Searching for a physiologically meaningful parameter for aortic biomechanics—is energy loss the way?

The authors reported no conflicts of interest. The Journal policy requires editors and reviewers to disclose conflicts of interest and to decline handling or reviewing manuscripts for which they may have a conflict of interest. The editors and reviewers of this article have no conflicts of interest. We read with interest the article by Nightingale and colleagues,1Nightingale M. Gregory A. Sigaeva T. Dobson G.M. Fedak P.W.M. Appoo J.J. et al.Biomechanics in ascending aortic aneurysms correlate with tissue composition and strength.J Thorac Cardiovasc Surg Open. 2022; 9: 1-10Scopus (6) Google Scholar who introduced 2 biomechanical parameters, low strain tangential modulus (LTM) and onset stress of the transition zone (TZO), for assessing ascending aortic aneurysms. It is critically important that parameters measuring aortic biomechanics (1) can be measured in the physiologic range and therefore translate clinically, (2) correlate with failure properties as these are the true metrics of interest, and (3) correlate with underlying aortic wall microstructure. Fulfilling these criteria sets up the biomechanical parameters for noninvasive in vivo testing that can segregate patients at low and high risk of aortic complications (Figure 1). We were enthusiastic that the authors were tackling some of these issues but are left with many questions. LTM and TZO were measured assuming that physiological loading of the ascending aorta ranges from zero stress to the early phase in the transition zone (Figure 1, D, in Nightingale and colleagues1Nightingale M. Gregory A. Sigaeva T. Dobson G.M. Fedak P.W.M. Appoo J.J. et al.Biomechanics in ascending aortic aneurysms correlate with tissue composition and strength.J Thorac Cardiovasc Surg Open. 2022; 9: 1-10Scopus (6) Google Scholar). However, the aorta is always under non-zero stress due to blood pressure and its ring configuration. This is immediately intuitive to surgeons. When starting stress and variable maximum stress are accounted for, we cannot assume that only the early linear region is important. Further, the aorta is strained unequally along different axes, and this was not considered. Energy loss, which we introduced in 2014,2Chung J. Lachapelle K. Wener E. Cartier R. De Varennes B. Fraser R. et al.Energy loss, a novel biomechanical parameter, correlates with aortic aneurysm size and histopathologic findings.J Thorac Cardiovasc Surg. 2014; 148 (discussion 1088-9): 1082-1088https://doi.org/10.1016/j.jtcvs.2014.06.021Abstract Full Text Full Text PDF PubMed Scopus (43) Google Scholar was used by the authors as a main comparator biomechanical property. Contrary to what the authors mentioned, energy loss has been tested under both supraphysiological and physiological displacement/loads.3Tang M. Eliathamby D. Ouzounian M. Simmons C.A. Chung J.C. Dependency of energy loss on strain rate, strain magnitude and preload: towards development of a novel biomarker for aortic aneurysm dissection risk.J Mech Behav Biomed Mater. 2021; 124: 104736https://doi.org/10.1016/j.jmbbm.2021.104736Crossref PubMed Scopus (4) Google Scholar Notably, we showed that energy loss is relatively insensitive to a wide range of loading conditions, including strain rate, starting and maximum strain, and strain ratios.3Tang M. Eliathamby D. Ouzounian M. Simmons C.A. Chung J.C. Dependency of energy loss on strain rate, strain magnitude and preload: towards development of a novel biomarker for aortic aneurysm dissection risk.J Mech Behav Biomed Mater. 2021; 124: 104736https://doi.org/10.1016/j.jmbbm.2021.104736Crossref PubMed Scopus (4) Google Scholar This is exciting, as it supports the robustness of energy loss to be measured in vivo for clinical translation. The authors evaluated each biomarker against rupture strength. Energy loss correlated better than LTM and TZO in the circumferential direction; data were more limited in the axial direction making conclusions less clear. Previously, energy loss was shown to also correlate strongly with delamination strength,3Tang M. Eliathamby D. Ouzounian M. Simmons C.A. Chung J.C. Dependency of energy loss on strain rate, strain magnitude and preload: towards development of a novel biomarker for aortic aneurysm dissection risk.J Mech Behav Biomed Mater. 2021; 124: 104736https://doi.org/10.1016/j.jmbbm.2021.104736Crossref PubMed Scopus (4) Google Scholar,4Chung J.C. Wong E. Tang M. Eliathamby D. Forbes T.L. Butany J. et al.Biomechanics of aortic dissection: a comparison of aortas associated with bicuspid and tricuspid aortic valves.J Am Heart Assoc. 2020; 9: e016715Crossref PubMed Scopus (23) Google Scholar which measures the forces to peel apart the aortic wall as occurs during aortic dissection. Therefore, rupture strength is the second failure criteria that energy loss correlates well with. We demonstrated correlations with elastin and collagen in our previous studies on energy loss.2Chung J. Lachapelle K. Wener E. Cartier R. De Varennes B. Fraser R. et al.Energy loss, a novel biomechanical parameter, correlates with aortic aneurysm size and histopathologic findings.J Thorac Cardiovasc Surg. 2014; 148 (discussion 1088-9): 1082-1088https://doi.org/10.1016/j.jtcvs.2014.06.021Abstract Full Text Full Text PDF PubMed Scopus (43) Google Scholar,4Chung J.C. Wong E. Tang M. Eliathamby D. Forbes T.L. Butany J. et al.Biomechanics of aortic dissection: a comparison of aortas associated with bicuspid and tricuspid aortic valves.J Am Heart Assoc. 2020; 9: e016715Crossref PubMed Scopus (23) Google Scholar The current study showed that, across several domains, correlations with microstructural components were stronger with energy loss than with either LTM and TZO.1Nightingale M. Gregory A. Sigaeva T. Dobson G.M. Fedak P.W.M. Appoo J.J. et al.Biomechanics in ascending aortic aneurysms correlate with tissue composition and strength.J Thorac Cardiovasc Surg Open. 2022; 9: 1-10Scopus (6) Google Scholar One of the limitations listed in this paper was the lack of healthy controls. Previous studies with healthy controls may be useful. Our previous work demonstrated that energy loss can distinguish between patients with normal aortas, aneurysms, and acute aortic dissection.4Chung J.C. Wong E. Tang M. Eliathamby D. Forbes T.L. Butany J. et al.Biomechanics of aortic dissection: a comparison of aortas associated with bicuspid and tricuspid aortic valves.J Am Heart Assoc. 2020; 9: e016715Crossref PubMed Scopus (23) Google Scholar This interesting paper hits the nail on the head on important criteria necessary for development of successful aortic biomechanics parameters. Energy loss appears to fulfill these criteria. We also look forward to future studies on LTM and TZO exploring these parameters in more detail. Biomechanics in ascending aortic aneurysms correlate with tissue composition and strengthJTCVS OpenVol. 9PreviewThis study correlates low strain tangential modulus (LTM) and transition zone onset (TZo) stress, biomechanical parameters that occur within the physiological range of stress seen in vivo, with tissue strength and histopathologic changes in aneurysmal ascending aortic tissue. Full-Text PDF Open AccessReply from authors: Toward physiologically meaningful biomechanical parameters from ex vivo biaxial testingJTCVS OpenVol. 10PreviewStemming from the Commentary by Tang and colleagues1 on our article,2 we were excited to be invited to elaborate more on the biaxial ex vivo biomechanical parameters for assessing ascending aortic aneurysms. Particularly, Tang and colleagues2 raise an excellent point regarding the limitations of ex vivo testing. Full-Text PDF Open Access

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