The role of elastin on the mechanical properties of the anterior leaflet in porcine tricuspid valves.

Elastin is present in the extracellular matrix (ECM) of connective tissues, and its mechanical properties are well documented. In Marfan syndrome, however, the inability to properly code for the protein fibrillin-1 prematurely leads to the degradation and loss of elastin fiber integrity in the ECM. In this study, the role of elastin in the ECM of the anterior leaflet of the tricuspid valve was investigated by examining the biomechanical behavior of porcine leaflets before and after the application of the enzyme elastase. Five loading protocols were applied to the leaflet specimens in two groups (elastase-treated and control samples). The mechanical response following elastase application yielded a significantly stiffer material in both the radial and circumferential directions. At a physiological level of stress (85 kPa), the elastase group had an average strain of 26.21% and 6.32% in the radial and circumferential directions, respectively, at baseline prior to elastase application. Following elastase treatment, the average strain was 5.28% and 0.97% in the radial and circumferential directions, respectively. No statistically significant change was found in the control group following sham treatment with phosphate-buffered saline (PBS). Two-photon microscopy images confirmed that after the removal of elastin, the collagen fibers displayed a loss of undulation. With a significant reduction in radial compliance, the ability to withstand physiological loads may be compromised. As such, an extracellular matrix that is structurally deficient in elastin may hinder normal tricuspid valve function.

Dilation of tricuspid valve annulus immediately after rupture of chordae tendineae in ex-vivo porcine hearts.

PURPOSE: Chordae rupture is one of the main lesions observed in traumatic heart events that might lead to severe tricuspid valve (TV) regurgitation. TV regurgitation following chordae rupture is often well tolerated with few or no symptoms for most patients. However, early repair of the TV is of great importance, as it might prevent further exacerbation of the regurgitation due to remodeling responses. To understand how TV regurgitation develops following this acute event, we investigated the changes on TV geometry, mechanics, and function of ex-vivo porcine hearts following chordae rupture. METHODS: Sonomicrometry techniques were employed in an ex-vivo heart apparatus to identify how the annulus geometry alters throughout the cardiac cycle after chordae rupture, leading to the development of TV regurgitation. RESULTS: We observed that the TV annulus significantly dilated (~9% in area) immediately after chordae rupture. The annulus area and circumference ranged from 11.4 ± 2.8 to 13.3 ± 2.9 cm2 and from 12.5 ± 1.5 to 13.5 ± 1.3 cm, respectively, during the cardiac cycle for the intact heart. After chordae rupture, the annulus area and circumference were larger and ranged from 12.3 ± 3.0 to 14.4 ± 2.9 cm2 and from 13.0 ± 1.5 to 14.0 ± 1.2 cm, respectively. CONCLUSIONS: In our ex-vivo study, we showed for the first time that the TV annulus dilates immediately after chordae rupture. Consequently, secondary TV regurgitation may be developed because of such changes in the annulus geometry. In addition, the TV leaflet and the right ventricle myocardium are subjected to a different mechanical environment, potentially causing further negative remodeling responses and exacerbating the detrimental outcomes of chordae rupture.

Anisotropic and nonlinear biaxial mechanical response of porcine small bowel mesentery.

Intestinal malrotation places pediatric patients at the risk of midgut volvulus, a complication that can lead to ischemic bowel, short gut syndrome, and even death. Even though the treatments for symptomatic patients of this complication are clear, it is still a challenge to identify asymptomatic patients who are at a higher risk of midgut volvulus and decide on a suitable course of treatment. Development of an accurate computerized model of this intestinal abnormality could help in gaining a better understanding of its integral behavior. To aid in developing such a model, in the current study, we have characterized the biaxial mechanical properties of the porcine small bowel mesentery. First, the tissue stress-strain response was determined using a biaxial tensile testing equipment. The stress-strain data were then fitted into a Fung-type phenomenological constitutive model to quantify the tissue material parameters. The stress-strain responses were highly nonlinear, showing more compliance at the lower strains following by a rapid transition into a stiffer response at higher strains. The tissue was anisotropic and showed more stiffness in the radial direction. The data fitted the Fung-type constitutive model with an average R-squared value of 0.93. An averaging scheme was used to produce a set of material parameters which can represent the generic mechanical behavior of the tissue in the models.

Surface Strains of Porcine Tricuspid Valve Septal Leaflets Measured in Ex Vivo Beating Hearts.

Quantification of the tricuspid valve (TV) leaflets mechanical strain is important in order to understand valve pathophysiology and to develop effective treatment strategies. Many of the traditional methods used to dynamically open and close the cardiac valves in vitro via flow simulators require valve dissection. Recent studies, however, have shown that restriction of the atrioventricular valve annuli could significantly change their in vivo deformation. For the first time, the porcine valve leaflets deformation was measured in a passive ex vivo beating heart without isolating and remounting the valve annuli. In particular, the right ventricular apexes of porcine hearts (n = 8) were connected to a pulse-duplicator pump that maintained a pulsatile flow from and to a reservoir connected to the right atrium and the pulmonary arteries. This pump provided a right ventricular pressure (RVP) waveform that closely matched physiological values, leading to opening and closure of the tricuspid and pulmonary valves (PVs). At the midsection of the valve leaflets, the peak areal strain was 9.8 ± 2.0% (mean±standard error). The peak strain was 5.6 ± 1.1% and 4.3 ± 1.0% in the circumferential and radial directions, respectively. Although the right ventricle was beating passively, the leaflet peak areal strains closely matched the values measured in other atrioventricular valves (i.e., the mitral valve (MV)) in vivo. This technique can be used to measure leaflet strains with and without the presence of valve lesions to help develop/evaluate treatment strategies to restore normal valve deformation.

On the Biaxial Mechanical Response of Porcine Tricuspid Valve Leaflets.

Located on the right side of the heart, the tricuspid valve (TV) prevents blood backflow from the right ventricle to the right atrium. Similar to other cardiac valves, quantification of TV biaxial mechanical properties is essential in developing accurate computational models. In the current study, for the first time, the biaxial stress-strain behavior of porcine TV was measured ex vivo under different loading protocols using biaxial tensile testing equipment. The results showed a highly nonlinear response including a compliant region followed by a rapid transition to a stiff region for all of the TV leaflets both in the circumferential and in the radial directions. Based on the data analysis, all three leaflets were found to be anisotropic, and they were stiffer in the circumferential direction in comparison to the radial direction. It was also concluded that the posterior leaflet was the most anisotropic leaflet.

A region-based anatomical landmark configuration for sinus surgery using image guided navigation system: a phantom-study.

PURPOSE: To evaluate the current beliefs about the ways to reduce target registration error (TRE) values in image guided Sinus surgery by rearranging the fiducial configuration, and investigating the best configurations for various surgical fields in a phantom study. METHODS: A new CT-compatible skull phantom consisting of implanted targets was designed to enable direct measurement of TRE in four fields of sinus surgery, Frontal, Ethmoid, Sphenoid and Maxillary. The effects of different landmark configurations on TRE values, measured by the Parsiss-IV navigation system were investigated to find the best landmark arrangement for each region, and compared to the TRE prediction formula to assess the clinically accepted landmark selection approaches based on this formula. RESULTS: It was shown that smaller values of TRE could be attained by arranging the center of the fiducials to be more focused on the surgery target. The addition of more fiducials and keeping non-linear arrangement of landmark would not necessarily decrease the TRE value. CONCLUSION: Optimizing the landmark configuration is important for increasing the localization accuracy in image guided sinus surgery. The common beliefs accepted in the clinical community about the ways to reduce the TRE are very general and should be adapted to specific field of image guided surgery.

Minimization of target registration error for vertebra in image-guided spine surgery.

PURPOSE: The accuracy of pedicle screw placement during image-guided spine surgery (IGSS) can be characterized by estimating the target registration error (TRE). The major factors that influence TRE were identified, minimized, and verified with in vitro experiments. MATERIALS AND METHODS: Computed-tomography-compatible markers are placed over anatomical landmarks of lumbar vertebral segments in locations that are feasible and routinely used in surgical procedures. TRE was determined directly for markers placed on the pedicles of vertebra segments. First, optimum selections of landmarks are proposed for different landmarks according to the minimum achievable TRE values in different configurations. These anatomical landmarks are feasible and accessible to overcome constraints that may be imposed during surgical procedures. Second, the effect of fiducial weighting on corresponding points to overcome anisotropic localization error based on maximum likelihood approach is evaluated. Third, an experimental model for fiducial localization error (FLE) is derived to obtain the weights. At the end, an error zone was obtained for each marker to indicate the possible acceptable deviation from the marker's exact location in practice. This study was performed in vitro on a spine phantom. RESULTS: Optimal landmark selection led to a 30% reduction in TRE. In addition, optimum weighting of the fiducials in an FLE model that incorporates anisotropic localization error in the registration algorithm led to a 28% reduction in the TRE. CONCLUSION: Landmark configuration, transformation parameters, and fiducial localization error are factors that significantly affect the total TRE. These factors should be optimized to minimize the TRE. Both the optimum configuration of landmarks and the anisotropic weighing of fiducials have significant impact on the registration accuracy for IGSS.