DOI:10.2214/AJR.08.1150
AJR 2009; 192:468-472
© American Roentgen Ray Society
Distal Attachment of the Brachialis Muscle: Anatomic and MRI Study in Cadavers
Hatice Tuba Sanal1,2,
Lina Chen1,
Pedro Negrao3,
Parviz Haghighi1,
Debra J. Trudell1 and
Donald L. Resnick1
1 Department of Radiology, VA San Diego Medical Center, San Diego, CA
92161.
2 Present address: Radiology Department, Gulhane Military Medical School, 06108
Etlik, Ankara, Turkey.
3 Department of Orthopedia, Santo Antonio's Hospital, Porto, Portugal.
Received May 2, 2008;
accepted after revision July 10, 2008.
Address correspondence to H. T. Sanal
(tubasanal{at}yahoo.com).
Abstract
OBJECTIVE. The purpose of this study was to use MRI and anatomic
correlation in cadavers to delineate the anatomic features of the distal
attachment of the brachialis muscle.
MATERIALS AND METHODS. MRI was performed on 13 cadaveric elbows. The
MRI findings were compared with those in anatomic sections and histologic
preparations. The brachialis muscle of one cadaver was dissected.
RESULTS. The dissected brachialis muscle had two heads, superficial
and deep. The attachment of the superficial head to the ulnar tuberosity was
farther distal than that of the deep head. The attachments of all aspects of
the muscle included a tendinous layer rather than purely muscular structures.
Histologic analysis showed no direct communication between the brachialis and
biceps brachii tendons or between the brachialis tendon and joint capsule.
CONCLUSION. Familiarity with the anatomic features of the distal
brachialis muscle and tendon is essential for accurate assessment of these
structures.
Keywords: anatomy • brachialis muscle • distal attachment • insertion • MRI
Introduction
The literature and anatomy textbooks
[1,
2] contain discrepancies
concerning the anatomic features of the insertion of the brachialis muscle. In
these sources, the brachialis muscle at its insertion site is described as
either a thick, broad tendon or as having two heads, one with a tendinous and
the other with an aponeurotic attachment. Furthermore, any relation of the
brachialis muscle and tendon with the biceps tendon and joint capsule has not
been described in great detail. The purpose of our study was to document the
precise anatomic and morphologic features of the distal attachment site of the
brachialis muscle. A better understanding of this attachment with respect to
the surrounding structures not only clarifies disease processes in this region
but also aids the surgeon in preoperative planning.
Materials and Methods
Thirteen freshly frozen elbow joints obtained from cadavers of seven men
and six women were studied. The mean ages of the persons at death was 76.7
years (range, 60–96 years). The specimens were deep-frozen at
–40°C (Bio-Freezer, Forma Scientific). No evidence of previous
surgical intervention around the elbow was observed visually. The specimens
were thaw ed for 24 hours at room temperature before MRI. They had been
amputated at the midhumeral level. Thus, to make the brachialis tendon taut,
the free end of the muscle was sutured and anchored with a string to one end
of the table.
Imaging was performed with a 1.5-T MRI system (Signa, GE Healthcare) with
the extremity coil in all cases and with a 5-inch (12.7 cm) surface coil in
five cases. With the extremity coil, images were obtained with full extension
of the elbow with the hand supinated in each case. Images were acquired with a
T1-weighted fast spin-echo sequence (TR/TE, 467/10; field of view, 130 x
130 mm; section thickness, 2.5 mm; gap, 0.5 mm; number of signals acquired, 2;
matrix size, 256 x 224) in the axial, coronal, and sagittal planes. For
imaging with the 5-inch surface coil, the coil was placed under the elbow. The
elbow was flexed to 90° with the forearm in a supinated position. This
position has been referred to as the flexed, abducted, and supinated position
[3].
For visualization of the brachialis tendon in the sagittal plane, coronal
localizer images were initially used. The images were acquired along the long
axis of the brachialis muscle if the tendon was seen on the localizer images.
When the tendon was not clearly seen, the images were acquired almost
perpendicular to the ulna. This view showed the full length of both the
brachialis and the biceps tendons (Fig.
1A,
1B). Images were obtained with
T1-weighted sequences with the parameters used for the extremity coil. In all
images, the field of view was adjusted to include the distal and proximal
thirds of the arm and forearm, respectively.
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Fig. 1A —Cadaver, 83 years old at death. T1-weighted MR images (TR/TE,
467/10) with elbow in flexed, abducted, supinated position show distal
brachialis tendon with straight course to insertion site in ulna (U)
(A) and biceps brachii tendon (Bi) and brachialis tendon attachments to
radius (R) and ulna (B). BR = brachialis muscle and tendon.
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Fig. 1B —Cadaver, 83 years old at death. T1-weighted MR images (TR/TE,
467/10) with elbow in flexed, abducted, supinated position show distal
brachialis tendon with straight course to insertion site in ulna (U)
(A) and biceps brachii tendon (Bi) and brachialis tendon attachments to
radius (R) and ulna (B). BR = brachialis muscle and tendon.
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For anatomic and histologic study, a band saw with a fine-toothed metal
blade (Exact, Apparatebau) was used to obtain tissue slices 3 mm thick from
eight cadaveric elbows in the axial (n = 2), sagittal (n =
5), and coronal (n = 1) planes. As each layer of tissue was removed,
a photograph of its surface was obtained. These photographs were compared with
the MR images to confirm the attachment of the brachialis muscle to the ulna,
its relation to the joint capsule, and any possible connection between the
biceps tendon and the brachialis tendon. A representative portion of the
attachment of the brachialis muscle to the ulna (n = 6), the
brachialis tendon next to the joint capsule (n = 7), and the biceps
tendon and brachialis tendon in front of the radioulnar joint (n = 2)
was excised from the 15 specimens that were histologically analyzed. The
preparation was fixed in 10% neutral buffered formalin for at least 72 hours;
decalcification was then performed. The excised specimen was embedded in
paraffin wax. Slices with a thickness of 4 µm were obtained with a sliding
microtome.
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Fig. 2A —Cadaver, 86 years old at death. Photographs of cranial
(A) and medial (B) aspects of brachialis muscle show two heads
and their attachments to ulnar tuberosity (T). Asterisk (A) indicates
brachioradialis muscle. SH = superficial head of brachialis muscle, DH = deep
head of brachialis muscle, R = radial nerve, U = ulnar nerve.
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Fig. 2B —Cadaver, 86 years old at death. Photographs of cranial
(A) and medial (B) aspects of brachialis muscle show two heads
and their attachments to ulnar tuberosity (T). Asterisk (A) indicates
brachioradialis muscle. SH = superficial head of brachialis muscle, DH = deep
head of brachialis muscle, R = radial nerve, U = ulnar nerve.
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The individual cross sections were mounted on slides, stained with H and E,
and examined with a light microscope in consensus by the same pathologist and
musculoskeletal radiologists who had analyzed the initial specimens. The
brachialis muscle was evaluated for a tendinous or muscular attachment at its
points of insertion on the ulna. The presence of any fibers from the
brachialis tendon to the joint capsule and biceps tendon was identified. The
elbow joint of one cadaver was dissected. The gross morphologic
characteristics of the brachialis muscle and tendon were recorded.
Results
The dissected brachialis muscle had two heads that were easily separated
from each other. The muscle belly of the superficial head was anterior and
ulnar in relation to the belly of the deep head
(Fig. 2A). The attachment of
the superficial head was more distal to the ulnar tuberosity than was the
attachment of the deep head. The deep head was fan-shaped and attached more
broadly to the ulnar tuberosity (Fig.
2B).
In all cases in which images were obtained in the flexed, abducted, and
supinated position, the full length of the brachialis tendon and its
attachment to the ulnar tuberosity were well visualized. This view also showed
the complete length of the tendon of the biceps brachii muscle and its
insertion on the radial tuberosity. In all imaging planes, the ulnar
tuberosity, where the brachialis tendon attaches, was easily identified owing
to its bumpy, irregular surface in the anteromedial aspect of the ulna. Axial
images showed both a tendinous and a muscular component of the distal
brachialis attachment. On such images, an imaginary triangle with its base on
the anterior aspect of the proximal radius and ulna and its tip pointing to
the antecubital fossa was a good landmark for identification of the distal
brachialis tendon (Fig. 3A,
3B). In the sagittal plane, the
fan-shaped muscle fibers attaching to the ulnar tuberosity also were well
visualized (Fig. 4). Coronal
images were useful for visualization of the tendinous part of the superficial
head of the brachialis muscle (Fig.
5A,
5B).
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Fig. 3A —Cadaver, 77 years old at death. T1-weighted axial image
(A) and drawing (B) show imaginary triangle with base on bones
and tip pointing to antecubital fossa serving as landmark for identification
of distal brachialis tendon. S = supinator muscle, Bi = biceps brachii tendon,
BR = brachialis muscle and tendon, Pt = ulnar head of pronator teres
muscle.
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Fig. 3B —Cadaver, 77 years old at death. T1-weighted axial image
(A) and drawing (B) show imaginary triangle with base on bones
and tip pointing to antecubital fossa serving as landmark for identification
of distal brachialis tendon. S = supinator muscle, Bi = biceps brachii tendon,
BR = brachialis muscle and tendon, Pt = ulnar head of pronator teres
muscle.
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Fig. 4 —Cadaver, 71 years old at death. Sagittal T1-weighted image
shows brachialis attachment. T = triceps muscle, BR = brachialis muscle and
tendon, Bi = biceps muscle, U = ulna, Br = brachioradialis muscle, FDp =
flexor digitorum profundus muscle.
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Fig. 5A —Cadaver, 81 years old at death. Consecutive T1-weighted
coronal images of elbow show tendinous insertion of superficial head of
brachialis muscle in ulnar tuberosity (asterisk). RH = radial head,
Pt = pronator teres muscle, BR = brachialis muscle and tendon.
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Fig. 5B —Cadaver, 81 years old at death. Consecutive T1-weighted
coronal images of elbow show tendinous insertion of superficial head of
brachialis muscle in ulnar tuberosity (asterisk). RH = radial head,
Pt = pronator teres muscle, BR = brachialis muscle and tendon.
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At histologic examination, the brachialis muscle was found to attach as a
tendon at its more distal part and as a musculotendinous structure in its more
proximal aspect. Although the muscle fibers were intimate with bone, a true
muscular attachment was not present, but a thin tendinous layer was evident
between these muscle fibers and bone. At the osseous attachment site of the
brachialis muscle, a thin layer of calcified cartilage was seen (Fig.
6A,
6B,
6C).
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Fig. 6B —Cadaver, 64 years old at death. Photograph of histologic
specimen including musculotendinous portion of attachment site shows muscle
fibers extend close to bone (arrows), but there is no direct
attachment. Attachment to calcified layer of bone is by tendinous fibers (t).
(H and E, x4)
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No direct communication between the brachialis and biceps brachii tendons
was observed at histologic examination. Instead, loose irregular connective
tissue, which corresponded to strands of low signal intensity, was found
between these two tendons on axial MR images (Fig.
7A,
7B,
7C). No fibers from the
brachialis tendon that attached to the capsule were seen at histologic
examination (Fig. 8A,
8B).
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Fig. 7A —Cadaver, 85 years old at death. BR = brachialis muscle and
tendon, Bi = biceps brachii tendon, U = ulna, R = radius. Axial section
photograph (A) and axial T1-weighted MR image (B) show no clean
cleavage plane between brachialis and biceps tendons.
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Fig. 7B —Cadaver, 85 years old at death. BR = brachialis muscle and
tendon, Bi = biceps brachii tendon, U = ulna, R = radius. Axial section
photograph (A) and axial T1-weighted MR image (B) show no clean
cleavage plane between brachialis and biceps tendons.
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Fig. 7C —Cadaver, 85 years old at death. BR = brachialis muscle and
tendon, Bi = biceps brachii tendon, U = ulna, R = radius. Histologic
photograph shows no direct communication or blending between tendinous fibers
of brachialis and biceps muscles. Instead, irregular connective tissue
(asterisk) is present. (x4)
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Fig. 8A —Cadaver, 87 years old at death. M = brachialis muscle, C =
joint capsule, H = humerus, U = ulna. Photograph of gross specimen in sagittal
plane shows brachialis muscle in front of ulnotrochlear joint.
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Fig. 8B —Cadaver, 87 years old at death. M = brachialis muscle, C =
joint capsule, H = humerus, U = ulna. Histologic photograph shows muscle
fibers lie parallel to joint capsule without violating it. (x4)
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Discussion
The anatomic and histologic appearances of the attachment of the brachialis
muscle to the ulna have not been well characterized
[1,
2,
4]. In various sources
[1,
2] the insertion has been
described as either a thick, broad tendon or as having two heads, one with a
tendinous and the other with an aponeurotic attachment. In our histologic
examination, we did not observe a pure muscular attachment. Muscle fibers were
seen to lie very close to bone, but these fibers did not directly attach to
the bone. Instead there was tendinous tissue between the muscle and bone
(Figs. 6A,
6B,
6C).
On axial images, an imaginary triangle with its base on the anterior aspect
of the proximal radius and ulna and its tip pointing to the antecubital fossa
is a useful landmark for identifying the distal brachialis muscle and tendon.
The ulnar head of the pronator teres muscle forms the medial border, and the
supinator muscle forms the lateral extent of this triangle. Inside the
triangle, the brachialis muscle is adjacent and deep in relation to the belly
of the pronator teres muscle, and the biceps tendon is radial in relation to
the brachialis muscle and adjacent to the supinator muscle (Fig.
3A,
3B). Owing to its bumpy,
irregular surface, the ulnar tuberosity where the brachialis tendon attaches
to it below the radioulnar joint can easily be identified in the anteromedial
aspect.
The brachialis muscle has been described as having more than one component
[1,
2]. In our study, the one
dissected brachialis muscle had two separate heads with a single attachment to
the ulna. As previously described
[1], the attachment of the
superficial head is more distal to the ulnar tuberosity in the form of a thick
tendinous structure compared with the deep head, which is musculoaponeurotic,
fan-shaped, and broad at its site of attachment. Although they differ at their
distal ends, the two heads attach to the ulnar tuberosity as a single
contiguous structure in a blended manner. The brachialis muscle is a pure
flexor of the elbow joint [2,
5]. This blended morphologic
nature of the tendon and aponeurotic fibers at the insertional site of the
brachialis muscle is postulated to be ideal for initiation and continuation of
elbow flexion [1].
We found that the flexed, abducted, and supinated position, previously
described as an optimal position for visualization of the distal biceps
brachii tendon, also is useful for delineation of the distal brachialis muscle
and tendon. In cases of biceps brachii tendon rupture, attaching the
brachialis muscle to the radius with transosseous sutures and suturing the
biceps brachii tendon to the brachialis tendon are surgical options
[6]. Use of the flexed,
abducted, and supinated position, which shows the two muscle insertions, can
aid the surgeon in choosing the optimal technique. In addition to being used
in procedures for biceps brachii tendon rupture, the brachialis muscle is used
after brachialis plexus injury to restore finger flexion and wrist extension
[7,
8]. The flexed, abducted, and
supinated view shows the continuity of the brachialis tendon.
Coronoid fractures, which can predispose the elbow to instability, have
been studied and classified
[9–11].
According to the proposed classification systems, type III fractures are basal
and involve more than 50% of the height of the coronoid process. These
fractures are large enough to include the brachialis tendon insertion on the
proximal portion of the ulna. Detecting these fractures on radiographs should
alert the radiologist to look for brachialis tendon injuries on MR images,
although such injuries are rare.
On axial MR images, the separation between the biceps brachii and
brachialis tendons often is not clear
(Fig. 7B), suggesting the
possibility of connecting fibers between the two. At histologic analysis,
however, no tendinous fibers were seen to extend between these two structures.
Rather, in all specimens, there was loose, irregular connective tissue between
the two tendons (Fig. 7C).
A small number of fibers, designated the articularis cubitus, have been
reported to extend from the deep aspect of the deep head of the brachialis
muscle and to attach directly to the anterior capsule of the elbow joint
[1,
12]. In our study, no such
fibers extending from the brachialis muscle to the joint capsule were found
anatomically or histologically. Rather, a thin layer composed of loose
connective tissue was present between the musculotendinous portion of the
brachialis muscle and the joint capsule
(Fig. 8B). Our findings are
similar to those reported by Tubbs et al.
[13]. Those investigators
found that most of the fibers of the brachialis muscle simply pass
superficially to the anterior capsule of the elbow joint via connective tissue
and that no direct muscle attachment is evident.
Our study had limitations. First, we used only T1-weighted MRI sequences,
which are susceptible to magic angle effect related to the short TE. Second,
the anatomic study was conducted with a small number of cadaveric specimens
derived from elderly donors, and only a limited number of histologic specimens
were used for analysis. Thus, the absence of capsular fibers derived from the
brachialis muscle and of connecting fibers between the biceps and brachialis
tendons, as concluded in our investigation, might have represented a sampling
error. Despite these limitations, our investigation yielded anatomic
information that should prove useful to radiologists and surgeons.
We conclude that knowledge of the anatomic features of the distal
brachialis muscle attachment site aids in understanding the functional and
anatomic characteristics of this muscle and serves as a guide for orthopedic
surgeons considering the use of the brachialis tendon as a treatment
option.
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Abstract
Introduction
