Although the incidence of sternal-related complications after sternotomy is low, the mortality rate is as high as 47% , and good sternal healing is closely related to a good quality of life . Schimmer  reported that BMI > 30 kg/m2, heart function greater than NYHA III, renal insufficiency, peripheral arterial disease, an immunosuppressive state, surgical assistant closure of the chest, postoperative bleeding, infusion of plasma-reduced blood > 5 U, secondary surgery hemostasis, and surgery posterior malleous were risk factors for post-operative sternal complications. In addition, age > 42 years, a history of sternal surgery, > 2 arterial grafts (> 2 arterial conduits), internal mammary artery-free, BMI, chronic heart failure, diabetes, respiratory failure, and unexpected secondary surgery could lead to sterile sternal dehiscence . A meta-analysis also indicated that a number of risk factors for sternal complications (OR values: 1.98 for female gender, 1.28 for smoking, 3.31 for diabetes, 2.59 for obesity, 3.11 for bilateral internal mammary artery grafting, 8.92 for secondary surgery, and 2.84 for transfusion) . Furthermore, off-midline sternotomy was the cause of sternal dehiscence .
Regarding the challenges of sternal complications, many researchers are working to reduce the occurrence. A cadaveric experiment performed by McGregor  confirmed that displacement of the xiphoid end under pressure is greater than the manubrium end. Dasika  also showed that the lower sternum is the most unstable site of the sternum in artificial sternal models. Additional reinforcement of the site could enhance sternal stability, and there was no difference between a figure-of-eight wire closure and a single wire closure. In addition, double-strand steel wires close the sternum with higher stability than single steel wire . The cadaveric experiment suggested that reinforcement of sternal wires with stainless steel coils improves sternal stability . Prophylactic sternal weave closure reduces the risk of postoperative sternal dehiscence in patients with diabetes-related obesity and concurrent coronary artery bypass surgery ; however, prophylactic sternal weave closure did not present an advantage for patients with one risk factor . With the development of science and technology, a variety of new sternum closure devices can significantly improve sternal healing. The early sternal healing rate is significantly improved, while the cost is not dramatically increased, including sternum plates and thermo-reactive nitrilium clips, which are quite important in high-risk patients [16, 17]. For low risk patients, the sternal plate did not present values in hospital stay, cost, and incidence of sternal complications . In addition, a thoracic support vest was beneficial for healing the sternum, and also reduced mechanical sternal complications and shortened the hospital stay .
After the sternal incision, there is more bleeding from the inner and outer periosteum and bone marrow. Bone wax is conventionally used to control bleeding, but absorption of non-degradable bone wax is difficult for patients. Water-soluble polymer wax is more beneficial to sternal healing than bone wax .
As part of the sternum, the role of the xiphoid cannot be overlooked. In comparison to a traditional median sternotomy, a xiphoid-sparing median sternotomy is effective in reducing the risk of wound and mediastinal infections . Intramedullary incarceration of a cancellous portion of the autologous xiphoid can also promote healing of the sternum and reduce the probability of pseudo-joint formation .
Platelet-rich plasma can promote bone healing . Shibata  reported that controlled release of platelet-rich plasma from locally-applied gelatin hydrogel improves early sternal healing in patients undergoing sternotomies. Intra-sternal injection of autologous platelet-rich plasma also reduces the incidence of sternal complications and reduces the probability of rehospitalization .
Loosening of the sternum may lead to sternal dehiscence, and secondary incision and mediastinal infections. Mediastinal infections or sternal defects usually require surgery for debridement, drainage, and reconstruction . For patients with a loosened sternum who do not need surgery or cannot undergo sternal reconstruction, low level laser therapy could be used for the treatment of upper sternal loosening, and trunk stabilization exercises are more suitable for the treatment of lower sternal loosening .
In this study we found that the mid-term sternal union proportion after sternotomy was 65.9%, which was unexpected, especially because the sternal union proportion reached 98.2% between 24 and 48 months after surgery . Inferior to the manubrium, the healing rate of the sternal segments gradually decreased, suggesting that poor healing mainly occurred in the lower sternum, which was consistent with the findings from in vitro experiments [11, 12]. Given that our subjects were low risk patients, no difference in healing scores existed between the Plates and Wires groups, and the results were consistent with the report by Peigh . In the subgroup analysis of the Wires group, we found that by using 6–7 wires for closure, the sternal healing rate was slightly better than the use of five wires for closure, suggesting that increasing the number of suture wires improved sternal healing, especially in the lower sternum. Univariate analysis suggested that patient age > 45 years was an independent risk factor for poor sternal healing, which was consistent with other studies. Other risk factors were not identified due to the small sample size in this study.
There were several limitations in this study. First, the sample size was small and no other risk factors, including diabetes, smoking, and obesity, were identified. Second, the study only analyzed low risk patients, while high risk patients could have worse sternal healing in the midterm stage. Third, 10 patients in the study only received five wires for sternal closure, which was not up to the standard and affected the results of sternal healing. We expect a study with a larger sample size to assess the progress of sternal healing after sternotomy.