Determination of the longitudinal median nerve mobility in different neurodynamic techniques

Abstract

Background

Neurodynamic techniques which include tensioning and gliding techniques are being employed in the treatment of carpal tunnel syndrome. There are few in vivo studies that have assessed the longitudinal mobility of the nerve using these techniques. The objective of this study was to determine the longitudinal excursion of the median nerve at the level of the wrist and distal arm by having the cervical spine lateral flexion or the wrist extension as terminal movement.

Method

Twenty healthy participants were included in the study. Techniques 1 and 2 had wrist extension as its terminal movement while techniques 3 and 4 had ipsilateral and contralateral neck lateral flexion as its terminal motion. Median nerve longitudinal excursion was determined using dynamic ultrasound and was measured by a motion tracking analysis program employing a fast template tracking method.

Results

Regardless of neurodynamic techniques, longitudinal mobility is highest at the wrist and arm level if the terminal movement is wrist extension. Median nerve excursion at the wrist and arm levels is 15.53 ± 7.04 mm and 6.82 ± 2.97 mm for technique 1 and 13.43 ± 5.64 and 5.33 ± 2.37 mm for technique 2, respectively. There was a significant decrease in median nerve excursion at the wrist level when the terminal movement was at the cervical spine.

Conclusion

The largest median nerve excursion in the arm and wrist occurred when wrist extension is the terminal movement. Contralateral cervical lateral flexion with a prepositioned extended wrist produced the least motion of the median nerve at both sites.

Keywords

Median nerve mobility dynamic ultrasound motion tracking analysis program (MTAP)

Introduction

Carpal tunnel syndrome (CTS) is a common entrapment neuropathy affecting the median nerve as it passes under the flexor retinaculum at the wrist.^1,2 The prevalence of carpal tunnel syndrome is 3.8% and is more common in women than men.^3–6 There are anatomic, systemic, and occupational factors which are putative factors associated with its cause but the precise etiology is unknown.⁷ A hypothesis on its etiology is that inflammatory and non-inflammatory processes could lead to fibrosis of the subsynovial connective tissue and intraneural tissues which alter the longitudinal and transverse motion of the median nerve and flexor tendons during hand movement. The increase in local strain may impair nerve conduction velocity and blood flow which may result in symptoms.^4,8–12

Neurodynamic techniques are being employed in the treatment of CTS.⁴ It has been suggested that these techniques alleviate the symptoms by improving intraneural circulation, axoplasmic flow, neural connective tissue viscoelasticity; and by reducing sensitivity of abnormal impulse generating sites.¹³ These techniques attempt to move the nerve in tension and stretch it by appropriate mobilization through different postures, followed by applying slow, rhythmic movements directed towards the spinal cord and peripheral nerves.¹⁴ These mechanisms of actions still require robust validation and are mainly based on anecdotal evidence.¹⁵ However, few studies have looked indirectly at improvement of intraneural circulation in using neurodynamic techniques.^16,17 There are two techniques being used which is either a tensioning or a gliding technique. The tensioning technique creates strain within the nerve by pulling both ends of the nerve simultaneously.¹⁵ However, there could be an exacerbation of symptoms because of production of ectopic discharges from mechanosensitive abnormal impulse generating sites when the nerve is maximally strained.³ An alternative to this is the gliding technique which is a combination of movements where elongation of the nerve bed at one joint is simultaneously counterbalanced by a reduction in the length of the nerve bed at an adjacent joint. The gliding technique produces larger longitudinal excursion of the nerve as compared with the tensioning technique but causes less nerve tension.^3,18 This technique is less aggressive as compared with the tension technique.^3,18 Some studies have examined the longitudinal mobility of the median nerve using nerve mobility techniques with dynamic musculoskeletal ultrasound.^18,19 A study by Coppieters et al.¹⁸ assessed the longitudinal excursion of the median nerve across the upper arm comparing median nerve gliding and tension techniques. It showed that the excursion associated with the gliding technique was significantly larger than the excursion with any other techniques while the tensioning technique resulted in the smallest excursion. A study by Brockwicz et al.¹⁹ determined the effect of cervical movements in the longitudinal mobility of the nerve at the level of distal and proximal forearm and showed minimal excursion of the nerve on both levels. Furthermore, it has been postulated that the nerve behaves like a continuous spring under tension and stretch is distributed along its entire length when mobilization techniques are applied.¹⁸ However, studies have shown that the nerve does not act as a uniform spring because strain is not evenly distributed in the entire nerve and studies have reported varying results on which of the segment of the nerve had the greatest strain.^20–23 Nevertheless, only the study by Dilley et al.²⁰ was an in vivo study using dynamic ultrasound.²⁰ The other studies were cadaveric studies where tension was measured using either a microstrain gauge or buckle force transducers attached to the median nerve.^21–23

There is no study to our knowledge that has reported the effects of the cervical and wrist movement on the different segments of the median nerve. This would be of interest because the results would support the clinical decision-making on which technique provides the optimum longitudinal excursion of the median nerve. Likewise, it would test the hypothesis that different segments of the nerve have varying lengths of longitudinal excursion. Our study aims were to determine the amount of longitudinal nerve excursion in vivo of two segments of the median nerve using high-resolution ultrasound during different neurodynamic techniques and to examine the association of different anthropometric measures with the longitudinal excursion of the median nerve.

Methods

The study obtained approval from the Ethics Committee of the College of Rehabilitation Sciences, University of Santo Tomas. Informed consent was obtained from the participants prior to the investigation.

Participant selection

To determine the sample size for the study, the differences and the variability in longitudinal nerve excursion reported in the study by Coppetiers et al.¹⁸ were used. Sample size required was 18 to detect differences in the excursion of the median nerve in techniques with a power of 90% an α of 0.01.

Twenty healthy participants were recruited over a period of 6 months at the Department of Rehabilitation Medicine, University of Santo Tomas, Manila, Philippines. The participants were men and women aged 18–50 years without any symptoms in the neck and upper extremities.

Anthropometric measures

The following anthropometric measures were obtained: height, weight, arm length (measured from the distance between the acromion process and the tip of the middle finger), and elbow circumference which was taken at the level of the elbow crease with the elbow fully extended.

Wrist and hand measurements included the wrist circumference which was taken between the radial styloid process and the distal wrist crease; wrist depth and the wrist width, measured at the distal flexor wrist crease which is the anatomical landmark of the proximal edge of the flexor retinaculum using a Vernier caliper; and the palm length measurement taken from the distal flexor crease of the wrist to the middle finger metacarpophalangeal crease.

From the anthropometric measurement, body mass index, wrist ratio, and wrist palm ratio were calculated. Body mass index was calculated by dividing weight in kilos by height in meters.² The wrist ratio was defined as the wrist depth divided by the wrist width while the wrist palm ratio was calculated by dividing wrist depth by palm length.²⁴ All the anthropometric measures were done three times and the mean of the three recordings calculated.

Nerve mobility techniques

The participants laid supine on a plinth with the right upper extremity resting on a specially designed frame which provided stabilization and control of the movement of the extremity during the testing. The device had the following: (i) stabilization board; (ii) range of motion control device for the neck and the wrist; (iii) a fabricated T-bar for shoulder depression; and (iv) a brace where the upper extremity was supported. The head of the participant was in a neutral position on a stabilization board with a strap over the forehead which had a lock that limited the range of motion of neck to 45° lateral flexion on the contralateral and ipsilateral side once measured by a standard goniometer.

The shoulder was abducted to 90° and was externally rotated. Then, the fabricated T-bar was adjusted in order that the shoulder was depressed for about 3.8 centimeters. The elbow, metacarpophalangeal, and interphalangeal joints of fingers and thumb were kept in full extension. The frame had a hinge located at the wrist joint which allowed 60° of extension (Figure 1).

Figure 1.

Frame used in performing the neurodynamic technique.

Four techniques were utilized as follows (Figure 2): techniques 1 and 2 passively extended the wrist up to 60° with the cervical spine prepositioned at 45° of ipsilateral lateral flexion (technique 1) or at 45° of contralateral lateral flexion (techniques 2). Technique 3 passively flexed the cervical spine to the ipsilateral side up to 45° with the wrist prepositioned at 60°, while technique 4 passively flexed the cervical spine to the contralateral side up to 45° with the wrist prepositioned at 60°.

Figure 2.

The four neurodynamic techniques utilized in the study.

All the movements were performed and recorded three times. The sequence of techniques was randomly performed using Microsoft Excel random sequence generator.

Testers

The two sonologists who performed dynamic sonographic image acquisition of the median nerve were Rehabilitation Medicine specialists who had been performing musculoskeletal ultrasound for 6 years. The two assessors who measured the movement of the median nerve using motion tracking analysis program (MTAP) were Rehabilitation Medicine residents.

Prior to the study, a reliability study to assess inter-rater and intra-rater reliabilities of measuring longitudinal mobility of the median nerve using MTAP was done and has been published.²⁵ There was moderate agreement between the two sonologists for the median nerve mobility at the level of the arm and the wrist while there was a moderate to almost perfect agreement between the two assessors’ readings in the mobility of the nerve at both levels.

Sonographic image acquisition

Prior to the study, the sonologists discussed the anatomical landmarks that were used in identifying the median nerve in the wrist and distal arm. For the wrist, the surface landmark was the distal wrist crease while the sonographic landmark was the distal end of the radius.²⁶ For the arm, the surface landmark was the elbow crease. Since there is no bony landmark that could be visualized in sonography, the median nerve was identified in cross section as it courses with the brachial artery.

A Sonosite M-Turbo ultrasound machine (FUJIFILM SonoSite, Inc., Bothell, WA) with a 38 mm linear array transducer set to 10 MHz excitation frequency was used in this investigation. The resolution of the ultrasound image was 1280 × 1024 pixels.

The transducer was applied on the volar aspect of wrist. To visualize a longitudinal image of the median nerve, the nerve was initially detected in the cross section and captured in the center of the screen. The probe was then rotated to 90° until a clear longitudinal image of the nerve became visible. The median nerve was identified as it lies superficial to the tendons of flexor digitorum superficialis. The neurodynanic techniques were then performed and the median nerve excursion during these techniques was recorded.

The transducer was then placed at the distal third of the arm. The median nerve was identified in the cross section where it was medial to the brachial artery.²⁷ An area where movement of the transducer was minimal or absent during the wrist movement was identified. This was essential because transverse movement can cause the nerve to move out of plane when the nerve is scanned longitudinally.²⁰ The transducer was then rotated 90° and was aligned along the course of the median nerve to capture the video image. The test movement was performed and repeated three times. Only the right upper extremity was used in the study.

Median nerve image analysis

The image was converted to a DivX image with a resolution of 320 × 240 pixels at 25 frames per second. The video was then converted to 8-bit grayscale. A 151 frame sequence was analyzed using the Motion Tracking Analysis Program (MTAP).

This program uses a fast template tracking method developed and implemented in MatLab™ by Nicoud et al.²⁸ at the Laser Light Scattering & Materials Science Laboratory of the University of South Australia. The method utilizes two-dimensional (2D) normalized cross-correlation analysis and an adaptive template which enables researchers to perform an in vivo measurement of median nerve longitudinal mobility.

The assessors agreed on the feature of interest (FOI) of the median nerve at the level of wrist and distal arm. At the wrist, the feature of interest (FOI) was placed 2–5 cm proximal to the midpoint of the image of the radial head. At the distal arm, the FOI was recorded 2–6 cm from the left lateral edge of the screen of the laptop computer because there was no visible bony landmark that could be used as a reference point. The feature of interest was uniformly measured with a size of 0.3 × 0.3 cm². which only included the nerve fascicles.²⁵

A total of 120 recordings (three repetitions for the wrist and arm for 20 participants) were saved and exported as digital images into a personal computer as uncompressed audio–video interleave (.avi, or “AVI” herein) files. The uncompressed AVIs were then imported into MATLAB (7.5, R2010b; The MathWorks, Inc., Natick, MA) and analyzed with the MTAP application. This adaptive template approach accommodated the changes which occur in the feature being tracked over several frames and improves the tracking²⁸ (Figure 3).

Figure 3.

Fast template tracking of the median nerve at the level of the wrist.

The tracking trajectory generated by the MTAP application was exported to a Microsoft Excel (MS Excel) spreadsheet. The MS Excel spreadsheet stored the trajectory of the template containing the X and Y coordinates for each frame. The initial and final X,Y values were the initial and final pixel positions of the selected template within the video frames. The linear displacement (pixels) between frames was readily calculated from the difference in X,Y pixel values between subsequent frames. The displacement was converted from pixels to millimetres wherein one millimetre is equal to 3.8 pixels. Distal excursion of the nerve was assigned a positive value while a proximal excursion of the nerve was assigned a negative value.

Statistical analysis

All data were entered in a custom-made Excel file. SAS Version 9.2 (2013) was used for analysis. Means and standard deviation were used for the descriptive data. Means, standard deviation, 95% confidence interval, and standard error of measurement were calculated for the mobility of the median nerve. Independent t-test was utilized to determine if there was a statistical difference between genders on the anthropometric measures; and to determine if there is a difference of the median nerve excursion at the wrist and arm for each of the four techniques. One-way repeated measure analysis of variance was utilized to assess the difference in the longitudinal median nerve excursion with the four mobilization techniques at the level of wrist and arm separately. A post-hoc pairwise comparison of means was performed with Bonferroni adjustment to determine if there was statistical difference between techniques. Pearson’s correlation was used to determine the correlation between anthropometric measures and the mobility of the median nerve. A p value of <0.01 was considered significant.

Results

There were 10 men and 10 women in the study with a mean age of 25.5 ± 1.7 and 25.4 ± 2.7, respectively. The men’s anthropometric measures were significantly greater as compared with the women except for body mass index, elbow circumference, wrist ratio, and wrist: palm ratio (Table 1).

Table 1.

Characteristics of participants.

	Male, n = 10 (mean ± SD)	Females, n = 10 (mean ± SD)	p
Age	25.5 ± 1.7	25.4 + 2.7	NS
Height (cm)	1.7 ± 0.08	1.6 ± 0.07	0.0007
Weight (kg)	76.3 + 13.3	55.2 ± 11.5	0.0013
BMI (kg/m²)	25.3 ± 2.3	22.2 ± 3.2	NS
Arm length (cm)	74.0 ± 3.4	66.4 ± 3.0	<0.0001
Elbow circumference (cm)	25.7 ± 1.8	23.3 ± 3.9	NS
Wrist depth (mm)	34.15 ± 2.2	28.8 ± 1.8	<0.0001
Wrist width (mm)	51.5 + 5.9	42.1 + 2.7	0.0006
Wrist ratio (wrist depth/wrist width)	0.66 ± 0.07	0.69 ± 0.03	NS
Palm length (cm)	11.5 ± 0.9	10.2 ± 0.7	0.001
Wrist:palm ratio (wrist depth/palm length)	0.29 ± 0.02	0.29 ± 0.03	NS

Median nerve mobility

Table 2 shows the median nerve excursion at the level of arm and wrist. There was a significant difference in the excursion of the median nerve at both levels with technique 1 having the greatest excursion. All techniques produced movement of the nerve distally except for technique 4 which moved the nerve proximally.

Table 2.

Measurements of median nerve excursions at the level of the arm and wrist, in millimeters (mm).

	Tech1	Tech 2	Tech3	Tech 4	P value	Post hoc test
Wrist level (mm)
Mean ± SD 95% CI SEM	15.53 ± 7.04 12.23–18.83 1.57	13.43 ± 5.64 10.79–16.07 1.26	5.98 ± 2.73 4.71–7.26 0.61	−3.80 ± 0.31 −4.45 to −3.15 0.31	<0.0001	Tech 1 vs. Tech 2 Tech 1 vs. Tech 3 Tech 1 vs. Tech 4 Tech 2 vs. Tech 3 Tech 2 vs. Tech 4 Tech 3 vs. Tech 4	NS p < 0.0001 p < 0.0001 p < 0.0001 p < 0.0001 p < 0.0001
ARM LEVEL (mm)
Mean ± SD 95% CI SEM	6.82 ± 2.97 5.42–8.21 0.66	5.33 ± 2.37 4.22–6.44 0.53	3.52 ± 1.45 2.83–4.20 0.32	−4.87 ± 2.49 −6.93 −3.70 0.56	0.0005	Tech 1 vs. Tech 2 Tech 1 vs. Tech 3 Tech 1 vs. Tech 4 Tech 2 vs. Tech 3 Tech 2 vs. Tech 4 Tech 3 vs. Tech 4	p = 0.001 p = 0.0001 p < 0.0001 p = 0.005 p < 0.0001 p < 0.0001
p value (difference arm and wrist movement in each tech)	<0.0001	<0.0001	0.0008	NS

SEM, standard error of measurement; NS, not significant.

With post-hoc tests, all techniques were statistically different from each other (p value <0.0001) except between techniques 1 and 2 at the level of wrist. At the level of the arm, all techniques were significantly different from each other (Tech 1 versus Tech 2, p = 0.001; Tech 1 versus Tech 3, p = 0.0001; Tech 1 versus Tech 4, Tech 2 versus Tech 4 and Tech 3 versus Tech 4, p < 0.0001; Tech 2 versus Tech 3, p = 0.005). Comparing the excursion of the median nerve at both levels, there was a larger excursion at the wrist as compared with the arm (Tech 1 and Tech 2, p value < 0.0001; Tech 3, p value = 0.0008) except for technique 4 where there was no statistical difference in the excursion of the nerve.

There is no anthropometric measure which correlated with the nerve excursion on arm. Only the arm length had an almost significant correlation with median nerve mobility at the level of the wrist for techniques 2 and 3 with an r = 0.45 (p = 0.06) and r = 0.60 (p = 0.06). Separate analysis for men and women was not done because of the small sample size.

Discussion

Our results clearly showed that regardless of neurodynamic techniques, longitudinal mobility is highest at the wrist and arm level, if the terminal movement is at the wrist. Our study also showed that there is significant decrease in the excursion of the median nerve at the wrist level when the terminal movement occurred in the cervical spine. Our results are similar to the in vitro studies of Dilley and Coppieters that used ultrasonography to measure longitudinal mobility of the nerve.^18,20 In the study by Dilley,²⁰ the median nerve excursion was greater in the mid-forearm as compared with the mid-arm when the wrist was extended to 60° with shoulder in abduction and elbow in an extended position.²⁰ The study also showed that excursion of the median nerve at both levels was least when the neck was flexed to the contralateral side. The report of Coppieters et al.¹⁸ determined the effect of cervical and elbow movement with median nerve excursion at the level of the upper arm. Single joint movement of elbow extension with the neck in contralateral and ipsilateral lateral flexion produced an excursion of 5.6 and 5.5 mm, respectively. Cervical contralateral lateral flexion with the elbow in a more extended position and in a more flexed position produced lesser excursion of −3.3 and −3.4 mm. They concluded that elbow movements resulted in larger excursions of the median nerve at the humerus than movements of the neck (p > 0.0001). The magnitude of nerve excursion could be explained by the effect of the anatomical relationship between the nerve and the axis of rotation in the moving joint. Greater longitudinal mobility of a nerve segment is observed if the nerve segment is adjacent to the moving joint and least in nerve segments distant from the moving joint.²⁹ Furthermore, greater nerve compliance is seen in segments that cross joints than in segments that do not cross the joint.²⁹ Less excursion of the median nerve at the proximal arm may be due to the difference in the strain.³⁰ Kleinrensink et al. determined transmitted force measurements on different segments of the median nerve of cadavers with upper limb tension test. The percent strain was higher in the wrist as compared with the elbow (8.34% versus 3.72%) with wrist extension up to 60° causing more excursion in the wrist as compared with the lower arm.³⁰

We hypothesized that there would be a greater excursion of the nerve at the distal arm as compared with the wrist when the terminal movement was in the cervical spine. However, our study proved otherwise. In technique 3, nerve excursion was significantly greater in the wrist than in the distal arm. While in technique 4, there was no significant difference in the nerve excursion on the two areas. In both techniques, there was a significant decrease in excursion as compared with the techniques where wrist extension was the terminal movement. Our study had similar findings with the studies of Brochwicz et al. and Dilley et al. where minimal excursion of the median nerve was observed with cervical movement.^19,20 The study by Brochwicz et al.¹⁹ looked into the effect of contralateral cervical lateral flexion and contralateral cervical lateral gliding on median nerve excursion at the distal forearm and middle forearm. It showed an excursion of 1.9 and 2.5 mm at the distal forearm and 2.3 and 3.3 mm excursion at the middle forearm for the cervical lateral flexion and cervical lateral gliding, respectively. The in vivo study of Dilley showed that there was 1.3 mm and 0.8 mm excursion of the median nerve at the level of distal upper arm and mid-forearm, respectively, with a percentage strain of 0.2% when the cervical spine was flexed up to 35° to the contralateral side^.20 There could be another mechanism that could explain the results such as the strain of the median nerve in the different locations. Studies have consistently shown that strain in the elbow is less compared with the wrist which could explain the lesser nerve excursion at the elbow and distal arm.^20,23,31 This could be due to the anatomy of the median nerve. The median nerve at the distal arm is found between the brachialis and biceps muscles. At the level of the wrist, the median nerve is within the carpal tunnel, a very compact area where tendons of the flexor digitorum superficialis, flexor digitorum profundus, and flexor pollicis longus are also housed. Because of this anatomical relationship, it is possible that there is more slack in the distal arm as compared with the wrist. According to Dilley, transmission of stretching forces would only occur if there is already significant tension of the nerve.²⁰ Furthermore, with cervical contralateral and ipsilateral lateral flexion as the terminal movement, the nerve bed that is more elongated is at the nerve roots and brachial plexus. It would have been ideal to determine the mobility of cervical nerve root or brachial plexus to test this hypothesis. However, it is not feasible to measure its excursion at these levels using sonography because of technical difficulties. Systematic reviews showed that mobilization exercises have limited evidence of effectiveness.^15,32 They concluded that outcome measures should not only be clinical objective measures such as range of motion and pain but should include an objective in vivo measurements of neural movement using dynamic musculoskeletal ultrasound. Recent randomized controlled trials using neurodynamic techniques showed conflicting results. The study of Heebner and Roddey⁶ showed that mobilization exercises were no more effective than the standard of care which consisted of the use of prefabricated volar wrist splint and tendon gliding exercises. The study of Manchanda³³ showed that mobilization exercises with splint were more effective than a splint alone in patients with CTS. It is recommended that more clinical studies be done on the effect of gliding and tensioning techniques in CTS.

Correlation of anthropometric measures with nerve mobility

Studies have shown the correlation with of wrist and hand dimensions contribute to the development of carpal tunnel syndrome.^24,34–36 A wrist ratio (wrist depth/wrist width) ≥0.72 was associated with the development of carpal tunnel syndrome.^24,34,35 The study of Moghtaderi et al.³⁵ showed that 0.01 unit increase in the wrist ratio increases the odds to 1.12 times of having CTS. The study of Claes showed that cross-sectional area of the median nerve using ultrasound measurement at the wrist generally correlated significantly with age, gender, height, and weight.³⁷ With these findings, the researchers hypothesized that there may be a correlation with anthropometric measure and longitudinal mobility of the median nerve. However, our study only showed that arm length had an almost significant correlation with median mobility at the level of the wrist. Our findings were similar to the study of Echigo et al.³⁸ where no correlation has been found among the upper limb, forearm, and hand lengths and the amount of nerve gliding during passive extension of the wrist and fingers. However, our study only included normal individuals which may not be able to detect the correlation between median nerve mobility and upper limb measurements.

Limitations

Although our study was able to determine the mobility of the median nerve in both distal arm and forearm with the different neural mobilization techniques, the sample size may still be limited in determining the correlation of the anthropometric measures with the varying neural mobilization technique. Furthermore, the research was performed only in normal individuals and did not include participants with carpal tunnel syndrome. The anthropometric measures of the symptomatic participants may have been different from the normal individuals which could have had an effect in the mobility of the median nerve.

Conclusions

On the basis of the data obtained in this study, we conclude that the largest median nerve excursion in the arm and the wrist occurred when the wrist extension is the terminal movement. Contralateral cervical lateral flexion with a prepositioned wrist in extension produced the least motion of the median nerve in the arm and wrist.

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

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