Study on accessory thenar branches of median nerve in hand: its surgical importance in carpal tunnel syndrome

Median nerve enters the palm through the carpal tunnel deep to flexor retinaculum of wrist. The median nerve gives off recurrent motor branch (thenar) just distal to flexor retinaculum to innervate thenar muscles of the hand. Objective: In present study the objective was to study the presence of accessory thenar branches and their side of origin. Materials and Methods: The study included 51 human cadaver hands at the Department of Anatomy, Mysore Medical College & Research Institute, Mysore during 2011-13. These specimens fixed in 10% formalin were finely dissected and variations were noted. Results: Out of 51 specimens, accessory thenar branches were present in 26 specimens (51%) i.e. 12 on right (23.5%) and 14 (27.45%) on left. Level of origin was observed within the retinaculum in 5 specimens (9.8%) on right and 2 specimens (3.9%) on left, distal to retinaculum in 7 specimens (13.7%) on right and 12 specimens (23.5%) on left side. Accessory thenar branch was originating from 1 st common digital nerve in 5 specimens (9.8%) on right and 1 specimen (1.96%) on left side, from radial proper digital nerve to thumb in 7 specimens (13.72%) on right, 13 specimens (25.49%) on left side. Conclusion: Knowledge of branching patterns & unusual variations helps in proper treatment of disorders of the median nerve. The possibility of multiple thenar innervations must be considered by hand surgeons in preoperative evaluation to reduce the hazards of iatrogenic injury or postsurgical complications.


Introduction
Anatomic variations of the median nerve are frequent and they are significant for wrist surgery particularly in the treatment of carpal tunnel syndrome. Median nerve enters the palm through the carpal tunnel deep to flexor retinaculum of wrist. Recurrent motor (thenar) branch originates at the distal border of the flexor retinaculum to innervate the thenar group of muscles namely abductor pollicis brevis, flexor pollicis brevis and opponens pollicis. 1 The recurrent motor branch is important as it supplies muscles which are important for grasping, holding, writing, threading needle, wielding a hammer and assessing quality of fabric. 2 The normal hand function is the balance between the extrinsic and intrinsic muscles. Much of the importance of the hand is de pendent on the action of the thumb. Functionally, the thumb is one half of the hand, which acts in the opposite direction to the fi ngers in grasping any object. The thumb mobility is under the control of long and short muscles. The long flexor tendons of the fingers provide the power of the grip , the short intrinsic muscles of the hand are responsible for adjusting the position and carrying out the finer skilled movements of the digits and specialized motion. 3 The presence of accessory thenar branches is of great importance for hand surgeons in order to maintain the thenar muscle innervation for ade quate functions of the whole hand. The present study aims at making the following observations in the specimens, to note the accessory thenar branches of median ner ve and its level of origin with respect to flexor retinaculum and site of origin. IJBR (2014) 05 (08) www.ssjournals.com

Discussion
Presence of accessory thenar branch of median nerve was 7.5% (246 hands) by Lanz 5 2 But the percentage is high in present study when compared to others. Knowledge of variable anatomy of the nerve could help to avoid incomplete decompression at operations for carpal tunnel entrapment and injury to the accessory thenar branch of median nerve.
The origin of accessory thenar branch of median nerve was originating proximal to retinaculum in studies done by Lanz 5 (1.62%) and Pushpalatha et al. 2 (20%), within retinaculum in studies done by Vashistha 11 (53.12%) and distal to retinaculum in studies done by Lanz 5 (7.3%), Pushpalatha et al. 2 (10%) and Vashistha 11 (46.87%). In present study the accessory thenar branch took origin from within the retinaculum in 13.7% and distal to retinaculum in 37.2%. This observation is similar to the study done by Vashistha 11 . Hence the accessory branches to the thenar muscles should be reviewed before undertaking any surgical procedure which will help the patient in getting effective treatment and avoids post-surgical complications.
Accessory thenar branches were originating from first common digital nerve in 40.62% in Vashistha 11 ,25% in Mumford et al. 12 and from radial proper digital nerve to thumb 5% in Pushpalatha et al. 2 , 59.37% in case of Vashistha 11 , and 50% in Mumford et al. 12 In the present study accessory thenar branches originated more from radial proper nerve to thumb than from common digital nerve which correlates with the study of Pushpalatha et al. 2 , Vashistha 11 and Mumford et al. 12 Knowledge of multiplicity of thenar branches would help in avoiding inadvertent injury and incomplete decompression during carpal tunnel release.
Lanz 5 U in 1977 studied on 246 hands in which the carpal tunnel was explored at operation, 29 variations in the course of median ne rve were found. Accessory branches at the distal portion were found in 18 hands, a high division of the median nerve in seven, and accessory branches proximal to the tunnel in four. Based on these findings and including published reports of others, the variations were classified into 4 grou ps. Group Ivariation in the course of recurrent motor branch of median nerve in 46%, Group IIpresence of accessory branch in the distal part of carpal tunnel in 7.5% and proximal to retinaculum in 1.6%, Group IIIhigh division or duplication of median nerve in 2.9%, and Group IVaccessory branch proximal to the carpal tunnel in 2.9%. The findings emphasize the importance of approaching the median nerve from the ulnar side when opening the carpal tunnel.

Conclusion
Median nerve is called as labourer's nerve and its lesion or injury will cause physical disability affecting work ability o f a person. Median nerve is often injured at wrist, accidentally or in an attempted suicide. In such cases, patient will be asked abduct or oppose the thumb against tight resistant, the thenar muscles can be felt to contract if the nerve is intact. Impairment of function affects not only employment but also a wide range of day today tasks like tying laces, putting up buttons and screwing the lids off jars.
Knowledge of branching patterns & unusual variations helps in proper treatment of disorders of the median nerve. The possibility of multiple thenar innervations must be considered by hand surgeons in preoperative evaluation to reduce the hazards of iatrogenic injury or postsurgical complications.