Bone healing is mainly affected by macroscopic factors (biomechanics and blood supply) as well as microscopic factors (such as molecular biology)[13]. BitKover found that there was no sternal healing 3 months after median sternotomy in a prospective computed tomography scan study[14]. Advanced age as an independent risk factor is not only related to sternal postoperative complications, but also affects sternum healing. Studies have confirmed the importance of immune mechanisms and inflammatory response in bone healing[15]. The cells that is important in inflammatory responses (macrophages, T cells, and mesenchymal stem cells) are related to age. The number and activity of osteochondrocytes and their progenitor cells in the bone marrow of the elderly are lower than young. In addition, advanced age is a high-risk factor for vascular diseases which is closely related to the ability of blood vessels to transport blood flow and it leads to poor blood perfusion of sternum. The choice of bypass vessels is important during coronary artery bypass surgery. Puskas[16] confirmed that transplantation of the internal mammary artery as a bypass vessel significantly improved the long-term survival rate. In clinical practice, almost all cardiothoracic surgeon choose the internal mammary artery as the first choice for transplantation. Interception of the internal mammary artery will not only increase the chance of sternal complications, such as mediastinal infection and sternal opening, but also affect sternum healing[17]. BMI > 35kg/m² as an independent risk factor influencing the score of sternum have been reported in previous literature[18], but the BMI of Asians is generally lower than that of Europeans[19]. In this study, only five patients had a BMI greater than 30kg/m². All of them were fixed with sternum plate, so this study failed to get positive results related to BMI. During follow-up we found that a small number of patients with dysplasia of the lower sternal segment had difficulty in achieving sternal healing based on imaging scan after surgery. The cortical bone and the thickness of the sternum was used as an indicator to measure sternum development. We found that whether the thickness of the sternum or the cortical bone thickness was not related to sternal healing score.
Several forces such as breathing and coughing act on the sternum and load the sternum through a combination of lateral shear and transverse shear. Losanoff et al.[20] confirmed that the lateral tension of the thorax is mainly concentrated in the lower part of the sternum. It is due to the confluence of multiple ribs in the lower sternum and the greater mobility of the chest wall in the area during breath and exercise[21]. Therefore, healing of the lower sternum is poor. Minimal anteroposterior movement of the sternal halves will lead to the result that the cortical bone on one side of the sternum enter the cancellous bone of the other side of sternum. That can affect bone fusion even without frank dissociation[22]. Therefore, the method of sternal closure seems to be important to long-term sternal healing. Mechanical studies demonstrated that rigid plate fixation of the sternum results in superior mechanical properties compared with wire fixation[23]. COL David J. Cohen[24] compared the biomechanics of different sternal closure techniques and found that the plates were stiffer than the figure-of-eight wire constructs in the transverse shear direction.
The main reasons for the differences between the results of this study and the conclusion of COL David J. Cohen are as follows: The sternum plates used in the two experiments were different. Failure of the wire system usually involves the wire cutting into the bone under loads. Within the maximum shear force that steel wire can bear, the stability of eight-shaped steel wire fixation sternum may be stronger than that of sternum plate. The vitro experiment was difficult to imitate the complex biomechanics of human body completely by the force of simple direction.
Titinium plates provide more stability than wire cerclage alone, but it lack the posterior sternal stabilization. 360-degree rigid sternal fixation with combination of plates and wire cerclage described here has been performed in 2 patients to date, with good sternal healing (sternal healing score ≥ 3). Taylor M. James once proposed a similar method to fix the sternum and this method had been employed in 40 patients, with a zero incidence of deep sternal wound infection[25]. Wire cerclage along with rigid sternal plates that helps stabilize the sternum not only in the posterior plane but also in the lateral plane may reduce the amount of mechanical stress put on the plate. Owing to the complication of this method, we recommend considering this technique for especially high-risk patient who are old people, or those with morbid obesity. According to biomechanics, we can use this method to fix the lower part of the sternum, while the upper part of the sternum could be fixed with titanium plate or eight-shape wire alone (Fig. 5).
The limitations of this study inherent was a retrospective review. There was obvious selection bias regarding whether a patient received titanium plate reinforcement, because this technique was usually used in old people,or those with morbidly obesity. We attempted to control for this bias by PSM. Lastly, because of the small number of patients fixed sternum with this new method, our ability to examine its stiff was limited.
In summary, advanced age and internal mammary artery interception are risk factors that affect sternal healing based on imaging findings. this new method of sternal closure provides an effective way of ensuring sternal stability of both sternal plates and reduces the risk for complication after cardiac surgery in high-risk patients.