Anatomical comparison of sciatic nerves between adults and newborns: clinical implications for ultrasound guided block

Authors


Abstract

The sciatic nerve (SN) is easily blocked under ultrasound guidance by identifying either the SN common trunk or its two components: the tibial nerve (TN) and the common peroneal nerve (CPN). The authors investigate whether there are anatomical differences between newborns and adults. The SN, TN and CPN of both lower extremities in 24 (11 neonatal and 13 adults) formolized cadavers were dissected. Distances were measured from the origin of the SN (passing under the piriformis muscle) to its division into TN and CPN, and from there to the popliteal crease. The sciatic/thigh coefficient (proportion relating SN length to thigh length) and the variation coefficient for the SN were calculated. The distance from the popliteal crease to the SN division was significantly shorter in neonates than in adults (1.04 ± 0.9 cm vs. 5.6 ± 5.1 cm, P = 0.0003). In addition, the neonatal SN divided at a proportionally more distal position in the thigh than it did in adults (86 ± 13 vs. 74 ± 15%, P = 0.0059). However, the coefficient of variation between the SN-division distances was not statistically different in infants and adults (12.8 vs. 18.2%, P = 0.4345). The variations in the point of SN division seen in the adult SN are already seen in the neonatal period, but in newborns the SN divided in a more distal position in relation to the thigh than in adults, so this finding of anatomical variability in neonates suggests that ultrasound guidance can be useful when performing a SN block in these small patients.

Introduction

The use of regional block techniques, especially since the introduction of ultrasound (US), has expanded rapidly, even in very young children (de Josemaría et al. 2009).

Sciatic nerve (SN) blocks at the popliteal fossa are very useful in the pediatric population, primarily in orthopedic and plastic surgery interventions (Van Geffen et al. 2010). It has recently been reported that performing a successful popliteal sciatic block involves the injection of local anesthetic drugs around both nervous components of the SN bifurcation: the posterior tibial nerve (TN) and the common peroneal nerve (CPN; deTran et al. 2011). In any case, the intention is usually to block both components of the SN with a single injection. Unfortunately, there is significant anatomic variability in the division of the SN, since, in the general population, the nerve can be found dividing into its two components, from as proximal a level as the gluteal area (shortly after passing through the piriformis muscle) in 16% of cases, to one as distal as the popliteal fossa in 34% of the cases (Prakash et al. 2010). This wide anatomic variability is why US is currently recommended to identify the exact location of sciatic nerve components when performing an anesthetic block (Perlas et al. 2008).

Additionally, applying regional anesthesia techniques in children requires knowledge of the physiological, pharmacological and anatomical differences between children and adult patients (Ivani & Mosseti, 2009). Attention to anatomical detail remains the key element for successful outcome in pediatric regional anesthesia (Bosenberg, 2012). However, the specific anatomical differences between children and adults have been little studied insofar as the division of the SN in the thigh, which usually occurs near the popliteal crease. This paper examines these differences and attempts to clarify whether this variability in the division of the nerve is already present at birth, in which case this age group would also benefit from imaging techniques like US when receiving a sciatic block.

Methods

The lower extremities of 13 adult and 11 newborn cadavers were obtained for dissection of the posterior aspect of the thigh. The individuals had died 6 months previously and were free of gross or congenital malformations. They had been embalmed for anatomical purposes in a solution of phenol (13%) as the primary fixative and glycerin (28%) for water content retention. The cadavers had been donated by their respective families to the School of Medicine for use in anatomical studies.

Prior to dissection, and in a prone position, the following measurements were recorded in all bodies: height (distance between the most superior point of cranium and the calcaneus bone), leg length (distance between coccyx and calcaneus bone), and thigh length (distance between coccyx and popliteal crease).

The SN was dissected from its exit below the piriformis muscle to its arrival at the tibia, in the case of the posterior tibial nerve component of the TN, or until the fibula for the CPN. A vertical incision was made in the posterior midline of the thigh together with another three horizontal incisions, the first at the level of the posterior iliac spine, the second in the sub-gluteal line, and the third at the sub-popliteal level. The skin and subcutaneous tissue of the buttock, thigh and upper third of the calf were displaced laterally. Once the muscular plane was exposed, the gluteus maximus and gluteus minimus were dissected and displaced medially (leaving their insertion in the sacrum and coccyx bones). In the same way, the semimembranosus and semitendinosus muscles were disconnected from their insertions on the ischial tuberosity and displaced medially. Finally, the femoral biceps muscle was dissected from the femoral trochanter and displaced laterally to reveal the nerve. The SN was dissected, resecting the epineural sheath, to locate the exact point at which the neural component clearly split into a medial TN and a lateral CPN component: this point was defined as the division of the SN. Distances were measured with a caliper between the SN exit below the inferior edge of the piriformis muscle and the division point (length of the SN) and from the point of division to the popliteal crease. The coefficient of variation was calculated with the following formula: standard deviation/mean × 100.

With the purpose of comparing the two age-groups, the following coefficients were calculated in all patients: leg/height coefficient (coccyx–calcaneus distance/cranium–calcaneus distance × 100), thigh/leg coefficient (coccyx–popliteal crease distance/coccyx–calcaneus distance × 100), sciatic/thigh coefficient (distance between the lower edge of the piriformis muscle and the SN division/distance between coccyx and popliteal crease × 100).

Data were compared between the two groups (adults and infants) with a multifactor anova test or the Fisher test. P-values ≤ 0.05 were considered statistically significant. All analyses were performed with statview for Windows™ (Version 4.5).

Results

Forty-eight legs from 24 bodies were examined (demographic data are shown in Table 1). In the case of adult cadavers, the specimens mainly corresponded to elderly individuals who had died of natural causes; in the case of children, they belonged to infants who had died within hours or days after a term delivery.

Table 1. Characteristics of specimens dissected.
 AdultsNeonates
  1. Data are expressed in number of patients or as mean ± standard error for age and weight and median ± interquartile values for distances.

n 1311
Sex (male/female)6/73/8
Age78 (7) years7 (6) days
Weight (kg)65 (8.2)3.3 (0.5)
Height (skull–calcaneus; cm)147.8 (137–159.1)48.4 (47.8–50.9)
Leg length (coccyx–calcaneus; cm)80.5 (75.2–87)20.2 (19–21.7)
Thigh length (coccyx–popliteal crease; cm)40.1(38.8–43.4)10.2 (10–10.5)

The dissection of muscle and nerve tissues was carried out without difficulty, so the TN, the CPN, and the division of the SN were easily identified in all dissected specimens (Fig. 1). The distance between the exits of the SN under the piriformis muscle and its division into TN and CPN, and the distance between these divisions of the SN to the popliteal crease on the 48 legs are shown in Fig. 2. The mean distance between the exits of the SN under the piriformis muscle and its division into TN and CPN was significantly shorter in neonates than in adults (8.6 ± 1.1 cm vs. 29.8 ± 5.4 cm, P < 0.0001) and the mean distance from the popliteal crease to the SN division was significantly shorter in neonates than in adults (1.04 ± 0.9 cm vs. 5.6 ± 5.1 cm, P = 0.0003). The variations in the length of the sciatic nerve were not statistically different between adult and neonatal cadavers (coefficients of variation of 18.2 vs. 12.8%, P = 0.4345).

Figure 1.

Division of the sciatic nerve in the thigh of two cadaveric specimens: (A) neonatal, (B) adult. Posterior gluteal area, posterior femoral area and popliteal fossa were dissected showing piriformis muscle (Pi. M), sciatic nerve (SN) dividing into tibial (TN) and common fibular nerve (CFN) at different distances above the popliteal crease (Po. crease). Scale bar: 5 cm.

Figure 2.

Division of the sciatic nerve in the posterior femoral region in individual leg specimens. Horizontal bars indicate distances in sciatic nerve between the piriformis muscle, the SN division and popliteal crease.

The thigh length was half of the leg length in both adults and infants, so the thigh/leg coefficient was identical in both groups (51 ± 3%, P = 0.7284). However, adults had proportionally longer lower extremities, so the leg/height coefficient was greater in adults than in neonates (55 ± 3 vs. 42 ± 3%, P < 0.0001). Finally, when studying the relative position of the division of the SN in the length of the thigh, it was observed that the SN split more proximally in adults, as the sciatic/thigh coefficient was significantly lower in adults compared with neonates (74 ± 15 vs. 86 ± 13%, P = 0.0059; Fig. 3).

Figure 3.

Relative position between the piriformis muscle and the popliteal crease at which the sciatic nerve was found divided in each specimen of both groups. *P = 0.0059.

Discussion

Popliteal sciatic nerve block has many indications for intraoperative (Van Geffen et al. 2010) and postoperative analgesia (Ponde et al. 2011) in children, but there is an increasing trend toward highlighting the importance of identifying the SN bifurcation by US in order to place the local anesthetic drugs either proximally or exactly in the division itself to improve the efficacy of the block (Ponde et al. 2010). However, little or no evidence exists about what the basic anatomy of the SN division in children is at the moment of birth. Information derived from anatomic dissection studies in this age group could be relevant for specialists involved in anesthesia and/or surgery on children's legs.

The same methodology was used for distance measurement and cadaver dissection in both age groups in this study to make comparison between adult and neonatal specimens possible. We wished to ascertain whether the known variability in the site of the sciatic division in adults was congenital (observed from birth) or acquired (observed only after adulthood). The results seem to indicate that the variability seen in adults may be provoked by a combination of at least two mechanisms. First, when comparing the distribution of the SN division (Fig. 2) there was a comparable variability in adults and in infants because the variation coefficients of the SN length (before division) were quite similar, although slightly higher in adults (18.2%) than children (12.8%). This means there is some variability in the structure of the SN at birth that is maintained in adulthood. But, in addition, there was also an age-dependent phenomenon: a more proximal nerve division in adults as compared with newborns (Fig. 3), because the sciatic/thigh coefficients were 74 ± 15 vs. 86 ± 13%, respectively. Thus, children are born with an SN that splits into its two components 12% closer to the popliteal crease than in adults. The limb innervation accompanies the growth of the leg to adult size (leg/height coefficient in adults was 55 ± 3 vs. 42 ± 3% in neonates) and, in most cases, this nerve elongation seems to occur in the most distal components, CPN and TN, rather than the proximal undivided SN.

The results of this study may have important implications for block use. The fact that variability already exists at birth (Fig. 2) would also explain the incidence of block failures (up to 14%) when using ‘blind techniques’ in children (Vas, 2005). This finding also explains the relative clinical usefulness of mathematical formulas based on direct external anatomical references (Bernière et al. 2008) or derived from imaging techniques (Suresh et al. 2007). It is obvious that the greater the known anatomical variability of a nervous structure, the greater the need for US guidance in individual cases (Lönnqvist, 2010).

The more distal division (Fig. 3) and the resulting short distance from the SN split to the popliteal crease in newborns, barely 1 cm, suggests that a distal subgluteal approach in neonates would be as effective as other more proximal but more complex sciatic block approaches (Dalens et al. 1990).

Nevertheless, this study has its logical limitations. These data were obtained from embalmed corpses, and there is a possibility that the embalming technique might provoke anatomical distortions, although, overall, cadaveric anatomical data generally agree with the clinical information (Smoll, 2010) and with data obtained by other, more accurate, imaging techniques such as MRI (Suresh et al. 2007). In addition, the specimens belonged to people who had died in old age or from perinatal problems, so they were obviously not healthy, which might imply some form of bias.

In spite of all the above, it seems appropriate for future verification of the results of this study to systematically perform similar measurements with ultrasonography that compare the sciatic nerve division simultaneously in living infants and adults.

In conclusion, the site of the sciatic nerve division presents a high variation from birth, although in newborns it lies much closer to the popliteal fossa than in adults. The position still varies, so US guidance for sciatic nerve block in this group of the pediatric population would also be highly recommended.

Acknowledgements

The authors would like to acknowledge Ms Carol F. Warren for her language assistance. The authors declare they have not received any financial support, nor do they have any conflict of interest.

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