The sternalis muscles: incidence and imaging findings on MDCT

We studied the incidence and appearance of the sternalis muscles on multidetector computed tomography (MDCT) to permit the differentiation of the sternalis from significant pathologic condition.

We retrospectively evaluated consecutive contrast-enhanced 16 row MDCT scans of the chest obtained in 1387 Korean patients (790 males and 597 females) between September 2003 and January 2005. All computed tomography scans were obtained in the supine position. Age ranges were 16 to 87 years and average was 59.2 years. Imaging analysis was based upon distribution (unilateral or bilateral, size, shape, and location). Statistical differences were evaluated by chi tests and Independent Samples t tests.

The sternalis muscle was present in 86 (6.2%) of 1387 patients. This muscle was more common in female (44 of 597, 7.3%) than in male (42 of 790, 5.3%) patients, but statistical significance was not noted (P=0.058). Twenty-three patients showed bilateral sternalis muscles. Among 63 patients with unilateral sternalis muscles, 28 patients showed left sternalis muscles and 35 patients showed right sternalis muscles. Unilateral distribution was more common. The height, AP diameter, and width of the sternalis muscle were not significantly different between unilateral and bilateral muscles (P=0.182, 0.911, and 0.114, respectively). The height and AP diameter showed no significant difference between male and female (P=0.470, 0.329, respectively) patients, but the width was wider in male (P<0.001) patients. All sternalis muscles showed flat appearance, except for 1 case. The sternalis muscles were located longitudinally in parasternal position in all cases.

The sternalis muscle is an unusual normal variant of the chest wall musculature running parallel to the sternum with various sizes and most sternalis muscles are flat on MDCT. Unilateral distribution is more common, and the size of muscle is larger in males. Its incidence is 6.2% in Korean population. Radiologists should be familiar with the image findings of the sternalis muscle to avoid any confusing pathologic lesions and facilitate its clinical use such as flap.