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Original Report |
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
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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.
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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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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.
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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.
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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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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.
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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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