Fascial structure of the anterior mediastinum: the surgical figure of the sternopericardial ligament and the role of affixing the thymus to the pericardium

Background: The fascial anatomy of the anterior mediastinum has not been thoroughly investigated. Only the sternopericardial ligament (SPL) has been defined and is often described as two independent tendons. However, these descriptions differ from our observations of thoracic surgery. Here, we aimed to precisely investigate the fascial anatomy, particularly the SPL, of the anterior mediastinum. Methods: We surgically and radiologically observed and analyzed the fascial anatomy of the anterior mediastinum in patients who underwent surgery for anterior mediastinal disease at our institution between July 2009 and August 2018. Results: A total of 55 thoracoscopic surgeries were performed. We observed a sagittal layer of fibrous tissue bridging the pericardium and sternum, in addition to the well-known fascial layers. Behind the lower bifurcation of the thymic glands, the thymic capsule was firmly fixed to the pericardium around the anterior aortopulmonary trunk. It thickened locally, formed a small white stripe, and occasionally deformed the pericardium by pulling it anteriorly. No other tendon-like structure was observed. Conclusions: Our observation of the SPL showed differences from classical descriptions. The SPL is a sagittal fibrous collection showing a unique figure. Part of the SPL is thickened behind the thymus, anchoring the thymus to the pericardium in the midline. The SPL is part of the bottom of Grodinsky's Space 3 in the thorax. The SPL is visible in radiologic images, such as the anterior junction line on a plain chest radiograph.