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Journal of the Anatomical Society of India

Lumbrical Muscles and Carpal Tunnel

Author(s): Joshi S.D., .Joshi S.S., Athavale S.A.

Vol. 54, No. 1 (2005-01 - 2005-03)

Rural Medical College, PIMS-DU Loni, Ahmednagar. M.S.


Lumbrical muscles, though small in size, have significantly greater role to play in the intricate movements of the fingers. It is quite unique in its position as it connects the flexors of the digits to the extensors and that both of its attachments are mobile. The articular system in the digits is connected by mechanical links and lumbrical muscle is one of the links of this system that produces dynamic controlled extension of interphalangeal joints. Lumbricals may significantly contribute to the aetiology of carpal tunnel syndrome. In cases of carpal tunnel syndrome and in persons whose job requires repetitive finger movements the origin of lumbricals is proximal. In the literature large numbers of morphological variations of lumbrical muscles are described. The great functional significance and variations of these muscles as described in the literature, plus their contribution in the production of carpal tunnel syndrome, prompted us to undertake a detailed study of these muscles.

A total of seventy normal hands (from 35 cadavers) were carefully dissected. In 50% cases the first lumbrical was quite bulky, grooved on its medial surface and it lodged the tendon of flexor digitorum superficialis. In the majority, this muscle was attached to the anterior surface of flexor digitorum profundus tendon besides its lateral margin. In approximately 20% proximal attachment of this muscle extended into carpal tunnel. The second lumbrical was bipennate in 45% and in approximately 1/3 rd cases its proximal attachment extended into the carpal tunnel. In 4% fourth lumbrical was absent.

As proximal attachment of the lumbricals can extend into the carpal tunnel (specially first and second) it may predispose to the development of carpal tunnel syndrome, especially in those individuals whose occupation requires repetitive finger movements.

Keywords: carpal tunnel, flexor digitorum superficialis et profundus, lumbrical muscle.


There are four lumbricals (resembling the shape of earthworm) in each hand. Compared to their small size, they play a very significant role in the dynamics of the intricate movements of the fingers required for the precision work. The muscles arise from the bare areas of tendons of flexor digitorum profundus (FDP) about the middle of palm (Dutta, I995), pass distally along the radial side of metacarpo-phalangeal joints anterior to deep transverse metacarpal ligament. The narrow tendon of insertion joins the radial margins of extensor expansion as distal wing tendon (Williams et al., 1995). Normally the first and the second lumbricals are unipennate and third and fourth are bipennate. There exists an articular system in fingers in which there are two sets of three longitudinal bones linked by two joints. Each of these two systems has a mechanical link that, passing from the flexor to the extensor side of finger, plays an important role in extension of the digits. The lumbrical muscles are the link of the proximal system and provide dynamic controlled extension of the interphalangeal joints. The more distal system has a rigid fascial link known as the oblique retinacular ligament. This is inextensible. These linkages assist, and are integrated with the functions of the extrinsic flexor and extensor tendons (Flatt et al., 1983). Throughout the flexor movement, the balance of forces within the fingers must be maintained by a synergistic paying out of tendon by the lumbrical muscles, the interossei, and the extensor digitorum.

Thus even the simplest action of hand employs all the constituent parts of the limb in a concert which is very complex and is not fully understood (Flatt et al.,1983).

Lumbricals produce flexion at metacarpophalangeal joint and extension at interphalangeal joints. Pinching the index finger against the thumb without a lumbrical would result in a nail-to-nail contact; the addition of the lumbricals increases the interphalangeal joint extension resulting in pulp-to-pulp pinch (Williams et al.). These muscles also play a significant role in proprioception. A large number of variations have been described in the literature, ranging from complete absence to reduction in their numbers or presence of accessory slips (Bryce, 1923).

Siegel et al. in 1995 have described the origin of lumbricals being significantly proximal in patients with carpal tunnel syndrome (CTS). If the lumbricals are large and more proximal in origin then in persons doing the jobs requiring repetitive hand motions there would result hypertrophy and compression of the median nerve. Cobb et al. in 1994 have described that lumbrical muscle incursion into the carpal tunnel (CT) during finger flexion is a normal occurrence. Thus, the lumbrical muscles should not be regarded as trivial in their functional role. Keeping these reported variations in the literature in mind and the possible role the lumbricals may play in the production of CTS it was thought prudent to undertake a detailed study of the origin of lumbricals and their relation to CT.

Material and Methods:

Lumbrical muscles of both hands were dissected in 35 cadavers (a total of 70 hands). Variations in the origins of individual lumbricals were noted. Proximal extent and their incursion into the CT during finger flexion was observed.


Individual lumbrical muscles were observed for their origin, attachment on the anterior and posterior surfaces of FDP, proximal extent of the origin and its relation to the carpal tunnel; grooving of the muscle belly and attachment to flexor digitorum superficialis (FDS). First lumbrical in all cases was unipennate. The second lumbrical in about 45% was found to be bipennate. Third and fourth lumbricals in all the cases were bipennate. The fourth lumbrical in 4% cases was absent. Other features like the attachment on the anterior and posterior surfaces of FDP and the attachment to FDS were noted and are summarized in tables I and II. The second lumbrical was bulkiest amongst all the lumbricals. In half of the cases the first lumbrical was very bulky (fig. 1). In three hands (two right and one left) lumbricals formed a continuous sheet on the anterior surface of the FDP tendons (fig. 2). The extent of proximal attachment of lumbricals into the CT is shown in table III. It was observed that the proximal attachment of the second lumbrical, in a larger percentage of cases, was extending into the CT compared to the rest of the lumbrical muscles. In the present series the proximal attachment of lumbricals was found to extend only upto the intermediate and distal parts of the CT in extended position of fingers. It extended into CT more in the left hand. All the lumbricals which were extending into the CT as well as those that extended upto its distal margin were seen to extend further proximally into the CT when passive finger flexion was performed. In some cases the first lumbrical not only extended into the CT but was found to become continuous with the fleshy fibres of FDS (fig. 3).

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Fig. 1: Shows a bulky first lubrical which is getting attached to the anterior surface of FDS for index (Arrow)

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Fig. 2: Shows attachment of lubricals on the anterior surface of FDP which are converging to form acontinuous muscular sheet in their proximal partwhich is extending into the CT (Arrow)


CTS is manifested by characteristic signs and symptoms resulting from an entrapment neuropathy of the median nerve at the wrist. It is also considered as a cumulative trauma disorder. There are a large number of causes cited in the literature which produce this syndrome e.g. i) trauma related (like lunate dislocation, post traumatic arthritis etc.), ii) in certain systemic diseases (e.g. Rheumatoid arthritis, hypothyroidism, amyloidosis etc.), iii) hormonal (e.g. pregnancy, menopause), iv) neoplasms (e.g. lipoma, ganglion), v) anomalous anatomical structures (e.g. muscles like lumbricals extending into CT) along with mechanical overuse (e.g. highly repetitive wrist and finger movements). Cobb et al. in 1995 described 'fist test' to ascertain the CTS due to lumbricals muscle incursion in CT. According to this test if a person is asked to keep the hand in sustained fist position for 45 seconds it would result in numbness in the area of distribution of median nerve.

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Fig. 3: Shows proximal attachment of first lubricalextending into the CT and becoming continuouswith the fleshy fibers of FDS (Arrows)

Pre-eminent position that the man enjoys amongst the animals is partly due to functional specialization of the human hand that can perform very intricate, highly skilled precision movements. The intrinsic muscles of hand are responsible for very coordinated intricate movements of the digits. It is in this connection that Wood Jones in 1946 stated "we shall look invain if we seek for movements that a man can do and a monkey cannot, but we shall find much if we see for purposive action that a man can do and a monkey cannot" (Flatt et al.). Lumbricals and interosseii with the long flexors bring about a coordinated activity so that there is normal arc of flexion at the digits and its all the three joints are angulated to the same degree (Williams et al.).

In the present series first lumbrical, in all cases, was unipennate; whereas in 45%, the second lumbrical was bipennate. Williams et al. (1995) state that any of them may be unipennate or bipennate. All the third and the fourth lumbricals were found to be bipennate. The attachment of the lumbricals on the anterior surface of FDP tendon was present in a higher percentage of cases in first and second lumbricals as compared to third and fourth (Table I and II). In a few cases these merged together forming a continuous sheet of muscle on the anterior surface of FDP tendon (fig.2). Kaplan and Hunter (1984) have described the blending of muscle fibers of the middle, ring and little fingers and have called this as lumbrical complex.

In approximately 50% cases first lumbricals besides being bulky showed a distinctly grooved medial surface which lodged the tendon of FDS, so much so that the FDS tendon is completely covered anteriorly at this site (Fig. 1). Similar findings have been reported by Kaplan and Hunter (1984).

The origin of the lumbricals, besides the margins and the anterior surface, also extended on the posterior surface of FDP tendons. This was more marked in case of second lumbrical (Table I and II). In about 7-8% the first lumbrical showed an additional attachment to FDS which was much less for other lumbricals. In the present series origin of lumbricals was observed to encroach into the CT in the extended position of fingers (Table III). The second lumbrical was most commonly found (30-37%) in the CT followed by first (22-26%), third and fourth (14-22%). Kaplan and Hunter (1984) while describing the proximal end of the lumbricals, have stated that the lumbricals for the middle, ring and little fingers reach the transverse level of the distal end pisiform bone when the fingers are extended, which, with the fingers flexed, are pulled into the carpal canal and may reach the level of distal end of the radius. They have also described a variable proximal extent of the first lumbrical which may even arise from forearm muscles. Bryce (1923) has mentioned that the origin of one or the other lumbricals may be displaced proximally arising from flexor retinaculum, FDS, FDP or Flexor pollicis longus.

Table-I: Proximal attachment of first and second lumbricals and their relation to FDS.

First Lumbrical Second Lumbrical
Right Left Right Left
Antero-lateral surface 91.30* 100 95.65 92.59
Posterior surface 13.04 22.20 52.17 44.44
Grooving 43.50 51.90 —- 11.11
Attachment to FDS 8.70 7.40 4.34

*All values are in percentages.

Table-II: Proximal attachment of third and fourth lumbricals.

Third Lumbrical FourthLumbrical
Right Left Right Left
Attachment on margins 60.86* 55.55 47.82 44.44
Extension on anterior surface 30.43 44.44 43.47 51.85
Extension on posterior surface 17.39 18.50 13.04 18.51

Lumbrical Muscles and Carpal Tunnel : Joshi S.D. et al.

Table-III: The extent of proximal attachment of lumbricals into the CT and upto the distal margin of flexor retinaculum in extended position of fingers.

Right Left
Proximal extent I II III IV I II III IV
Up to the distal margin of CT 60.86* 65.21 69.56 65.21 70.37 55.55 62.96 66.66
Into the CT 26. 30.4 17.39 17.39 22.22 37.03 22.22 14.86

Cobb et al.(1994) have studied the lumbrical muscle incursion into the CT during finger flexion and have suggested that incursion is a normal occurrence and is a possible cause of work related CTS. Siegle et al.(1995) have stated that in cases of CTS and in those performing repetitive hand motions lumbricals had significantly larger and proximal origins in CT which they consider as a contributory factor in the causation of CTS. After reviewing 1280 cases of CTS, Iob et al. (2000) found an unexpectedly high frequency (11%) of transverse muscular fibers within the CT. Singer and Ashworth (2001) in their series found 4% anatomic soft tissue variations in CTS which they consider to predispose to median nerve compression.


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Lumbrical Muscles and Carpal Tunnel: Joshi S.D. et al.

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