible evaluation and quantification of regional changes in blood flow secondary to onset of nerve block. This technique could also be useful in surgeries that benefit from increased regional circulation, such as vascular procedures or vascular grafts.20 Further studies will determine the differences between different brachial plexus approaches and will provide us will more information to enable us to choose the best anaesthesia strategy for each patient. Our study has several limitations. It is observational with a limited sample size, and our results need to be corroborated in larger series. It would also be advisable to carry out comparative studies to determine whether our approach is superior to other brachial plexus block techniques. Although none of our patients presented any intra- or postoperative complications, our sample was too small to characterise the safety of this approach. Perfusion changes should ideally be monitored in all territories in order to detect specific local changes. This was not done in our study. In conclusion, this new brachial plexus block using 20 ml of 1.5% mepivacaine proved effective in achieving anaesthesia of the upper extremity. Changes secondary to the sympathetic blockade can be monitored by measuring brachial plexus blood flow, PI and skin temperature. In view of the associated sympathetic blockade, this technique should be considered useful, together with motor and sensory response, for determining onset of blockade.

D. Nieuwveld et al.

Ethical responsibilities Protection of human and animal rights The authors declare that the procedures followed were in accordance with the ethical standards of the responsible committee on human experimentation and with those of the World Health Organisation and the Helsinki Declaration. Confidentiality of data. The authors declare that they have followed the protocols implemented in their place of work regarding the use of patient data in publications. Right to privacy and informed consent. The authors have obtained the informed consent of all patients and/or subjects included in this manuscript. The informed consent forms can be obtained from the author for correspondence.

Authorship Dr Daniela Nieuwveld, contribution: study design, patient recruitment, monitoring and data collection, analysis of results, preparation of the text and final approval of the manuscript. Dr Viviana Mojica, contribution: monitoring and data collection, preparation of the text and final approval of the manuscript. Dr Ana Eugenia Herrera, contribution: monitoring and data collection, preparation of the text and final approval of the manuscript. Dr. Jaume Pomés, contribution: data collection (cadaver) and final approval of the text. Professor Alberto Prats, contribution: data collection (cadaver) and final approval of the text. Dr. Xavier Sala-Blanch, contribution: study design, monitoring and data collection, analysis of results, preparation of the text and final approval of the manuscript.

Conflict of interests The authors declare they have no conflicts of interest.

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