Distribution of Primary Motor Nerve Branchesand Terminal Nerve Entry Points to the Forearm Muscles

JIE LIU, ROBERTW.H. PHO,* BARRY P. PEREIRA, HUI-KING LAU,AND V. PREM KUMAR

Department of Orthopaedic Surgery, National University of Singapore, Singapore

ABSTRACT Background: The information available on innervationpattern of the human forearm muscles in standard anatomy texts, al-though adequate for routine procedures, is not detailed enough forsurgical reconstruction in complex injuries of the limb and for paralyticconditions of the forearm from peripheral nerve and spinal cord injuries.Methods: The innervation pattern in 10 cadaveric forearms was studied.

The contributions of the main nerve trunks to each forearm muscle wasexamined. The location and number of the primary motor nerve branch-ing points and of the terminal nerve entry points to each muscle wereinvestigated. The location of both the primary nerve branching points andterminal nerve entry points was presented as a percentage of forearmlengthmeasured from the lateral humeral epicondyle to the radial styloid.Results: Seven of 19 forearm muscles were innervated from a single

branch from the main nerve trunk. The remaining 12 received more thanone primary branch. Two of 19 forearm muscles had only one terminalnerve entry point. The others had two or more each. In 13 of 19 forearmmuscles, the statistical median location of the primary motor nervebranching points was within the proximal one-third of the forearm lengthand either more proximally or distally for the remainder. The statisticalmedian location of the terminal nerve entry points was within theproximal one-third in 9 forearm muscles and within the middle one-thirdof the forearm in 8 forearmmuscles. In two, it was located proximal to theelbow and in the distal one-third of the forearm, respectively.Conclusions: In lacerations across the forearm, where main nerve

trunks are divided, mere repair of the nerve trunks would not address thedenervation of muscle or segments of muscle by the division of theprimary (or secondary) nerve branches traversing the wound and whichtook origin proximal to the laceration either from the divided nervetrunks or from other undamaged nerve trunks. Although the main nervetrunks may be intact, segmental crush injuries will defunction muscles bydirect muscle damage or by damage to the terminal nerve entry points tothe muscle. Knowledge of the location of the nerve branches and theterminal nerve entry points facilitates the insertion of electrodes at themotor points of forearm muscles for functional electrical stimulation inupper motor neuron lesions. The information in this study may also beusefully applied in selective denervation procedures to balance musclesin spastic upper limbs. Anat. Rec. 248:456–463, 1997. r 1997 Wiley-Liss, Inc.

Key words: innervation pattern; primary nerve branches; terminal nerveentry points; forearmmuscles

The detailed innervation pattern of the human fore-arm muscles, which is lacking in standard anatomytexts (Mortensen and Pettersen, 1966; Hollinshead,1983; Williams et al., 1989), has been supplemented bythe works of Brash (1955), Hollinshead and Markee(1946), Linell (1921), Reid (1920), Seddon et al. (1943),and Sunderland (1968). However, practicing surgeons

Contract grant sponsor: National Medical Research Council; Con-tract grant number: NMRC/96/0066; contract grant sponsor: ShawFoundation; contract grant number: GR05988N; contract grant spon-sor: National University of Singapore; contract grant number:RP950330.*Correspondence to: Robert W.H. Pho, Department of Orthopaedic

Surgery, National University of Singapore, 5 Lower Kent Ridge Road,Singapore 119260.Received 4 November 1996; accepted 10 February 1997.

THE ANATOMICAL RECORD 248:456–463 (1997)

r 1997 WILEY-LISS, INC.

still face great variations in the locations of branchingpoints from the nerve trunks and terminal nerve entrypoints, which can be frustrating when they attempt tolocate motor points for insertion of electrodes for func-tional electrical stimulation. Denervation procedures(Garland et al., 1980; Mertens and Sindou, 1991) forspastic muscles may be confounded by the variation inthe location of the branching points in different pa-tients and even in different limbs in the same patient.Assessment of any remaining undenervated and undam-aged muscles in clean lacerations of the main nervetrunks in the forearm or in segmental crush injuries ofthe forearm is also difficult if knowledge of the branch-ing pattern and its variation to the various muscles islacking. Subsequent reconstruction is also difficult.Our purpose is to present succinctly the innervation

pattern of the forearm muscles with emphasis on thenumber of nerve branches and terminal nerve entrypoints to each muscle and the range of location of theseanatomical and surgically important entities with re-spect to fixed anatomical landmarks in different cadav-eric specimens.

MATERIALS AND METHODS

Ten fresh upper limbs from adult human cadavers ofAsian descent were used in this study. In all, the causeof death did not affect the muscles or nerves in theforearm. During dissection, the limbs were extended atthe elbow, with the wrist in the neutral position.Midline volar and dorsal incisions were made fromabove the elbow to below the wrist. The main trunks ofthe median, ulnar, and radial nerves and the anteriorinterosseous and posterior interosseous nerves wereexposed. Under a 332 magnification operating micro-scope (Carl Zeiss, Germany), the nerve trunks and theirbranches were individually followed distally to theterminal nerve entry points in the forearm muscles.Nineteen forearm muscles (8 flexors and 11 extensors)were studied in each limb. The number of motor nervebranches arising from the main nerve trunk to theindividual muscles and the locations of these branchingpoints were recorded. These branches were referred toas the primary nerve branches and the branching point

as the primary nerve branching point. Secondarybranches were defined as the subdivisions of the pri-mary branch into two or more branches. The number ofthe branches into the muscle belly and the locations oftheir entry points into the epimysium were also re-corded. These branches were referred to as the terminalnerve branches, and the entry points into the epimy-sium were referred to as the terminal nerve entrypoints (Liu et al., 1994, 1995).All measurements were done in situ by using a

vernier caliper (Mitutoyu, Japan). The number of pri-mary motor nerve branches to the various muscles andthe number of terminal nerve entry points are reportedas the mean with one standard deviation based on 10samples for each muscle. The distribution of the loca-tions is reported as a statistical median location alongthe length of the forearm. The location is expressed as apercentage of the forearm length, which was defined asthe distance between the lateral humeral epicondyleand the styloid process of the radius (Fig. 1). Locationsproximal to the lateral epicondyle were expressed asnegative values.

RESULTS

A total of 190 forearm muscles, 50 forearm mainnerve trunks, 289 primary motor nerve branches withtheir branching points from the main nerve trunks, and775 terminal nerve branches with their entry pointswere observed.

Contributions From the Nerve Trunks to Each Muscle

The main nerve contribution to each muscle is shownin Table 1. Most muscles, with the exception of theflexor digitorum sublimis (FDS), flexor digitorum pro-fundus (FDP), and extensor carpi radialis brevis (ECRB)were innervated by branches arising from one nervetrunk. In the case of FDS, innervation was from themedian nerve alone in 9 specimens (90%); in 1 specimen(10%), there was an additional innervation from theanterior interosseous nerve. For the FDP muscle, 8specimens (80%) were innervated by both the anteriorinterosseous nerve and the ulnar nerve, and 2 (20%)were innervated solely by the anterior interosseousnerve. Where the FDP was innervated by both nerves,the anterior interosseous nerve was noted to give moreprimary branches and terminal nerve entry points thanthe ulnar nerve (average ratio 5 3:1; Fig. 2). For theECRB, 9 specimens (90%)were innervated by the posteriorinterosseous nerve and 1 (10%) by both the posteriorinterosseous and the superificial branch of the radialnerve.

Number of Primary Nerve Branches and Terminal NerveEntry Points to Muscles

The mean number of primary branches and terminalnerve entry points to eachmuscle is also summarized inTable 1.Three flexors (flexor carpi radialis [FCR], palmaris

longus [PL], and pronator quadratus [PQ]) had only oneprimary branch, and the remaining 5 flexors (pronatorteres [PT], FDS, FDP, flexor pollicis longus [FPL], andflexor carpi ulnaris [FCU]) each received a mean of twoor more primary branches. The PL shared its primarynerve branch from the median nerve with the FDS (Fig.

Abbreviations

AIN anterior interosseous nerveAPL abductor pollicis longusBR brachioradialisECRB extensor carpi radialis brevisECRL extensor carpi radialis longusECU extensor carpi ulnarisEDC extensor digitorum communisEDM extensor digiti minimiEI extensor indicisEPB extensor pollicis brevisEPL extensor pollicis longusFCR flexor carpi radialisFCU flexor carpi ulnarisFDP flexor digitorum profundusFDS flexor digitorum sublimisFPL flexor pollicis longusMN median nervePIN posterior interosseous nervePL palmaris longusPQ pronator quadratusPT pronator teresSUP supinatorUN ulnar nerve

457DISTRIBUTION OF NERVE BRANCHES TO FOREARM MUSCLES

3). This sharing was also seen in one specimen whereboth FDP and FPL shared the primary branch from theanterior interosseous nerve. In addition, the secondarynerve branch to PL always passed through the musclebelly of FDS before innervating PL (Fig. 3).Four extensors (extensor digiti minimi [EDM], exten-

sor carpi ulnaris [ECU], extensor pollicis brevis [EPB],and extensor indicis [EI]) consistantly had only oneprimary branch, whereas the other 7 (brachioradialis[BR], extensor carpi radialis longus [ECRL], ECRB,supinator [SUP], extensor digitorum communis [EDC],aabductor pollicis longus [APL], and extensor pollicislongus [EPL])may have had one, two, or three branches,depending on the specimen. Only the SUP displayed amean of two or more branches. In about 50% of thespecimens, EDM and EDC, ECU and EDC, and EPLand EI were found to receive the secondary branchesfrom the same primary branch from the posteriorinterosseous nerve. Similarly, occurrence of twomusclesbeing innervated from the same primary branch wasseen in BR and ECRL, ECRL and ECRB,APL and EPB,and EPB and EPL. EI in one specimen, besides sharing

its primary branch with EPL, had its secondary branchfirst passing through the EPL muscle belly beforeinnervating the EI.Fourteen of 19 forearm muscles (7 flexors and 7

extensors) each had two or more terminal nerve entrypoints. EI and EPB had only one entry point, and thenumber of terminal entry points on PL (Fig. 3), EPL,and EDMwas between one and two.

Distribution of the Primary Nerve Branching Points andTerminal Nerve Entry Points

The distribution of the primary nerve branchingpoints and terminal nerve entry points in the forearm isschematically represented in Figure 4. The location ofeach point was expressed as a percentage of the fore-arm length. In our series, the mean forearm length was260.0 6 16 mm.The primary nerve branching points in all the BR and

ECRL specimens were located proximal to the trans-verse plane of the lateral epicondyle. Three of 10specimens of PT and 1 of 10 ECRB also had branchingpoints in this region. All FCR, PL, FCU, SUP, and EDM

Fig. 1. Measurement details. The distances from the lateral epicondyle of the humerus to the primarymotor nerve branching points and the terminal nerve entry points were recorded for each individualmuscle. These distances were expressed as a percentage of the length of the forearm, which is the distancebetween the lateral epicondyle and the styloid process of the radius. Details for FPL are given.

458 J. LIU ET AL.

specimens had all their branching points located withinthe proximal one-third of the forearm, whereas FDP,FPL, EDC, ECU, APL, EBP, EPL, and EI specimenshad their branching points within the proximal andmiddle one-thirds of the forearm. Two specimens ofFDS also had their branching points located in thedistal one-third. All PQ specimens had their branchingpoints in the distal one-third.Most forearm muscles, with exception of PQ, EPL,

EPB, and EI, had the statistical median occurrence ofthe branching points located within the proximal one-third.Although most of the muscles had their primary

nerve branching points more proximally located thanthe corresponding terminal nerve entry points, EDChad some of its primary nerve branching points moredistally located than the corresponding nerve entrypoints.Five BR specimens (50%) had all of their terminal

entry points located proximal to the transverse plane ofthe lateral epicondyle and the remaining 5 specimens(50%) had entry points both proximal to the epicondyleand in the proximal third region. Seven ECRL speci-mens (70%) had their entry points scattered in theproximal one-third of the forearm, and the other three(30%) had their nerve entry points located proximal tothe lateral epicondyle. PT and SUP had all their entrypoints within the proximal one-third of the length offorearm, and FCR, PL, FPL, FCU, ECRB, EDC, ECU,andAPL had entry points within both the proximal andmiddle one-thirds. However, FDS and FDP had termi-nal entry points scattered in all three regions of theforearm. Entry points of EPB, EPL, and EI were found

within the middle and distal one-thirds, whereas thoseof EDM were found only within middle one-third andPQ only within the distal one-third.Muscle from 9 forearms had the statistical median

occurrence of the terminal nerve entry points locatedwithin the proximal one-third of the forearm and 8 hadtheirs located in the middle one-third; BR and PQ hadtheirs proximal to the elbow and in the distal one-thirdof the forearm, respectively.

DISCUSSION

The innervation pattern of the forearm musclespresented in this study, especially the number (Table 1)and the location of the primary motor nerve branchingpoints, and the terminal nerve entry points (Fig. 4) willhave clinical application for clinicians and perhaps willcomplement previous works (Brash, 1955; Hollinsheadand Markee, 1946; Linell, 1921; Reid, 1920; Seddon etal., 1943; Sunderland, 1968).Sunderland (1968) discussed details of the motor

nerve branching points from themain nerve trunks andthe length of the branches to their entry points from anerve regeneration point of view. Reid (1920) fastidi-ously located the surface marking of the terminal nerveentry points for EMG analysis.Linell (1921) reported a constant relationship be-

tween the length of forearm and the distance of motornerve entry points from the lateral epicondyle andobtained constant figures for each muscle based on anaverage limb length. However, considerable variationsin locations of primary nerve branching points andterminal nerve entry points can occur, as shown in ourstudy, and even between the two sides of the samesubject. Thus, the locations expressed as a range with astatistical median may be more useful.With regard to the contribution of the main nerve

trunks to muscles, the data generally agreed with thoseof previous reports (Linell, 1921; Seddon et al., 1943;Mortensen and Pettersen, 1966; Sunderland, 1968;Hollinshead, 1983; Spinner, 1984; Tountas and Bergman,1993;Williams et al., 1989). There were, however, interest-ing variations in the innervation of FDP, ECRB, and SUP.In general, FDP received nerve supply from both the

ulnar nerve and themedian/anterior interosseous nerve(some investigators did not differentiate these twonerves), as in 8 of our specimens (80%). Sunderland(1968) showed that this double innervation pattern tothe muscle was quite constant in 19 of 20 (95%)cadaveric limbs and in 37 of 38 (97%) clinical cases.Furthermore, his clinical observation also suggestedthat the median nerve innervation is responsible forflexing the radial two fingers and the ulnar innervationfor the ulnar two fingers in 50% of cases and thatdifferent portions of the muscle power different fingersin the other 50%. Brand and Hollister (1993) docu-mented that FDP is composed of four individual musclebellies to the four fingers, but there is no report on theinnervation to each belly. Based on their description, itis easy to understand that the innervation to FDP maybe very complex and have more than one pattern.ECRB was generally understood to be innervated by

the posterior interosseous nerve (Linell, 1921; Cricenti

TABLE 1. Contribution of themain nerve trunks to eachmuscle and the number (mean 6 S.D.) of primarymotor

nerve branches and terminal nerve entry points

MuscleMain nervecontribution

Numberof primarymotor nervebranches

Numberof terminalnerve entrypoints

FlexorsPT MN 2.0 6 0.67 3.4 6 0.70FCR MN 1.0 3.1 6 1.10PL MN 1.0 1.1 6 0.33FDS MN (90%);MN and

AIN (10%)4.2 6 1.23 5.6 6 1.65

FDP AIN andUN (80%);AIN (20%)

3.7 6 1.49 5.8 6 1.23

FPL AIN 2.0 6 0.82 3.0 6 0.67PQ AIN 1.0 2.4 6 0.52FCU UN 2.0 6 0.94 2.5 6 0.71

ExtensorsBR Radial nerve 1.2 6 0.42 2.4 6 0.70ECRL Radial nerve 1.2 6 0.42 3.3 6 0.82ECRB PIN (90%); PIN and

RN (10%)1.1 6 0.32 2.5 6 0.71

SUP PIN 2.1 6 1.20 3.1 6 0.35EDC PIN 1.1 6 0.32 4.3 6 0.67EDM PIN 1.0 1.2 6 0.67ECU PIN 1.0 2.89 6 0.78APL PIN 1.1 6 0.32 2.0EPB PIN 1.0 1.0EPL PIN 1.4 6 0.52 1.7 6 0.67EI PIN 1.0 1.0

459DISTRIBUTION OF NERVE BRANCHES TO FOREARM MUSCLES

et al., 1994). However, the main radial nerve or thesuperficial branch of the radial nerve (Brand andHollister, 1993) may also innervate it in differentspecimens. One of 10 specimens in this study received abranch from the superficial radial nerve.Although Sunderland (1968) reported that 20% of

SUP in his series had their nerve branches from theradial nerve, this was not observed in the present study(all of these were innervated by the posterior interosse-ous nerve) nor in other reports (Mortensen and Pet-tersen, 1966; Hollinshead, 1983; Spinner, 1984; Toun-tas and Bergman, 1993; Williams et al., 1989).Flexors in general tended to have more primary

motor nerve branches and terminal nerve entry pointsthan the extensors. This finding may be related to theneed to generate more power in the flexors and, in thecase of FDS and FDP, the need for complex function.Recent reports (English and Letbetter, 1982; Segal etal., 1991; Segal, 1992; Thomson et al., 1991) havesuggested the relationship of terminal nerve entrypoints to subdivisions of muscles and neuromuscularcompartments. Each entry point may correspond to aneuromuscular compartment that can function indepen-

dently from other compartments (English and Letbet-ter, 1982). Thus, muscles with complex functions suchas FDS, FDP, and EDC have a greater number ofterminal nerve entry points than do other forearmmuscles (Schieber, 1991).Our study, like previous reports (Mortensen and

Pettersen, 1966; Hollinshead, 1983; Tountas and Berg-man, 1993), showed that PL always shares one primarymotor nerve branch with FDS from the median nerveand that the secondary or terminal branch to PL passesthrough the proximal part of the muscle belly of FDSbefore innervating it. This finding may be related to theevolutionary development of PL and FDS because PLmay substitute for the ring-finger slip of FDS (Tountasand Bergman, 1993). The same reasoning can be ap-plied to EDM because it shares a nerve branch withEDC from the posterior interosseous nerve and othernerve sharing instances. In one of our cases, theterminal nerve branch to EI also passed through thebelly of EPL first before innervating EI. This phenom-enon has not been observed in previous reports(Mortensen and Pettersen, 1966; Hollinshead, 1983;Sunderland, 1968) andmay be a less common variation.

Fig. 2. FDP innervated by the anterior interosseous nerve, which gives off three primary motor nervebranches (white arrows) to the FDP; the ulnar nerve gives off one (black arrow). The anterior interosseousnerve gives off five terminal nerve entry points (white arrowheads); the ulnar nerve gives off only two(black arrowheads).

460 J. LIU ET AL.

Segmental crush injuries of the forearm over theproximal one-third will effectively defunction most ofthe flexor compartment (Fig. 4), whereas some of theextensor muscles (BR, ECRL, EPB, EPL, EI) and PQwill survive with intact innervation. Middle one-thirdforearm crush injuries may defunction FDS, FDP, FPL,and some extensors (EDM, APL, EPB, EPL or EI),

whereas the rest of the flexor muscles will survive onlymarginally harmed. Distal one-third injuries essen-tially leave the forearm musculature intact. In theclinical situation, however, the actual damage to softtissue is usually not confined to the zone of injury butextends proximally and distally as the energy of theforce is dissipated.

Fig. 3. Primary motor nerve branch (white arrow) shared by PL and FDS. Secondary nerve branch to PL(black arrow) passing through the belly of FDS before innervating it. In this specimen, PL has twoterminal nerve entry points (white arrowheads). The black arrowhead indicates one of the terminal nerveentry points on FDS.

461DISTRIBUTION OF NERVE BRANCHES TO FOREARM MUSCLES

Figure 4 also enables assessment of muscle denerva-tion with lacerations involving the major nerve trunks.Laceration over the proximal one-third of the forearmwill effectively denervate all the muscles innervated bythe major nerve trunk. Mid one-third forearm lacera-tions of the median and ulna require repair essentiallyfor hand intrinsic muscle function and sensation be-cause almost no forearm muscles will be completelydenervated with the exception of PQ. However, merelyrepairing a major nerve divided in a transverse lacera-tion of the forearm will not address the division of theprimary and secondary nerve branches traversing thelaceration having arisen from the divided major nerveand other major nerves proximal to the laceration.Recovery in such a situation will be expected to beincomplete.The information on the number and the locations of

motor nerve branching points and terminal nerve entrypoints is also useful in selective denervation of spasticmuscles to equalize tone between agonist and antago-nist muscles (Garland et al., 1980; Mertens and Sindou,1991). This surgical procedure enables restoration offunction, improves cosmesis, andmay facilitate nursingcare with better positioning of the spastic limbs. Infor-mation on the neuromuscular compartment supplied

by each terminal nerve entry point will probably berequired before deciding which terminal nerve branchshould be sectioned to restore optimal muscle function.The location of the terminal nerve entry points to the

different muscles is useful for electrophysiological stud-ies (Reid, 1920) and facilitates the application of elec-trodes to activate muscles paralyzed from upper motorneuron lesions (Hoshimiya et al., 1989; Kilgore et al.,1989; Lau et al., 1995; Liu et al., 1995; Nathan, 1989),which applies to patients with traumatic spinal cordinjury, cerebravascular accidents, and even cerebralpalsy. Electrodes may be surface (percutaneous), beingintroduced into the terminal nerve entry points throughthe skin, or may even be applied by open surgery bydirect suture to terminal nerve entry points. By stimu-lating the points electrically with computer control,useful hand function can be achieved. Although thebranches to the different muscles have been outlined indetail in this study, additional physiological studies areindicated to determine the extent of each muscle’sneuromuscular compartment innervated by a particu-lar branch and its terminal nerve entry point (Englishand Letbetter, 1982; Thomson et al., 1991). Only thencan the ideal branches or motor nerve entry points beselected for neuromuscular stimulation.

Fig. 4. Distribution (vertical bars) of location of the primary motornerve branching points and distribution of location of the terminalnerve entry points expressed as a percentage of the forearm length(lateral epicondyle of humerus to the styloid process of radius). Theshorter horizontal bars represent the sites where the most distal and

most proximal primary motor nerve branching points and terminalnerve entry points in the different muscles. The longer horizontal barsrepresent the statistical median location of the primary motor nervebranching points and the terminal nerve entry points.

462 J. LIU ET AL.

The present study, unlike previous work (Linell,1921; Sunderland, 1968), has illustrated the greatvariation in the innervation pattern of the forearmmusculature. Such information may prove useful to thepracticing surgeon who may find information fromstandard texts (Mortensen and Pettersen, 1966; Wil-liams et al., 1989) and even those designated forsurgeons (Hollinshead, 1983) incomplete. Further stud-ies extended to other sections of the human limbmay beindicated for a complete picture.

ACKNOWLEDGMENTS

We are grateful to Mr. E.L. Leow for the illustration,Associate Professor E.H. Yap for the use of the cadaverdissection room, and Mr. Tan Boon Kiat for the photog-raphy.

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463DISTRIBUTION OF NERVE BRANCHES TO FOREARM MUSCLES