Keywords
supernumerary muscles - musculotendinous abnormalities - extensor digitorum brevis
manus
Introduction
In the literature, we can find descriptions of a wide variety of aberrant muscles
around the hand. These anomalies can be classified as follows: variations in the location,
shape, or size of a muscle considered normal; existence of aberrant supernumerary
muscles; and absence of one or more usually existing muscles.[1]
Musculotendinous anatomical variations are usually asymptomatic. If they manifest,
they can give rise to neuropathies due to compression, pain due to the presence of
tenosynovitis and/or of a ganglion associated with the anomaly, or, in the case of
agenesis, absence of the function corresponding to said muscle.[2]
If we focus on supernumerary muscles, we could divide them into those found on the
dorsal side, and those found on the volar side.
Supernumerary muscles on the dorsal surface
There are multiple studies about the evolutionary development of the extensor muscles.[3] The precursor muscle mass in the forearm differentiates embryologically into three
distinct parts:
-
A radial portion that differentiates into the brachioradialis muscle and the extensor
carpi radialis longus and brevis muscles (ECRL and ECRB).
-
A superficial portion, which forms the extensor digitorum communis (EDC), the extensor
carpi ulnaris (ECU), and the extensor digiti minimi (EDM) muscles.
-
And a deep portion, which gives rise to the abductor pollicis longus (APL) and the
extensor pollicis brevis (EPB) muscles on the radial side, and the extensor pollicis
longus (EPL) and extensor indicis proprius (EIP) muscles on the ulnar side.
Anatomical studies[4]
[5] suggest that the radial and superficial portions are more stable in different species,
while the deep portion is highly unstable and has undergone considerable evolutionary
change. Therefore, it is to be expected that most anomalies happen within this deeper
portion.
Supernumerary muscles in the volar aspect
The abnormal muscles in the volar aspect of the wrist can be divided into three groups.
The first group is composed of those muscles that cross the carpal tunnel, while the
second and third groups are made up of those that cross the Guyon canal. The muscles
in the second group originate from the antebrachial fascia or from the tendons of
the flexor carpi radialis (FCR), flexor carpi ulnaris (FCU), or palmaris longus (PL)
muscles, and they insert themselves in the common origin of the flexor brevis and
the abductor of the fifth finger. The muscles that form the third group share the
same origin as the previous ones, but inserted themselves by fusing with the hypothenar
musculature, so they could be considered aberrant muscle bellies with a proximal origin
of the muscles that make up the hypothenar musculature.[6]
Due to their location, the muscles in the first group can cause a compression syndrome
of the median nerve, while those in the second and third groups usually cause compressive
neuropathy of the ulnar nerve.
Materials and Methods
We herein present a descriptive study in which we compiled surgical and radiographic
images of 23 cases of supernumerary muscles, treated by the authors between 1990 and
the present (33 years), associating them with different clinical presentations.
Results
A summary of the cases is presented in [Table 1].
Table 1
|
Aberrant muscle
|
n
|
Presentation
|
|
Extensor digitorum brevis manus (EDBM)
|
7 cases
|
2 asymptomatic;
3 with pain and claudication;
2 with dorsal ganglion
|
|
Palmaris longus profundus (PLP)
|
5 cases
|
5 with carpal tunnel syndrome
|
|
Supernumerary abductor of the 5th finger (aberrant abductor digiti minimi, aADM)
|
5 cases
|
5 with cubital tunnel syndrome
|
|
Flexor carpi radialis brevis (FCRB)
|
3 cases
|
3 with pain and claudication
|
|
Extensor of the 3rd finger (extensor medii proprius, EMP)
|
1 case
|
1 with pain and claudication
|
|
Aberrant palmar muscles
|
2 cases
|
1 with pain and claudication;
1 paucisymptomatic
|
|
23
|
|
Discussion
The cases reviewed are of the following types of aberrant muscles:
–
Extensor digitorum brevis manus
(EDBM): we found seven cases (five men and two women) of EDBM, accompanied by a dorsal ganglion
in two cases. The discomfort they caused led to their removal in five cases (four
men and one woman), with resolution of the symptoms.
The EDBM is a rare aberrant muscle on the back of the hand that it was described for
the first time by Albinus in 1734. The frequency of appearance ranges from 1 to 10%,
and it is bilateral in 30% of the cases.[7]
It originates on the dorsum of the lunate and capitate bones, and can sometimes also
originate in the scaphoid, the dorsal intermetacarpal ligaments, the distal radius
or the ulna. From there, it is directed in the form of a fusiform muscle mass (with
approximately 5 to 7 cm in length and 2 to 3 cm in diameter[8]) between the extensor tendons of the second and third fingers, inserting itself
at the level of the metacarpophalangeal (MCP) joint of the second finger in the form
of a single tendon, ulnar to the EIP, although it can also give rise to several tendons
and insert itself over the second to 4th fingers. The EDBM is vascularized by the
posterior branch of the posterior interosseous artery, and it is innervated by branches
that come from the posterior interosseous nerve.[9]
The differential diagnosis should also be made with extensor tenosynovitis.[10] It usually causes pain, especially in the dominant hand. In this case, the treatment
is surgical and consists of excision of both the muscle and the ganglion if it is
also present. The treatment does not cause significant functional sequelae, and complete
disappearance of the symptoms is usually achieved ([Figures 1]
[2]).
Fig. 1 Anatomical diagram and appearance of the extensor digitorum brevis manus (EDBM) muscle
without a ganglion (A) and with an associated ganglion (B).
Fig. 2 Appearance of the EBDM on MRI images, during surgery and after removal.
– Extensor medii proprius: we present an extremely rare case of extensor of the middle finger in a teenager,
who required its removal due to discomfort that increased with hand activity.
The extensor of the third finger is an anomalous muscle, which originates on the dorsum
of the forearm and runs ulnarly to the EIP, to insert itself into the extensor mechanism
of the third finger. There are many anatomical variations[11]
[12] that may share a common origin with EIP. The vascularization, innervation, symptoms,
and treatment are similar to those of the EDBM, and the treatment is surgical excision
when it causes pain. ([Figure 3]).
Fig. 3 Appearance of an extensor muscle of the middle finger, which was removed. *EMP muscle
once removed. Abbreviations: P, proximal; D, distal.
-
– Aberrant palmar muscles: their clinical expression depends on their size and location. In our series, we found
two cases of aberrant palmar muscles, both detected by magnetic resonance imaging
(MRI). One case (male) underwent surgery because he reported incapacitating discomfort
in his manual work. The other case (female) was not operated on and was left undetermined,
as the patient considered that her discomfort was tolerable.
The aberrant lumbrical muscle is shown, which causes irritation in the ulnar collateral
branch of the second finger. The surgical approach led to the definitive diagnosis
and treatment, after a long period of clinical evolution ([Figure 4]).
-
– Flexor carpi radialis brevis
(FCRB): it may present a significant volume. We show three cases of FCRB, all in men with
a high-demand work activity, which were detected in the clinical examination as they
presented a significant volume. The discomfort that existed before surgery disappeared
after its removal ([Figures 5]
[6]
[7]).
-
– Palmaris minor accessory muscle (palmaris longus profundus, PLP): we present five cases of PLP, in three men and two women. All five were operated
on for carpal tunnel syndrome, and we were unaware of the presence of PLP, whose removal
was associated with the section of the anterior annular ligament of the carpus. In
all five cases, a clear improvement in the symptoms was achieved. This muscle is normally
not detected before surgery, due to the lack of imaging tests required for the diagnosis
and treatment of carpal tunnel syndrome.
Fig. 4 Aberrant lumbrical muscle, irritating the radial branch of the index finger.
Fig.s 5, 6, and 7 Intraoperative images of three cases of flexor carpi radialis brevis (FCRB).
Fig.s 8 and 9 Cases of palmaris longus profundus (PLP) muscle, all of them detected intraoperatively.
Fig. 10. Aberrant abductor muscle of the little finger.
It is a muscle that originates from the epitrochlea, runs parallel to the palmaris
longus, and inserts itself into the palmar aponeurosis, on its deep surface.[13]
[14] It can cause symptoms of compression of the median nerve at the level of the carpal
tunnel because it takes up space. Muscle resection resolves the symptoms ([Figures 8]
[9]).
Fig. 11 Aberrant abductor muscle of the little finger.
-
– Aberrant abductor digiti minimi muscle
(aADM): it can cause compression of the ulnar nerve. Its presence was confirmed in five cases
in our (two men and three women). In the three oldest cases, surgery was performed
without obtaining a prior ultrasound or MRI. In two more recent cases with a clinical
picture of severe compression of the ulnar nerve proven by nerve conduction studies,
MRIs were obtained that confirmed the presence of the aberrant muscle. Its removal
and opening of the Guyon canal resulted in a progressive improvement in the clinical
condition, which enabled both patients to return to work.
The ADM usually inserts itself into the ulnar aspect of the base of the proximal phalanx
and into the extensor aponeurosis, collaborating in the flexion and abduction of the
metacarpophalangeal joint and in the extension of the proximal interphalangeal joint
of the fifth finger.[15]
The aADM usually originates in the antebrachial fascia, although both its origin and
insertion are very variable, and it crosses the Guyon canal to insert itself next
to the ADM in the proximal phalanx ([Figures 10]
[11]).
Apart from Dr. Cantero's series of 58 cases,1 we have not found any such extensive published series in the literature on this type
of pathology, since most publications are series of cases.
Since asymptomatic patients with supernumerary muscles do not schedule consultations,
it is difficult to know what their real incidence is, so we focus on associating certain
clinical presentations with a specific supernumerary muscle.
Hand claudication with intense or repetitive activities could be associated with aberrant
muscles on the dorsal or volar aspects of the forearm, although it implies a diagnostic
challenge due to the low reproducibility of the clinical symptoms in consultations
and the broad differential diagnosis.
The cases of EDBM, PLP, and aADM are those that have a direct association with common
clinical syndromes; therefore, their recognition is especially interesting.
In cases of dorsal ganglion, it is advisable to check the possible presence of an
EDBM on an MRI or ultrasound, since its non-removal may be a cause of recurrence.
It is reasonable to expect that the presence of the EDBM would be less likely to cause
compressive symptoms because its muscular belly lies distal to the edge of the extensor
retinaculum. Symptoms related to these abnormal muscles have been attributed to mechanical
problems, namely an increase in volume within a small, rigid compartment that can
cause pain due to muscle ischemia or inflammatory synovitis.
The presence of a PLP can be a cause of compression at the level of the median nerve
before it exits through the carpal tunnel, and its intraoperative recognition is important,
since unroofing the median nerve could be an insufficient treatment for these patients.
The presence of a supernumerary ADM is one of the known causes, which must be ruled
out in cases of compression of the ulnar nerve or of its branches as they pass through
the Guyon canal. This suspicion should arise even in cases of unremarkable imaging
studies if there are clinical symptoms and/or electromyography findings consistent
with it.
