The unilateral muscular slip of the two-bellied Serratus Posterior Inferior muscle: a case report and a review of literature

Abstract

Purpose: The two-bellied serratus posterior inferior (SPI) muscle with a muscular slip is a rare variation of the back region, which can cause significant discomfort to patients. Patients typically present with symptoms of chronic pain syndrome, radiating back pain, myofascial pain, or lower back pain. This report describes a case of a female cadaver with a two-headed SPI muscle and a right muscular slip, along with a literature review.

Methods: A case of an unusual back muscle was observed in a female cadaver during advanced cadaver dissection of the back region. The SPI muscle was found deep to the latissimus dorsi muscle and superficial to the erector spinae and thoracolumbar fascia. Its oblique arrangement and insertion onto the 8th-11th costae were consistent with its known anatomy, but the presence of two separate fibrotendinous heads and an uncommon variance between the erector spinae and latissimus dorsi muscles was observed.

Results: The SPI muscle fibers were found to be attached to the 8th costa on the right side and displayed two heads on both sides. In our study, the presence of muscular and tendinous digitations towards the twelfth rib was not detected, which aligns with the characteristics described for types D and E. However, we observed a separation between the digitations. Therefore, it is appropriate to classify our findings as type E according to the established categorization. Simultaneously, an anomalous muscular slip, which did not conform to any of the aforementioned classifications, was identified as extending towards the 8th rib.

Conclusion: The underlying cause of unilateral oblique muscular fiber extension is thought to result from aberrant muscle migration during embryonic development or alterations in tendon attachment sites. Differential diagnosis of unidentified lower back pain should consider the various types and alterations of the SPI muscle.

Keywords: Cadaver; Case report; Serratus posterior inferior; Variations; Vertebra.