Dissection flap fenestration can reduce re-apposition force of the false lumen in type-B aortic dissection: a computational and bench study.

Thoracic endovascular aortic repair (TEVAR) has been widely adopted as a standard for treating complicated acute and high-risk uncomplicated Stanford Type-B aortic dissections. The treatment redirects the blood flow towards the true lumen by covering the proximal dissection tear which promotes sealing of the false lumen. Despite advances in TEVAR, over 30% of Type-B dissection patients require additional interventions. This is primarily due to the presence of a persistent patent false lumen post-TEVAR that could potentially enlarge over time. We propose a novel technique, called slit fenestration pattern creation, which reduces the forces for re-apposition of the dissection flap (i.e., increase the compliance of the flap). We compute the optimal slit fenestration design using a virtual design of experiment (DOE) and demonstrate its effectiveness in reducing the re-apposition forces through computational simulations and benchtop experiments using porcine aortas. The findings suggest this potential therapy can drastically reduce the radial loading required to re-appose a dissected flap against the aortic wall to ensure reconstitution of the aortic wall (remodeling).

Effect of proximal fixation length on complications after endovascular repair of type B aortic dissection.

OBJECTIVE: We evaluated the effect of the achieved proximal seal length on the outcomes after endovascular repair of acute type B aortic dissection (aTBAD). METHODS: A post hoc analysis was performed using data from two prospective, multicenter investigational studies of the Zenith Dissection Endovascular System (STABLE I and II). Patients treated for aTBAD within 14 days of symptom onset were included if complete preoperative and postoperative imaging data were available for review. The patients were divided into four groups according to the length of the achieved proximal seal according to the centerline imaging findings: ≥20 mm, ≥10 to <20 mm, ≥0 to <10 mm, and <0 mm. The outcomes stratified by the achieved proximal seal length were evaluated. All imaging findings were based on core laboratory analysis. RESULTS: A total of 110 patients were included in the present analysis; 51 were from STABLE I and 59 from STABLE II. Although the study protocol criteria required a ≥20 mm length of nondissected aorta distal to the left common carotid artery to serve as a proximal seal zone, an achieved proximal seal length of ≥20 mm was observed in only 19 of the 110 patients (17.3%) according to the location of stent-graft placement. After a mean follow-up duration of 41.6 ± 21 months, the cumulative rate of the composite device outcome (ie, proximal entry flow, retrograde dissection, transaortic growth, and stent-graft migration) was lowest in patients with an achieved proximal seal length of ≥20 mm (15.8%; 3 of 19). The cumulative rate increased as the seal length decreased (32.0% [8 of 25], 55.6% [20 of 36], and 60.0% [18 of 30] with a proximal seal length of ≥10 to <20 mm, ≥0 to <10 mm, and <0 mm, respectively; P < .01, Cochran-Armitage trend test). CONCLUSIONS: A clear inverse relationship was found between the proximal seal length achieved and associated adverse outcomes. This finding underscores the importance of landing the stent-graft in healthy, nondissected aorta to minimize the risk of complications and provide a durable repair in patients with aTBAD.

Validated Computational Model to Compute Re-apposition Pressures for Treating Type-B Aortic Dissections.

The use of endovascular treatment in the thoracic aorta has revolutionized the clinical approach for treating Stanford type B aortic dissection. The endograft procedure is a minimally invasive alternative to traditional surgery for the management of complicated type-B patients. The endograft is first deployed to exclude the proximal entry tear to redirect blood flow toward the true lumen and then a stent graft is used to push the intimal flap against the false lumen (FL) wall such that the aorta is reconstituted by sealing the FL. Although endovascular treatment has reduced the mortality rate in patients compared to those undergoing surgical repair, more than 30% of patients who were initially successfully treated require a new endovascular or surgical intervention in the aortic segments distal to the endograft. One reason for failure of the repair is persistent FL perfusion from distal entry tears. This creates a patent FL channel which can be associated with FL growth. Thus, it is necessary to develop stents that can promote full re-apposition of the flap leading to complete closure of the FL. In the current study, we determine the radial pressures required to re-appose the mid and distal ends of a dissected porcine thoracic aorta using a balloon catheter under static inflation pressure. The same analysis is simulated using finite element analysis (FEA) models by incorporating the hyperelastic properties of porcine aortic tissues. It is shown that the FEA models capture the change in the radial pressures required to re-appose the intimal flap as a function of pressure. The predictions from the simulation models match closely the results from the bench experiments. The use of validated computational models can support development of better stents by calculating the proper radial pressures required for complete re-apposition of the intimal flap.

Role of Re-entry Tears on the Dynamics of Type B Dissection Flap.

Mortality during follow-up after acute Type B aortic dissection is substantial with aortic expansion observed in over 59% of the patients. Lumen pressure differential is considered a prime contributing factor for aortic dilation after propagation. The objective of the study was to evaluate the relationship between changes in vessel geometry with and without lumen pressure differential post propagation in an ex vivo porcine model with comparison with patient clinical data. A pulse duplicator system was utilized to propagate the dissection within descending thoracic porcine aortic vessels for set proximal (%circumference of the entry tear: 40%, axial length: 2 cm) and re-entry (50% of distal vessel circumference) tear geometry. Measurements of lumen pressure differential were made along with quantification of vessel geometry (n = 16). The magnitude of mean lumen pressure difference measured after propagation was low (~ 5 mmHg) with higher pressures measured in false lumen and as anticipated the pressure difference approached zero after the creation of distal re-entry tear. False lumen Dissection Ratio (FDR) defined as arc length of dissected wall divided by arc length of dissection flap, had mean value of 1.59 ± 0.01 at pressure of 120/80 mmHg post propagation with increasing values with increase in pulse pressure that was not rescued with the creation of distal re-entry tear (p < 0.01). An average FDR of 1.87 ± 0.27 was measured in patients with acute Type B dissection. Higher FDR value (FDR = 1 implies zero dissection) in the presence of distal re-entry tear demonstrates an acute change in vessel morphology in response to the dissection independent of local pressure changes challenges the re-apposition of the aortic wall.

Total Endovascular Treatment of Aortic Arch Disease Using an Arch Endograft With 3 Inner Branches.

PURPOSE: To report early experience with a new endovascular graft developed for aortic arch aneurysm repair in patients unfit for open surgery. CASE REPORT: Three consecutive men (62, 74, and 69 years old) at high risk for open repair were treated for postdissection aortic arch aneurysms using a custom-made 3 inner branched endovascular graft. The 2 proximal branches are antegrade and perfuse the innominate artery and the left common carotid artery; the third branch is retrograde and perfuses the left subclavian artery. The latter is preloaded with a catheter and wire to aid cannulation. Technical success was achieved in each case. The mean procedure time, fluoroscopy duration, and contrast volume were 180 minutes, 35 minutes, and 145 mL, respectively. The perioperative period was uneventful. All branches were patent on 6-month computed tomography and duplex ultrasound imaging. CONCLUSION: This new patient-specific device allows total endovascular revascularization of the supra-aortic trunks during arch repair. These encouraging results support its more widespread use.

Early experience with a modified preloaded system for fenestrated endovascular aortic repair.

OBJECTIVE: Preloaded endovascular delivery systems expand the anatomic eligibility for complex aortic repair by requiring only one iliac access vessel and providing a stable platform for guiding sheaths into challenging target vessels. This article reports the lessons learned and early clinical outcomes using a modified preloaded delivery system for fenestrated endovascular aneurysm repair (FEVAR) in three aortic centers in Europe. METHODS: From October 2015 to March 2016, consecutive patients presenting with extensive aortic aneurysm treated with a modified preloaded FEVAR were prospectively enrolled from three high volume European aortic centers. The new design is a modification of previous designs of preloaded fenestrated stent grafts and of the p-branch device platform. The technical details of implantation are described and perioperative outcomes, including the learning curve, are collected and reported. RESULTS: All patients (30 patients; 80% men; 70.2 years old) presented for nonurgent repair of either a type Ia endoleak (3/30; 10%), a type I-II-III thoracoabdominal (8/30; 27%), or a type IV thoracoabdominal or pararenal (19/30; 63.%) aneurysm repair of a mean size of 64 ± 13 mm using a custom made device. Primary technical success was achieved in 28 of 30 patients (93%) and assisted primary technical success in 29 of 30 patients (97%). The two technical failures included open conversion to repair a ruptured iliac artery and restenting of a dissected superior mesenteric artery which was recognized hours after the index procedure had finished. The mean procedure time was 277 ± 153 minutes, fluoroscopy time 79 ± 36 minutes, dose area product 112 ± 90 Gy cm2, and contrast volume 87 ± 46 mL. All renal fenestrations were successfully stented without type III endoleak on completion angiogram; the preloaded guiding sheaths were used for 53 of 58 renal arteries (91%). Challenges related to learning to the use of the modified preloaded system were experienced early and had no clinical consequences. Major complications occurred in seven cases (23%), including two perioperative deaths because of stroke and sepsis following primary conversion attributable to iliac rupture. There were no target vessel occlusions or type I/III endoleaks found on postoperative imaging. CONCLUSIONS: Based on early experience, the modified preloaded system can be safely and effectively used during FEVAR, with good technical result and a short period of learning. This device expands treatment to patients with compromised iliac access, thus, additional patients and more follow-up will be required to determine unique risks of operating in this patient population.

Role of Pulse Pressure and Geometry of Primary Entry Tear in Acute Type B Dissection Propagation.

The hemodynamic and geometric factors leading to propagation of acute Type B dissections are poorly understood. The objective is to elucidate whether geometric and hemodynamic parameters increase the predilection for aortic dissection propagation. A pulse duplicator set-up was used on porcine aorta with a single entry tear. Mean pressures of 100 and 180 mmHg were used, with pulse pressures ranging from 40 to 200 mmHg. The propagation for varying geometric conditions (%circumference of the entry tear: 15-65%, axial length: 0.5-3.2 cm) were tested for two flap thicknesses (1/3rd and 2/3rd of the thickness of vessel wall, respectively). To assess the effect of pulse and mean pressure on flap dynamics, the %true lumen (TL) cross-sectional area of the entry tear were compared. The % circumference for propagation of thin flap (47 ± 1%) was not significantly different (p = 0.14) from thick flap (44 ± 2%). On the contrary, the axial length of propagation for thin flap (2.57 ± 0.15 cm) was significantly different (p < 0.05) from the thick flap (1.56 ± 0.10 cm). TL compression was observed during systolic phase. For a fixed geometry of entry tear (%circumference = 39 ± 2%; axial length = 1.43 ± 0.13 cm), mean pressure did not have significant (p = 0.84) effect on flap movement. Increase in pulse pressure resulted in a significant change (p = 0.02) in %TL area (52 ± 4%). The energy acting on the false lumen immediately before propagation was calculated as 75 ± 9 J/m2 and was fairly uniform across different specimens. Pulse pressure had a significant effect on the flap movement in contrast to mean pressure. Hence, mitigation of pulse pressure and restriction of flap movement may be beneficial in patients with type B acute dissections.

Branched versus fenestrated endografts for endovascular repair of aortic arch lesions.

OBJECTIVE: Endovascular repair of the aortic arch represents a formidable challenge because of aortic diameter, angulation, elasticity, and greater distance to the femoral access vessels. Whereas both fenestrated and branched endografts have been customized to accommodate complex pathologic processes of the arch, no data comparing the techniques are available. The aim of this study was to compare the outcomes of custom-made fenestrated vs branched thoracic endovascular aortic repair (fTEVAR vs bTEVAR). METHODS: This was a single-center, retrospective comparative study of all consecutive patients treated with fTEVAR and bTEVAR for aortic arch diseases. All patients were considered unsuitable for open surgical therapy and treated with customized stent grafts (Cook Medical, Bloomington, Ind). RESULTS: Within 42 months, 29 patients underwent fTEVAR and bTEVAR (66 ± 9 years; nine female patients). The fTEVAR patients (n = 15) had no differences in comorbidities compared with the bTEVAR patients (n = 14). Dissection or postdissection aneurysm was the indication in 6 of 15 fTEVARs and 5 of 14 bTEVARs (40% vs 36%; P = NS); the remaining procedures were performed for aneurysms. Six (40%) fTEVAR patients underwent previous cervical debranching compared with all bTEVAR patients. In all patients with bTEVAR, two arch vessels were targeted (innominate, 13; left carotid artery, 14; left subclavian artery, 1), whereas fTEVAR targeted 1.6 ± 0.5 arch vessels (bovine trunk, 4; innominate artery, 1; left carotid artery, 10; left subclavian artery, 9). Technical success was achieved in all but one case of a fenestrated endograft that was displaced, resulting in major stroke and death of the patient. Strokes occurred in two fTEVAR patients and one bTEVAR patient (P = NS). The 30-day mortality was 20% in the fTEVAR patients (n = 3) vs 0% in the bTEVAR patients (P = NS). The causes of early mortality were major stroke (n = 1), access complication (n = 1), and myocardial infarction (n = 1). Mean follow-up was 8 (1-35) and 10 (2-22) months for fTEVAR and bTEVAR, respectively. No branch occlusions occurred, and two patients underwent coil embolization for endoleaks (P = NS). One patient was readmitted with infected branched endograft 4 months after intervention and has so far been successfully treated with aneurysm sac drainage and antibiotics. There was one late nonaneurysm-related death in each group. CONCLUSIONS: Both fTEVAR and bTEVAR are feasible for the treatment of aortic arch diseases in high-risk patients. Results are promising, although fTEVAR was associated with higher mortality in this early experience. bTEVAR was more commonly used in Ishimaru zone 0.

International experience with endovascular therapy of the ascending aorta with a dedicated endograft.

OBJECTIVE: The objective of this study was to evaluate the safety and feasibility of a novel stent graft specifically designed for treatment of the ascending aorta. METHODS: This was a multicenter, retrospective analysis of all consecutive patients treated with the dedicated Zenith Ascend TAA Endovascular Graft (William Cook Europe, Bjaeverskov, Denmark) for pathologic processes requiring stent grafting of the ascending aorta. The graft is short (6.5 cm), with a delivery system designed for transfemoral placement in the ascending aorta. RESULTS: In 10 patients (five men; age, 67 years; range, 26-90 years), the Zenith Ascend graft was implanted for the following indications: dissection (n = 5) and aneurysm (n = 4) of the ascending aorta and fixation of an intraprocedural dislocated aortic valve (n = 1). All patients were judged to be at high risk for open surgery (nine patients were classified as American Society of Anesthesiologists class 3 or class 4). A transfemoral approach was selected in eight cases and a transapical approach in two. All endografts were successfully deployed without intraoperative adverse events at the targeted landing zone. Clinical success in coverage of the lesions was achieved in all cases with the exception of an attempted treatment of an intraprocedural aortic valve implantation dissection that resulted in early mortality. The 30-day survival was 90%. Early neurologic events included one patient with stroke and paraplegia and one patient with a transient ischemic attack. One patient underwent early evacuation of a hemopericardium. There were two late reinterventions for persisting endoleaks. At a mean follow-up of 10 months (range, 1-36 months), three late deaths occurred, with one treatment related, as a result of graft infection. CONCLUSIONS: Despite the fact that in this first published series the graft was frequently used as a "rescue tool" outside its intended indication, treatment with the Zenith Ascend graft in this early experience appears to be safe and feasible for repair of ascending aorta pathologic processes in high-risk patients unsuitable for open repair.

Endovascular Repair of Saccular Ascending Aortic Aneurysm After Orthotopic Heart Transplantation Using an Investigational Zenith Ascend Stent-Graft.

PURPOSE: To report the use of an investigational stent-graft to treat an ascending aortic aneurysm in a patient with a heart transplant. CASE REPORT: A 48-year-old man presented with a 3.5×1.5-cm saccular aneurysm in the mid anterior ascending aorta, abutting the sternum. The patient's history was notable for placement of a left ventricular assist device followed by orthotopic heart transplantation 2 years prior to treat end-stage familial dilated cardiomyopathy. Under compassionate use, a custom-designed ascending aortic stent-graft (Zenith Ascend) was successfully delivered via an 18-F system and deployed just distal to the origin of the left main coronary artery under pulmonary artery catheter-guided rapid ventricular pacing. The patient was discharged the next day, and 6-month follow-up was unremarkable. Imaging at 5 months showed an excluded aneurysm sac with no endoleak or migration. CONCLUSION: The ideal ascending aortic stent-graft should be low profile, conformable to the arch anatomy, with short tip delivery system and a stepwise deployment mechanism that allows precise placement relative to the ostia of the coronary arteries and the innominate artery. This case illustrates the advancement of endovascular techniques to the most challenging segment of the aorta to decrease morbidity and mortality in high-risk patients.

Global experience with an inner branched arch endograft.

BACKGROUND: Branched endografts are a new option to treat arch aneurysm in high-risk patients. METHODS AND RESULTS: We performed a retrospective multicenter analysis of all patients with arch aneurysms treated with a new branched endograft designed with 2 inner branches to perfuse the supra aortic trunks. Thirty-eight patients were included. The median age was 71 years (range, 64-74 years). An American Society of Anesthesiologists score of 3 or 4 was reported in 89.5% (95% confidence interval [CI], 79.7-99.3) of patients. The 30-day mortality rate was 13.2% (95% CI, 2.2-24.2). Technical success was obtained in 32 patients (84.2% [95% CI, 72.4-95.9]). Early secondary procedures were performed in 4 patients (10.5% [95% CI, 0.7-20.3]). Early cerebrovascular complications were diagnosed in 6 patients (15.8% [95% CI, 4.0-27.6]), including 4 transient ischemic attacks, 1 stroke, and 1 subarachnoid hemorrhage. The median follow-up was 12 months (range, 6-12 months). During follow-up, no aneurysm-related death was detected. Secondary procedures during follow-up were performed in 3 patients (9.1% [95% CI, 0.0-19.1]), including 1 conversion to open surgery. We compared the first 10 patients (early experience group) with the subsequent 28 patients. Intraoperative complications and secondary procedures were significantly higher in the early experience group. Although not statistically significant, the early mortality was higher in the early experience group (30% [95% CI, 0.0-60.0]) versus the remainder (7.1% [95% CI, 0.0-16.9]; P=.066). Being part of the early experience group and ascending aortic diameter≥38 mm were found to be associated to higher rates of combined early mortality and neurologic complications. CONCLUSIONS: Our preliminary study confirms the feasibility and safety of the endovascular repair of arch aneurysms in selected patients who may not have other conventional options. CLINICAL TRIAL REGISTRATION INFORMATION: Thoracic IDE NCT00583817, FDA IDE# 000101.

Endovascular repair of a chronic arch dissecting aneurysm with a branched endograft.

Acute type A dissection requires emergent surgery that often includes ascending aorta replacement. Regular follow-up with computed tomography (CT) can identify aneurysmal dilatation of the false lumen distal to the open repair. Because redo operations can be high risk, we performed an endovascular repair on a patient deemed unfit for open operation using an aortic arch branched device with 2 internal branches for the innominate trunk and the left common carotid artery. This endovascular approach is a possible alternative to open surgical procedures in selected cases.

Prediction of equibiaxial loading stress in collagen-based extracellular matrix using a three-dimensional unit cell model.

Mechanical signals are important factors in determining cell fate. Therefore, insights as to how mechanical signals are transferred between the cell and its surrounding three-dimensional collagen fibril network will provide a basis for designing the optimum extracellular matrix (ECM) microenvironment for tissue regeneration. Previously we described a cellular solid model to predict fibril microstructure-mechanical relationships of reconstituted collagen matrices due to unidirectional loads (Acta Biomater 2010;6:1471-86). The model consisted of representative volume elements made up of an interconnected network of flexible struts. The present study extends this work by adapting the model to account for microstructural anisotropy of the collagen fibrils and a biaxial loading environment. The model was calibrated based on uniaxial tensile data and used to predict the equibiaxial tensile stress-stretch relationship. Modifications to the model significantly improved its predictive capacity for equibiaxial loading data. With a comparable fibril length (model 5.9-8µm, measured 7.5µm) and appropriate fibril anisotropy the anisotropic model provides a better representation of the collagen fibril microstructure. Such models are important tools for tissue engineering because they facilitate prediction of microstructure-mechanical relationships for collagen matrices over a wide range of microstructures and provide a framework for predicting cell-ECM interactions.

Off-the-shelf fenestrated endografts: a realistic option for more than 70% of patients with juxtarenal aneurysms.

PURPOSE: To evaluate the suitability of a standardized fenestrated endograft in patients with juxtarenal abdominal aortic aneurysms. METHODS: High resolution computed tomographic angiograms from 100 consecutive patients (96 men; mean age 72 years) with juxta- (n = 78) and pararenal (n = 22) aneurysms treated electively between 2005 and 2010 with custom-made fenestrated endografts were reviewed. A centerline of flow reconstruction was carried out in a 3D imaging workstation to precisely define the aortic morphology, including aortic diameters and distances between visceral and renal arteries. The applicability of 2 different "off-the-shelf" standardized fenestrated endografts designed by the manufacturer was evaluated in this cohort. Both designs included 2 fenestrations for the renal arteries, 1 for the superior mesenteric artery (SMA), and a scallop for the celiac trunk. The designs differed in the lengths of the SMA to renal fenestration and renal to renal fenestration. RESULTS: Endovascular treatment with one or both "off-the-shelf" endografts was deemed possible in 72 patients (56 with design 1, 52 with design 2, and 36 with both endografts). Of the 28 patients who were not candidates for a standardized fenestrated stent-graft of either design, the primary cause was a right renal artery that did not match the position of its corresponding fenestration. CONCLUSION: Standardized fenestrated designs suitable for endovascular treatment of >70% of patients with juxta- and pararenal aneurysms currently treated with custom-made fenestrated endografts will soon be available. This new generation of endografts will permit rapid treatment of a large majority of patients requiring fenestrated endograft repair.

Development of a three-dimensional unit cell to model the micromechanical response of a collagen-based extracellular matrix.

The three-dimensional microstructure and mechanical properties of the collagen fibrils within the extracellular matrix (ECM) is now being recognized as a primary factor in regulating cell proliferation and differentiation. Therefore, an appreciation of the mechanical aspects by which a cell interacts with its ECM is required for the development of engineered tissues. Ultimately, using these interactions to design tissue equivalents requires mathematical models with three-dimensional architecture. In this study, a three-dimensional model of a collagen fibril matrix undergoing uniaxial tensile stress was developed by making use of cellular solids. A structure consisting of thin struts was chosen to represent the arrangement of collagen fibrils within an engineered ECM. To account for the large deformation of tissues, the collagen fibrils were modeled as hyperelastic neo-Hookean or Mooney-Rivlin materials. The use of cellular solids allowed the fibril properties to be related to the ECM properties in closed form, which, in turn, allowed the estimation of fibril properties using ECM experimental data. A set of previously obtained experimental data consisting of simultaneous measures of the fibril microstructure and mechanical tests was used to evaluate the model's capability to estimate collagen fibril mechanical property when given tissue-scale data and to predict the tissue-scale mechanical properties when given estimated fibril stiffness. The fibril tangent modulus was found to be 1.26 + or - 0.70 and 1.62 + or - 0.88 MPa when the fibril was modeled as neo-Hookean and Mooney-Rivlin material, respectively. There was no statistical significance of the estimated fibril tangent modulus among the different groups. Sensitivity analysis showed that the fibril mechanical properties and volume fraction were the two input parameters which required accurate values. While the volume fraction was easily obtained from the initial image of the gel, the fibril mechanical properties were not readily available. Therefore the fibril mechanical properties were estimated in the leave-one-out cross-validation (LOOCV) analysis. The LOOCV analysis showed that the model was able to predict the ECM stress-stretch curve with an average mean squared error of 9.71 kPa(2). The three-dimensional architecture expands on previous continuum models and two-dimensional representations to provide a useful model for studying the hierarchical effects of ECM microstructure on cell function. This model can be used as a design tool to engineer the optimum microstructure for cells to function.

Fibril microstructure affects strain transmission within collagen extracellular matrices.

The next generation of medical devices and engineered tissues will require development of scaffolds that mimic the structural and functional properties of the extracellular matrix (ECM) component of tissues. Unfortunately, little is known regarding how ECM microstructure participates in the transmission of mechanical load information from a global (tissue or construct) level to a level local to the resident cells ultimately initiating relevant mechanotransduction pathways. In this study, the transmission of mechanical strains at various functional levels was determined for three-dimensional (3D) collagen ECMs that differed in fibril microstructure. Microstructural properties of collagen ECMs (e.g., fibril density, fibril length, and fibril diameter) were systematically varied by altering in vitro polymerization conditions. Multiscale images of the 3D ECM macro- and microstructure were acquired during uniaxial tensile loading. These images provided the basis for quantification and correlation of strains at global and local levels. Results showed that collagen fibril microstructure was a critical determinant of the 3D global and local strain behaviors. Specifically, an increase in collagen fibril density reduced transverse strains in both width and thickness directions at both global and local levels. Similarly, collagen ECMs characterized by increased fibril length and decreased fibril diameter exhibited increased strain in width and thickness directions in response to loading. While extensional strains measured globally were equivalent to applied strains, extensional strains measured locally consistently underpredicted applied strain levels. These studies demonstrate that regulation of collagen fibril microstructure provides a means to control the 3D strain response and strain transfer properties of collagen-based ECMs.

Local, three-dimensional strain measurements within largely deformed extracellular matrix constructs.

The ability to create extracellular matrix (ECM) constructs that are mechanically and biochemically similar to those found in vivo and to understand how their properties affect cellular responses will drive the next generation of tissue engineering strategies. To date, many mechanisms by which cells biochemically communicate with the ECM are known. However the mechanisms by which mechanical information is transmitted between cells and their ECM remain to be elucidated. "Self-assembled" collagen matrices provide an in vitro-model system to study the mechanical behavior of ECM. To begin to understand how the ECM and the cells interact mechanically, the three-dimensional (3D) mechanical properties of the ECM must be quantified at the micro-(local) level in addition to information measured at the macro-(global) level. Here we describe an incremental digital volume correlation (IDVC) algorithm to quantify large (>0.05) 3D mechanical strains in the microstructure of 3D collagen matrices in response to applied mechanical loads. Strain measurements from the IDVC algorithm rely on 3D confocal images acquired from collagen matrices under applied mechanical loads. The accuracy and the precision of the IDVC algorithm was verified by comparing both image volumes collected in succession when no deformation was applied to the ECM (zero strain) and image volumes to which simulated deformations were applied in both ID and 3D (simulated strains). Results indicate that the IDVC algorithm can accurately and precisely determine the 3D strain state inside largely deformed collagen ECMs. Finally, the usefulness of the algorithm was demonstrated by measuring the microlevel 3D strain response of a collagen ECM loaded in tension.

Simultaneous mechanical loading and confocal reflection microscopy for three-dimensional microbiomechanical analysis of biomaterials and tissue constructs.

At present, mechanisms by which specific structural and mechanical properties of the three-dimensional extracellular matrix microenvironment influence cell behavior are not known. Lack of such knowledge precludes formulation of engineered scaffolds or tissue constructs that would deliver specific growth-inductive signals required for improved tissue restoration. This article describes a new mechanical loading-imaging technique that allows investigations of structural-mechanical properties of biomaterials as well as the structural-mechanical basis of cell-scaffold interactions at a microscopic level and in three dimensions. The technique is based upon the integration of a modified, miniature mechanical loading instrument with a confocal microscope. Confocal microscopy is conducted in a reflection and/or fluorescence mode for selective visualization of load-induced changes to the scaffold and any resident cells, while maintaining each specimen in a "live," fully hydrated state. This innovative technique offers several advantages over current biomechanics methodologies, including simultaneous visualization of scaffold and/or cell microstructure in three dimensions during mechanical loading; quantification of macroscopic mechanical parameters including true stress and strain; and the ability to perform multiple analyses on the same specimen. This technique was used to determine the structural-mechanical properties of three very different biological materials: a reconstituted collagen matrix, a tissue-derived biomaterial, and a tissue construct representing cells and matrix.