Bone healing is mainly affected by macroscopic factors (biomechanics and blood supply), as well as microscopic factors (such as molecular biological factors)[13]. Bit Kover found that there was no sternal healing 3 months after median sternotomy in a prospective computed tomography scan study[14]. As an independent risk factor, advanced age is not only related to sternal postoperative complications, but it also affects sternum healing. Studies have confirmed the importance of immune mechanisms and the inflammatory response in bone healing[15]. Cells that are 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 elderly individuals are lower than those in young individuals. 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 leads to poor blood perfusion of the sternum. Furthermore, the choice of bypass vessels is important during coronary artery bypass surgery. Puskas[16] confirmed that the transplantation of the internal mammary artery as a bypass vessel significantly improved the long-term survival rate. In clinical practice, almost all cardiothoracic surgeons choose the internal mammary artery as the first choice for transplantation. The interception of the internal mammary artery will not only increase the chance of sternal complications (such as mediastinal infections and sternal opening), but will also affect sternum healing[17]. As an independent risk factor influencing sternum scores, BMI > 35 kg/m² has been reported in the previous literature[18]; however, the BMI of Asians is generally lower than that of Europeans[19]. In this study, only five patients had a BMI greater than 30 kg/m². All of the patients were fixed with sternum plates; thus, this study failed to obtain positive results related to BMI. During the follow-up, we found that a small number of patients with dysplasia of the lower sternal segment had difficulty achieving sternal healing based on imaging scans after surgery. The cortical bone and the thickness of the sternum were used as indicators for measuring sternum development. We found that either the thickness of the sternum or the cortical bone thickness was not related to the 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. This is due to the confluence of multiple ribs in the lower sternum and the greater mobility of the chest wall in the area during breathing and exercise[21]. Therefore, healing of the lower sternum is poor. Minimal anteroposterior movement of the sternal halves will lead to the cortical bone on one side of the sternum entering the cancellous bone of the other side of the sternum. This can affect bone fusion even without frank dissociation[22]. Therefore, sternal closure seems to be important for long-term sternal healing. Mechanical studies have 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-eight wire constructs used 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 include the fact that the sternum plates that were used in the two experiments were different. The failure of the wire system usually involves wire cutting into the bone under loads. Within the maximum shear force that steel wires can bear, the stability of the eight-shaped steel wire fixation sternum may be stronger than that of the sternum plate. In vitro experiments have difficulty in completely imitating the complex biomechanics of the human body by the force of a simple direction.
Titanium plates provide more stability than wire cerclage alone, but they lack posterior sternal stabilization. The 360-degree rigid sternal fixation (with a combination of plates and wire cerclage described in this study) has been performed in 2 patients to date, with good sternal healing being observed (with a sternal healing score ≥ 3). Taylor M. James once proposed a similar method for fixing the sternum, and this method was employed in 40 patients, with a zero incidence of deep sternal wound infection[25]. The use of wire cerclage in combination with rigid sternal plates that helps to stabilize the sternum both in the posterior plane and in the lateral plane may reduce the amount of mechanical stress that is placed on the plate. Due to the complications of this method, we recommend considering this technique for especially high-risk patients who are older or those patients with morbid obesity. According to biomechanics, we can use this method to fix the lower part of the sternum, whereas the upper part of the sternum can be fixed with a titanium plate or with the sole use of an eight-shape wire (Fig. 5).
The inherent limitations of this study include its retrospective nature. There was an obvious selection bias regarding whether a patient received titanium plate reinforcement because this technique is usually used in older people or those individuals with morbid obesity. We attempted to control for this bias by using PSM. Lastly, due to the small number of patients with fixed sternums with this new method, our ability to examine its stiffness 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 the sternal stability of both sternal plates and reduces the risk for complications after cardiac surgery in high-risk patients.