Correlation between compression, tensile and tearing tests on healthy and calcified aortic tissues
Introduction
Cardiovascular disease (CVD) is responsible for approximately 30% of all deaths worldwide and claims more lives each year than the next six main causes of death combined [1], [2]. Atherosclerosis is the most common disease of the arterial wall [3]. The atherosclerotic lesion partially or totally blocks the lumen, and decreases the blood supply to distal tissues. Several invasive and noninvasive procedures are available to revascularize an occluded artery [4], [5], [6], [7], e.g. bypass operations where the narrowing caused by atherosclerosis is circumvented.
Anastomosis, i.e. a technique to connect healthy sections of tubular structures in the body after the diseased portion has been surgically removed, is performed during most cardiovascular bypass surgeries. The advent of surgeon-aiding laparoscopic suturing devices, e.g. EndoStitch (US Surgical Inc., Norwalk, CT, USA) and robot-assisted laparoscopy, e.g. the master–slave system Da Vinci (Intuitive Surgical, Mountain View, CA, USA), allows a surgeon to perform anastomosis quicker than with manual laparoscopy [8], [9], [10], [11], although opinions are sometimes contradictory [12], [13].
One of the challenges in the automatization of suturing is calcified atherosclerosis [14], [15]. Tissues that suffer from calcified atherosclerosis often become more brittle [16], [17]. Calcified plaques have unique mechanical properties [18], [19]. They lower the tearing resistance and increase the risk of leaks [20], [21], [22]. To prevent tearing, the surgeon has to locate positions where the suture, clip or staple can be placed safely [23]. In classical open surgery, the surgeon locates the calcifications by manual palpation or by using a needle. However, due to the lack of haptic feedback inherent to laparoscopic suturing devices and master– slave systems, the surgeon is not yet capable of discriminating healthy from diseased tissues [24], [25]. Several parameters have been proposed for automatic discrimination. The stiffness is probably the best parameter because its relative ease of acquisition. The static radial compressive modulus of atherosclerotic plaques is, unlike its circumferential tangential equivalent, significantly affected by the degree of calcification [26].
To date, no studies have been reported that compared healthy to calcified atherosclerotic aortic tissue. Studies of the overall mechanical behavior of excised atherosclerotic plaques are valuable [3], [18], [27], but a discrimination method cannot be developed by merely testing excised plaques.
After cyclic preloading, i.e. preconditioning, soft tissues take on a steady-state behavior where the stiffness and the hysteresis in successive cycles is constant [28]. However, surgeons do not precondition tissues before operating [29]. Single-cycle loading has not been frequently reported. This study focuses on one cycle loading to simulate the clinical reality.
The purpose of this project was to develop and validate a non-destructive method that is able to distinguish healthy from calcified aortic tissue. Such a validated method can be of considerable use in future development of automated suturing devices and, moreover, contribute to an increased understanding of the biomechanics of diseased soft tissues.
Section snippets
Specimen preparation
Forty-five porcine abdominal aortas segments from healthy 6-to-8-month old pigs were obtained from a slaughter house. The pigs were sacrificed according to standard slaughtering procedure. After removal, each aorta was transported in isotonic physiological solution (Baxter, 0.9% sodium chloride). At the laboratory the aorta was dissected and the surrounding tissue removed. The control group was kept in physiological solution for 80 min, while the test group I was perfused for 80 min in a
Uniaxial unconfined compression
Fig. 5 shows the relationship between the stress and strain, for healthy porcine aortic tissue (control group), artificially calcified porcine aortic tissue (test group I), and calcified human aortic tissue (test group II). The stress–strain curves are concave upward containing no linear portion from which a meaningful global elastic modulus could be determined.
The mean wall thickness of the samples of the control group and test group I were mm and mm, respectively. The mean
Discussion
The purpose of this study was to develop a non-destructive method that is able to distinguish healthy from calcified aortic tissue. To verify whether the samples classified as calcified actually had a lower resistance to tearing, tensile tests and tearing tests validated the method.
Many published data on the mechanical properties of aortic tissues is available. They postulate that the stress–strain relationship of aortic tissues is non-linear, viscoelastic and different under tension and
Conclusion
This study showed that a non-destructive uniaxial unconfined compression test is able to significantly discriminate healthy from calcified aortic tissue. The samples classified as calcified had a lower tensile strength and a lower resistance to tearing.
Conflict of interest
None.
Acknowledgments
The authors gratefully acknowledge the support of Jan Demol and Stefan Vinckier.
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