Bony ingrowth potential of 3D-printed porous titanium alloy: a direct comparison of interbody cage materials in an in vivo ovine lumbar fusion model.

Background contextThere is significant variability in the materials commonly used for interbody cages in spine surgery. It is theorized that three-dimensional (3D)-printed interbody cages using porous titanium material can provide more consistent bone ingrowth and biological fixation.PurposeThe purpose of this study was to provide an evidence-based approach to decision-making regarding interbody materials for spinal fusion.Study designA comparative animal study was performed.MethodsA skeletally mature ovine lumbar fusion model was used for this study. Interbody fusions were performed at L2-L3 and L4-L5 in 27 mature sheep using three different interbody cages (ie, polyetheretherketone [PEEK], plasma sprayed porous titanium-coated PEEK [PSP], and 3D-printed porous titanium alloy cage [PTA]). Non-destructive kinematic testing was performed in the three primary directions of motion. The specimens were then analyzed using micro-computed tomography (µ-CT); quantitative measures of the bony fusion were performed. Histomorphometric analyses were also performed in the sagittal plane through the interbody device. Outcome parameters were compared between cage designs and time points.ResultsFlexion-extension range of motion (ROM) was statistically reduced for the PTA group compared with the PEEK cages at 16 weeks (p-value=.02). Only the PTA cages demonstrated a statistically significant decrease in ROM and increase in stiffness across all three loading directions between the 8-week and 16-week sacrifice time points (p-value≤.01). Micro-CT data demonstrated significantly greater total bone volume within the graft window for the PTA cages at both 8 weeks and 16 weeks compared with the PEEK cages (p-value<.01).ConclusionsA direct comparison of interbody implants demonstrates significant and measurable differences in biomechanical, µ-CT, and histologic performance in an ovine model. The 3D-printed porous titanium interbody cage resulted in statistically significant reductions in ROM, increases in the bone ingrowth profile, as well as average construct stiffness compared with PEEK and PSP

Alterations of biaxial viscoelastic properties of the right ventricle in pulmonary hypertension development in rest and acute stress conditions

Introduction: The right ventricle (RV) mechanical property is an important determinant of its function. However, compared to its elasticity, RV viscoelasticity is much less studied, and it remains unclear how pulmonary hypertension (PH) alters RV viscoelasticity. Our goal was to characterize the changes in RV free wall (RVFW) anisotropic viscoelastic properties with PH development and at varied heart rates.Methods: PH was induced in rats by monocrotaline treatment, and the RV function was quantified by echocardiography. After euthanasia, equibiaxial stress relaxation tests were performed on RVFWs from healthy and PH rats at various strain-rates and strain levels, which recapitulate physiological deformations at varied heart rates (at rest and under acute stress) and diastole phases (at early and late filling), respectively.Results and Discussion: We observed that PH increased RVFW viscoelasticity in both longitudinal (outflow tract) and circumferential directions. The tissue anisotropy was pronounced for the diseased RVs, not healthy RVs. We also examined the relative change of viscosity to elasticity by the damping capacity (ratio of dissipated energy to total energy), and we found that PH decreased RVFW damping capacity in both directions. The RV viscoelasticity was also differently altered from resting to acute stress conditions between the groups—the damping capacity was decreased only in the circumferential direction for healthy RVs, but it was reduced in both directions for diseased RVs. Lastly, we found some correlations between the damping capacity and RV function indices and there was no correlation between elasticity or viscosity and RV function. Thus, the RV damping capacity may be a better indicator of RV function than elasticity or viscosity alone. These novel findings on RV dynamic mechanical properties offer deeper insights into the role of RV biomechanics in the adaptation of RV to chronic pressure overload and acute stress

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Background: Rotator cuff tears are a common source of shoulder pain. High rates (20%-94%) of structural failure of the repair have been attributed to multiple factors, including poor repair tissue quality and tendon-to-bone integration. Biologic augmentation using growth factors has potential to promote tendon-to-bone integration, improving the function and long-term success of the repair. One such growth factor is platelet-derived growth factor-BB (PDGF-BB), which has been shown to improve healing in tendon and bone repair models

QUASI-LINEAR VISCOELASTIC THEORY IS INSUFFICIENT TO COMPREHENSIVELY DESCRIBE THE TIME-DEPENDENT BEHAVIOR OF HUMAN CERVICAL SPINE LIGAMENTS

ABSTRACT Stress relaxation experiments were conducted on cervical spine ligaments at multiple strain magnitudes to determine the validity and applicability of the quasi-linear viscoelastic (QLV) theory to model their dynamic behavior. The results indicate that the shape of the stress relaxation curve is dependent upon the magnitude of the applied strain. Thus, a more general, nonlinear formulation is required to model these ligaments within the physiological strain range. Keywords: quasi-linear, viscoelasticity, cervical spine, ligament INTRODUCTION Finite element (FE) analysis is a useful tool to study cervical spine biomechanics Spinal ligaments play an important role in spinal biomechanics. Although spinal ligaments display viscoelastic material behavior, little research has been conducted to define these properties. Well defined viscoelastic properties of spinal ligaments are requisite for the development of dynamic FE models of the cervical spine. Previous work [2] has used the quasi-linear viscoelastic (QLV) theory proposed by Fung [3] to model viscoelastic behavior of cervical spine ligaments. The QLV formulation is commonly written by invoking the convolution integral: where ( ) is the reduced relaxation function, ( ) is the instantaneous elastic stress, denotes strain, and is a dummy variable of integration MATERIALS AND METHODS Experimental Methods Eight C5-C6 functional spinal units (FSU) were dissected from frozen human cadaveric cervical spines. To separate the ligaments, each FSU vertebrae was transected at the mid-coronal plane of the vertebral body and at the pedicles. The anterior longitudinal ligament (ALL, n=8), posterior longitudinal ligament (PLL, n=8), and ligamentum flavum (LF, n=6) were isolated, resulting in boneligament-bone preparations for each ligament. The cranial and caudal bones were potted in polymethylmethacrylate (PMMA). Full hydration was maintained during specimen preparation via periodic physiologic saline spray. All experiments were conducted in an environmental chamber, filled with physiologic saline heated to 37 °C, that was attached to a translation (x-y) table and rigidly fixed to the base of a servo-hydraulic materials testing machine (Bionix 858, MTS, Minneapolis, MN). A single degree of freedom load cell was placed between the MTS actuator and a custom upper fixture. The load cell force was zeroed. The cranially potted bone was attached and the crosshead was positioned to this zero force configuration for 1 hr to assure specimen equilibration. The specimens were ramped at 0.05 mm/s to 5 N of pretension and the resulting displacement was used as the reference configuration. Subsequently, each ligament was preconditioned at 10% engineering strain, applied at 1 Hz for 120 cycles, and was returned to its reference configuration for 600 s

The development and validation of a numerical integration method for non-linear viscoelastic modeling.

Compelling evidence that many biological soft tissues display both strain- and time-dependent behavior has led to the development of fully non-linear viscoelastic modeling techniques to represent the tissue's mechanical response under dynamic conditions. Since the current stress state of a viscoelastic material is dependent on all previous loading events, numerical analyses are complicated by the requirement of computing and storing the stress at each step throughout the load history. This requirement quickly becomes computationally expensive, and in some cases intractable, for finite element models. Therefore, we have developed a strain-dependent numerical integration approach for capturing non-linear viscoelasticity that enables calculation of the current stress from a strain-dependent history state variable stored from the preceding time step only, which improves both fitting efficiency and computational tractability. This methodology was validated based on its ability to recover non-linear viscoelastic coefficients from simulated stress-relaxation (six strain levels) and dynamic cyclic (three frequencies) experimental stress-strain data. The model successfully fit each data set with average errors in recovered coefficients of 0.3% for stress-relaxation fits and 0.1% for cyclic. The results support the use of the presented methodology to develop linear or non-linear viscoelastic models from stress-relaxation or cyclic experimental data of biological soft tissues

Non-linear stress-relaxation fits.

<p>The proposed numerical integration <i>direct fit</i> method for non-linear viscoelastic characterization was able to accurately fit the idealized stress-relaxation experimental data, including the non-linear stress-strain behavior during the ramping phase and the strain-dependent relaxation indicative of non-linear viscoelastic behavior.</p