The gracilis is a spiral unipennate muscle in the medial thigh compartment, which is innervated by the anterior branch of the obturator nerve and receives its vascular supply from the medial circumflex femoral artery, superficial femoral artery, deep femoral artery, descending genicular artery, and the anterior branch of the obturator artery. The gracilis assists with hip adduction, knee flexion, and knee internal rotation. The gracilis may suffer strain injury when patients have an adductor strain, which makes up 46.5% of hip and groin injuries in collegiate men's soccer, and is also common in hockey, football, and basketball athletes. Surgeons commonly use the gracilis for breast, craniofacial, lower limb, perineal, and anterior cruciate ligament (ACL) reconstruction, as well as other reconstructive surgeries.[1][2][3][4][5][6]

The gracilis is a long, thin muscle located in the medial compartment of the thigh. It originates on the medial aspect of the ischiopubic ramus and joins together with the sartorius and semitendinosus muscle tendons to form the pes anserine, which inserts on the superior medial tibia, medial to the tibial tuberosity. The gracilis has a spiral arrangement of muscle fiber bundles and the muscle fibers insert onto the anterior aspect of the gracilis tendon obliquely, making it a spiral unipennate muscle. There are 5 to 7 muscle fiber bundle compartments in the gracilis muscle, with nerve branches coursing along each compartment, which may indicate independent neuromuscular compartment functioning. Crossing both the hip and knee joints, the gracilis muscle performs hip adduction, hip flexion, hip internal rotation, knee flexion, and knee internal rotation.[1][4][3][7][8]

The gracilis muscle derives from the mesoderm and can be found in the fetus starting from the eighteenth week. It is complete within the twenty-eighth week.

The gracilis obtains its vascular supply from the medial circumflex femoral artery, superficial femoral artery, deep femoral artery, descending genicular artery, and the anterior branch of the obturator artery. The anterior division of the internal iliac artery, the inferior epigastric artery, or the external iliac artery can give off the obturator artery, which traverses the ilioischial line, enters the obturator foramen, and splits into the anterior and posterior branch of the obturator nerve as it exits the obturator foramen. The external iliac artery turns into the common femoral artery as it passes the inguinal ligament. The common femoral artery splits into the superficial and deep femoral artery. The deep femoral artery then gives rise to the medial and lateral circumflex femoral arteries, while the superficial femoral artery gives rise to the descending genicular artery near its terminal end.[3][5][9][10]

When describing the neurovascular supply of the gracilis muscles, the terms major and minor pedicles are utilized. Major pedicles are areas where one artery (the major artery), two veins (venae comitantes), and the anterior branch of the obturator nerve enter the muscle, while minor pedicles are where one artery and two vena comitantes enter the muscle. The medial circumflex femoral artery, deep femoral artery, and the anterior branch of the obturator artery are the major arteries which may be present in major pedicles. Patients usually have one major pedicle going into the proximal anterior aspect of the gracilis; however, some patients may have two major pedicles. Once the major pedicle artery goes into the gracilis, it assumes one of three intramuscular distribution patterns:

• Type I (Dichotomous): The major artery splits into a proximal and distal branch. The proximal branch runs proximally from the pedicle insertion and ramifies to supply the proximal gracilis. The distal branch starts running distally in the muscle, and branches into two branches, each of which branch into an additional two branches.
• Type II (Array): The major artery enters the gracilis, and it runs diagonally (slightly towards the distal end) in an anterior-to-posterior direction, from which arrays of ascending and descending branches arise and supply the gracilis.
• Type III (Dual Pedicle): In patients with two major pedicles, both of the pedicles are present on the proximal anterior aspect of the gracilis. However, they do not have any connections between the major arteries or their branches. One pedicle major artery and its branches supply the proximal gracilis, and one pedicle major artery and its branches supply the distal gracilis.

The major artery typically supplies the posterior (deep) portion of the muscle, while the other branches of the major artery supply the anterior (superficial) portions of the gracilis.

The minor pedicle artery can be the superior femoral artery, deep femoral artery, or the descending genicular artery. Most of the minor pedicles are distal to the major pedicle(s). The artery of minor pedicles enter the gracilis anteriorly, and within the muscle, divides into ascending and descending branches. The ascending and descending branches form anastomoses with adjacent vessels, especially along the anterior aspect of the gracilis.

In the major and minor pedicles, each artery has two venae comitantes running alongside them, with one vein distal and anterior to the artery, and one vein proximal and posterior to the artery. Each vein has a distinct draining region. Veins only anastomose with minor pedicle arteries outside of the muscle, not within the muscle. When patients had two major pedicles, each major artery only had one vena comitante.[5][3]

The obturator nerve originates from the L2 to L4 spinal nerve roots, and it traverses along the iliopectineal line, enters the obturator canal, and exits from the obturator forearm and splits into the anterior and posterior branches. The gracilis muscle receives innervation from the anterior branch of the obturator nerve. The major pedicle(s) contain the anterior branch of the obturator nerve, and typically the anterior branch of the obturator nerve forms two divisions, one which goes superficial to the pedicle, and one deep to the pedicle. From there, the nerve divides into two to six descending branches, which distally traverse the length of the muscle. From the main branch of the obturator nerve that entered the muscle, one or two branches go proximally and supply the proximal gracilis. There are variations with the paths of the vasculature (arteries and vena comitantes) and the nerves; they usually follow each other intramuscularly, but not always.[2][3]