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Named for its curved shape, an arcuate ligament is any of four such ligaments in the body: the median, medial, and lateral arcuate ligaments of the diaphragm, and the arcuate ligament of the pubis, also known as the inferior pubic ligament. Like any ligament, all are bands of dense, fibrous connective tissue largely made up of collagen fibers. The first three are found along the underside of the diaphragm near where it attaches to the spinal column, while the arcuate pubic ligament is found on the underside of the pelvis, linking the two pubic bones at its center.
A parachute-shaped muscle filling the space at the very bottom of the ribcage and separating the digestive organs from the heart and lungs, the diaphragm features several small circular openings through which the aorta, esophagus, several muscles, and other vertical structures pass. Each arcuate ligament of the diaphragm partially encircles one of these structures. Rather than connecting one bone to another bone, then, as most ligaments do, the arcuate ligaments attach to a bone or bones — in this case the vertebrae — curve around one of these openings, and attach at their other end to the same bone or bones.
Nearest to the midline of the body in the diaphragm is the median ligament, which links the right and left crura of the diaphragm. The crura are tendon-like structures connecting the posterior aspect of the diaphragm to the spine; the left crus links to several of the lumbar vertebrae along the left side of the anterior spine and then partially encircles the aortic hiatus, the hole through which the aorta passes, as the median arcuate ligament. As it curves around the right side of the hiatus, it forms the right crus, which descends to attach to the anterior lumbar spine along its right side.
On either side of the median ligament is the medial arcuate ligament. This ligament arches around the opening in the diaphragm through which the psoas major, a muscle of the trunk and anterior hip, passes. It arises from the top or sometimes second lumbar vertebra just beside the crura on the vertebral body, arches up and around the psoas, and returns to the same vertebra, where it attaches to the transverse process. The transverse process is a bony eminence that projects sideways from the body of the vertebra.
To the outside of these two ligaments in the diaphragm is the lateral arcuate ligament. This ligament lies just alongside the medial ligament and forms an arch around the quadratus lumborum, a muscle of the trunk. The quadratus lumborum does not actually pass through the diaphragm but rather lies just behind its rear border, running vertically between the lumbar spine and ribcage at its top and the pelvis at its bottom. As such, the lateral ligament forms a portion of the rear border of the diaphragm, arising from the transverse process of L1, the first lumbar vertebra, curving around the quadratus lumborum along its anterior side, and attaching to the underside of the 12th rib.
The last of the arcuate ligaments is the inferior pubic ligament. This arcuate ligament forms the superior or topmost border of the pubic arch at the very bottom center of the pelvis. Here the pubic bones, which are centrally located along the bottom of the pelvis and curve downward to either side, meet at the midline of the body, just above the genitalia. This joint, which is held together by the inferior pubic ligament, is known as the pubic symphysis. It is the joint that allows the pelvic bones to move apart slightly during childbirth.