AJR 2003; 181:1511-1517
© American Roentgen Ray Society
Side Strain: A Tear of Internal Oblique Musculature
David A. Connell1,
Ash Jhamb and
Trefor James
1 All authors: MRI Department, Victoria House Hospital, 316 Malvern Rd.,
Prahran, Victoria 3181, Australia.
Received February 11, 2003;
accepted after revision June 11, 2003.
Address correspondence to D. A. Connell
(dconnell{at}netspace.net.au).
Abstract
OBJECTIVE. Our objective was to describe the normal MRI anatomy of
the musculature of the lateral abdominal wall and the findings in athletes
with side strain injury.
CONCLUSION. MRI can delineate the sheets of musculature that make up
the lateral abdominal wall. Side strain injury is caused by tearing of the
internal oblique muscle from the undersurface of one of the lower four ribs or
costal cartilages. MRI can document the site of a muscle tear, characterize
the severity of injury, and monitor healing.
Introduction
Side strain is a clinical diagnosis characterized by sudden onset of pain
and point tenderness over the rib cage. Common activities associated with this
type of injury include cricket, javelin throwing, rowing, and ice hockey
[1,
2]. Although an uncommon
injury, it is significant for elite athletes because it results in exclusion
from competition and prolonged convalescence. Somewhat surprisingly, the
anatomic basis of the injury has not been described in the literature. After
performing MRI in a group of patients who presented with side strain, we
believe it is caused by a tear of the internal oblique muscle from its rib or
costal cartilage origin.
The aim of this study was to describe normal MRI anatomy of the musculature
of the lateral abdominal wall and the imaging findings in a group of athletes
who presented at our institution with side strain injury. We attempted to
identify which muscle was injured and to characterize the location and the
degree of muscle injury. To our knowledge, this has not been previously
reported.
The internal oblique muscle forms part of the superficial covering of the
anterolateral abdominal wall. It is one of three large flat muscles in this
region that lie under cover of the external oblique muscle. Fleshy fibers
arise from the upper surface of the lateral two thirds of the inguinal
ligament, the anterior two thirds of the iliac crest, and the thoracolumbar
fascia. The posterior fibers pass upward and forward to be inserted into the
inferior border of the lower four ribs and costal cartilages and thereafter
become continuous with the internal intercostal muscles (Fig.
1A,
1B). The upper fibers form a
short free superomedial border
[3].
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Fig. 1A. —Normal anatomy of anterolateral abdominal wall. Diagram shows
internal oblique muscle arising from iliac crest and inserting into lower
fourth rib under cover of external oblique muscle.
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The fibers arising from the inguinal ligament arch medially across the
round ligament in women or spermatic cord in men to form, along with the
transverses abdominis, the conjoint tendon. The conjoint tendon inserts into
the pubic crest and medial side of the pecten pubis
[4]. The remaining fibers of
the internal oblique muscle diverge and end in an aponeurosis that broadens
superiorly. The upper two thirds of the aponeurosis split into two lamellae
that ensheathe the rectus abdominis and reunite at the linea alba. The
posterior layer fuses with the aponeurosis of the transversus abdominis muscle
and its upper portion inserts into the seventh, eighth, and ninth costal
cartilages. The internal oblique muscle derives its nerve supply from the
ventral rami of the lower six thoracic and the first lumbar nerves.
Subjects and Methods
From February 2001 through October 2002, nine patients were referred to our
institution by sports physicians and orthopedic surgeons for MRI of the
lateral thoracic wall. An additional patient was referred from another
institution, and the information was thought to be of sufficient quality for
inclusion in our study group. Hence, 10 patients made up the study cohort.
There were nine men and one woman with ages ranging between 23 and 36 years
(mean, 28.0 years) with eight acute injuries of the left-sided musculature and
two of the right. Seven of the injuries occurred in cricket players: six were
associated with bowling and one was from a fielding injury. One injury
occurred in a javelin thrower, one occurred in a golfer, and the final injury
occurred in a female rower. The injury occurred on the nondominant side in the
cricket players and javelin thrower and on the left side in the right-handed
golfer.
The initial diagnosis was based on the clinical history, and examination
findings were subsequently confirmed on MRI. The interval from injury to MRI
was 1 day to 3 weeks, with eight of 10 patients being scanned within the first
5 days after the injury. In addition, the patient referred from another
institution also underwent sonography.
Each patient was placed in the supine position, encouraged to breathe with
diaphragmatic rather than chest wall respiration, and scanned with a 1.5-T
magnet (LX Horizon, General Electric Medical Systems, Milwaukee, WI). A phased
array surface coil (SHOPA, Medrad, Indianola, PA) was strapped over the
patient's lateral abdominal wall and centered over the area of point
tenderness. An axial localizing image was obtained, after which we performed
the following sequences: axial and sagittal fast spin-echo imaging performed
through the anterolateral abdominal wall (TR/TEeff, 4,000/30;
matrix, 512 x 256; signals acquired, 2; field of view, 18 cm; section
thickness with no intersection gap, 3 mm; echo-train length, 8) and axial and
sagittal oblique muscle STIR imaging performed through the anterolateral chest
wall (TR/TE, 5,300/38; inversion time, 120 msec; matrix, 256 x 224;
signals acquired, 3; field of view, 18 cm; section thickness with no gap, 4
mm; echo-train length, 10).
The abdominal wall musculature was identified and evaluated with respect to
morphology and signal intensity. Specifically, site of injury and degree of
tearing were noted. Acute injuries were characterized by high signal on STIR
images at the muscle, rib, or costal cartilage interface. A complete tear was
defined as separation of muscle fibers creating a space beneath the
undersurface of the rib or costal cartilage or discontinuity of fibers at the
site of injury. Partial tears were defined as feathery patterns of T2
hyperintensity, representing myofibril disruption with blood or fluid tracking
between myofibrils. Interpretation was made by a musculoskeletal radiologist
and a fellow by consensus.
The study was approved by our institutional review board and informed
consent was obtained from all 10 patients. In addition, institutional review
board approval and informed consent were obtained for three volunteers who
were imaged to identify normal abdominal wall musculature and to illustrate
normal anatomy. For the three patients who subsequently returned for further
MRI studies, we used the same sequence parameters to monitor resolution of the
hematoma and healing. Two players underwent imaging 6 weeks after the initial
injury as a prelude to return to playing cricket. The third player underwent
repeated imaging at 3 months when symptoms failed to resolve with conservative
management.
Results
On MRI, the normal internal oblique muscle has a sheet-like appearance of
striated intermediate signals running upward and forward to insert into the
lower ribs and costal cartilages (Fig.
2A,
2B,
2C). This is in
contradistinction to the striated fibers of the external oblique muscle that
run downward and forward, perpendicular to the internal oblique muscle. These
two layers of muscle are separated by a thin layer of fat, which is best seen
on axial images. The muscle layers were thicker in athletes compared with
volunteers, in whom this fatty layer was more prominent. The internal oblique
muscle lies immediately superficial to the intercostal neurovascular bundles,
which run along the inside of the lower rib margin. Because the internal
oblique muscle inserts into the undersurface of the rib, both the muscle and
the rib are usually seen on the same sagittal oblique muscle section. The
external oblique muscle lies superficially again, and because of its
attachment to the outer surface of the rib, the muscle and rib are rarely seen
together on the same image.
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Fig. 2A. —Normal anatomy in 21-year-old volunteer. Surface marker has
been placed over region of clinical concern and axial fast spin-echo image
(TR/TE, 4,000/30) has been obtained. External oblique muscle (open
arrow) lies superficial to internal oblique muscle (solid
arrow). Sagittal oblique muscle scans are plotted from axial image.
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Fig. 2B. —Normal anatomy in 21-year-old volunteer. Sagittal oblique
muscle fast spin-echo image (4,000/30) shows external oblique muscle
(asterisk) running downward and forward from 11th rib and costal
cartilage (arrow).
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Fig. 2C. —Normal anatomy in 21-year-old volunteer. Sagittal oblique
muscle image (4,000/30) of slice adjacent to that shown in B shows
internal oblique muscle (asterisk) passing upward and forward
(small arrows) to insert into 11th rib (large arrow). These
fibers run almost perpendicular to external oblique muscle
(star).
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The patient results are summarized in
Table 1. Of the 10 injuries,
all occurred where the muscle inserted into the rib or costal cartilages (Fig.
3A,
3B). Two injuries involved the
ninth rib, three injuries involved the 10th rib, and four injuries showed the
muscle tearing from the 11th rib alone. In one patient, the injury involved
tearing of muscle fibers from both the 10th and 11th ribs. The acute tears
showed edema and hemorrhage, with hematoma tracking between the myofascial
coverings of the internal and external oblique muscles (Fig.
4A,
4B,
4C). The extent of the muscle
tear ranged from 6 to 35 mm in length. Stripping of periosteum from the
undersurface of the rib was observed in four patients (Fig.
5A,
5B). In one patient, high
signal extended from the internal oblique muscle into the external muscle,
suggesting concomitant injury. The MRI findings were consistent with the
clinical findings. All patients were treated conservatively.
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Fig. 3A. —23-year-old male javelin thrower with point tenderness and
pain during competition. Axial STIR image (TR/TE, 5,300/38; inversion time,
120 msec) shows increased signal (solid arrow) around 10th rib
(open arrow) corresponding to clinical site of tenderness.
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Fig. 3B. —23-year-old male javelin thrower with point tenderness and
pain during competition. Sagittal oblique muscle STIR image (5,300/38;
inversion time, 120 msec) shows high signal where internal oblique muscle
arises from undersurface of 10th rib (arrow). Hematoma tracks along
muscle fibers of internal oblique muscle (asterisk).
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Fig. 4A. —33-year-old male cricketer with bowling injury. Axial STIR
image (TR/TE, 5,300/38; inversion time, 120 msec) identifies site of tear of
internal oblique muscle (open arrow) with hematoma tracking between
internal and external oblique muscles (solid arrows).
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Fig. 4B. —33-year-old male cricketer with bowling injury. Sagittal
oblique muscle STIR image (5,300/38; inversion time, 120 msec) shows
periosteal stripping (arrows) and hematoma filling defect (beneath
rib).
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Fig. 4C. —33-year-old male cricketer with bowling injury. Axial fast
spin-echo image (400/30) obtained 3 months after B shows hypertrophied
mass of scar tissue (arrow) that was subsequently resected at
surgery.
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Fig. 5A. —31-year-old male cricket bowler with onset of chest wall pain
after completing bowling action. Sagittal oblique muscle fast spin-echo image
(TR/TE, 4,000/30) shows detachment of internal oblique muscle fibers
(short arrows) from undersurface of left 11th costal cartilage
(long straight arrow). Hematoma fills defect created by detachment
(open arrow). External oblique (asterisk) is shown.
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Fig. 5B. —31-year-old male cricket bowler with onset of chest wall pain
after completing bowling action. Sagittal oblique muscle STIR image (5,300/38;
inversion time, 120 msec) shows defect (open arrow) and hematoma
tracking into internal oblique muscle (solid arrow).
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Three patients returned for follow-up MRI. High-signal tracking between
muscle fibers was no longer visible on STIR imaging, nor was the hemorrhage
between the internal and external muscles on the axial scans seen. The gap
created by the detachment of muscle fibers from the undersurface of the rib or
costal cartilage was filled with intermediate signal intensity suggestive of
fibrosis and scar tissue. One patient who had failed treatment and was imaged
gain at 3 months showed a hypertrophied mass of intermediate signal lying
beneath the rib (Fig. 4C).
This was later shown to be scar tissue when resected at surgery.
Discussion
Rib stress fractures that have previously been reported in rowers,
swimmers, golfers, and canoeists are thought to result from the repetitive
forces exerted by the external oblique and serratus anterior muscles
[1,
5]. Fractures typically occur
in the anterolateral to posterolateral aspects of the fifth to ninth ribs
[6]. In addition, external
oblique muscle strains have been described in the groin in elite ice hockey
players; when correctly diagnosed and treated, prompt recovery and return to
full function result [2].
Our study shows that side strain is caused by an acute tear of the internal
oblique musculature where it inserts into the undersurface of the ninth, 10th,
or, most commonly, the 11th rib. Six patients from our study cohort had
detachment of muscle fibers from the cartilaginous cap or adjacent costal
cartilage, suggesting that this may be a weak point of attachment. Athletes
typically present with a history of acute pain in the anterolateral or
posterolateral thoracic wall. Movements similar to that causing the initial
injury often reproduce the pain, as does deep inspiration. In one patient
there was concomitant tearing of the external oblique muscle, although
clinical findings were similar to those of the other injuries and there was no
apparent increase in recovery time.
We postulate that the mechanism of injury for internal oblique muscle
strain is sudden eccentric contracture with rupture of muscle fibers.
Movements associated with bowling (cricket), rowing, swimming, and golf cause
lengthening of the muscle, which is then subjected to superimposed eccentric
contraction, making it vulnerable to rupture. Six of the 10 injuries in our
study occurred in bowlers, with the muscle tear occurring on the
non–bowling arm side. For example, in a right-handed bowler, the left
arm is initially hyperextended and then forcefully pulled through to allow the
right arm to follow through and release the ball. In the hyperextended
position, the internal oblique muscle on the left side can be assumed to be at
maximum tension or eccentric contraction. The sudden vigorous motion from this
eccentric contraction or pull through that allows the dominant shoulder to
flex and release the ball is the probable point at which the internal oblique
muscle is likely to rupture. A similar mechanism can be proposed for other
throwing sports [7,
8]. One hypothesis on the
mechanism of injury is based on clinical history and in two cases observation
of video replay showing the incident. We have not shown this mechanism of
injury in the laboratory. A high percentage of type II or fast twitch fibers
may also be a predisposing factor to tearing
[9,
10].
The mechanism for injury in rowing is different: the shoulder is behind the
hips and the scapula is fully retracted. During this action and at exhalation,
the internal oblique muscle is assumed to be at maximum tension, again leaving
it susceptible to injury. This is also the case for the serratus anterior
muscle, which, at a different phase of the stroke, undergoes an eccentric
contraction and has been shown to be avulsed at its origin at the ribs via a
similar mechanism [1]. Similar
mechanisms of injury can be proposed for golfers, swimmers, and javelin
throwers.
MRI appears to be a sensitive test for evaluating side strain injury,
showing an abnormality in all patients who had a clinical suspicion of a
muscular tear. We found that sagittal oblique muscle images were the most
useful for assessing the degree of muscle injury. These are best plotted from
the axial scans. Because of the wide origin and varied fiber direction of the
muscle, it does not retract far. Muscle defects at the rib or costal cartilage
ranged from 10 to 35 mm in size, and the length of tear equated clinically to
the severity of muscle injury. Stripping of the periosteum occurs as the
muscular attachment is avulsed from the osseous or cartilaginous origin; this
can result in excessive hemorrhage even though the muscle tear may be low
grade. The presence of hematoma often aids in the identification of the site
of muscle injury, although the signal intensity of hemorrhage is altered with
different sequences and time
[11].
Certain technical aspects of the study should be considered. Tearing of the
internal oblique muscle results in diaphragmatic splinting, which in turn
prevents excessive respiratory excursion. This minimizes degradation of image
quality possible through motion artifact. In asymptomatic individuals and
patients with chronic injuries, respiration can interfere with image quality.
Fast spin-echo techniques help to decrease respiratory artifact through
multiple rephasing, in addition to decreasing imaging time, without loss of
resolution or contrast. Use of a surface coil increases the signal-to-noise
ratio, enhances spatial resolution, and increases the conspicuity of the
lesion. The neurovascular bundle runs beneath the rib, and this linear band of
high signal should not be confused with an internal oblique muscle tear.
We find the most useful way of discriminating internal from external
oblique muscles is the orientation of muscle fibers. External oblique muscle
fibers run forward and downward, almost perpendicular to the orientation of
the internal oblique muscle, which runs downward and backward. Because it
arises from the outer surface of the rib, the external oblique muscle lies
superficially to the internal oblique muscle and is not usually seen in the
same imaging plane as the rib. Both muscles are thin sheets, even in athletes,
and accurate imaging requires a precise technique. In difficult cases,
recourse to sonography may be useful, as we found in one case.
A major limitation of this study was the lack of surgical or pathologic
correlation. However, the MRI abnormality corresponded to the region of point
of tenderness and pain. In the three patients who were monitored, the muscle
defect was filled in with low signal compatible with scar tissue. Nine of the
10 patients in our study cohort returned to competition without impairment of
function or apparent significant loss of strength. The typical recovery time
was between 6 and 10 weeks. One patient had surgical excision of a mass of
scar tissue that had formed beneath the rib.
In conclusion, we believe side strain is an acute injury caused by tearing
of the internal oblique muscle from the rib cage. Side strain tears are rarely
a diagnostic dilemma for clinicians. However, MRI can be used to document
these injuries, identifying the site and degree of tearing. MRI may
occasionally reveal additional injuries such as tears of the external oblique
muscle or rib fractures. Follow-up MRI can be used to monitor healing and to
evaluate muscle quality and the formation of scar tissue in patients who fail
to respond to treatment.
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Abstract
Introduction
