Letter to the Editor regarding a study titled "A modified straight leg raise test to differentiate between sural nerve pathology and Achilles tendinopathy. A cross sectional cadaver study" by Coppieters et al. (2015)
I would like to commend Coppieters et al. (2015) for investigating the variations in strain and excursion of the sural nerve and Achilles tendon during a modified straight leg raise test. Indeed, this article makes an important contribution to the literature on lower limb neural biomechanics as there are not many cadaveric studies in this area. The authors have used a robust and feasible method to answer the research question. Pragmatically, positioning the cadaver in side lying appears suitable for doing a modified straight leg raise test. If the reliability of goniometer readings in measuring hip and ankle angles would have been analysed then this would have been an added strength to the study because these measurements are the predictors of strain and excursion in the sural nerve and Achilles tendon in this study. Whether calibration of the strain gauges and digital calliper was done before and/or during the study to determine the accuracy/precision of the equipment was not reported. The cumulative increase in strain or mm? induced in the sural nerve was 1.2 (0.6) (mean (SD) with ankle dorsiflexion and 2.1 (0.5) with hip flexion and ankle dorsiflexion. The mean excursion (proximal sliding) of the sural nerve with ankle dorsiflexion was found to be 1.2 mm with a standard deviation of 1.8 mm. Why the standard deviation value (1.8 mm) was larger than the mean value (1.2 mm) should be discussed. As three repetitions were used for documenting strain and excursion of the sural nerve and Achilles tendon, would reporting ICC (2,3) be adequate rather than calculating both ICC (2,1) and ICC (2,3) values? SEM repeated formula (standard error of measurement [SEM] value multiplied by the square root of the number of repetitions) has been recommended for calculating measurement error associated with repeated measures (Domholdt, 2005) This value will be larger than SEM values reported by the authors. Further, calculating smallest real difference or minimal detectable change values at a 95% confidence interval will clarify whether the measured values (outcome measures) which were relatively small in the current study, show a real difference that is free from measurement error. The authors have discussed that one of the reasons for the poor reliability of the strain measurement of the Achilles tendon following ankle dorsiflexion during the modified straight leg raise test was small inter cadaver variability when compared to the within cadaver variation. Was there a lack of consistent and linear relationship between the applied tension or tendon stretch with ankle dorsiflexion and corresponding change in the strain of the Achilles tendon in the cadavers? From Fig. 2, it is evident that the mean strain value of the Achilles tendon with ankle dorsiflexion is negligible (1.4% [0.1% If so, discussing the biomechanical behaviour of the Achilles tendon in embalmed cadavers will be useful. In Fig. 2, the scale of the vertical (Y) axis reads as cumulative increase in strain in percentage however, in the discussion section, the standard deviation values of the strain measurements have been reported in mm. This needs further clarification.
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