The Use of Nitinol Clips in Cardiac Surgery: A New Method of Sternal Closure

Healing complications after cardiac surgery such as dehiscence, osteomyelitis, mediastinitis, and superficial wound infection may occur. This video shows a new and potentially superior method of sternal closure with thermoreactive NITINOL clips that provide semirigid fixation.

• NITINOL belongs to a family of intermetallic materials that contain a nearly equal mixture of nickel and titanium. Nitinol is characterized by shape memory and superelasticity depending on temperature. The clips are becoming malleable at low temperatures (<10°C) and returning to their original shape when the temperature increases. The memory effect begins at temperatures exceeding 27°C, and the definitive shape is retained at 35°C.
• To apply the clips, 3 steel wires are applied on the sternum: 2 in the manubrium and 1 near the xiphoid and are tighten. The clips are mounted onto an applicator that splays them open and then applied around the sternum in a groove already created with the diathermy. In body temperature, the clips return to their original shape and strength, clasping the sternum.

Deep sternal wound complications (DSWC) after cardiac surgery have an incidence of 2 to 8% and carry a significant mortality. The technique of sternal closure is an important factor affecting the incidence of dehiscence and wound infection.

We describe a new method of sternal closure with thermoreactive NITINOL clips (Flexigrips). Nitinol is characterized by shape memory and superelasticity depending on the temperature. The clips are becoming malleable at low tempera­tures (<10°C) and returning to their original shape when the temperature increases.

To apply the clips, two steel wires are applied in the manubrium and one near the xiphoid and they are tightened. The clips are mounted onto an applicator that splays them open and then applied around the sternum. At body temperature, the clips return to their original shape and rigidity.

The clips are currently only available between 20 to 40 mm sizes. The first 2 intercostal spaces are too far apart in most male patients to allow insertion of a clip that is available, also there the internal mammary artery lies closer to the medial edges of these spaces and may be at higher risk on insertion. The lower intercostal spaces are generally too small to allow the insertion of a 4th nitinol clip; if there is sufficient space then a clip may be inserted. Due to anatomical and nitinol clip size limitations, the above technique has been developed, which can be utilised safely in the vast majority of patients.

Over a one-year period 1,119 consecutive patients underwent major cardiac surgery via median sternotomy in our centre. Sternal closure was performed using nitinol clips in 235 patients (Group I) and standard stainless steel wires in the remaining 884 patients (Group II). The mean values for risk factors between Group I and Group II were respectively; age 70.1±10.9 and 67.1±12.9 yr (p<0.01), EuroSCORE 6.5±3.4 and 6.0±3.5 (p=0.07), body mass index (BMI) 29.5±6.1 and 27.5±8.1 (p<0.01), diabetes 19.2% and 16.8% (p<0.01) and pulmonary comorbidity 17 % and 11% (p<0.01). The two groups were comparable in terms of gender, left ventricular function and reoperations.

The overall incidence of DSWC was 2.2% (25/1,119). The incidence of DSWC was higher in Group II (2.3%) compared to Group I (1.7%) (p=0.8). Mechanical sternal dehiscence without infection occurred in 2 patients in Group II. All patients in both groups needed further procedures (average 1.96/patient) which included sternal rewiring or application of vacuum pump or plastic reconstruction. Mortality from Group II related to SWC was 14% (3 out of 21) whereas in Group I there was no death related to SWC (non significant). Despite a higher risk profile, patients undergoing sternal closure with nitinol clips had a lower incidence of DSWC and a lower mortality from DSWC compared to patients undergoing conventional sternal closure. The abstract was presented at the 58th ESCVS International Congress 30/04/2009 - 02/05/2009 Warsaw, Poland.

Further literature supports the benefits from the use of nitinol clips for sternal closure [1,2]. Nitinol clips in both papers had superior results compared to standard stainless steel wires. The incidence of sternal wound complications was statistically significant higher in the group where stainless steel wires were used.

In conclusion this technique has several advantages. It allows semi­rigid compression, with better and more physiologic sternal stability, as opposed to the rigid compression of the steel wire due to its mixture with nickel (55 wt %) and titanium. These clips are thicker than steel wires and confer a lower risk of bone cutting especially in obese patients were shearing forces between sternal edges are higher. Nitinol has a higher biocom­patibility than that of steel again due to its consistency. It is fast and easy to implant, as well as to remove (Nitinol clips do not integrate into bone). Finally, nitinol clips are MRI compatible.

We believe that this method of sternal closure is easy and safe and it should be used at least in the high risk group of patients for sternal wound complications. The cost of management of sternal wound complications can double or triple the primary cost of hospitalization. The cost of each clip is higher than the steel wire but since it reduces the infection/dehisence incidence it makes nitinol clips more cost effective.

References

1. Negri A, Manfredi J, Terrini A, et al. Prospective evaluation of a new sternal closure method with thermoreactive clips. Eur J Cardio-thorac Surg 2002;22:571-75.

2. Centofanti P, La Torre M, Barbato L, Verzini A, Patanè F, di Summa M. Sternal closure using semirigid fixation With thermoreactive clips. Ann Thorac Surg 2002;74:943-45.