Sternal Angle Anatomy Redefined by Insights from CT Thorax

Purpose: The manubriosternal angle (SA), commonly known as “Angle of Louis” (AoL), has been an important surface landmark in thoracic anatomy. Universally accepted as corresponding to T4–T5 intervertebral disc (IVD) level as well as to major mediastinal landmarks such as tracheal bifurcation (TB), main pulmonary artery (MPA) branching, and 'azygos superior vena cava (A-SVC) junction, and having clinical relevance in applied surgical anatomy, the arrival of modern era cross-sectional imaging modalities such as CT and magnetic resonance imaging have unequivocally demonstrated these thoracic landmarks in live subjects, providing objective real-time data to revisit traditionally held anatomical concepts. Materials and Methods: An observational study was done, including 155 adult patients undergoing CT thorax at the department of radiodiagnosis and imaging at our hospital. Results: The vertebral level of SA was seen at T4–T5 IVD only in 34.2% patients, whereas in 72.6% patients, it was located between T4 vertebral body (VB) and T4–T5 IVD. In addition, the location of mediastinal structures corresponding to AoL was observed as follows: TB (30.3%), A-SVC junction (38.1%) and MPA branching (9.0%). However, in majority of patients, these structures were seen located between T4–T5 IVD and T5 VB, namely TB (69.6%), A-SVC junction (63.3%), and MPA branching (45.8%). Comparison of our results with similar studies, national and international, provided correlatable inputs. Conclusion: It is evident from our study results that significant variation from traditionally held anatomical precepts exists in real-time thoracic anatomy at the level of SA, when live subjects undergo cross-sectional imaging. Notwithstanding the impact of limited sample size, findings of this study highlight the universal need to review and revise traditionally accepted thoracic anatomy landmarks at the AoL.