Anatomic study of the innervation of Extensor Carpi Radialis Longus and Extensor Carpi Radialis Brevis. Interest in nerve transfers.

Abstract

Purpose: This study pretends to describe the innervation pattern of Extensor Carpi Radialis Longus (ECRL) and Extensor Carpi Radialis Brevis (ECRB) to use ECRL branches as a first choice in nerve transfers to restore flexion and extension of wrist and fingers due to its easier identification and its redundant innervation.

Methods: 9 cryopreserved and anonymous specimens were dissected to identify the radial nerve and describe the number of branches, distance between its origin and a line connecting both epicondyles, length of each branch and type of innervation of ECRL and ECRB according to Taylor’s classification.

Results: 6 out of 9 specimens presented two branches from the radial nerve to innervate the ECRL (Taylor type 3). 3 out of 9 specimens only had one branch to innervate the ECRL, originating from the radial nerve (two of them classified as a Taylor type 1 and the other as a Taylor type 2). All the specimens had only one branch to innervate the ECRB (eight showing a Taylor type 1 pattern, and one classified as Taylor Type 2); in 7 out of 9 this branch emerges from the deep branch of the radial nerve, emerging in the other 3 specimens from the superficial branch of the radial nerve.

Conclusion: The use of the ECRL branches could be considered as the first choice in nerve transfers to restore the flexion and extension of elbow, wrist and fingers in nerve injuries, because of its constant origin after the brachioradialis branch and in most of the cases its redundant innervation.

Introduction

Nerve transfers as a treatment for nerve injuries, either at the level of the brachial plexus or the terminal branches, are gaining popularity due to its excellent results in restoration of muscular function. To restore the extension of wrist and fingers in radial nerve injuries, nerve transfers from median to radial nerve are the most common used, transferring median nerve branches to the flexor digitorum superficialis (FDS), flexor carpi radialis (FCR) and palmaris longus (PL) to radial nerve branches to the extensor carpi radialis brevis (ECRB) and posterior interosseous nerve (PIN) [7]. Although there is little literature regarding the use of nerve branches to the ECRL in nerve transfers, in the last 10 years some authors reported its potential into this field according to its anatomical characteristics and easier identification in comparison with other alternatives as branches to the ECRB. Charlotte Waxweiler et al. [11] describe the use of the branch to the ECRL in nerve transfers from the brachioradialis (BR) to the ECRL in spastic upper limb to reduce spasticity and recover extension of the wrist. Antonio Garcia-López [5] et al. described a nerve transfer to restore wrist extension in radial nerve palsy or posterior cord lesions using the pronator teres (PT) branch from the median nerve to the ECRL branch. Regarding the excellent results achieved in the mentioned studies, we anatomically characterize the branches of the radial nerve to the ECRL and ECRB to determine if there is redundant innervation and if there is variance in its origin. In our study we describe the innervation pattern of the ECRL and ECRB according to its origin and number of branches, its distance of emergence to a line connecting both epicondyles, the length of these branches and the type of innervation according to Taylor’s classification [10] to use it as a first choice in nerve transfers for the restoration of flexion and extension of the elbow and wrist and fingers.

Methods

This is an anatomical descriptive study to characterize the branches of the radial nerve to the ECRL and ECRB. Nine cryopreserved and anonymous specimens were dissected with magnifying loops x4.2 to identify the branches emerging from the radial nerve. The incision was made over the interval between the brachialis and the brachioradialis. After the radial nerve was identified in this interval, using blunt dissection we exposed the branches emerging from the radial nerve from proximal to distal, taking as the proximal limit the last branch to the BR and the distal limit the entrance of the deep branch of the radial nerve into the Frohse’s arcade. We registered where the ECRL and ECRB branches emerged from, especially if the ECRB branches originated from the superficial or deep branch of the radial nerve, and evaluated the number of branches for the ECRL and ECRB. We measured the distance between the emergence point of each mentioned branch to a line connecting the two epicondyles (intercondylar line), getting negative values if the measure was proximal to the line and positive if they were distal to it. We also measured the length of each branch from its emergence point to its entry into the muscle. All measures were taken with a calibrated ruler. Photographical record was done for all the specimens.

Results

In all nine specimens we dissected the radial nerve and individualized its motor branches to the ECRL and ECRB.

Six out of nine specimens presented two branches to innervate ECRL emerging both from the radial nerve after the distal branch to the BR and before the bifurcation into superficial and deep branches, except for one specimen where the distal branch emerged from the deep branch of the radial nerve. Three specimens had only one motor branch for the ECRL emerging from the radial nerve.

All specimens except from one presented the origin of the motor branches to the ECRL before the nerve crossed the intercondylar line. The measured mean point of origin was -17,73mm ± 13,04. If we measure the mean point of origin only accounting for the specimens with two branches to ECRL, we obtain that the mean emergence point for the proximal and distal branches at -26,33mm ± 10,05 and -11,67mm ± 7,31 respectively.

Regarding its type of innervation according to Taylor’s classification, we classified the specimen with two branches to the ECRL as a Taylor type 3, two specimens with one motor branch to ECRL were classified as Taylor type 1, and one specimen was classified as Taylor type 2 (Table 1).

Table 1

ECRL innervation characteristics including: the specimen, number of branches, origin of motor branches, distance from the origin to the intercondylar line, length from its origin to the entrance into the muscle and Taylor type.

All specimens only had one motor branch to the ECRB. We found variability in its origin, emerging in seven specimens from the deep branch of the radial nerve, just before its division into the supinator motor branches and its entry into the Frohse’s arcade, and in two specimens emerging from the superficial branch.

In all the specimens the motor branch to the ECRB emerged distal to the intercondylar line, with a mean distance of 23,89mm ± 8,37.

All specimens were classified as Taylor type 1, except from one specimen that was classified as Taylor type 2 (Table 2).

Table 2

 ECRB innervation characteristics including: the specimen, number of branches, origin of motor branches, distance from the origin to the intercondylar line, length from its origin to the entrance into the muscle and Taylor type.

We measured the length of each motor branch to ECRL and ECRB to estimate with further studies the possibility of a tension free nerve transfer. For the motor branches to the ECRL we obtained a mean length of 44,47mm ± 12,26, measuring both the proximal and the distal branches apart we obtained a mean length of 37,80mm ±13,72 and 48,67mm ± 11,83 respectively. In reference to the motor branch to the ECRB we obtained a mean length of 55,40 ± 17,90 from its emergence point to its entrance into de muscle belly.

Discussion

In this study we describe the innervation pattern of ECRL and ECRB to use its branches in nerve transfers for restoration of flexion and extension of the wrist and elbow. Hyejin Cho, et al. [2] reported the ECRL was innervated by one branch from the radial nerve in the 70% of the specimens, by two branches in the 27,5% and by three branches in a 2,5% of them; in all the specimens above the intercondylar line. We describe that the ECRL have double innervation from the radial nerve in six out of nine (66,7%) specimens, finding a higher incidence of redundant innervation in our specimens. This redundant innervation allows the use of one of these branches to restore flexion or extension of the wrist or elbow depending on the nerve transfer, without a total loss of function of the ECRL when using it as a donor. Classically nerve transfers reported in the literature for the restoration of flexion and extension of the wrist did not include the branches of the radial nerve to the ECRL neither as donor nor receptors. Antonio Garcia-López et al. [5] described the use of the ECRL branches as receptor in the restoration of wrist extension, using a nerve transfer from pronator teres branches to ECRL branches and FCR branch to the PIN. They advocate for the ECRL instead of the ECRB branch because of its easier identification, in specimens where the ECRB branch arises from the deep branch of the radial nerve it can easily be confused with the PIN, or even with the supinator branches. As we mentioned before, Charlotte Waxweiler et al. [11] describe the use of ECRL in nerve transfers using a branch from the BR to the ECRL to reduce spasticity and recover extension of the wrist. They found a single motor branch innervating the ECRL in a 70% of the specimens and two branches in a 30% of cases, a lower percentage of redundant innervation of that we found. As we also found, they mention that all ECRL branches were originated distally to the BR nerve branches. Jan Fridén and Andreas Gohritz [4] reported a case of nerve transfer from brachialis motor nerve to an ECRL nerve branch for restoration of wrist extension in a patient with C5 tetraplegia with conservation of elbow flexion by biceps brachialis (C5/6) and brachialis muscles (C5/6/7) and abolition of wrist extension but without ECRL denervation (C5/6) with good clinical results (M3 strengh of wrist extension) by five months after surgery. Regarding its use as donor, we didn’t find any reports in the literature. As we reported, its anatomical location proximal to the intercondylar line allows it to be a potential donor for nerve transfer to musculocutaneous nerve branches in restoration of elbow flexion, without a complete loss of function of the ECRL due its redundant innervation. We reported three patterns of ECRL motor nerve branching: two branches innervating the ECRL from the radial nerve, two branches innervating the ECRL, the proximal originating from the radial nerve and the distal one from the deep branch of the radial nerve, and one single branch for the ECRL originating from the radial nerve. Regarding ECRB innervation, we found in concordance with Hyejin Cho, et al [2], that ECRB was only innervated by one nerve branch in all the specimens, emerging in seven out of nine specimens from the deep branch of the radial nerve and in two out of nine from the superficial branch. The use of the ECRB branch for nerve transfer is well reported in the literature. Its commonly used for median to radial nerve transfers in restoration of wrist and fingers extension, but it’s also reported its use as donor in restoration of pronation of the forearm using PT branches as receptors [1, 7, 8]. Classically for median to radial nerve transfer it is used FDS nerve to ECRB nerve and FCR nerve to PIN because of its synergistic function [8]. Koji Sukegawa, et al [9] reported the feasibility of the median to radial nerve transfer from FDS nerve to ECRB nerve. They reported that the stump of the FDS branch reached the ECRB nerve branch without tension in all the specimens they studied, and also reported that the number of axons in the FDS nerve branch was superior to 30% of axons of the ECRB needed for the nerve transfer to be functional [3, 6]. In our study we found that the ECRB was innervated by a single branch from the radial in all the specimens dissected, and all of them below the intercondylar line. We reported two branching patterns to innervate the ECRB. In seven specimens the ECRB branch was originated from the deep branch of the radial nerve, just before it’s entrance into de Frohse’s arcade. The other two specimens gave the ECRB branch from the superficial branch of the radial nerve, suggesting that injury in the superficial branch of the radial nerve can be accompanied by an ECRB paralysis.

Conclusion

To conclude, we reported different patterns of ramification of the radial nerve to innervate the ECRL and ECRB, finding three different patterns for the ECRL and two for the ECRB. We described its anatomical characteristics regarding its origin and its length in order to easily identify them to its use as nerve transfers. We describe both ECRL and ECRB are reasonable options to undergo nerve transfer either as donor or recipient nerve. ECRB nerve transfer is already stablished for median to radial nerve transfer, so we described its anatomic parameters to facilitate its identification. We also reported the potential of ECRL motor branches to use it as a donor or receptor in nerve transfers for the restoration of flexion/extension of elbow and wrist due its constant origin after the BR motor branches and in most of the cases its redundant innervation.

Declarations

Author Contribution

Pablo Martínez Collado: Protocol/project development, data collection, data analysis, manuscript writing/editing.

Guillermo-José Tarnawski Español: Data collection, data analysis, manuscript writing/editing.

Anton Fornieles Espinel: Data collection, data analysis, manuscript writing/editing

Júlia Benítez Flores: Data collection, data analysis, manuscript writing/editing

Mª Rosa Morro Martí: Protocol/project development, manuscript writing/editing.

Manuel Llusá Pérez: Protocol/project development, manuscript writing/editing.

Ethical Approval 

Not applicable

Competing interests 

All authors certify that they have no affiliation with or involvement in any organization or entity with any financial interest or non-financial interest in the subject matter discussed in this manuscript.

Authors' contributions 

P.M.C: Protocol/project development, data collection, data analysis, manuscript writing/editing, dissections; GJ.T.E: Data collection, data analysis, manuscript writing/editing; A.F.E: Data collection, data analysis, manuscript writing/editing; J.B.F: Data collection, data analysis, manuscript writing/editing; MR.M.M: Protocol/project development, manuscript writing/editing; M.L.P: Protocol/project development, manuscript writing/editing.

Funding 

The authors did not received support from any organization for the submitted work.

Availability of data and materials 

All data was collected by our team and is reported in Table 1 and Table 2 in the manuscript.

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