Rupture of the pectoralis major muscle is an uncommon clinical entity. Although the diagnosis is usually suspected clinically, marked edema, pain, and decreased range of motion make clinical assessment of the location and extent of injury difficult. MR imaging has been advocated as a useful means of examining the injury in the acute phase, thus aiding clinical treatment [1,2,3]. To date, only three studies have attempted to review the MR imaging and CT findings in a total of 20 patients with surgically confirmed rupture of the pectoralis major muscle [1,2,3].

The complex anatomy of the pectoralis major muscle, particularly at its insertion in the humerus, makes imaging and diagnosis of injuries to the tendon difficult. This study attempts to define the MR imaging characteristics of the normal pectoralis major tendon insertion and of a small series of surgically proven cases of acute rupture.

Six recently frozen cadaveric shoulder specimens from two women and one man were thawed to room temperature for 24 hr, then imaged using a 1.5-T MR magnet (General Electric Medical Systems, Milwaukee, WI).

The cadavers were imaged in the supine position, with the shoulder in the anatomically neutral position. A 5-inch surface coil was placed over the axilla. Using a coronal localizer, the inferior aspect of the humeral head and the deltoid tuberosity were identified and used to define the superior and inferior axial landmarks, respectively, for imaging. Axial spin-echo images (TR/TE, 500/minimum) and T2-weighted spin-echo axial images (4000/85) were obtained. All sequences included a 12-cm field of view, 3-mm section thickness, 1-mm intersection gap, two acquisitions, and a 256 × 192 matrix. Additional oblique coronal T1-weighted images were obtained using the same parameters.

After imaging, the cadavers were dissected using an anterior deltopectoral surgical approach to isolate the myotendinous junction and the insertion of the pectoralis major tendon. A 3-mm-diameter gadolinium marker, which is a 1:50 dilute of gadopentetate dimeglumine (Omniscan; Nycomed, Princeton, NJ) in 5 cm of standard 3-mm-diameter IV tubing, was then sewn along the superficial surface of the humeral insertion as identified by the orthopedic surgeon. A second 5-mm-diameter gadolinium marker was placed along the anterior course of the myotendinous junction. The cadaveric shoulders were then imaged again using the same parameters. Additional images were obtained with the joint in external rotation in three of the specimens.

Six healthy volunteers (four men and two women) were imaged with a 5-inch surface coil placed over the axilla. Using the inferior aspect of the humeral head and the deltoid tuberosity as anatomic landmarks, imaging parameters were obtained as follows: axial gradient-echo localizer; oblique coronal double-echo (proton density- and T2-weighted) images (TR/TE, 2500/minimum of 70); oblique sagittal two-dimensional fast spin-echo T2-weighted images (4000/90); axial proton density-weighted images (1500/minimum); and axial two-dimensional fast spin-echo T2-weighted images (4000/90). All sequences were obtained with 16-cm field of view, 3-mm section thickness, 0.5-mm intersection gap, two acquisitions, and a 256 × 192 matrix. The healthy volunteers were imaged in both internal and external rotation.

A dedicated search of the database at our hospital identified five cases of surgically proven rupture of the pectoralis major muscle. Because the injury was suspected clinically by the referring orthopedic surgeon in each case, a routine shoulder protocol followed by dedicated images in the region of pectoralis major insertion was obtained. The protocols varied slightly according to the ordering radiologist; however, in general, T1- and T2-weighted axial, coronal, and sagittal images centered over the axilla and extending inferiorly to the level of the deltoid tuberosity were obtained.

All MR images were reviewed by two experienced musculoskeletal radiologists. A consensus interpretation was obtained in cases of discrepancy.

The pectoralis major muscle has a broad crescentic origin from the clavicle, sternum, and the fifth through seventh costal cartilages. The muscle fibers converge like a fan into three laminae (clavicular, manubrial, and abdominal), which twist 180° before coalescing into a single tendon of insertion in the bicipital groove of the humerus, just lateral to the biceps tendon. The tendon of insertion measures approximately 5 mm in transverse dimension and 5 cm in cephalocaudal dimension. The tendon and myotendinous junction are more accurately described as a thin coalescence of the anterior and posterior investing fascia than a true tendon [4] (Fig. 6).

The tendon can be further divided into two separate components consisting of a clavicular head and a sternal head. The clavicular head consists mainly of fibers from the clavicular laminae, while the sternal head arises from the manubrial and abdominal laminae. As a result of the 180° twist, the clavicular and upper sternal fibers insert most distally, while the lower sternal fibers arising from the manubrial and abdominal laminae insert most superiorly. Manktelow et al. [5] have reported that the sternal portion of the pectoralis major muscle has a discrete neurovascular supply that is completely independent from the neurovascular supply to the clavicular portion.

Rupture of the pectoralis major muscle and tendon is a rare entity, with fewer than 100 reported cases in the literature [1,2,3,4,5,6,7,8,9,10,11,12]. However, the injury appears to be increasing in prevalence as the number of high-performance athletes in both the professional and recreational sector increases [3].

To date, the injury has been reported exclusively in men and is most common among weight lifters and high-performance athletes, particularly football players. Certain injuries predispose the muscle to rupture in different locations [2, 6,7,8]. The most common mechanism of injury is related to excessive stress during weight-lifting, particularly while bench-pressing [1, 2, 4, 6, 8,9,10,11,12]. Another less common mechanism of injury is a direct blow, such as may occur in an automobile accident or during a football tackle [6, 7, 9]. Partial tears of the muscle belly are common in this type of injury, with the injury typically occurring at the site of direct contact [2]. Other reported causes of injury include injury suffered by attempting to grasp an object while falling, senile degeneration, and various other sporting activities [1,2,3,4,5,6,7,8,9,10,11,12]. Incomplete ruptures are more common than complete ruptures [1, 9]. In general, complete ruptures tend to occur distally at the humeral insertion or myotendinous junction, and incomplete ruptures tend to occur more proximally or within the muscle belly [2, 7, 9, 11].

Clinically, patients present with the characteristic history of excessive muscle stress or a direct blow to the shoulder region while the arm is in abduction and extension. Patients frequently report a “pop” at the time of the injury with associated marked ecchymosis and swelling. Range of motion in adduction, flexion and internal rotation of the shoulder is limited because of pain and weakness.

Treatment of patients presenting with acute ruptures varies according to patient activity. Nonoperative treatment of pectoralis major rupture will not result in significant functional loss for most patients and is generally recommended for patients who are inactive [6, 7, 9]. However, a cosmetically disfiguring bulge will persist, and repair for cosmetic reasons is considered in patients who are unwilling to accept a poor cosmetic outcome.

In general, early surgical repair is associated with a better outcome for the athlete, with earlier return to full strength and range of motion. Immediate diagnosis avoids surgical delay, which has the advantage of avoiding adhesions, muscle retraction, and atrophy, which can occur as early as 6 weeks after the initial injury [2, 8, 9]. Several groups of researchers have reported worse functional outcome after surgical repair in patients with chronic tears more than 6 weeks old [2, 9, 11]. Repair in the acute phase has also been associated with a better cosmetic outcome [8, 11]. McEntire et al. [9] has reported that accurate knowledge of the exact site of injury is important in determining treatment. Incomplete tears of the muscle belly and injuries occurring proximally tend to respond well with conservative treatment, except when a large hematoma is involved. However, patients with distal injuries, either at the humeral insertion or myotendinous junction, tend to have a better outcome with surgical repair [3, 9].

Because clinical diagnosis may be limited by pain and swelling, the limited number of cases reviewed in the literature have asserted that MR imaging is extremely beneficial in the acute setting [1,2,3]. Early diagnosis and treatment is aided by evaluation with MR imaging during the acute phase to assess for extent of injury, to detect the presence of associated injuries or hematoma, and to aid in treatment decisions. Furthermore, MR imaging allows exact localization of injury to the muscle belly, tendon, or the myotendinous junction by revealing abnormally high T2-weighted signal intensity (edema) at the site of injury.

If injury to the pectoralis major insertion is suspected, a dedicated study of this region is necessary; most routine shoulder imaging does not extend caudally enough to include the pectoralis insertion. Initially, a routine shoulder MR imaging examination should be performed to exclude associated injuries. This should be followed by imaging in the axial plane, using the quadrilateral space as the superior boundary and the deltoid tuberosity as the inferior boundary to insure the entirety of the pectoralis major insertion and myotendinous junction are identified. T1-weighted images, proton density images, and T2-weighted axial images should be obtained. If the patient is able to tolerate it, imaging with the arm in the external rotation may improve visibility of the tendon insertion. We obtained most of our diagnostic information from axial imaging; however, oblique coronal images may provide additional information regarding the extent of the tear at the humeral insertion [3].

In the axial plane, the tendon of the pectoralis major should be routinely identified within 1-1.5 cm inferior to the quadrilateral space, and 1 cm superior to the origin of the lateral head of the triceps muscle. In an acute trauma patient, failure to identify a normal low-signal-intensity humeral insertion within these boundaries should prompt a search for rupture with retraction of the tendon medially. Abnormally increased signal intensity can be identified on T2- weighted images at the leading edge of the muscle. Abnormal fluid surrounding the biceps tendon in the bicipital groove is an associated finding.

In conclusion, the normal pectoralis major muscle has a characteristic appearance and location on MR imaging. Axial imaging provides the most diagnostic information. The quadrilateral space and the lateral head of the triceps are reliable landmarks for the superior aspect of the tendon insertion; the deltoid tuberosity defines the inferior boundary. Understanding the normal anatomy of the pectoralis major improves diagnostic evaluation of the pectoralis major in cases of acute rupture, where clinical examination may be limited by marked ecchymosis, swelling, and tenderness. Early diagnosis is particularly important in high-performance athletes, in whom early treatment with surgical repair has been shown to result in better functional outcome with earlier return to full strength and function of the muscle.

Address correspondence to J. Lee.