THEORETICAL MODEL OF STERNUM EXTERNAL FIXATION FUNCTIONING IN PHYSICAL THERAPY OF PATIENTS FOLLOWING CARDIAC SURGERY VIA STERNOTOMY
Authors: Vitomskyi V. ‌V. 
Published: October 2020

Abstract. The purpose is to develop and analyze a theoretical model of sternum external fixation functioning as sternal precaution after cardiac surgery via sternotomy.
Methods: theoretical modeling based on literature data.
Results. The literature data, measuring the impact of sternum external fixation on the development of complications after cardiac surgery via sternotomy, do not present a proper report of the functioning mechanism or mechanical model of the interaction between the sternum and sternum external fixation. The first stage of theoretical model development included selecting the criteria based on the sternum anatomy, physiology of respiration, results of scientific research, which enabled to define key aspects of the theoretical model.The second stage included studying interaction of non-elastic SEF and sternum during a deep breath and a cough acting as main elements of inspiratory muscle training after cardiac surgeries; performing a similar algorithm of studying elastic type of SEF, which enabled to investigate and analyze preventive potential of SEF in relation to sternal dehiscence in the lateral direction. The third stage included the analysis of SEF restricting potential for anteroposterior stabilisation of the sternum. Since the use of sternum external fixation should not impede pulmonary function recovery after cardiac surgery and cannot restrict chest circumference increase with inhalation, sternum external fixation cannot properly function as sternal precaution when chest circumference is increased due to sternal edge dehiscence as well. The restricting effect of inelastic sternum external fixation will be possible only in case of a large dehiscence, when its size is bigger than the chest circumference increase during normal and deep breathing. Only when the circumference of inelastic sternum external fixation corresponds to the chest circumference after a full exhalation, the effect restricting dehiscence development will be possible. However, this condition is not practically feasible and does not comply with the need for pulmonary function recovery. As an example a barrel with iron rings that prevent it from expanding and emerging of dehiscence between the boards. However, this is not practically possible and is inconsistent with physiology of respiration. The restricting effect of elastic sternum external fixation will be possible in case the force of compression is greater than the force expanding the sternum during a cough, which will completely restrict inhaling and disable its practical use. The use of sternum external fixation must be biomechanically justified. The fact that the sternum is covered with soft tissues (muscles, which are joined with the bones of the sternum, shoulder blade and humerus; subcutaneous fat, which increases with excessive body weight) also reduces sternum external fixation effectiveness, as the existence of a soft and movable layer between the fixing parts and fixing means is a negative factor. On the other hand, dehiscence is a rare case among patients who do not use SEF. This confirms the priority of the sternal closure stability after sternotomy and the factors affecting it: the strength of bone tissue, the diameter of the wire, used during sternotomy, and the number of sutures.
Conclusions. The theoretical model analyzed in this study confirms either the insignificant role of sternum external fixation or its complete absence in the prevention of sternal dehiscence in the lateral direction and anteroposterior displacement of sternum edges after sternotomy.