Introduction
The radial nerve (RN) is the main nerve among those originating from the posterior
fascicle of the brachial plexus. It innervates all muscles in the posterior compartment
of the arm and forearm. It passes from the posterior to the anterior compartments,
bypassing the RN groove in the humerus. It passes through the intermuscular septum
between the brachialis muscle (BM) medially, and the brachioradialis (BR) muscle laterally.
It follows distally, emerging between the BR and the extensor carpi radialis longus
(ECRL). It is divided into the superficial branch of the radial nerve (SBRN) and posterior
interosseous nerve (PIN), which is also called the deep branch of the radial nerve
(DBRN). The radial tunnel is a musculoaponeurotic structure through which the PIN
progresses, extending from the lateral epicondyle of the humerus to the distal edge
of the supinator muscle (SM).[1 ]
[2 ] Knowledge of the anatomy of the motor branches of the RN in the forearm is important
when performing surgical procedures in the region, such as the approach of the proximal
third and the head of the radius, the release of compressive syndromes of the PIN
and radial tunnel, and to understand the order of recovery of muscle function after
a nerve injury.[1 ]
[2 ] The information in the present study can also be applied in selective denervation
procedures to balance muscles in spastic upper limbs.[3 ]
[4 ] The branches of the RN can be transferred to restore digital flexion in cases of
proximal lesion to the median nerve (MN) or lesions to the brachial plexus involving
predominantly lateral and medial cords, with preservation of the posterior cord.[5 ]
[6 ] The motor points of the forearm muscles are defined as the place of entry of each
nerve branch into the muscular body.[3 ] The studies consulted show that the order of motor innervation, the number of branches,
the motor points, and the anatomical variations of the RN branches present controversies.[3 ]
[4 ]
[5 ]
[6 ]
[7 ]
[8 ]
[9 ]
[10 ]
[11 ] The aim of the present study was to analyze the anatomical variations of the motor
branches of the RN in the elbow region, considering origin, course, length, branches,
motor points and relationships with neighboring structures.
Materials and Methods
In total, 30 limbs from 15 cadavers, all adult and male, prepared by intra-arterial
injection of a solution with 10% of glycerin and formaldehyde, were dissected. Each
forearm was dissected with the elbow in extension, the wrist in neutral position,
and the forearm in pronation. The cadavers showed no evidence of deformities, previous
surgical procedures, or traumatic injuries in the studied area. We removed the skin
and fascia from the distal third of the arm, forearm and wrist. The RN was identified
in the arm between the BM and BR and dissected from proximal to distal. The tendons
from the BR, ECRL and extensor carpi radialis brevis (ECRB) muscles were sectioned
in their distal thirds, and separated from their fibrous connections, to facilitate
the identification of the nerve branches. The division of the RN into its branches,
SBRN and PIN, was identified and related to the intercondylar humeral line (IHL).
The branches destined to the BR, ECRL, ECRB, SM and PIN muscles were dissected. The
vascular structures were not preserved to facilitate nerve dissection. We used, at
certain stages of dissection, a magnifying glass with 2.5 times of magnification.
We analyzed the distance from the IHL both from the RN division point and from the
emergence point of the RN in the arm, between the BM and BR muscles. The order of
innervation of each muscle, the number of branches, and the number of motor points
were recorded. With a digital caliper and a millimeter ruler, we measured the diameter
and length of the branches to the BR, ECRL, ECRB, PIN, and SM. The present study was
approved by the Ethics in Research Committee under opinion number 3,339,423.
Results
The RN crossed the lateral intermuscular septum, between the BM and BR, at an average
of 9.2 cm (8.5 cm to 10.3 cm) proximal to IHL. The division into SBRN and PIN occurred
at an average of 1.2 cm (0 cm to 2.3 cm) proximal to the IHL. We didn't record any
RN division distal to the IHL.
Branches to the BM: we identified one branch from the RN to the BM in three limbs,
and two branches in another limb, all of them above the IHL and proximal to the origin
of the branches to the BR muscle ([Figure 1A ]).
Fig. 1 (A) branch to the BM (a); branch to the BR (b); branches to the ECRL (c); branch
to the ECRB (d); SBRN (e); PIN (f). (B) radial nerve (a).
Branches to the BR muscle: they detached to the other forearm muscles proximally in
relation to the branches in all limbs. The presence of only 1 branch to the BR muscle
was recorded in 23 limbs (76.5%) ([Figure 1A ]); in 7 (23.5%), we identified 2 branches ([Figure 1B ]). We did not identify more than two branches to the BR in any limb. The length of
the branch to the BR was of 3.2 ± 0.8, and the mean number of motor points was 2.4 ± 0.8.
Branches to the ECRL: they originated in the RN before its division into 26 limbs
([Figure 2A ]). In two limbs, they originated at the division point of the RN; in two others,
from the PIN. In 21 limbs (70%), we identified 1 branch to the ECRL ([Figure 2A ]), and there were 2 branches in 9 limbs (30%) ([Figure 1A ]). We did not identify more than teo branches to the ECRL. In 6 limbs (20%), the
branches to the BR and ECRL muscles originated from a common trunk ([Figure 1B ]). In one limb, they originated from a common trunk to the BR and ECRB ([Figure 2B ]). The length of the branch to the ECRL muscle was of 3.2 ± 1.0, and the mean number
of motor points was 2.9 ± 1.0.
Fig. 2 (A) Radial nerve (a); branch to the BR (b); branch to the ECRL (c); branch to the
ECRB (d); PIN (e); SBRN (f); branch to the SM (g). (B)
Branches to the ECRB: In all limbs, we identified only one branch to the ECRB. It
originated in the PIN in 14 limbs (46.5%) ([Figure 2A ]), in the SBRN in 12 (40%) ([Figure 3A ]), and in the RN only in 4 (13.5%): 3 at the same point of division of the RN into
SBRN and PIN ([Figure 3B ]), and 1 in the proximal division. The branch to the ECRB was divided, penetrating
the anterior surface of the muscular body into at least two motor points. In 15 limbs,
we recorded a distance between the proximal and distal motor points greater than 3 cm
([Figure 4A ]). The length of the branch to the ECRB muscle was of 4.5 ± 2.5, and the mean number
of motor points was 2.7 ± 1.2.
Fig. 3 (A) Radial nerve (a); branch to the BR (b); branch to the ECRL (c); branch to the
ECRB (d); SBRN (e); PIN (f); branches to the SM (g). Fig.
Fig. 4 (A) Radial nerve (a); branches to the BR (b); branch to the ECRL (c); branch to the
ECRB (d); SBRN (e); ECRB motor points (f); PIN.
Branches to the SM: They showed great variability, and we identified 2 to 5 branches,
all from the PIN, with at least one branch destined to each of the superficial and
deep heads ([Figure 4A ] and [4B ]). We identified branches to the SM proximal to the arcade of Frohse in 6 limbs,
2 branches to the SM in 11 limbs, and 3 branches in 4 limbs ([Figure 5A ] and [5B ]). In 7 limbs, we did not identify branches to the SM proximal to the arcade of Frohse.
In them, the PIN emitted branches to the SM while passing through the muscle ([Figure 6A ] and [6B ]). In two limbs, only one branch detached from the PIN, but it was duplicated proximally
to the arcade of Frohse. The length of the branches to the SM was of 1.0 ± 0.8, and
the mean number of motor points was 2.7 ± 1.2. The number of motor points from the
muscles innervated by the RN in the proximal, middle and distal thirds of the forearm
are described in [Table 1 ].
Fig. 5 (A) branches to the MS (a); PIN (b); arcade of Frohse (c); branch pair ECRB (d);
SBRN (e); median nerve (f); nerve branches.
Fig. 6 (A) PIN (a); arcade of Frohse (b); SBRN (c); branch to the ECRB (d). (B) branches
to the MS (a); PIN (b); arcade of Frohse (c).
Table 1
Muscle
1 motor point
2 motor points
3 motor points
4 motor points
5 motor points
Average motor points
Brachioradialis
10
16
4
-------------
-------------
2.4 ± 0.8
Extensor carpi radialis longus
7
17
5
1
-------------
2.9 ± 1.0
Extensor carpi radialis brevis
3
12
10
5
-------------
2.7 ± 1.2
Supinator
------------
4
16
7
3
3.4 ± 0.9
Discussion
Our results are in agreement with those of some authors,[7 ]
[8 ]
[9 ]
[10 ]
[11 ] who report that the order of innervation of the forearm muscles by the RN is very
variable. In the present study, we observed that the branch from the RN to the BR
detached from the RN proximally to its division into the SBRN and PIN. The branches
to the ECRL detached from the RN proximally to its division into 26 limbs, in 2, at
the point of division, in 2 others, to the PIN. We did not identify the division of
the RN distally to the IHL.
Abrams et al[8 ] studied 20 limbs from cadavers and identified more than 1 branch to the BM in 10
limbs. In 8 of these 10 limbs, the innervation of the BM occurred proximally in relation
to the BR. Sunderland[7 ] observed that, in 18 of 20 limbs, the BM received radial innervation. Our findings
were different: we identified that in only 4 out of 30 limbs there was 1 branch of
the RN to the BM, all of them proximal to the branches destined to the BR muscle.
Fuss and Wurzl[10 ] studied 50 cadaver limbs and recorded that the BR muscle received 1 branch in 22
limbs, 2 branches in 12 limbs, and 3 branches in 16 limbs. In the present study, the
first branch of the RN to the forearm muscles was to the BR muscle in all limbs. We
identified only 1 branch in 23 limbs, 2 branches in 7 limbs, and we did not identify
more than 2 branches to the BR. The reason for the discrepancy must have occurred
due to the way of interpreting. We considered the number of branches that detached
from the RN: those that branched out after their origin, forming several motor points,
were considered as a single branch ([Figure 4B ]). Fuss and Wurzl[10 ] also report that sometimes branches to the BR detached from the RN from a common
trunk with branches to the BM or ECRL.
Branovacki et al[11 ] dissected 60 limbs from cadavers, and reported that the BR was the first forearm
muscle to be innervated in 42 limbs (70%); in 12 (20%), the innervation of the BR
and ECRL originated at the same point; in 6 (10%), the branch to the ECRL originated
proximally to that of the BR. In the present study, we identified in 6 limbs (20%)
that the BR and ECRL originated from a common trunk. We did not identify, however,
the ECRL innervated before the BR in any limb. Fuss and Wurzl[10 ] reported an extremely rare case in which a branch to the BR carried motor and sensory
fibers: the motor fibers penetrated the BR, and the sensory ones joined the SBRN.
Regarding the ECRL muscle, Fuss and Wurzl[10 ] reported that they identified 1 branch in 22 limbs, 2 branches in 14, and 3 branches
in 12 limbs. Our findings were divergent: we identified 2 branches only in 9 limbs;
in the remaining 21, there was only 1 branch to the ECRL. The reason behind such different
results must have been the way of interpretation, as occurred in relation to the branches
to BR muscle. There are widely divergent conclusions regarding the origin of the branch
to the ECRB. The origin in the RN itself, the SBRN, or the PIN has been described.
The differences in incidence found in the literature are significant.[8 ]
Salsbury[12 ] dissected 50 limbs and stated that the innervation came from the SBRN in 56%, from
the PIN in 36%, and from the point where the PIN and SBRN branch out in 8%. Cricenti
et al[13 ] dissected 30 limbs and found the origin of the ECRB in the PIN in 28 (93%) limbs,
and in the SBRN in 2 (7%) limbs. Nayak et al[14 ] reported that they dissected 72 limbs from cadavers and found that the branch destined
to the ECRB originated in the RN in 11 limbs (15.2%), in the PIN in 36 limbs (50%),
and in the SBRN in 25 limbs (34.7%). Abrams et al[8 ] recorded the origin of the ECRB in the PIN in 45%, in the SBRN in 25%, and in the
RN in 30% of the limbs. Branovacki et al[11 ] recorded the origin of the ECRB in the PIN in 45%, in the SBRN in 25%, and in the
bifurcation of the SBRN and PIN in 30% of the limbs.
We agree with the explanation provided by Abrams et al,[8 ] that the discrepancies among studies can be explained by the inconsistency of dissection
and measurement techniques. The ECRB branch is often a set of separate fascicles,
but adherent to the SBRN or the PIN. Variation may occur depending on how much the
nervous branch is proximally dissected before the measurement[8 ] ([Figure 5B ]). We identified the origin of the ECRB in the PIN in 14 limbs (46.5%), in the SBRN
in 12 (40%), and in the RN only in 4 (13.5%) limbs, and, in 3 of these, at the same
point in which the RN divides into the PIN and SBRN, and, in another, the origin was
proximal to the division of the RN.
Branovacki et al[11 ] identified more than 1 branch to the SM in 73% of the limbs, and, Cricenti et al,[13 ] in 87%. In the present study, we identified two to five branches to the SM, with
at least one branch destined to each of the superficial and deep heads, all originating
in the PIN. Although Sunderland[7 ] reported in his series that 20% of the branches to the SM originated from the RN,
this was not observed in the present study. We agree with Spinner[2 ] and Liu et al[4 ] that the branches to the SM originated from the PIN. Bradovaki et al[11 ] observed that in 12% of the limbs the branches to the SM detached from the PIN inside
the muscle mass of the SM, and the authors reported that this variation, as far as
they were aware, had not been previously reported. We identified this variation in
7 (23%) limbs ([Figure 6A ] and [6B ]).
We analyzed the number of motor points of the BR, ECRL, ECRB and SM muscles, which
are defined as entry points of the nerve branches into the muscular body. We recorded
that most motor points are located in the proximal third of the muscles ([Table 2 ]).
Table 2
Muscle
Average motor points
Proximal third of the muscle
n (%)
Proximal and middle thirds
n (%)
Distal third
n (%)
Full extension of the muscular body n (%)
Brachioradialis
2.4 ± 0.8
30
(100%)
--------------
---------------
--------------
Extensor carpi radialis longus
2.9 ± 1.0
30
(100%)
--------------
---------------
--------------
Extensor carpi radialis brevis
2.7 ± 1.2
17
(56.5%)
13
(43.5%)
-------------
-------------
Supinator
3.4 ± 0.9
6
(20%)
19
(63.5%)
---------------
5
(16.5%)
Segal et al[15 ] suggested the relationship between the number of motor points and neuromuscular
compartments. Each motor point corresponds to a neuromuscular compartment with a function
independent from other compartments. This explains why muscles with more complex functions,
such as flexors and finger extensors have a greater number of motor points compared
to other forearm muscles.[16 ] Knowledge of the location of nerve branches and motor points facilitates the insertion
of electrodes in the motor points of the forearm muscles for functional electrical
stimulation in lesions to the upper motor neurons.[3 ] The information in the present study can also be applied usefully in selective denervation
procedures to balance spastic muscles.[4 ]
[11 ]
Liu et al[4 ] reported that forearm injuries, although the main nerve trunks may be intact, segmental
crush injuries will damage muscles by direct muscle damage or damage to their motor
points.
Fuss and Wurzl[10 ] reported that the attempt to correlate clinical signs and symptoms with surgical
anatomy may cause some confusion because of the controversies regarding the innervation
sequence recorded in the literature. For example, the sequences described by Clara[16 ] (BM; BR; ECRL; ECRB; SM; and ECRL) and by Roseinstein[17 ] (BR; ECRL; SM, extensor digitorum communis (EDC); and ECRB) may be criticized because
they suggest that each muscle does not receive more than a single branch, and that
there is a logical sequence of branches. We recorded that in most of our cases the
sequence of motor branches was: BM; BR; ECRL; ECRB; PIN; and branches to the SM. However,
in two limbs the branch to the ECRL originated from the PIN; in four, the branch to
the ECRB originated from the RN: in three of these, at the same point the RN divides
into SBRN and PIN, and only one was proximal to the division. In six limbs, the branches
to the BR and ECRL originated from a common trunk; in seven limbs, we identified more
than one branch to the BR; and, in nine limbs, more than one branch to the ECRL. We
agree with Fuss and Wurzl[10 ] that the sequence reported by Rosenstein[17 ] is very difficult to occur.
We suggest that the surgical approach to the branches of the RN in the elbow region
can be performed with the forearm in pronation and the elbow in extension. The incision
should have approximately 13 cm in length, starting from a point 3 cm proximal to
the lateral epicondyle, accompanying the axis of the radius. We incise the fascia
in the distal region of the arm and forearm and identify the space between the brachial
and brachioradial muscles. Deepening the dissection in this space enables the identification
of the RN with the branches to the BM, BR, ECRL and ECRB. The space between the ECRB
and the EDC is more distally identified. The dissection is deepened in this space,
enabling the identification of the SM and the arcade of Frohse. The PIN, proximally
to the arcade of Frohse, can be identified by palpation against the diaphysis of the
radius. The superficial head of the SM should be sectioned, following the path of
the PIN, thus exposing the intramuscular portion of the PIN and the branches destined
to the SM.
Conclusion
Knowledge of the anatomy of the RN branches to the forearm muscles is important when
performing surgical procedures in the region (such as the approach of the proximal
third and head of the radius, the release of compressive syndromes of the radial tunnel
and PIN, and distal nerve transfers) in order to understand the order of recovery
of muscle function after a nerve injury. Our data conflict with several anatomical
studies, and the variability in this region should be emphasized.
