Lumbrical Muscles Neural Branching Patterns: A Cadaveric Study With Potential Clinical Implications

Abstract

Background

Lumbrical muscles originate in the palm from the 4 tendons of the flexor digitorum profundus and course distally along the radial side of the corresponding metacarpophalangeal joints, in front of the deep transverse metacarpal ligament. The first and second lumbrical muscles are typically innervated by the median nerve, and third and fourth by the ulnar nerve. A plethora of lumbrical muscle variants has been described, ranging from muscles’ absence to reduction in their number or presence of accessory slips. The current cadaveric study highlights typical and variable neural supply of lumbrical muscles.

Materials

Eight (3 right and 5 left) fresh frozen cadaveric hands of 3 males and 5 females of unknown age were dissected. From the palmar wrist crease, the median and ulnar nerve followed distally to their terminal branches. The ulnar nerve deep branch was dissected and lumbrical muscle innervation patterns were noted.

Results

The frequency of typical innervations of lumbrical muscles is confirmed. The second lumbrical nerve had a double composition from both the median and ulnar nerves, in 12.5% of the hands. The thickest branch (1.38 mm) originated from the ulnar nerve and supplied the third lumbrical muscle, and the thinnest one (0.67 mm) from the ulnar nerve and supplied the fourth lumbrical muscle. In 54.5%, lumbrical nerve bifurcation was identified.

Conclusion

The complex innervation pattern and the peculiar anatomy of branching to different thirds of the muscle bellies are pointed out. These findings are important in dealing with complex and deep injuries in the palmar region, including transmetacarpal amputations.

Introduction

Lumbrical muscles (LMs) of the hand are named after the Latin word “Lumbricus” because of their worm-like appearance. Lumbrical muscles have no bony attachments, as they originate in the palm from the 4 tendons of the flexor digitorum profundus (FDP) and course distally along the radial side of the corresponding metacarpophalangeal joints, in front of the deep transverse metacarpal ligament. Each LM forms a narrow tendon and reaches the dorsal surface of the proximal phalanx, where it joins the radial margin of the dorsal digital expansion as the distal wing tendon.

The LM₁ and LM₂ (the most radial) are typically innervated by the median nerve (MN), while the LM₃ and LM₄ (the most ulnar) are innervated by the ulnar nerve (UN). ¹ Henle considered the above arrangement as the rule in the hand, but he mentioned that LM₃ and LM₄ may also be supplied by the MN and the UN superficial branch (UNsb), respectively, ² as well as Tountas and Bergman supported that LM₁ and LM₂ may also be supplied by the UN motor branches. ³

A plethora of LM variants has been described ranging from muscles’ absence to reduction in their number (3 or 2 LMs) or presence of accessory slips. Usually, LM variability is accompanied by neural aberrations.^3-8

From the clinical point of view, any sign of L hypertrophy may justify possible compression of the adjacent (radial and ulnar collateral) arteries, leading to chronic sub-ischemia. ⁹

The present cadaveric study highlights LMs morphological variants and clarifies typical and variable innervation by identifying the origin of the nerve fascicles supplying LMs. This information will be helpful for hand surgeons when operating in the palmar area, as well as for clinicians when interpreting findings from electromyography studies and evaluating clinical manifestations.

Materials and Methods

Eight (5 right and 3 left) fresh frozen cadaveric hands were dissected in the Laboratory of Anatomy of the Padua University (Italy) and in the Department of Anatomy and Surgical Anatomy of the Medical School of the Aristotle University of Thessaloniki (Greece) by using magnifying loupes (3.5x and 5.0x). After the removal of the skin and subcutaneous tissue of the palm, the palmar aponeurosis, the tendons of the flexor digitorum superficialis, and the palm arteries were exposed and the LMs and FDP tendons were identified. From the palmar wrist crease, the MN and UN were followed distally to their terminal branches. Lumbrical muscle superficial innervation was recorded and the FDP tendons were cut at the wrist and reflected distally to reveal the LM deep surface. After dissecting the UN deep branch (UNdb), LM innervation patterns were carefully recorded (Figure 1[a]).

Figure 1.

(a) First and second lumbrical muscles (1, 2 above the blue background) innervation by the median nerve (MN) collaterals, in a left cadaveric hand. (b) A single case of the first lumbrical by the MN via 2 branches. (c) Flexor tendon and MN package have been folded and elevated. Third lumbrical muscle double innervation by ulnar nerve branches. The second lumbrical muscle is innervated by a branch of the MN (arrow) and of the ulnar nerve (above the light blue background). (d) Double branches innervation of the fourth lumbrical muscle by the ulnar nerve. (e) Innervation of the fourth lumbrical muscle by the ulnar nerve superficial and deep branch.

Note. UNsb = ulnar nerve superficial branch; UNdb = ulnar nerve deep branch.

Each lumbrical nerve (LN) was traced to its origin from the parent nerve. Lumbrical nerve diameters were measured using a digital sliding caliper (Mitutoyo ABSOLUTE 500-196-20, accurate to 0.01 mm, Mitutoyo Europe GMBH) and branching pattern was recorded up to their entrance point in LMs: proximal, middle, or distal third; radial or ulnar; and dorsal or palmar side. The measurements were taken by the Department of Engineering of the University of Messina. The study was approved by the review board for anatomical research of the Italian Association for Surgery of the Hand and by the ethics committee of the Aristotle University of Thessaloniki.

Results

Lumbrical Muscle and Lumbrical Nerve Morphometry

Mean lengths of LM₁, LM₂, LM₃, and LM₄ were 5.16 ± 0.93 (min 3.88 to max 6.49 cm), 4.91 ± 0.93 (3.67-6.20), 41 ± 0.84 (3.87-6.37), and 4.80 ± 0.54 (4.06-5.56) cm, respectively, and mean distances (L-transverse carpal ligament [TCL]) are condensed in Table 1. The LM₃ was consistently longer than the others. Lenghts of LM₁ and LM₂ were positively strongly correlated by Pearson (P = .002 and r = 0.904). Male hands had higher mean values of lengths for all LMs compared to female hands and the highest mean length for both genders was recorded in LM₃ (Table 1). The LM₁-TCL mean distance was the shorter one. Lumbrical muscle origin was more proximal in female than male hands (Table 1). Lumbrical nerve diameters ranged from 0.67 to 1.38 mm (mean value, 1.02 ± 0.35 mm). Mean diameters of LN₁, LN₂, LN₃, and LN₄ were 1.14 ± 0.41, 1.01 ± 0.33, 1.27 ± 0.87, and 1.01 ± 0.33 mm, respectively. The thickest branch (1.38 mm) supplied LM₃ and originated from the UN, while the thinnest one (0.67 mm) supplied LM₄ and emanated from the UN. The LN mean lengths are summarized in Tables 2 to 5. The thickest branch (LN3) was also the most elongated.

Table 1.

Lumbrical Muscle (LM) Distance From the Transverse Carpal Ligament (LM-TCL) Lumbrical Muscles Length—Lumbrical Muscle Length (LMl) in Centimeters.

	Distance				Length
Hands/S/G	First LM-TCL	Second LM-TCL	Third LM-TCL	Fourth LM-TCL	First LMl	Second LMl	Third LMl	Fourth LMl
1/L/F	1.70	2.54	1.46	1.99	6.49	6.00	6.14	4.21
2/L/M	1.66	2.21	1.72	1.97	5.99	5.12	5.60	5.50
3/R/F	0.77	1.27	1.26	1.76	4.71	4.25	4.95	4.91
4/R/M	3.49	3.59	4.72	4.43	3.88	3.67	6.37	4.81
5/L/F	−1.19	−0.34	1.31	1.27	6.18	6.20	3.87	4.49
6/L/F	3.38	3.05	3.9	3.72	4.51	4.18	5.02	4.06
7/R/M	2.34	1.17	1.27	1.40	5.03	5.56	5.16	4.88
8/L/F	2.27	1.34	2.18	2.24	4.53	4.36	6.23	5.56
	Mean value ± SD				Mean value ± SD
Total hands = 8	1.80 ± 1.50	1.85 ± 1.24	2.22 ± 1.33	2.34 ± 1.12	5.16 ± 0.93	4.91 ± 0.93	5.41 ± 0.84	4.80 ± 0.54
Right hands = 3	2.20 ± 1.36	2.01 ± 1.36	2.41 ± 1.99	2.53 ± 1.65	4.54 ± 0.59	4.49 ± 0.96	5.49 ± 0.76	4.86 ± 0.51
Left hands = 5	1.56 ± 1.68	1.76 ± 1.32	2.11 ± 1.05	2.23 ± 0.90	5.54 ± 0.94	5.17 ± 0.92	5.37 ± 0.96	4.76 ± 0.71
Male hands = 3	2.49 ± 0.92	2.32 ± 1.21	2.57 ± 1.87	2.60 ± 1.61	4.96 ± 1.05	4.78 ± 0.98	5.71 ± 0.61	5.06 ± 0.37
Female hands = 5	1.38 ± 1.72	1.57 ± 1.31	2.02 ± 1.11	2.19 ± 0.92	5.28 ± 0.96	4.99 ± 1.01	5.24 ± 0.97	4.64 ± 0.60

Note. Negative values (-) mean that the LMs origin is situated within the carpal tunnel. S = side; L = left side; R = right side; G = gender.

Table 2.

Motor Innervation of the First Lumbrical Muscle (LM) in 8 Cadaveric Hands and Associated Morphologic and Morphometric Details of the Lumbrical Nerve (LN), Lumbrical Nerve Length (LNl), Median Nerve (MN), and UN.

Hands/S/G	n of branches	Side of the muscle where the nerve enters	Branch third of the muscle belly where the first LN enters	First LNl	Distance MN bifurcation-collateral nerve	Distance MN bifurcation-LM entry point
1/L/F	1	External	1/3 middle	0.98	2.01	2.75
2/L/M	1	Internal	1/3 middle	1.99	0.83	0.75
3/R/F	1	External	1/3 middle	1.44	1.19	2.62
4/R/M	1	External	1/3 middle	1.31	2.18	3.47
5/L/F	First branch	External	1/3 middle	0.74	2.03	2.48
	Second branch			0.67	2.51	2.93
6/L/F	1	External	1/3 proximal	1.90	0.87	2.88
7/R/M	1	External	1/3 middle	1.15	2.52	3.16
8/L/F	1	External	1/3 proximal	0.68	1.10	1.39
N = 8	N = 9	8/9 external 1/9 internal	7/9 middle 2/9 proximal
				First LNl	Distance MN bifurcation-collateral nerve	Distance MN bifurcation-LM entry point
				Mean value ± SD
Total hands = 8				1.20 ± 0.49	1.69 ± 0.69	2.49 ± 0.87
Right hands = 3				1.55 ± 0.31	1.41 ± 0.68	2.99 ± 0.43
Left hands = 5				1.10 ± 0.52	1.98 ± 0.68	2.41 ± 0.96
Male hands = 3				1.73 ± 0.36	1.29 ± 0.76	2.36 ± 1.43
Female hands = 5				0.99 ± 0.31	2.05 ± 0.54	2.78 ± 0.26

Note. All distances were expressed in centimeters. S = side; L = left side; R = right side; G = gender; n = number of cases; N = total number of cases.

Table 5.

Data of the Motor Innervation of the Fourth Lumbrical Muscle (LM) by the Ulnar Nerve (UN) in 8 Cadaveric Hands and Associated Morphologic and Morphometric Details.

Hands/S/G	n of motor branches	Entry point of motor branch (relation to the muscle third)	Fourth LNl	Distance origin UN motor palmar branch entering muscle belly	Ulnar palmar branch-origin of the lumbrical muscle
1/L/F	First branch Second branch	1/3 distal	2.34	3.57	1.39
			2.42	3.50	1.19
2/L/M	1	1/3 middle	2.79	3.46	2.15
3/R/F	1	1/3 distal	1.59	2.87	1.60
4/R/M	1	1/3 middle	3.15	3.80	0.88
5/L/F	1	1/3 distal	2.36	2.97	0.56
6/L/F	First branch Second branch	1/3 distal 1/3 distal	2.01	3.45	1.54
			0.93
7/L/F	First branch Second branch	1/3 middle	1.36	2.09	0.73
		1/3 distal
1/L/F	1	1/3 middle	1.95	3.12	1.62
N = 8	N = 11	6/11 distal 5/11 middle
			Fourth LNl	Distance origin UN motor palmar branch entering muscle belly	Ulnar palmar branch-origin of the lumbrical muscle
			Mean value ± SD
Total hands = 8			2.11 ± 0.45	3.02 ± 0.88	1.98 ± 1.10
Right hands = 3			2.13 ± 0.60	3.26 ± 0.33	1.76 ± 0.33
Left hands = 5			2.56 ± 0.38	3.46 ± 0.35	1.00 ± 0.36
Male hands = 3			2.00 ± 0.41	3.27 ± 0.35	1.44 ± 0.22
Female hands = 5			2.66 ± 0.38	3.45 ± 0.34	1.24 ± 0.69

Note. All distances were expressed in centimeters. S = side; G = gender; L = left side; R = right side; n = number of cases; N = total number of cases; LNl = lumbrical nerve length.

Lumbrical Muscle Type of Innervation

LM₁ and LM₂ were innervated by the MN through the first radial digital common trunk to the index finger and the second common digital nerve to the index and middle fingers (Figure 1[a]). Both LNs emanated from the MN, as third and fourth collaterals (first and second collateral were the thenar muscle branch and the common sensory trunk for the thumb, respectively). The LM₁ in a single case (12.5%) was supplied by the MN via 2 branches (Figure 1[b]). In 12.5%, the LM₂ was supplied by MN + UN (Figure 1[c]).

Lumbrical Nerve Bifurcation

In 54.5% (6/11 LNs) a bifurcation was identified. Two LNs (1LN₁ and 1LN₂) bifurcated before innervating the proximal and middle thirds of the muscle belly. Bifurcation of LN₁ and LN₃ was recorded in a hand (H₅). The LM₃ and LM₄ were innervated by double branches in 5 hands (2 for LM₃ and 3 for LM₄). Five LNs (2LN₃ and 3LN₄) bifurcated before innervating the middle and distal third of the muscle (Figure 1d and e).

Lumbrical Nerve Entrance Point in Lumbrical Muscles

Details of LM innervation, with reference to the entrance point into the muscle, are condensed in Tables 2–5. Specifically, LM₁ innervation occurred commonly (77.8%) in the middle third of the muscle with the LN entering the muscle from the radio-palmar surface (external) in 88.8% (Table 2). Neural branches supplied LM₂ entering the muscle in the middle (55.5%) and proximal third (44.4%) (Table 3). The lumbrical nerves LN₃ and LN₄ entered in the muscle in the distal (in 60% and 54.5%) and middle thirds (in 40% and 45.5%), respectively (Tables 4 and 5). Lumbrical muscles and LNs showed proportional lengths in 2 hands (H₄ and H₆), and the distance from the nerve trunk origin was longer for each of LNs in 1 hand (H₅) (2.93 cm) than in other cases. No multiple innervation was detected, except for a single case (LM₂) of double innervation from UN + MN (Table 3).

Table 3.

Data of the Motor Innervation of the Second Lumbrical Muscle by the Median Nerve (MN) in 8 Cadaveric Hands (9 Observations Concerning Neural Supply) and Associated Morphologic and Morphometric Details.

Hands/S/G	n of motor branches	Side of nerve entrance into the muscle	Entry point of motor branch (relation to the muscle third)	Second LNl	Distance bifurcation MN-collateral nerve	Distance bifurcation MN-entrance point of the muscle
1/L/F	1	Internal	1/3 middle	1.31	1.99	3.25
2/L/M	1	Internal	1/3 middle	2.20	1.46	1.42
3/R/F	1	External	1/3 middle	1.30	1.10	2.47
4/R/M	1	Internal	1/3 middle	2.66	1.19	3.31
5/L/F	1	Internal	1/3 middle	1.10	1.72	2.93
6/L/F	1	Internal	1/3 proximal	2.01	0.84	2.86
7/L/F	1	Internal	1/3 proximal	0.88	1.94	2.48
8/R/M	First branch from MN Second branch from UN	Internal Internal from UN	1/3 proximal 1/3 proximal	0.82 1.13	1.38 1.14 (from UN)	2.33 3.12 (from UN)
N = 8	N = 9	8/9 internal 1/9 external	5/9 middle 4/9 proximal
Mean value ± SD				Second LNl	Distance bifurcation MN-collateral nerve	Distance bifurcation MN-entrance point of the muscle
Total hands = 8				1.53 ± 0.67	1.45 ± 0.40	2.58 ± 0.65
Right hands = 3				1.61 ± 0.93	1.41 ± 0.46	2.75 ± 0.48
Left hands = 5				1.48 ± 0.59	1.47 ± 0.42	2.47 ± 0.76
Male hands = 3				1.91 ± 0.92	1.53 ± 0.37	2.40 ± 0.94
Female hands = 5				1.30 ± 0.43	1.40 ± 0.46	2.68 ± 0.43

Note. All distances were expressed in centimeters, S = side; L = left side; R = right side; G = gender; n = number of cases; N = total number of cases; LNl = lumbrical nerve length; UN = ulnar nerve.

Table 4.

Data of the Motor Innervation of the Third Lumbrical Muscle (LM) by the Ulnar Nerve (UN) in 8 Cadaveric Hands and Associated Morphologic and Morphometric Details.

Hands/S/G	n of motor branches	Entry point of motor branch (relation to the muscle third)	Third LNl	Distance origin UN motor palmar branch entering muscle belly	Ulnar palmar branch-lumbrical origin
1/L/F	1	1/3 middle	2.18	3.69	1.55
2/L/M	First branch Second branch	1/3 distal	2.58	3.45	1.88
			2.22	3.57	1.67
3/R/F	1	1/3 distal	1.97	3.47	1.86
4/R/M	1	1/3 middle	2.75	4.24	1.71
5/L/F	First branch Second branch	1/3 distal	2.35	1.48	3.64
			1.6	2.92	1.23
6/L/F	1	1/3 distal	2.48	1.93	4.28
7/L/F	1	1/3 middle	1.39	2.15	0.82
8/R/M	1	1/3 middle	1.62	3.34	1.16
N = 8	N = 10 nerves	6/10 distal 4/10 middle
			Third LNl	Distance origin UN motor palmar branch entering muscle belly	Ulnar palmar branch-lumbrical origin
				Mean value ± SD
Total hands = 8			2.11 ± 0.45	3.02 ± 0.88	1.98 ± 1.10
Right hands = 3			1.93 ± 0.31	3.32 ± 0.35	1.58 ± 1.23
Left hands = 5			2.46 ± 0.21	2.95 ± 1.18	2.61 ± 1.25
Male hands = 3			2.05 ± 0.49	3.2 ± 0.31	1.65 ± 0.36
Female hands = 5			2.39 ± 0.23	2.98 ± 1.20	2.57 ± 1.29

Note. L = left; R = right; S = side; G = gender; n = number of cases; N = total number; LNl = lumbrical nerve length.

Discussion

Lumbrical innervation patterns present a high range of variability ¹⁰ that can be explained based on the following 3 hypotheses:

1. Lumbrical muscles were usually all supplied on their superficial surface, and the deep nerve is gradually displacing the superficial nerve, as a priori has an advantage over the superficial nerve in cases in which there is constant exposure to pressure in the palmar area. ¹⁰

2. Significant variant neural patterns may result from altered signaling between mesenchymal cells and neuronal growth. ¹¹ Variant LM origins and insertions maybe of paramount surgical importance. ¹²

3. An increase in muscles’ complexity, accompanied by greater coordination of action, increases the nerves’ complexity supplying the muscles.^13,14 The LMs assist in metacarpophalangeal flexion and contribute mainly to the interphalangeal joint extension ¹⁵ helping in various object manipulating skills like writing, stitching, and any other forms of precision work. Lumbrical innervation is in high frequency typical (85.2%-92%).^16,17

In the current study, the LM₁, LM₂, LM₃, and LM₄ mean lengths were 5.16 ± 0.93, 4.91 ± 0.93, 5.41 ± 0.84, and 4.80 ± 0.54 cm, respectively, and LM₃ was consistently longer than the others. In the Koncilia et al. study, the LM₁, LM₂, LM₃, and LM₄ mean lengths had higher mean values, 6.2, 5.9, 5.1, and 4.6 cm, respectively, and LM₁ was longer than the others.

Regarding LM typical innervation patterns, data literature are summarized in Table 6, for a quick intra-comparison with the current study results. The LM₃ has the highest rate of variable innervation^8,20,21 due to the high variability of its insertions (split, displaced, and additional). ²⁰ The current study concluded that only a nerve branch innervates each LM (25%), while Lauritzen and Szabo ¹⁸ recorded a higher frequency (44.4%) similarly to Mehta and Gardner ⁹ and Homma and Sakai. ²² The present study recorded a double nerve branch in 62.5% and dual innervations (MN + UN) in 12.5%.

Table 6.

Innervation Patterns of the Lumbrical Muscles (LMs), Median Nerve (MN), Ulnar Nerve Deep Branch (UNdb), Ulnar Nerve Superficial Branch (UNsb), and Ulnar Nerve (UN).

LM	LM innervation	Frequencies range (%)
LM1	MN7,9,16,17	—
	MN + UNdb7,18
LM2	MN7,9,16,17	94.4-100
	UNdb5,10,16,19	5.5 16
	MN + UNdb16,18,19	—
LM3	MN1,7,10,15,16	2-57.11,7,10,15,16
	UNdb1,7,10,16,17,18	42.9-1001,7,10,16,17,18
	MN + UNdb3,4,6,7,10,16,18,21	4.8-643,4,6,7,10,16,18
	UNdb + UNsb9,10	—
LM4	UNdb7,10,16,17	—
	UNdb + UNsb7,9,10

Lumbrical Nerve Entrance Point Variability in Lumbrical Muscle

Lauritzen and Szabo ¹⁸ noted variations in number of nerve branches and in muscle entrance site. In the current study, in 5 specimens, LM₃ and LM₄ were innervated by multiple branches. Lumbrical muscle innervation occurred typically in the proximal to middle third junctions of the muscle. ²³ In the current study, a dual innervation was solely observed for LM₂, while Lauritzen and Szabo ¹⁸ observed LM dual innervation in 68% (of LM₃ in 64% and of LM₁ and LM₂ in 4%, per each). No dual innervation of LM₄ was observed in the current study.

In the Hur study, ⁷ 3 types of dual innervations (MN + UN) were detected: (1) MN + UNdb in 52%; (2) MN + UNsb + UNdb in 18%; and (3) MN + UNsb in 2%. In a palm, 2 LMs had a simultaneous dual innervation in 4%. The UNsb + UNdb innervated LM₃ (20%) and LM₄ (10%). When LM₂ was innervated by the UNsb, the UNdb did not innervate it. Typical LMs pattern, with the MN innervating LM₁ + LM₂ and the UNdb LM₃ + LM₄, was found in 26%, while the commonest pattern (60%), LM₁ and LM₂ innervation by MN, LM₃ by MN + UN, and LM₄ by UN. ⁷

Mehta and Gardner ⁹ and Hur ⁷ observed LM₁ and LM₂ dual innervation in 1.3% and 4%, respectively. The lumbrical muscle LM₃ received accessory innervation in 64%, ⁷ by MN in 37.3%, and by UNsb + MN in 19.7%. ⁹ The relatively high incidence of LM dual innervation found in the present study (12.5%), concerning only LM₂, contrasts with the results of Lauritzen and Szabo ¹⁸ and Hughes and Clarke, ²⁴ who reported typical L innervation in all specimens. Discrepancy in frequency might be due to sample’s differences, that is, difference in the numbers of studied specimens.

Sinnatamby ²⁵ described that each LM is supplied by the same nerve innervating the belly of its parent FDP tendon, irrespective of the FDP variable innervation. Oh et al ²⁶ reported that the MN innervated the fourth and fifth digits FDP muscles in 92% and 56%, respectively. Hur ⁷ highlighted that the MN innervated LM₃ attaching to the fourth digit in 72%, and there was no case in which the MN innervated LM₄ attaching to the fifth digit. Although the frequency of dual innervation differed between FDP muscles of the digits and the corresponding L, the FDP muscle of the fourth digit and the corresponding LM₃ showed the highest frequency of dual innervation.

The findings of the current study reproduce the above mentioned data in a consistent way. The nature of the solely lumbrical intrinsic minus hand has not been well described, nor has the impairment resulting from loss of lumbrical muscle function, largely because the principal mechanical actions of L are duplicated to some extent by synergistic interosseous muscles and extrinsic muscles. ¹⁸ Moreover, Johanson et al ²⁷ pointed out that different individuals habitually accomplish the same manual tasks by different strategies of muscle use, suggesting that a lumbrical muscle minus hand will not be the same in every subject. Rabischong ²⁸ supported that the LMs have the highest density of mechanoreceptors of any muscle, and proposed that they possibly play a critical role in proprioception in addition to their function as interphalangeal joint extensors. It would appear to be prudent to protect L innervation wherever possible, and in particular to avoid unnecessary mobilization of the digital nerves during area dissection.

Sparing of LM innervation in carpal tunnel syndrome has been described.^29,30 Unlike the motor branches to the thenar muscles which are superficially located and hence subject to compression against the flexor retinaculum, the LNs are more dorsal and therefore better protected from direct compression. ³⁰

The presence of MN-UN communicating branches could potentially alter clinical signs and manifestations, which may lead to misdiagnosis and iatrogenic injury.^31-33 Sensory loss may result after loss of interconnections, which would diminish the overall nerve supply to the affected digits. ³³ The functional loss resulting from traumatic or entrapment nerve lesions would be expected to differ in the presence of MN-UN interconnections. An accurate understanding of LM innervation requires the meticulous knowledge of the parts of a branch arising from the MN distal to the UNsb-MN communication.

A last clinical consideration is represented by the devastating scars after deep palmar trauma and transmetacarpal amputations; in these critical conditions, the complex occurrence of devascularization with denervation and transmuscular scars, as well as the surrounding fibrosis from soft tissues and bone, produce Finocchietto’s palsy. ³⁴ An active role in microsurgical repair of both vessels and nerves should be required in acute surgical care, but it looks like an impossible task for traditional microsurgery, as these structures are too thin. ³⁵ A role of supermicrosurgery ³⁶ could be claimed, even if this could be achieved with a significant increase in operative time. Another way could be represented by muscle repair and nerve transfers too, or LM neurotization. Current study limitations were the low number of dissected cadaveric (5 females and 3 male) hands that did not allow correlations with the observed side and the gender.

Conclusion

The current study demonstrates the diverse LM innervation patterns, in particular the relatively common presence of double branches’ innervation in 62.5% and the LM₂ dual innervation in 12.5%. These results will be helpful for accurate diagnosis, surgical procedures, and electrophysiological examinations in hand lesions affecting MN and UN. Despite the knowledge of variable innervation patterns, in the event of an injury it can still be difficult to discriminate on the basis of symptoms which structures are actually affected, and consequently it can be difficult to predict how a surgical intervention should be planned (because of the unpredictability of operation).