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Biphasic motion of the median nerve in the normal asian population

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The biomechanical interaction between the median nerve and the flexor tendons is an important consideration in Carpal tunnel syndrome (CTS). We aim to quantify the displacement and compressive deformation pattern of the median nerve in various stages
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  BIPHASIC MOTION OF THE MEDIAN NERVE IN THENORMAL ASIAN POPULATION C. H. Goh, *  B. H. Lee *  and Amitabha Lahiri † * Yong Loo Lin School of Medicine National University of Singapore ,  Singapore 523230 †  Department of Hand and Reconstructive Microsurgery National University Hospital  ,  Singapore 119074 Received 25 August 2014; Revised 7 October 2014; Accepted 8 October 2014; Published 20 January 2015 ABSTRACT Background:  The biomechanical interaction between the median nerve and the  fl exor tendons is an important consideration inCarpal tunnel syndrome (CTS). We aim to quantify the displacement and compressive deformation pattern of the median nerve in various stages of   fi nger   fl exion in the normal population at the inlet of the carpal tunnel.  Methods:  Transverse ultrasounds images were taken at the carpal tunnel inlet during full-extension, mid- fl exion and full  fl exion. The displacement, distance, Feret  ’ sdiameter, and perimeter of the median nerve were calculated and compared between each position.  Results:  Biphasic median nervemotion was observed, with a displacement of 2 : 84  3 : 49 mm in the ulnar direction from full-extension to mid- fl exion (Phase I)and a further 0 : 93  3 : 04 mm from mid- fl exion to full  fl exion (Phase II). Of 49 hands, 37 (75.5%) exhibited ulnar displacement in Phase I while 12 (24.5%) exhibited radial displacement. Feret  ’ s diameter (5 : 95  1 : 08 mm) and perimeter (13 : 28  2 : 09 mm)of the median nerve were greatest in the mid- fl exed position.  Conclusion:  In a healthy Asian population, the median nerve has a biphasic motion during   fi nger   fl exion, with maximal deformation in the mid- fl exed position.  Keywords  : Median; Nerve; Compression; Displacement; Carpal. INTRODUCTION Carpal tunnel syndrome (CTS) is the most common peripheralnerve entrapment syndrome, af  fl icting up to 3.8% of the pop-ulation. 1 Clinically, one in every   fi  ve subjects who complain of symptoms of pain, numbness and a tingling sensation in thehands is expected to have CTS based on clinical and electro-physiological testing. 1 Idiopathic CTS is the common diagnosisin patients with these symptoms, but despite the prevalence, itsexact etiology remains largely unclear. 2  Although  fi rst described by Paget in 1854, there is muchdebate behind the etiology of idiopathic CTS. Some studieshave investigated the  fi brosis of the subsynovial connectivetissue (SSCT) encasing the  fl exor tendons as a contributing factor in the etiology of primary CTS. 3 – 6 In 2004, Ettema  et al.  demonstrated marked proliferation and   fi brosis of theSSCT in patients with established primary CTS. It was sug-gested that this resulted in increased carpal tunnel pressuresthat could give rise to CTS. 3  A later study in 2007 again by Ettema   et al.  found alteration in the gliding characteristicsof the SSCT in patients with CTS, suggesting the role of abnormal  fl exor tendon mechanics in the etiology in CTS. 5 Despite SSCT  fi brosis being the most common  fi nding in CTS Correspondence to: Mr. Bing Howe Lee, Department of Hand and Reconstructive Microsurgery, National University Hospital, Level 11, NUHS TowerBlock, IE Kent Ridge Rd, Singapore 119228. Tel: ( þ 65) 9232-6282, E-mail: leebinghowe92@gmail.com Hand Surgery, Vol. 20, No. 1 (2015) 73 – 80© World Scienti fi c Publishing Company DOI: 10.1142/S021881041550010073    H  a  n   d   S  u  r  g .   2   0   1   5 .   2   0  :   7   3  -   8   0 .   D  o  w  n   l  o  a   d  e   d   f  r  o  m   w  w  w .  w  o  r   l   d  s  c   i  e  n   t   i   f   i  c .  c  o  m   b  y   N   A   T   I   O   N   A   L   U   N   I   V   E   R   S   I   T   Y   O   F   S   I   N   G   A   P   O   R   E  o  n   0   2   /   0   3   /   1   5 .   F  o  r  p  e  r  s  o  n  a   l  u  s  e  o  n   l  y .  patients, whether it is a cause or a consequence of CTS has yet to be determined. 4,6 It is a common observation that a number of patientsexperience episodic aggravation of symptoms of median nervecompression, particularly associated with activities involving sustained grip, such as holding the telephone. This indicatesthe possibility of a dynamic mechanism causing compression of the median nerves involving the long   fl exor tendons of thedigits. During such activities however, the wrist is usually inneutral or extended position and rarely in the  fl exed position, which is the position considered to create greatest compressionof the median nerve. 7 Some recent studies have studied the biomechanical inter-action of the median nerve with the adjacent   fl exor tendons.These studies have demonstrated displacement of the mediannerve due to volar displacement of the  fl exor tendons during  fi nger   fl exion. 8 – 13 It is also important to note that the displacement of themedian nerve is a normal biomechanical phenomenon ob-served in asymptomatic individuals and by itself not a direct cause of CTS. 8 In comparison to healthy controls, there are demonstrablechanges in transverse median nerve motion in patients withCTS during   fi nger   fl exion, although there is some inconsistency in the literature. 8,11,14 Nakamichi and Tachibana reported that symptomatic candidates exhibited reduced transverse motion(0 : 37  0 : 34 mm) compared to normal candidates (1 : 75  0 : 49 mm). 14 Conversely, van Doesburg   et al.  reported increased median nerve motion in CTS patients (2 : 20  1 : 80 mm versushealthy controls (1 : 93  1 : 48 mm). More recently, dynamicultrasonographic studies conducted by Liong   et al.  have simi-larly shown signi fi cantly reduced transverse displacement of the median nerve in CTS patients. 9 Unlike the aforementioned studies, which recorded the initial and   fi nal end point of thenerve, in this study the median nerve position was analysed using dynamic ultrasound. The authors reported a non-linear arc-likeulnar-volar displacement of the median nerve with greatest displacement and compressive deformation of the median nerveoccurring in the mid- fl exed position 9 of the fi ngers rather than inthe extended or fully   fl exed position in the region of the distalforearm at the inlet of the carpal tunnel. Although this particular study had a small number of subjects, it highlighted the role of dynamic events occurring at the inlet of the carpal tunnel, rather than within the carpal tunnel, which may have important contribution to the pathology of median nerve compression.Our study was designed to quantify the displacement and compressive deformation pattern of the median nerve in vari-ous stages of   fi nger   fl exion in the normal population at theinlet of the carpal tunnel. MATERIALS AND METHODSSubject Selection 33 healthy volunteers comprising 17 males and 16 females were included in this study. Mean age of the volunteers was21 : 0  0 : 47 years. Volunteers were excluded from the study if they possessed history of any neurologic symptom, systemicdisease, obesity, rheumatoid arthritis, or any trauma or pre- vious surgery of the upper limbs. All candidates provided verbal informed consent, and thisstudy was performed in accordance with the guidelines of the institutional review board and conforms to the HelsinkiDeclaration. Ultrasound Image Acquisition Transverse images of the carpal tunnel inlet were obtained using the Sonosite EDGE Ultrasound Machine, Sonosite Inc,USA). The transducer (HFL38X, Bandwidth 13-6 MHz) wasplaced transversely, at the distal wrist crease and perpendicular to the long axis of the forearm. The depth was adjusted to25 – 30 mm for optimal image acquisition. Volunteers were asked to sit in a standardized position withtheir hand supported on a   fl at table with the palm facing upwards and elbow   fl exed at a right angle (Fig. 1). The tendonof the abductor pollicis longus was identi fi ed and marked. Thetransducer was placed at the distal wrist crease, and perpen-dicular to the long axis of the forearm, using the marked abductor pollicis longus as the lateral border. The mediannerve and the surrounding tendons were identi fi ed.Ultrasound images were obtained in the following threepositions (Fig. 1): .  Position A (Full-extension of the digits): The wrist was held in the neutral position with all four   fi ngers held in full-extension, and the thumb held in full radial abduction inthe same plane as of the palm. A snapshot ultrasound image was acquired. .  Position B (Mid- fl exed position of digits): The volunteers were instructed to grasp a standardized cylinder (circum-ference 16.7 cm, diameter 5.2 cm, height 13.4 cm) while 74  C. H. Goh, B. H. Lee & A. Lahiri     H  a  n   d   S  u  r  g .   2   0   1   5 .   2   0  :   7   3  -   8   0 .   D  o  w  n   l  o  a   d  e   d   f  r  o  m   w  w  w .  w  o  r   l   d  s  c   i  e  n   t   i   f   i  c .  c  o  m   b  y   N   A   T   I   O   N   A   L   U   N   I   V   E   R   S   I   T   Y   O   F   S   I   N   G   A   P   O   R   E  o  n   0   2   /   0   3   /   1   5 .   F  o  r  p  e  r  s  o  n  a   l  u  s  e  o  n   l  y .  maintaining the position of the hand and the wrist. A second ultrasound snapshot was acquired. .  Position C (Full  fl exion): The cylinder was removed and the volunteers were asked to make a full  fi st while maintaining the position of the hand and the wrist. The third snapshot ultrasound image was acquired. .  This process was repeated with the contralateral hand. Image Analysis In total, 66 sets of images (each set comprising of three images,one image for each of the three  fi nger positions) were reviewed, with 49 sets (74.2%) deemed suitable for analysis. Seventeensets of images were rejected from the  fi nal analysis due to poor image quality or indistinct de fi nition of the median nerve inany one of the three positions (Fig. 2).Images were analysed using Image J 1.47 (NIH open sourcesoftware, USA).The median nerve was identi fi ed in each of the three imagesand the hyperechoic rim of the median nerve was outlined manually using a series of points to form a minimum-enclosing polygon (MEP).The following parameters were calculated with the software. .  Displacement of the median nerve (mm):  Centroid position of the median nerve was calculated in terms of coordinates (  x  ;  y ) in relation to a   fi  xed reference point (0,0). The displacement in the radial-ulnar direction wasrepresented on the  x  -axis and dorsal-volar displacement  was represented on the  y -axis. The displacement was thendetermined by calculating the difference in centroid posi-tions between the three hand positions. .  Direction of displacement of the median nerve:  Positivequadrant was assigned for the volar and ulnar direction while negative quadrant was assigned for the dorsal and radial direction. .  Absolute distance (mm):  The distance moved by themedian nerve irrespective of direction. .  Feret  ’ s diameter (mm):  Maximum diameter of the mediannerve in any plane. An increase in Feret  ’ s diameter indicated increased   fl attening of the nerve. .  Perimeter (mm):  Perimeter of the MEP automatically calculated by the software. Statistical Analysis Statistical analyses were performed by IBM SPSS Statistics Version 21. Paired, parametric student  ’ s  t  -tests were performed on the data sets to identify any signi fi cant changes inthe median nerve parameters between the three positions ineach set of hand images. All results were expressed as (A)(B)(C)Fig. 1  Ultrasound examination setup. These series of pictures depict thethree different positions  (A, B  and   C)  that the ultrasound images werecaptured. The ultrasound probe was placed transversely, just proximal tothe wrist crease and perpendicular to the long axis of the forearm.Position  (A) : Digits held in full-extension, with thumb in full radialabduction. Position  (B) : Digits held in mid- fl exion (grasping the cylinder,5.20 cm diameter) mid- fl exed position of digits. Position  (C) : Digits held infull  fl exion.  Biphasic Motion of the Median Nerve in the Normal Asian Population  75    H  a  n   d   S  u  r  g .   2   0   1   5 .   2   0  :   7   3  -   8   0 .   D  o  w  n   l  o  a   d  e   d   f  r  o  m   w  w  w .  w  o  r   l   d  s  c   i  e  n   t   i   f   i  c .  c  o  m   b  y   N   A   T   I   O   N   A   L   U   N   I   V   E   R   S   I   T   Y   O   F   S   I   N   G   A   P   O   R   E  o  n   0   2   /   0   3   /   1   5 .   F  o  r  p  e  r  s  o  n  a   l  u  s  e  o  n   l  y .  means  standard deviations. A   p -value of   <  0.05 was con-sidered statistically signi fi cant. RESULTSBiphasic Motion of Median Nerve The coordinates of the median nerve in the three digitalpositions ( n ¼ 49) were plotted in Fig. 4. Movement patterns were subdivided based on the predominant direction of motionof the median nerve.Two directionally distinct phases of motion were noted for the median nerve. There was a signi fi cantly larger displacement during the  fi nger motion from extended position (Position A) tomid- fl exed position (Position B) which was labeled as  ‘ Phase I ’ followed by a smaller displacement occurring during themovement of the  fi ngers from a mid- fl exed position (PositionB) to the fully- fl exed position (Position C) which was labeled as ‘ Phase II ’ .On observing the mean values of all measurement,there was an ulnar displacement of 2 : 84  3 : 49 mm from Fig. 3  Mean values of displacement and Feret  ’ s diameter of the median nerve. This  fi gure depicts the biphasic displacement and   fl attening of the median nervein the ulnar direction during digital  fl exion. The blue oval represents the median nerve. Displacement and compression (Phase I): Full-extension to mid- fl exion.Escape phase (Phase II): Mid- fl exion to full  fl exion. In Phase I, there is ulnar displacement of the median nerve of 2 : 84  3 : 49 mm. In Phase II, there is further ulnar displacement of 0 : 93  3 : 04 mm. Cross-sectional FD is represented by the black dotted line within the nerve. The changes in FD values are also annotated and are represented by the  fl attening of the oval median nerve in various positions. FD increases from 5 : 49  1 : 23 mm to a maximum of 5 : 95  1 : 08 mm inPhase I and decreases to 5 : 75  0 : 98 mm in Phase II. (A) (B) (C)Fig. 2  Example of ultrasound image. Cross-sectional view of the carpal tunnel inlet, with the left border indicating the radial side of the wrist. The hyperechoicmedian nerve is traced in yellow with a series of points to form a minimum-enclosing polygon (MEP). Snapshots of the median nerve in three positions are labeled  (A) ,  (B)  and   (C) . 76  C. H. Goh, B. H. Lee & A. Lahiri     H  a  n   d   S  u  r  g .   2   0   1   5 .   2   0  :   7   3  -   8   0 .   D  o  w  n   l  o  a   d  e   d   f  r  o  m   w  w  w .  w  o  r   l   d  s  c   i  e  n   t   i   f   i  c .  c  o  m   b  y   N   A   T   I   O   N   A   L   U   N   I   V   E   R   S   I   T   Y   O   F   S   I   N   G   A   P   O   R   E  o  n   0   2   /   0   3   /   1   5 .   F  o  r  p  e  r  s  o  n  a   l  u  s  e  o  n   l  y .  full-extension to mid- fl exion (Phase I). In Phase II, there was a further displacement of 0 : 93  3 : 04 mm in the ulnar dorsaldirection (Fig. 3). There was signi fi cant difference in dis-placement of the two phases (  p ¼ 0 : 017). Displacement in thedorsal-volar axis was not found to be signi fi cant (  p ¼ 0 : 127). Predominant Patterns of Median NerveMovement from Extended to Mid-FlexedPosition of Fingers Two patterns of motions were observed and analysed separately. Ulnar predominant displacement Of 49 volunteer hands, 37 (75.5%) exhibited an ulnar dis-placement of the nerve from full-extension to mid- fl exed po-sition (Fig. 5). Of these 37 hands, there were 23 (62.2%) maleand 14 (37.8%) female hands. In terms of transverse move-ment, there is a mean ulnar displacement of 4 : 20  2 : 87 mmfrom full-extension to mid- fl exion (Phase I).In the Phase II (mid- fl exed position of digits to full  fl exion),the median nerve displacement was 0 : 430  2 : 78 mm in theulnar direction. Fig. 4  Total median nerve movement. Scatter plot diagram of all the median nerve in the database ( n ¼ 49). Each line represents a different individual hand.The  fi rst point at the srcin represents the position of the median nerve at full-extension. The next point represents the position of the median nerve at mid- fl exion. The third point represents the positions of the median nerve at full  fl exion. Fig. 5  Ulnar predominant displacement of the median nerve. Scatter plot diagram of all the median nerve with an ulnar pattern of displacement in Phase Idatabase ( n ¼ 37). Each line represents a different individual hand. The  fi rst point at the srcin represents the position of the median nerve at full-extension. Thenext point represents the position of the median nerve at mid- fl exion. The third point represents the positions of the median nerve at full  fl exion.  Biphasic Motion of the Median Nerve in the Normal Asian Population  77    H  a  n   d   S  u  r  g .   2   0   1   5 .   2   0  :   7   3  -   8   0 .   D  o  w  n   l  o  a   d  e   d   f  r  o  m   w  w  w .  w  o  r   l   d  s  c   i  e  n   t   i   f   i  c .  c  o  m   b  y   N   A   T   I   O   N   A   L   U   N   I   V   E   R   S   I   T   Y   O   F   S   I   N   G   A   P   O   R   E  o  n   0   2   /   0   3   /   1   5 .   F  o  r  p  e  r  s  o  n  a   l  u  s  e  o  n   l  y .
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