Mechanical comparison of three sternotomy closure techniques using a polyurethane foam sternal model: Reply

Abstract

Casha and Gauci criticize our use of standardized polyurethane models rather than cadaver or animal specimens for testing various forms of sternal closure. Ozaki and colleagues tested cadaver specimens using a materials testing system similar to that which we employed. Large standard deviations in their measurements of lateral displacement and stiffness [1Ozaki W Buchman S.R Iannettoni M.D Frankenburg E.P Biomechanical study of sternal closure using rigid fixation techniques in human cadavers.Ann Thorac Surg. 1998; 65: 1660-1665Abstract Full Text Full Text PDF PubMed Scopus (65) Google Scholar] led us to develop a standard polyurethane model of the sternum. Our model simulates trabecular bone. Composite models of bone can be produced by applying a cortical analog of epoxy reinforced with short glass fibers on the surface of a polyurethane foam trabecular analog [2Heiner A.D Brown T.D Structural properties of a new design of composite replicate femurs and tibias.J Biomech. 2001; 34: 773-781Abstract Full Text Full Text PDF PubMed Scopus (241) Google Scholar]. These are much more expensive to produce. Since even Casha and associates noted that sheep sterna had the property of trabecular bone, the added expense did not seem indicated [3Casha A.R Gauci M Yang L Saleh M Kay P.H Cooper G.J Fatigue testing median sternotomy closures.Eur J Cardiothorac Surg. 2001; 19: 249-253Crossref PubMed Scopus (53) Google Scholar]. Casha and coworkers tested several wire closure techniques. Peristernal simple wires performed best and figure-of-eight wires placed through the sternal bone performed worst. Our wire technique was peristernal and figure-of-eight, combining the best and worst designs Casha and associates tested. This is the standard closure we have used in our clinical practice. Casha and Gauci cite work of McGregor and colleagues to claim that there are minimal forces in the transverse (anterior-posterior) and rostral-caudal directions [4McGregor W.E Trumble D.R Magovern J.A Mechanical analysis of midline sternotomy wound closure.J Thorac Cardiovasc Surg. 1999; 117: 1144-1150Abstract Full Text Full Text PDF PubMed Scopus (120) Google Scholar]. McGregor and coworkers applied unidirectional forces in each of three orthogonal directions to a cadaver chest using a mechanical system. They noted that “traction forces were observed to cause more sternal distraction in the lateral direction than similar forces applied along other orthogonal lines of motion.” Their closure was weakest relative to forces applied in this direction. In another part of their study, a large high-pressure balloon was placed centrally in the mediastinum and inflated to simulate coughing and the Valsalva maneuver while measuring pressure changes in both pleural spaces. This study did produce significant lateral separation of the sternal halves while producing little motion in the anterior-posterior and rostral-caudal directions. Coughing and Valsalva maneuvers would place great pressures in the pleural spaces from diaphragmatic contraction rather than applying the forces in the central mediastinum. The physiologic forces might be quite different from those produced in this model. During respiration, the rib cage moves in three directions. As the ribs move outwards (or laterally), a lateral distraction force acts on the sternotomy site. As the ribs move forward, away from the spine, a transverse shear force acts on the sternotomy site. Finally, as the ribs move upwards toward the head, a longitudinal shear force is placed on the sternotomy site. In addition, moving, supporting oneself, and pushing with the upper limbs to climb out of bed produce transverse and rostral-caudal forces on the sternal closure. Bitkover and associates, in their clinical study employing computed tomography of the sternum after median sternotomy, defined step-off as “ventro-dorsal malalignment of the sternal halves” [5Bitkover C.Y Cederlund K Aberg B Vaage J Computed tomography of the sternum and mediastinum after median sternotomy.Ann Thorac Surg. 1999; 68: 858-863Abstract Full Text Full Text PDF PubMed Scopus (53) Google Scholar]. They found that step-off was present in 15 of 20 patients studied, which suggests significant stress in the anterior-posterior direction. We believe that our model provides a highly standardized, reproducible, and inexpensive technique for testing various sternal closure techniques. It provides a good first order approximation of what would be expected clinically. More sophisticated models can be produced, but they are more expensive and their use for testing may not be any more instructive.