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Variations in the course of the thenar motor branch of the median nerve
VARIATIONS IN THE COURSE OF THE THENAR BRANCH OF THE MEDIAN NERVE
MOTOR
P. J. H U R W I T Z
From the Surgical Day Care Clinic, Plastic and Reconstructive surgery, Aarau, Switzerland
In a prospective study of 80 operations in 61 patients for carpal tunnel syndrome, special attention was given to the course of the thenar motor branch and its variations. An anomalous origin of the branch was found in 21%. Multiple motor branches occurred in 12.5%. Seventeen patients had operations on both hands, but anomalies were often found in one side only. Special attention is drawn t o an additional anomaly where the motor branch lies superficially to the retinaculum buried in a hypertrophic preligamentous muscle. If this anomaly is not borne in mind, the nerve can easily be injured during splitting of the flexor retinaculum. We found this variation in 9% of our patients, but it is rarely mentioned in the literature and in many large series it is not described at all.
Journal of Hand Surgery (British and European Volume, 1996) 2lB." 3:344-346 In its short course the thenar motor branch of the median nerve shows numerous variations. Since decompression of the median nerve for carpal tunnel syndrome is one of the most common operations in hand surgery, it is important to be familiar with the variations in its course, in order to achieve a complete neurolysis and also to avoid inadvertant damage to the nerve. The more important variations have been described in several studies based on intraoperative findings (Lanz, 1975 and 1977; Pfeiffer and Nigst, 1973; Tountas et al, 1987) and anatomical dissections (Poisel, 1974; Mackinnon and Dellon; 1988). Several rare anomalies have been published as individual case reports. (Mannerfelt and Hybbinette, 1972; Entin, 1968; Bennett and Crouch, 1982; Graham, 1973). We describe our observations made during 80 consecutive operations for carpal tunnel syndrome. Special emphasis is given to the rarely mentioned anomaly where the motor branch lies superficially to the flexor retinaculum within a hypertrophic preligamentous muscle. This hypertrophic muscle serves as a tell-tale sign for this preligamentous nerve course.
classification of Poise1 (1974): extraligamentous (origin distal to the flexor retinaculum); subligamentous (origin within the carpal canal, winding around the distal edge of the retinaculum); or transligamentous (piercing the retinaculum). Table 1 shows the incidence of these three different courses in our series, which are quite similar to the findings of Poisel (1974). A fourth course has been described by Mannerfelt and Hybbinette (1972). In this rare anomaly, the motor branch arises fi'om the anterior aspect of the median nerve, turns ulnarly- distally, and passes superficially to the flexor retinaculum to the thenar muscles. In its superficial-transverse course it is buried within a hypertrophic muscle mass directly overlying the flexor retinaculum. We found this anomaly in seven patients (9%). In all seven the thenar branch arose from the anterior surface of the median nerve within the carpal canal. There was always a horizontal hypertrophic muscle lying directly on the flexor retinaeulum. In four patients, the nerve branch pierced the flexor retinaculum (transligamentous course), and three times the nerve curved around the distal border of the retinaculum (subligamentous course; Fig 1). Three of the seven patients had bilateral disease, but the anomaly was not found in the contralateral hand. The thenar motor branch originates usually from the radial side of the median nerve or the beginning of its first division, but variations are quite frequent. In our series we encountered 17 (21%) anomalies of origin of the motor branch. In order of frequency they are: origin from anterior side of nerve; origin from the second division of the median nerve; origin from the ulnar side; and origin from the dorsal side of the first division of
METHODS A prospective study was done on 80 consecutive operations in 61 patients for carpal tunnel syndrome. In 19 patients (31%) the disease was bilateral and both hands were operated on. All operations were done by the same surgeon by an open method, and no endoscopic surgery was done. The median nerve was visualized from the distal flexion crease of the wrist to the distal branching of the nerve. In each case the thenar motor branch was observed from its origin to its entrance into the thenar muscles. Its course was marked on a standard hand sheet, and a short description was added when a variation was found.
Table 1--Course of thenar motor branch in 80 hands
RESULTS
Extratigamentous Subligamentous Transligamentous
Three different patterns can be distinguished for the course of the thenar motor branch according to the 344
Nr
%
(Poisel)
44 23 13
55% 29% 16%
(46%) (31%) ( 23%)
MEDIAN MOTOR BRANCH
345 Table 3--Incidence of multiple thenar branches
2 branches 3 branches 1 branch dividing distally into two
FR
Total
8 (10%) 1 (1.25%) 1 (1.25%) 10 (12.5%)
Table 4--Comparison of variations of motor branch in 17 patients operated on both hands
Nerve course in both hands Anomalies in origin and multiple branches Preligamentous course + hypertroph, muscles
Same
Different
11/17 (65%) 1/i0 (10%)
6/17 (35%) 9/10 (90%)
0
3/3 (100%)
DISCUSSION
b Fig 1
Preligamentous motor branch within hypertrophic muscle. (a) Transligamentous course. (b) Subligamentous course. HM = hypertrophic muscle, FR=flexor retinacuhim, M N - m e d i a n nerve.
the median nerve (Table 2). Multiple thenar branches were found in 12.5% (Table 3). 19 patients (24%) underwent operation on both hands. In two patients, only one hand had a complete record of the nerve course. In the remaining 17 patients we compared the left and right hands, as to the course and anomalies of the motor branch. The course of the nerve (extraligamentous, subligamentous, transligamentous, superficially to retinaculum) was in two-thirds identical in both hands and different in one-third. The anomalies in origin and number oof branches were mostly different between right and left (90%; Table 4). Table 2--Anomalous origin of thenar branch from median nerve
From anterior side From ulnar side From second division From dorsal side of of first division
10 (12.5%) 1 (1.25%) 5 (6.25%) 1 (1.25%)
Total
17 (21.25%)
The classification of Poisel into an extraligamentous, subligamentous, and transligamentous course o f the thenar motor branch is well established. B u t the frequency of these groups varies greatly with different authors (Poisel, 1974; Pfeiffer and Nigst, 1973; Tountas et al, 1987). This may partly be due to the fact that some reports are based on intraoperative observations, whereas others describe anatomical dissections. Clinical reports often have a greater incidence of the transligamentous course, since the passage of the motor branch through a separate opening in the retinaculum may predispose to nerve compression with ensuing thenar atrophy (Bennett and Crouch, 1982), which explains the greater incidence of transligamentous course in some clinical series. Pfeiffer and Nigst (t973) reported an operative series of 52 hands. In 25 they found a transligamentous course, and 24 had signs of thenar atrophy. In other clinical series there was a low incidence of transligamentous course (Tountas et al, 1987). The preligamentous course of the nerve within a hypertrophic muscle is of great clinical importance, since the nerve may inadvertently be severed during transsection of the retinaculum. Although a hypertrophic preligamentous muscle is sometimes found with a normal course of the nerve, the preligamentous course of the nerve always seems to be accompanied by a hypertrophic muscle. The presence of a hypertrophic muscle superficial to the retinaculum is therefore a sign to look for a preligamentous nerve within the muscle mass. Although we found this anomaly in 9%, it is only rarely reported in the literature. Mannerfelt and Hybbinette first published two cases in 1972. They thought that the hypertrophic muscle was either palmaris brevis or the superficial head of flexor pollicis brevis. Pfeiffer and Nigst (1973), in their series of 52 operated hands mentioned four patients with an abnormal muscle on the distal part of the retinaculum, the motor branch being extraligamentous and curving around this muscle.
346
They described the muscle as palmaris brevis. Palmaris brevis is actually a subcutanenous muscle which originates from the ulnar border of the palmar aponeurosis and sometimes from the pisiform bone, and inserts into the ulnar skin and into the fascia of abductor digiti minimi (Schmidt and Lanz, 1992). Although this muscle has many variations, ranging from a few fibres to a hypertrophic muscle mass, the hypertrophic muscle described in this paper is probably not palmaris brevis, since it lies deeper, directly over the flexor retinaculum. It is probably a hypertrophic flexor pollicis brevis or abductor pollicis brevis. The origin of this latter muscle can vary considerably and be as far ulnarwards as the pisiform bone (Schmidt and Lanz, 1992). Tountas et al (1987), in a clinical series of 821 hands, did not describe any cases of preligamentous nerve course at all. A slightly different variation was described by Lanz (1977) where an accessory motor branch arises proximally to the carpal tunnel. On its way to the thenar muscles it pierced the retinaculum and continued in a superficial position. It then either communicated with a distal median branch or entered the thenar muscles separately. Two motor branches are reported with varying frequency. Tountas found this variation in only 0.4%, Lanz in 4.4%, and Pfeiffer in 9.6%. Three motor branches are only sporadically encountered. In our series we found a double motor branch eight times, and three branches once. In an additional case we found a single motor branch dividing into two branches as a " Y " before entering the thenar muscles. The thenar motor branch originates normally from the radial side of the median nerve or from the beginning of its first division (Tountas et al, 1987). Origin from the anterior and ulnar side has also been reported (Mackinnon and Dellon, 1988; Entin, 1968; Graham, 1973). The origin from the ulnar side, although very rare, is of particular clinical importance, since it crosses the anterior aspect of the median nerve and can easily be injured when dividing the flexor retinaculum.
THE J O U R N A L OF H A N D SURGERY VOL. 21B No. 3 JUNE 1996
Additional anomalies were origin from the second division of the median nerve, and from the dorsal aspect of the first division. These last two variants have, to the best of our knowledge, not been described previously. In those patients who were operated on in both hands, anomalies were often found in one hand only (Table 4). Variations in the course of the thenar motor branch are quite frequent, but the incidence of the anomalies differ greatly between the various reports. Knowledge of these anomalies is important for achieving complete neurolysis and to avoid inadvertant damage to the motor branch. This is particularly true for the preligamentous course of the motor branch within a hypertrophic preligamentous muscle. This rarely reported anomaly was found in 9% of our patients. References BENNETT J B and C R O U C H C C (1982). Compression syndrome of the recurrent motor branch of the median nerve. Journal of H a n d Surgery, 7: 407-409. E N T I N M A (1968). Carpal tunnel syndrome and its variants. Surgical Clinics of North America, 48:1097-1112. G R A H A M W P (1973). Variations of the motor branch of the median nerve at the wrist. Plastic and Reconstructive Surgery, 51: 90-92. L A N Z U (1975). Variationen des Nervus medianus im Bereich des Karpalkanals. Handchirurgie, 7: 159-162. L A N Z U (1977). Anatomical variations of the median nerve in the carpal tunnel. Journal of Hand Surgery, 2: 44-53. M A c K I N N O N S E and D E L L O N A L (1988). Anatomic investigations of nerves at the wrist: I. Orientation of the motor fascicle of the median nerve in the carpal tunnel. Annals of Plastic Surgery, 2 1 : 3 2 35. M A N N E R F E L T L and HYBBINETTE C-H (1972). Important anomaly of the thenar motor branch of the median nerve. Bulletin of the Hospital for Joint Diseases, 33: 15-21. P F E I F F E R K M and NIGST H (1973). Ungew6hnliche Befunde bei der Karpaltunneloperation. Handchirurgie 5: 99-103. PO1SEL S (1974). Ursprung und Verlauf des Ramus muscularis des Nervus digitalis palmaris communis I (N.medianus). Chirurgische Praxis, 18: 471-474. SCHMIDT C P and L A N Z U. Chirurgische Anatomie der Hand. Stuttgart, Hippokrates, 1992:88 90; 124-127. TOUNTAS C P, BIHRLE B S, MACDONALD C J and B E R G M A N R A (1987). Variations of the median nerve in the carpal canal. Journal of Hand Surgery, 12A: 708-712.
Accepted after revision: 19 September 1995 Dr Peter Joel Hurwitz, Schanzweg 7, 5000 Aarau, Switzerland. © 1996 The British Society for Surgery of the Hand
MOTOR
P. J. H U R W I T Z
From the Surgical Day Care Clinic, Plastic and Reconstructive surgery, Aarau, Switzerland
In a prospective study of 80 operations in 61 patients for carpal tunnel syndrome, special attention was given to the course of the thenar motor branch and its variations. An anomalous origin of the branch was found in 21%. Multiple motor branches occurred in 12.5%. Seventeen patients had operations on both hands, but anomalies were often found in one side only. Special attention is drawn t o an additional anomaly where the motor branch lies superficially to the retinaculum buried in a hypertrophic preligamentous muscle. If this anomaly is not borne in mind, the nerve can easily be injured during splitting of the flexor retinaculum. We found this variation in 9% of our patients, but it is rarely mentioned in the literature and in many large series it is not described at all.
Journal of Hand Surgery (British and European Volume, 1996) 2lB." 3:344-346 In its short course the thenar motor branch of the median nerve shows numerous variations. Since decompression of the median nerve for carpal tunnel syndrome is one of the most common operations in hand surgery, it is important to be familiar with the variations in its course, in order to achieve a complete neurolysis and also to avoid inadvertant damage to the nerve. The more important variations have been described in several studies based on intraoperative findings (Lanz, 1975 and 1977; Pfeiffer and Nigst, 1973; Tountas et al, 1987) and anatomical dissections (Poisel, 1974; Mackinnon and Dellon; 1988). Several rare anomalies have been published as individual case reports. (Mannerfelt and Hybbinette, 1972; Entin, 1968; Bennett and Crouch, 1982; Graham, 1973). We describe our observations made during 80 consecutive operations for carpal tunnel syndrome. Special emphasis is given to the rarely mentioned anomaly where the motor branch lies superficially to the flexor retinaculum within a hypertrophic preligamentous muscle. This hypertrophic muscle serves as a tell-tale sign for this preligamentous nerve course.
classification of Poise1 (1974): extraligamentous (origin distal to the flexor retinaculum); subligamentous (origin within the carpal canal, winding around the distal edge of the retinaculum); or transligamentous (piercing the retinaculum). Table 1 shows the incidence of these three different courses in our series, which are quite similar to the findings of Poisel (1974). A fourth course has been described by Mannerfelt and Hybbinette (1972). In this rare anomaly, the motor branch arises fi'om the anterior aspect of the median nerve, turns ulnarly- distally, and passes superficially to the flexor retinaculum to the thenar muscles. In its superficial-transverse course it is buried within a hypertrophic muscle mass directly overlying the flexor retinaculum. We found this anomaly in seven patients (9%). In all seven the thenar branch arose from the anterior surface of the median nerve within the carpal canal. There was always a horizontal hypertrophic muscle lying directly on the flexor retinaeulum. In four patients, the nerve branch pierced the flexor retinaculum (transligamentous course), and three times the nerve curved around the distal border of the retinaculum (subligamentous course; Fig 1). Three of the seven patients had bilateral disease, but the anomaly was not found in the contralateral hand. The thenar motor branch originates usually from the radial side of the median nerve or the beginning of its first division, but variations are quite frequent. In our series we encountered 17 (21%) anomalies of origin of the motor branch. In order of frequency they are: origin from anterior side of nerve; origin from the second division of the median nerve; origin from the ulnar side; and origin from the dorsal side of the first division of
METHODS A prospective study was done on 80 consecutive operations in 61 patients for carpal tunnel syndrome. In 19 patients (31%) the disease was bilateral and both hands were operated on. All operations were done by the same surgeon by an open method, and no endoscopic surgery was done. The median nerve was visualized from the distal flexion crease of the wrist to the distal branching of the nerve. In each case the thenar motor branch was observed from its origin to its entrance into the thenar muscles. Its course was marked on a standard hand sheet, and a short description was added when a variation was found.
Table 1--Course of thenar motor branch in 80 hands
RESULTS
Extratigamentous Subligamentous Transligamentous
Three different patterns can be distinguished for the course of the thenar motor branch according to the 344
Nr
%
(Poisel)
44 23 13
55% 29% 16%
(46%) (31%) ( 23%)
MEDIAN MOTOR BRANCH
345 Table 3--Incidence of multiple thenar branches
2 branches 3 branches 1 branch dividing distally into two
FR
Total
8 (10%) 1 (1.25%) 1 (1.25%) 10 (12.5%)
Table 4--Comparison of variations of motor branch in 17 patients operated on both hands
Nerve course in both hands Anomalies in origin and multiple branches Preligamentous course + hypertroph, muscles
Same
Different
11/17 (65%) 1/i0 (10%)
6/17 (35%) 9/10 (90%)
0
3/3 (100%)
DISCUSSION
b Fig 1
Preligamentous motor branch within hypertrophic muscle. (a) Transligamentous course. (b) Subligamentous course. HM = hypertrophic muscle, FR=flexor retinacuhim, M N - m e d i a n nerve.
the median nerve (Table 2). Multiple thenar branches were found in 12.5% (Table 3). 19 patients (24%) underwent operation on both hands. In two patients, only one hand had a complete record of the nerve course. In the remaining 17 patients we compared the left and right hands, as to the course and anomalies of the motor branch. The course of the nerve (extraligamentous, subligamentous, transligamentous, superficially to retinaculum) was in two-thirds identical in both hands and different in one-third. The anomalies in origin and number oof branches were mostly different between right and left (90%; Table 4). Table 2--Anomalous origin of thenar branch from median nerve
From anterior side From ulnar side From second division From dorsal side of of first division
10 (12.5%) 1 (1.25%) 5 (6.25%) 1 (1.25%)
Total
17 (21.25%)
The classification of Poisel into an extraligamentous, subligamentous, and transligamentous course o f the thenar motor branch is well established. B u t the frequency of these groups varies greatly with different authors (Poisel, 1974; Pfeiffer and Nigst, 1973; Tountas et al, 1987). This may partly be due to the fact that some reports are based on intraoperative observations, whereas others describe anatomical dissections. Clinical reports often have a greater incidence of the transligamentous course, since the passage of the motor branch through a separate opening in the retinaculum may predispose to nerve compression with ensuing thenar atrophy (Bennett and Crouch, 1982), which explains the greater incidence of transligamentous course in some clinical series. Pfeiffer and Nigst (t973) reported an operative series of 52 hands. In 25 they found a transligamentous course, and 24 had signs of thenar atrophy. In other clinical series there was a low incidence of transligamentous course (Tountas et al, 1987). The preligamentous course of the nerve within a hypertrophic muscle is of great clinical importance, since the nerve may inadvertently be severed during transsection of the retinaculum. Although a hypertrophic preligamentous muscle is sometimes found with a normal course of the nerve, the preligamentous course of the nerve always seems to be accompanied by a hypertrophic muscle. The presence of a hypertrophic muscle superficial to the retinaculum is therefore a sign to look for a preligamentous nerve within the muscle mass. Although we found this anomaly in 9%, it is only rarely reported in the literature. Mannerfelt and Hybbinette first published two cases in 1972. They thought that the hypertrophic muscle was either palmaris brevis or the superficial head of flexor pollicis brevis. Pfeiffer and Nigst (1973), in their series of 52 operated hands mentioned four patients with an abnormal muscle on the distal part of the retinaculum, the motor branch being extraligamentous and curving around this muscle.
346
They described the muscle as palmaris brevis. Palmaris brevis is actually a subcutanenous muscle which originates from the ulnar border of the palmar aponeurosis and sometimes from the pisiform bone, and inserts into the ulnar skin and into the fascia of abductor digiti minimi (Schmidt and Lanz, 1992). Although this muscle has many variations, ranging from a few fibres to a hypertrophic muscle mass, the hypertrophic muscle described in this paper is probably not palmaris brevis, since it lies deeper, directly over the flexor retinaculum. It is probably a hypertrophic flexor pollicis brevis or abductor pollicis brevis. The origin of this latter muscle can vary considerably and be as far ulnarwards as the pisiform bone (Schmidt and Lanz, 1992). Tountas et al (1987), in a clinical series of 821 hands, did not describe any cases of preligamentous nerve course at all. A slightly different variation was described by Lanz (1977) where an accessory motor branch arises proximally to the carpal tunnel. On its way to the thenar muscles it pierced the retinaculum and continued in a superficial position. It then either communicated with a distal median branch or entered the thenar muscles separately. Two motor branches are reported with varying frequency. Tountas found this variation in only 0.4%, Lanz in 4.4%, and Pfeiffer in 9.6%. Three motor branches are only sporadically encountered. In our series we found a double motor branch eight times, and three branches once. In an additional case we found a single motor branch dividing into two branches as a " Y " before entering the thenar muscles. The thenar motor branch originates normally from the radial side of the median nerve or from the beginning of its first division (Tountas et al, 1987). Origin from the anterior and ulnar side has also been reported (Mackinnon and Dellon, 1988; Entin, 1968; Graham, 1973). The origin from the ulnar side, although very rare, is of particular clinical importance, since it crosses the anterior aspect of the median nerve and can easily be injured when dividing the flexor retinaculum.
THE J O U R N A L OF H A N D SURGERY VOL. 21B No. 3 JUNE 1996
Additional anomalies were origin from the second division of the median nerve, and from the dorsal aspect of the first division. These last two variants have, to the best of our knowledge, not been described previously. In those patients who were operated on in both hands, anomalies were often found in one hand only (Table 4). Variations in the course of the thenar motor branch are quite frequent, but the incidence of the anomalies differ greatly between the various reports. Knowledge of these anomalies is important for achieving complete neurolysis and to avoid inadvertant damage to the motor branch. This is particularly true for the preligamentous course of the motor branch within a hypertrophic preligamentous muscle. This rarely reported anomaly was found in 9% of our patients. References BENNETT J B and C R O U C H C C (1982). Compression syndrome of the recurrent motor branch of the median nerve. Journal of H a n d Surgery, 7: 407-409. E N T I N M A (1968). Carpal tunnel syndrome and its variants. Surgical Clinics of North America, 48:1097-1112. G R A H A M W P (1973). Variations of the motor branch of the median nerve at the wrist. Plastic and Reconstructive Surgery, 51: 90-92. L A N Z U (1975). Variationen des Nervus medianus im Bereich des Karpalkanals. Handchirurgie, 7: 159-162. L A N Z U (1977). Anatomical variations of the median nerve in the carpal tunnel. Journal of Hand Surgery, 2: 44-53. M A c K I N N O N S E and D E L L O N A L (1988). Anatomic investigations of nerves at the wrist: I. Orientation of the motor fascicle of the median nerve in the carpal tunnel. Annals of Plastic Surgery, 2 1 : 3 2 35. M A N N E R F E L T L and HYBBINETTE C-H (1972). Important anomaly of the thenar motor branch of the median nerve. Bulletin of the Hospital for Joint Diseases, 33: 15-21. P F E I F F E R K M and NIGST H (1973). Ungew6hnliche Befunde bei der Karpaltunneloperation. Handchirurgie 5: 99-103. PO1SEL S (1974). Ursprung und Verlauf des Ramus muscularis des Nervus digitalis palmaris communis I (N.medianus). Chirurgische Praxis, 18: 471-474. SCHMIDT C P and L A N Z U. Chirurgische Anatomie der Hand. Stuttgart, Hippokrates, 1992:88 90; 124-127. TOUNTAS C P, BIHRLE B S, MACDONALD C J and B E R G M A N R A (1987). Variations of the median nerve in the carpal canal. Journal of Hand Surgery, 12A: 708-712.
Accepted after revision: 19 September 1995 Dr Peter Joel Hurwitz, Schanzweg 7, 5000 Aarau, Switzerland. © 1996 The British Society for Surgery of the Hand