Original articleNormative sciatic nerve excursion during a modified straight leg raise test☆
Introduction
Neurodynamic techniques are used frequently in the assessment and treatment of patients with neural mechanosensitivity (Butler, 2000, Coppieters and Alshami, 2007, Bialosky et al., 2009, Nee et al., 2012). The straight leg raise test (SLR) is a long established measure of mechanosensitivity of the lumbosacral plexus, sciatic nerve and its distal extensions (Butler, 2000, Boyd et al., 2009, Walsh and Hall, 2009). However, there is limited research investigating normal sciatic nerve excursion during the test (Smith, 1956, Breig and Marions, 1963, Goddard and Reid, 1965, Coppieters et al., 2006). During SLR, cadaveric studies have found the sciatic nerve to move between 4 mm (Goddard and Reid, 1965) and 28 mm (Coppieters et al., 2006). Goddard and Reid (1965) investigated sciatic nerve excursion at the sacral ala in 30 cadavers and found the sciatic nerve to move distally between 4 and 7.5 mm. These figures were markedly less than Coppieters et al. (2006), who found 28 mm of proximal nerve excursion during the SLR with the ankle dorsiflexed in 8 male cadavers 2 cm distal to the ischial tuberosity. Greater excursion found by Coppieters et al. (2006) could be explained by two factors. Firstly, measurements were taken closer to the moving joint by Coppieters et al. (2006), where the nerve is more compliant (Dilley et al., 2003, Phillips et al., 2004, Boyd et al., 2005). Secondly, measurements taken by Goddard and Reid (1965) were close to where the sciatic nerve branches from the lumbosacral plexus, which could have resulted in less excursion (Millesi et al., 1995).
The major limitation of these latter two studies is their applicability to living humans. Since living tissue is likely to have marked differences in its response to loading and movement it is not possible to extrapolate such findings directly to living individuals.
A recent in vivo study using B mode ultrasound found less than 2 mm of distal tibial nerve excursion behind the knee with the limb positioned in SLR and active ankle movements performed (Boyd et al., 2012). Sciatic nerve excursion in the posterior thigh during a modified slump test has been found to be in the region of 3 mm (Ellis et al., 2012). Slightly greater excursion was found during a slider manoeuvre (movements in which elongation of the nerve bed at one joint is simultaneously counterbalanced by a reduction in the length of the nerve bed at a distant joint) than a tensioner manoeuvre (manoeuvres in which the combination of joint movements elongate the nerve bed), suggesting that pre-tensioning results in reduced excursion. However, the difference in excursion between the two manoeuvres was around 0.6 mm which is unlikely to be of clinical significance, and only just above the smallest detectable difference (SDD) (0.55 mm) (calculated from standard error of measurement (SEM) 0.2 mm in Ellis et al., 2012).
Whilst Ellis et al. (2012) and Boyd et al. (2012) have initiated some in vivo work on the sciatic and tibial nerves, the very small numbers used by Boyd et al. (2012) and choice of slump test in Ellis et al.'s (2012) study justify further investigation of SLR. Heterogeneity in lower limb range of movement is not uncommon in asymptomatic individuals and has been linked to demographic factors such as age (Nolan et al., 2010). In addition, slightly greater sciatic nerve excursion has been found in younger cadavers compared to older cadavers during SLR (Goddard and Reid, 1965). Another factor which has not been investigated in relation to nerve excursion is height. Whilst range of SLR was not found to be related to height (Boyd and Villa, 2012), this does not rule out a relationship between height and nerve excursion since factors other than nerve excursion could limit the overall range in SLR.
The aim of the present study was to provide preliminary normative data on longitudinal and transverse plane movements of the sciatic nerve measured in the mid-posterior thigh of asymptomatic individuals during a modified SLR. In addition it aimed to assess if demographic factors such as age or height affected the amount of nerve excursion available. The clinical interest of these values will be the comparison with symptomatic groups in future studies and the assessment of normal variation between individuals. In addition since the change in the initial position and second positions chosen (hip flexed to 30 and subsequently 60°) represent a more progressive SLR test, differences in amounts of nerve excursion between the 2 positions were explored. The results will add to the limited in vivo data available on excursion values during tensioners and sliders during SLR.
Section snippets
Subjects
Eighteen asymptomatic subjects (9 men, 9 women, mean age 28.9 (14.3), range 19–68 years) were recruited for the study. All subjects read an information sheet and signed a consent form prior to participating. Exclusion criteria included any neuromusculoskeletal conditions to the spine or lower limbs within the last year, systemic disorders such as rheumatoid arthritis and pregnancy. In addition, subjects were required to have normal neurological integrity test (detailed under procedure) and full
Results
The results from 2 subjects were excluded as the image quality was too poor for the motion analysis software to track longitudinal excursion of the nerve effectively. Table 1 shows the demographics of the remaining 16 subjects.
Discussion
This study demonstrates the wide variation in normative sciatic nerve excursion in both transverse and longitudinal planes. The most consistent transverse plane movement of the nerve during knee extension was superficial. Boyd et al. (2012) found a consistent superficial movement of the tibial nerve at the popliteal fossa during ankle dorsiflexion. The reason for this is unclear but could be related to relative positioning of the nerve to the femur or moving joint, or muscular effects on the
Conclusions
There is marked variability in the amount of longitudinal and transverse nerve movement between individuals. No significant association between age or height was found to explain these variations. A significant difference between the two positions was found indicating that with greater pre-loading, more nerve excursion occurred. This finding is in contrast to other studies which have demonstrated a decrease in excursion with preloading. However, only one aspect of the nerve was analysed, and it
Acknowledgements
The authors wish to thank Kayleigh Quaife for her role as research assistant.
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Work attributed to: Clinical Research Centre for Health Professions, University of Brighton, Aldro Building, 49 Darley Rd., Eastbourne, BN20 7UR, UK.