Roentgen Significance of the Pulmonary Ligament

Abstract

The inferior pulmonary ligament is unfamiliar to most radiologists because it blends with the mediastinum and is not visualized on the normal chest roentgenogram. The thoracic surgeon, however, encounters this structure during operative procedures involving the lower part of the thorax. Although the ligament does not affect the normally functioning lung, it does exert a restrictive influence under certain pathological circumstances and can affect the resultant roentgen appearance of the chest. In these instances, the ligament itself may become visible. It is the purpose of this paper to clarify the anatomy of this structure and its appearance in various disease states. Anatomy The inferior pulmonary ligament (5) consists of a double serosal sheath connecting the visceral pleura on the medial surface of the lower lobe of each lung to the parietal pleura covering the mediastinum (Fig. 1). It extends downward in a sheet-like manner from the inferior margin of the pulmonary hilus to the diaphragm and loosely attaches the medial surface of the lung to the mediastinum. The ligament forms a barrier between the anterior and posterior parts of the mediastinal portion of the pleural cavity below the root of the lung. When the lung is displaced laterally away from the mediastinum, the pulmonary ligament is stretched and assumes a triangular configuration (Fig. 2). It has, therefore, been referred to as the triangular ligament. The apex of the triangle is situated at the inferior margin of the root of the lung immediately below the inferior pulmonary vein. The lower margin or base of the ligament is usually continuous with the pleura over the diaphragm, leaving a linear bare area on the diaphragm between the two leaves of the ligament. In some cases, the ligament does not reach the diaphragm and terminates in a free falciform border (Fig. 2). The medial border of the pulmonary ligament arises from the pleura over the mediastinum immediately posterior to the heart on each side (Fig. 3). The left ligament lies adjacent to the esophagus and is bounded posteriorly by the descending aorta. The right ligament is related anteriorly to the inferior vena cava and posteriorly to the azygos vein. The lateral border is attached to the medial surface of the lower lobe. As the ligament courses downward from the hilus, it curves in a posterior direction (Fig. 1). The lower border of the ligament, therefore, pursues a slightly oblique course running posteriorly and laterally from the mediastinum to the medial surface of the base of the lower lobe. The two sheets of pleura which comprise the ligament enclose a network of connective and elastic tissue. At the hilus, this network blends intimately with the connective tissue surrounding the pulmonary vessels, the bronchi, and the esophagus. Within it, lymph nodes are occasionally found.