Orthopaedic anatomy of the sternum: A micro-CT study to guide the fixation of sternal fractures

Background: The rise of manubriosternal joint (MSJ) fractures and lack of literature has spurred an interest in surgical fixation. We hypothesize that locations exist within the sternum that will yield best screw purchase and more rigid fixation of sternal fractures. Methods: Nine cadaveric sternums were scanned using micro-CT and reconstructed. The anterior surface length and angle were measured. The sternum was divided into percentages from the jugular notch (0%) to the xiphisternal joint (100%). Three reference points were marked dividing the axial section into 4 equal quarters from right to left and the total width was measured. At each point of interest, total and anterior and posterior cortical thickness were measured. Values at each percentage were averaged and analyzed. Results: The average length of the manubrium and sternum were 5.1 cm and 9.8 cm respectively and the MSJ occurred near 35%. The sternal angle averaged 16.9°. The sternum was widest at 10%, averaging 6.6 cm and narrowest at the MSJ. The superolateral and inferomedial manubrium and superomedial sternum was the thickest. Additionally, the middle 50% of the manubrium and the superior sternal body exhibited small specimen variations and had small standard errors. The 95% confidence interval encompasses a maximum and minimum thickness value of 21.4 and 5.2 mm respectively. The results obtained, though clinically relevant, exhibited no statistical difference. Conclusions: Micro-CT is an effective tool for the evaluation of sternal anatomy. The middle 50% of the manubrium and superior sternal body exhibited the least variability and should be utilized for screw placement during fracture fixation. Screw lengths should be no greater than 21 mm and no less than 5.2 mm. This study predicts that the sternum is amenable to effective plate and screw fixation. A larger scale CT study of living patients warranted before areas of greatest screw purchase can be determined with certainty. Correspondence to: Jonathon R. Lindner M.D, M.S.E., 550 S. Jackson St., 1st Floor ACB, Louisville, KY 40202, USA; Tel: (502) 553-4963; E-mail: jrlin718@ gmail.com Received: May 14, 2017; Accepted: May 29, 2017; Published: May 31, 2017 Background The Angle of Louis, more commonly known as the sternal angle or the manubriosternal joint (MSJ), was first described by Pierre Charles Alexandre Louis, a 19th century Frenchman who postulated that increased angulation was associated with worsening progression of emphysema [1]. Since then, the anatomy of the sternum and the MSJ has remained essentially unstudied. This can be attributed to a multitude of factors, the most important of which is the historical infrequency of sternal fractures and low complication rates of cerclage wiring following median sternotomy for open thoracic surgery [2]. Today, sternal fractures are believed to be present in 0.64% of all motor vehicle trauma patients [3]. Furthermore, sternal fractures are present in 3% to 8% of all blunt force trauma patients [2]. The sternum is comprised of three bones: the manubrium, the sternal body, and the xiphoid process. It articulates with the clavicles and the cartilaginous portions of the first through seventh ribs and serves as a major attachment for the sternocleidomastoid and pectoralis major muscles. The MSJ, found at the insertion of the second rib, is connected by fibrocartilage [4]. It is comprised of cancellous bone encased in a thin layer of compact cortical bone. The anterior surface of the manubrium and proximal sternal body is convex and the posterior surface is concave. Further down the body, the shape transitions to biconcave before reaching the xiphoid [5]. These known anatomical properties of the sternum help explain an inherent weakness at the MSJ. In fact, Labbe et al (2009) found that 82% of sternal fractures were located at the MSJ and of those 56% were fracture-dislocations [6]. Recently, there has been increasing interest in fixation of MSJ fractures and dislocations due to increasing incidence of the fractures themselves combined with the desire to avoid the comorbidities of poorly healed fractures. This incidence has spiked in the last few decades as a result of increasing emphasis on seat belt usage and growing participation in contact sports. Also, the widespread use of CT imaging of trauma patients in emergency departments has increased the apparent incidence [3,4,7]. The indications for operative management include nonunion, chronic pain leading to respiratory compromise, damage to underlying organs, and failed closed reduction of displaced fractures [8]. However, at this time there has been no standard method proven to be superior to any other for the fixation of MSJ fracture-dislocations [9,10]. All comparative studies to date have focused primarily on sternal reduction following median sternotomy. In these studies, plating has been proven to provide lower rates of nonunion and mediastinitis, reduced pain, increased safety, closer sternal approximation, and required minimal soft tissue dissection Lindner J (2017) Orthopaedic anatomy of the sternum: A micro-CT study to guide the fixation of sternal fractures Biol Eng Med, 2017 doi: 10.15761/BEM.1000118 Volume 2(2): 2-4 as compared to cerclage wiring [2,11,12]. Additionally, preliminary studies of plate fixation of MSJ fracture-dislocations have shown low rates of complications and infection, excellent healing, and minimal damage to surrounding structures and vascular supply [10,13,14]. These studies along with the overwhelming success of rigid plate fixation in other orthopedic applications, place this method in the forefront for fixation of displaced MSJ fractures. The rising incidence of MSJ fracture-dislocations, lack of standardized fixation method, relative lack of anatomical knowledge, and the promising results of plate fixation warrant an anatomical study of the sternum. The objective of this study was to determine the general anatomy, areas of greatest total and cortical thickness, and regions with the least variation of cadaveric sternums using micro-CT imaging. Thickness patterns were used to determine if the sternum is amenable to plate fixation and will serve to guide the placement of screws. Materials and methods Cadaveric specimens with attached clavicles and ribs were obtained from the University of Louisville Fresh Tissue Dissection Laboratory. Each specimen was lightly embalmed using the technique described by Anderson [15]. Once obtained, the specimens were sealed and frozen at -20°C until scanned in groups of three in a cylinder with diameter of 175 mm.