Failure of sternal wires depends on the number of turns and plastic deformation: combined experimental and computational approach

Evolution of degradation mechanism and fixation strength of biodegradable Zn-Cu wire as sternum closure suture: An in vitro study

This work reports the first in vitro study on the in-situ biodegradation behaviour and the evolution of fixation strength of Zn-Cu alloy wires in a simulated sternum closure environment. Zn-Cu wires were used to reapproximate the partial bisected sternum models, and their fixation effect was compared with traditional surgical grade 316 L stainless steel (SS) wires in terms of fixation rigidity, critical load, first/ultimate failure characteristics. The metal sutures were then immersed in Hank's balanced salt solution for 12 weeks immersion period, and their corrosion behaviours assessed. Zn-Cu wires showed similar fixation rigidity at 70.89 ± 6.97 N/mm as SS, but the critical load, first failure and ultimate failure characteristics were inferior to SS. The key challenges that limited the fixation effect of the Zn-Cu wires were poor mechanical strength, short elastic region, and strain softening behaviours, which resulted in poor load-bearing capabilities and reduced the knot security of the sutures. The in-situ biodegradation of the Zn-Cu suture was accompanied by the early onset of localised corrosion within the twisted knot and the section located next to the incision line. Crevice corrosion and strain-induced corrosion were the dominant mechanisms in the observed localised corrosion. The localised corrosion on the Zn-Cu sutures did not lead to a significant shift in fixation rigidity, critical load and the first failure characteristics. The findings suggest that the Zn-based biodegradable metallic wires could be a promising sternum closure suture material once the limitations in mechanical characteristics are addressed.

Review of Biomechanical Studies and Finite Element Modeling of Sternal Closure Using Bio-Active Adhesives

The most common complication of median sternotomy surgery is sternum re-separation after sternal fixation, which leads to high rates of morbidity and mortality. The adhered sternal fixation technique comprises the wiring fixation technique and the use of bio-adhesives. Adhered sternal fixation techniques have not been extensively studied using finite element analysis, so mechanical testing studies and finite element analysis of sternal fixation will be presented in this review to find the optimum techniques for simulating sternal fixation with adhesives. The optimal wiring technique should enhance bone stability and limit sternal displacement. Bio-adhesives have been proposed to support sternal fixation, as wiring is prone to failure in cases of post-operative problems. The aim of this paper is to review and present the existing numerical and biomechanical sternal fixation studies by reviewing common sternal closure techniques, adhesives for sternal closure, biomechanical modeling of sternal fixation, and finite element modeling of sternal fixation systems. Investigating the physical behavior of 3D sternal fixation models by finite element analysis (FEA) will lower the expense of conducting clinical trials. This indicates that FEA studies of sternal fixation with adhesives are needed to analyze the efficiency of this sternal closure technique virtually.

In Silico Analysis of Elastomer-Coated Cerclage for Reducing Sternal Cut-Through in High-Risk Patients

BACKGROUND

AISI 316 L stainless steel wire cerclage routinely used in sternotomy closure causes lateral cut-through damage and fracture, especially in cases of high-risk patients, which leads to postoperative complications. A biocompatible elastomer (Pellethane®) coating on the standard wire is proposed to mitigate the cut-through effect.

METHODS

Simplified peri-sternal and transsternal, sternum-cerclage contact models are created and statically analyzed in a finite element (FE) software to characterize the stress-reduction effect of the polymer coating for thicknesses between 0.5 and 1.125 mm. The performance of the polymer-coated cerclage in alleviating the detrimental cortical stresses is also compared to the standard steel cerclage in a full sternal closure FE model for the extreme cough loading scenario.

RESULTS

It was observed via the simplified contact simulations that the cortical stresses can be substantially decreased by increasing the coating thickness. The full closure coughing simulation on the human sternum further corroborated the simplified contact results. The stress reduction effect was found to be more prominent in the transsternal contacts in comparison to peri-sternal contacts.

CONCLUSIONS

Bearing in mind the promising numerical simulation results, it is put forth that a standard steel wire coated with Pellethane will majorly address the cut-through complication.

Noncontact Measurement of the Deformation of Sternal Skin During Shoulder Movements and Upper Extremity Activities Restricted by Sternal Precautions

BACKGROUND

Existing variation has been identified in the rehabilitation programs for patients following cardiac surgery. Sternal precautions are believed to be overly restrictive and detrimental to patient recovery both physically and psychologically.

OBJECTIVE

The objective of this study was to determine the deformation of sternal skin during shoulder movements and upper extremity activities using a noncontact approach.

DESIGN

This was a cross-sectional, nonexperimental observational study.

METHODS

Two black dots were marked on participants' skin overlying sternoclavicular joints using an erasable marker. The coordinates of the dots were recorded using a digital camera and obtained using ImageJ, a public domain image processing program. Skin deformation between the 2 dots was quantified as biomechanical strain.

RESULTS

The sternal skin strain was - 15.3% (SD = 5.6) and - 12.0% (SD = 7.0) at 90 and 180 degrees of flexion; 0.0% (SD=0.0) and-12.8% (SD=5.8) at 90 and 180 degrees of abduction; and - 6.4% (SD=2.8), - 8.9% (SD=3.8), and - 9.8% (SD=4.6) when lifting the 0-, 5-, and 10-lb weights, respectively. The sternal skin strain was 7.9% (SD=3.9) for extension to the end range and-2.5% (SD=5.8) for pushing up from a chair. There is a trend of strain magnitude decrease with the increase of rhomboid strength, but no statistically significant association was found between them (R=0.12).

LIMITATIONS

Limitations included convenience sampling, small sample size, and using skin deformation as a proxy for mechanical loading of the bony structures.

CONCLUSIONS

The data do not support the restriction on most of the shoulder movements and upper extremity activities following cardiac surgery. The approach has the advantage of measuring skin deformation in the entire sternal region.