Lanny V. Griffin

A Biomechanical Comparison of Three Sternotomy Closure Techniques

BACKGROUND A biomechanical study of three sternotomy closure techniques (figure-of-eight stainless-steel wires, Pectofix Dynamic Sternal Fixation [DSF] stainless-steel plates, and figure-of-eight stainless-steel cables) was conducted to compare strength and stiffness variables in three clinically relevant loading modes (anterior-posterior shear, longitudinal shear, and lateral distraction). METHODS All tests were conducted on polyurethane foam sternal models that simulate the properties of cancellous bone. Each model was divided longitudinally and reconstructed using one of the sternotomy closure repair techniques. Tests were performed using a materials testing system that applies a continuously increasing amount of force in one direction to the model until it catastrophically breaks. A total of six trials of each fixation type in each of three test groups were prepared and tested, for a total of 54 tests. Strength and stiffness variables as well as a post-yield analysis of failure were evaluated. RESULTS Sternums repaired using the DSF plate system are a more rigid construct than sternums repaired using the stainless-steel wires or cables in the distraction and transverse shear modes and they are not significantly different from sternums repaired with wires or cables in the longitudinal shear mode. The DSF plate system offers a 25% improvement in resistance to failure (yield) compared to wires when a transverse shear force is applied to the model. The cable system had a higher resistance to failure than the wires in all modes although the differences were not statistically significant. Additionally, the DSF plate system provides substantial reduction of the implants cutting into the sternal model under loading as evidenced by the post-yield displacement when compared with either cables or wires for the distraction and longitudinal shear modes. For the transverse shear mode, the cables or wires would completely fail at the load for which cutting begins for the DSF. CONCLUSIONS Both the DSF plate system and the stainless-steel cable system offer important advantages over figure-of-eight wire for sternal closure.

Fatigue strength of common tibial intramedullary nail distal locking screws

BackgroundPremature failure of either the nail and/or locking screws with unstable fracture patterns may lead to angulation, shortening, malunion, and IM nail migration. Up to thirty percent of all unreamed nail locking screws can break after initial weight bearing is allowed at 8–10 weeks if union has not occurred. The primary problem this presents is hardware removal during revision surgery. The purposes of our study was to evaluate the relative fatigue resistance of distal locking screws and bolts from representative manufacturers of tibial IM nail systems, and develop a relative risk assessment of screws and materials used. Evaluations included quantitative and qualitative measures of the relative performance of these screws.MethodsFatigue tests were conducted to simulate a comminuted fracture that was treated by IM nailing assuming that all load was carried by the screws. Each screw type was tested ten times in a single screw configuration. One screw type was tested an additional ten times in a two-screw parallel configuration. Fatigue tests were performed using a servohydraulic materials testing system and custom fixturing that simulated screws placed in the distal region of an appropriately sized tibial IM nail. Fatigue loads were estimated based on a seventy-five kilogram individual at full weight bearing. The test duration was one million cycles (roughly one year), or screw fracture, whichever occurred first. Failure analysis of a representative sample of titanium alloy and stainless steel screws included scanning electron microscopy (SEM) and quantitative metallography.ResultsThe average fatigue life of a single screw with a diameter of 4.0 mm was 1200 cycles, which would correspond roughly to half a day of full weight bearing. Single screws with a diameter of 4.5 mm or larger have approximately a 50 percent probability of withstanding a week of weight bearing, whereas a single 5.0 mm diameter screw has greater than 90 percent probability of withstanding more than a week of weight bearing. If two small diameter screws are used, our tests showed that the probability of withstanding a week of weight bearing increases from zero to about 20 percent, which is similar to having a single 4.5 mm diameter screw providing fixation.ConclusionOur results show that selecting the system that uses the largest distal locking screws would offer the best fatigue resistance for an unstable fracture pattern subjected to full weight bearing. Furthermore, using multiple screws will substantially reduce the risk of premature hardware failure.

The effects of testing methods on the flexural fatigue life of human cortical bone.

A flexural model of four-point bending fatigue that has been experimentally validated for human cortical bone under load control was used to determine how load and displacement control testing affects the fatigue behavior of human cortical bone in three-point and symmetric four-point bending. Under load control, it was predicted that three-point bending produced no significant differences in fatigue life when compared to four-point bending. However, three-point bending produced less stiffness loss with increasing cycles than four-point bending. In four-point bending, displacement control was predicted to produce about one and a half orders of magnitude greater fatigue life when compared to load control. This prediction agrees with experimental observations of equine cannon bone tested in load and displacement control (Gibson et al., 1998). Displacement controlled three-point bending was found to produce approximately a 25% greater fatigue life when compared to load control. The prediction of longer fatigue life under displacement control may have clinical relevance for the repair of damaged bone. The model can also be adapted to other geometric configurations, including modeling of whole long bones, and with appropriate fatigue data, other cortical bone types.

Fatigue testing of three peristernal median sternotomy closure techniques

BackgroundFailure of a sternotomy closure because of closure system fatigue is a complication that may result in dehiscence and put the individual at risk for serious complications. The purpose of this study was to assess the fatigue performance of three peristernal median sternotomy closure techniques (figure-of-eight stainless-steel wires, figure-of-eight stainless-steel cables, or Pectofix Dynamic Sternal Fixation [DSF] stainless-steel plates) in order to quantify the potential risk of fatigue failure of these devices when subject to cyclic loads in physiologically relevant loading directions.Study DesignAll tests were conducted on polyurethane foam sternal models. A cardiothoracic surgeon divided each sternal model longitudinally and repaired it with a closure device. Tests were performed using a materials testing system that applied cyclic loading in a uniaxial direction until the test model catastrophically broke or data run-out occurred. For each loading direction (lateral distraction and longitudinal shear), five trials of each closure technique were tested. Life data and location of device failure (if present) were evaluated. Statistical analysis was performed using regression with life data allowed for correlation between life data and the various closure techniques to develop risk assessment curves for each device.ResultsThe data show that the figure-of-eight stainless-steel cable and the DSF plate systems are considerably less likely to fail under both lateral distraction and longitudinal shear cyclic loading conditions as compared to the figure-of-eight stainless-steel wire system. Moreover, the figure-of-eight stainless-steel cable system is the most resistant to failure, particularly for high cycle counts.ConclusionThis study in addition to Cohen and Griffins earlier published biomechanical comparison of the ultimate strength of these same three closure techniques provide extensive experimental evidence regarding the mechanical differences among these three peristernal median sternotomy closure techniques. All data support the hypothesis that both the DSF plate system and the stainless-steel cable system offer important advantages over figure-of-eight wire closure techniques; although twisted wires are the weak-link in the systems we tested.