Rationale and Objectives
Carpal tunnel syndrome (CTS) is the most common peripheral nerve entrapment syndrome. Strong pinch or grip with wrist flexion has been considered a risk factor for CTS. Studying median nerve displacement during wrist movements may provide useful information about median nerve kinematic changes in patients with CTS. The purpose of this study was to evaluate the deformability and mobility of the median nerve in patients with CTS compared to healthy subjects.
Materials and Methods
Dynamic ultrasound images were obtained in 20 affected wrists of 13 patients with CTS. Results were compared to complementary data obtained from both wrists of 10 healthy subjects reported in a previous study. Shape and position of initial and final median nerve were measured and analyzed for six defined wrist movements. The deformation ratios for each movement were defined as the median nerve area, perimeter, and circularity of the final position normalized by respective values assessed in the initial position. The median nerve displacement vector and magnitude were also calculated.
Results
The deformation ratio for circularity was significantly less in patients with CTS compared to healthy subjects during wrist flexion ( P < .05). The mean vector of median nerve displacement during wrist flexion was significantly different between patients with CTS and healthy subjects ( P < .05). The displacement magnitude of the median nerve was found to be less in patients with CTS compared to healthy subjects during most movements, with the exception of wrist extension with fingers extended.
Conclusions
Patients with CTS differ from normal subjects with regard to mobility and deformability of the median nerve.
Carpal tunnel syndrome (CTS) is the most common peripheral nerve entrapment syndrome, and its precise etiology remains largely unclear. Like other peripheral nerves, the median nerve is exposed to various mechanical stresses related to limb postures and movements. In the case of the median nerve in the carpal tunnel, the relevant movements are of the fingers and wrist. The median nerve response to these stresses is a combination of displacement and deformation . Both displacement and deformation are known to vary from normal in patients with CTS, because of increased carpal tunnel pressure, and due to fibrosis in the median nerve and the surrounding tissue in the carpal tunnel .
Actuated by finger or wrist motion, the median nerve can displace in three dimensions. Longitudinal sliding of the median nerve in the carpal tunnel has been observed both in vitro and in vivo . Reduced longitudinal excursion of the median nerve at the carpal tunnel has been identified in patients with CTS . Transverse motion of the median nerve has also been studied in the carpal tunnel in normal subjects and in patients with CTS . Nakamichi et al. found reduced transverse motion of the median nerve during passive flexion and extension of the index finger in patients with CTS. Erel et al. studied the transverse motion of the median nerve during passive extension of the digits at the metacarpophalangeal joint from 90° flexion to neutral. Their results showed that in patients with CTS there was a significant reduction in transverse movement on the more symptomatic side compared to the contralateral.
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Materials and methods
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Ultrasound Examination
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Image Analysis
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circularity=4πareaperimeter2 circularity
=
4
π
area
perimeter
2
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r=x2+y2−−−−−−√ r
=
x
2
+
y
2
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Normalization of Measurements
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Statistical Analysis
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Results
Maximal Wrist Angles during Wrist Movements
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Deformation Ratio of the Median Nerve
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Table 1
Median Nerve Deformation Parameters
Wrist Motions Area Deformation Ratio Perimeter Deformation Ratio Circularity Deformation Ratio Finger flexion Patients 1.00 ± 0.16 0.98 ± 0.08 1.05 ± 0.13 Healthy subjects 0.99 ± 0.12 0.98 ± 0.09 1.09 ± 0.15 Wrist flexion with fingers extended Patients 0.93 ± 0.20 0.87 ± 0.16 1.23 ± 0.16 † Healthy subjects 1.06 ± 0.24 0.86 ± 0.16 1.49 ± 0.38 Wrist flexion with fingers flexed Patients 0.98 ± 0.21 0.89 ± 0.13 ∗ 1.25 ± 0.28 † Healthy subjects 0.94 ± 0.22 0.79 ± 0.15 1.57 ± 0.36 Wrist extension with fingers extended Patients 1.01 ± 0.20 1.02 ± 0.14 0.99 ± 0.20 Healthy subjects 1.03 ± 0.15 0.97 ± 0.11 1.12 ± 0.19 Wrist extension with fingers flexed Patients 0.97 ± 0.20 1.0 ± 0.12 0.99 ± 0.20 Healthy subjects 0.98 ± 0.15 0.98 ± 0.12 1.05 ± 0.22 Wrist ulnar deviation with fingers extended Patients 0.97 ± 0.20 0.90 ± 0.12 1.19 ± 0.16 † Healthy subjects 1.02 ± 0.27 0.93 ± 0.20 1.53 ± 0.32
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Displacement Vector of the Median Nerve
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Table 2
Median Nerve Displacement Mean Vector and Confidence Limits in Patients with Carpal Tunnel Syndrome and Healthy Subjects
Mean ρ (NU) Mean θ a (NU) b (NU) ψ Finger flexion Patients 0.1 28.9° 0.5 0.2 9.1° Healthy subjects 0.2 117.3° 0.5 0.2 −16.0° Wrist flexion with fingers extended Patients 0.8 137.3° † 1.0 0.3 −6.2° Healthy subjects 1.5 267.1° 1.2 0.5 −11.9° Wrist flexion with fingers flexed Patients 1.0 331.4° † 1.0 0.4 4.5° Healthy subjects 1.8 295.0° 1.1 0.5 −11.7° Wrist extension with fingers extended Patients 0.2 144.4° ∗ 0.7 0.2 −12° Healthy subjects 0.4 225.6° 0.5 0.2 −7.8° Wrist extension with fingers flexed Patients 0.6 184.4° 0.5 0.2 −0.7° Healthy subjects 0.5 182.0° 0.5 0.2 −2.5° Wrist ulnar deviation with fingers extended Patients 1.8 190.9° † 0.8 0.3 14.6° Healthy subjects 2.8 188.5° 0.3 0.2 −5.2°
ρ = vector length; θ = vector angle; a = major ellipse axes for variation of the confidence limits; b = minor ellipse axes for variation of the confidence limits; ψ = inclination of the ellipse.
One normalized unit (NU) is ∼1.8 mm.
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Normalized Magnitude of the Median Nerve Displacement
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Table 3
Median Nerve Displacement Normalized Magnitudes
Magnitude of Median Nerve Displacement (NU) Finger flexion Patients 0.75 ± 0.44 Healthy subjects 0.82 ± 0.33 Wrist flexion with fingers extended Patients 1.74 ± 0.78 ∗ Healthy subjects 2.36 ± 0.79 Wrist flexion with fingers flexed Patients 1.71 ± 0.90 † Healthy subjects 2.46 ± 0.84 Wrist extension with fingers extended Patients 0.90 ± 0.68 Healthy subjects 0.77 ± 0.46 Wrist extension with fingers flexed Patients 0.85 ± 0.56 Healthy subjects 0.81 ± 0.58 Wrist ulnar deviation with fingers extended Patients 1.93 ± 1.23 † Healthy subjects 2.86 ± 0.51
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Discussion
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Acknowledgments
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