A Cadaveric, Biomechanical Analysis of Sternal Fixation Systems

Rigid fixation of bony structures has been introduced and implemented as a means to improve stability and reduce motion. Tight fixation has been reported as an essential factor for successful healing1. A variety of devices including plates, intramedullary nails, pedicle screws and interbody fusion devices are used in orthopaedic, spinal and cranial facial procedures to provide rigid fixation. The sternum, however, remains one of the few bones that is not rigidly fixated. The purpose of this study was to compare the mechanical properties of wire cerclage with rigid fixation using plates and screws in human cadaveric sterna. Thirty-one sterna were tested in lateral distraction, rostro-caudal shear, and anterior/posterior shear using a pneumatic test system to determine strength and stiffness. Rigid fixation with plates was shown to exhibit superior mechanical properties compared to wire cerclage. When tested in lateral distraction, the stiffness of the plates was more than 400% greater than peristernal wires (p<0.05). Similarly, the yield load (560N vs. 397N) was also significantly better in sterna that were rigidly fixated. Rigid fixation of the sternum following a median sternotomy with SternaLock plates results in superior stability, stiffness and strength when compared to peristernal wires. Introduction Immobilization supports bony union. This was recognized thousands of years ago by the Egyptians, who splinted fractures6. A clinical study demonstrated higher fusion rates when rigid fixation or metal to metal devices were used as compared to laminar fixation with hooks or wires6. A variety of rigid fixation systems have been developed and utilized in orthopedic applications, including intramedullary nails, spinal fusion devices, external fixators, and cranial and maxillofacial plates. These systems give the surgeon the necessary tools to stabilize the bone, allowing for bone remodeling and healing to progress. Sternal stability and union following a sternotomy is an important step in returning the patient to normal activity. One factor in the development of sternal wound infections is bony instability after sternotomy1. Wire cerclage has historically been used to reduce the sternum. Sternal separation and dehiscence following closure with wire cerclage has been reported to occur at a rate of 0.3–8%. These complications are often costly, and have been linked to mortality rates of 10-40%1,5. To date, few studies have been published comparing the mechanical properties of rigid fixation with sternal plates to the traditional method of wire cerclage. An in vitro study* demonstrated that rigid plates provide significantly more stable fixation that wires2. A separate study in baboons* demonstrated rigid fixation resulted in earlier union with primary osseous healing, suggesting greater inherent stability3. Objective To compare the mechanical properties (stiffness, yield load, and ultimate load) between wire cerclage and rigid sternal fixation using SternaLock plates in cadaveric sterna. Methods Thirty-one cadaveric sterna were used in this analysis. Each sternum sample was accompanied by CT scans, and the age and gender of the donor. Each sternum was divided along the midline and closed with peristernal wires or SternaLock plates, consistent with a sternotomy procedure. A cardiothoracic surgeon assisted in the wiring and plating. Group A (n=15; Fig.1a) consisted of divided sterna closed with three trans-sternal wires at the manubrium and five peristernal wires along the body of the sterna. Group B (n=16; Fig. 1b) used two “X” plates on the sternum body A Cadaveric, Biomechanical Analysis of Sternal Fixation Systems 1 Rigid Fixation with SternaLock® System Results in Superior Stability Compared to Wire Cerclage