Exploration of the influence of different biomimetic designs of 3D printed multi-material artificial spinal disc on the natural mechanics restoration

One of the great challenges of artificial spinal disc (ASD) design lies in the reproduction of the complex mechanics of an intervertebral disc (IVD) that is characterized by a viscoelastic, nonlinear, and anisotropic behavior. Although the development of multi-material additive manufacturing (AM) combined with biomimetic design provide new opportunities for the realization of ASDs with complex behavior, the influence of different biomimetic designs on the kinematics of ASD in conjunction with AM is not yet explored. Therefore, this study proposes and fabricates four types of biomimetic, multi-material ASD designs based on mimicking either the material stiffness gradient or the structure found in a natural IVD. The results show that all the designs exhibit a desired viscoelastic behavior, while the ASD design based on a chainmail-like structure exhibits a nature-mimicking nonlinear rotational load response. In terms of restoring the natural trend of IVD’s anisotropic behavior, the ASD design that mimics the structure in an IVD outperforms the design that solely mimics the IVD’s material stiffness gradient. Additionally, all the designs proposed in this study show comparable instant helical axis (IHA) and instant center of rotation (ICOR) to an IVD regarding their location and moving direction

Influence of aging on mechanical properties of the femoral neck using an inverse method.

Today, we are facing rapid aging of the world population, which increases the incidence of hip fractures. The gold standard of bone strength assessment in the laboratory is micro-computed finite element analysis (μFEA) based on micro-computed tomography (μCT) images. In clinics, the standard method to assess bone fracture risk is based on areal bone mineral density (aBMD), measured by dual-energy X-ray absorptiometry (DXA). In addition, homogenized finite element analysis (hFEA) constructed from quantitative computed tomography reconstructions (QCT) predicts clinical bone strength more accurately than DXA. Despite considerable evidence of degradation of bone material properties with age, in the past fifty years of finite element analysis to predict bone strength, bone material parameters remained independent of age. This study aims to assess the influence of age on apparent modulus, yield stress, and strength predictions of the human femoral neck made by laboratory-available bone volume fraction (BV/TV) and μFEA; and by clinically available DXA and hFEA. Using an inverse method, we test the hypothesis that FEA material parameters are independent of age. Eighty-six human femora were scanned with DXA (aBMD) and with QCT. The femoral necks were extracted and scanned at 16 μm resolution with μCT. The grayscale images were downscaled to 32 μm and 65 μm for linear and non-linear analyses, respectively, and segmented. The μFE solver ParOSolNL (non-linear) and a standard hFEA method were applied to the neck sections with the same material properties for all samples to compute apparent modulus, yield stress, and strength. Laboratory-available BV/TV was a good predictor of apparent modulus (R2 = 0.76), almost as good as μFEA (R2 = 0.79). However, yield stress and strength were better predicted by μFEA (R2 = 0.92, R2 = 0.86, resp.) than BV/TV (R2 = 0.76, R2 = 0.76, resp.). For clinically available variables, prediction of apparent modulus was better with hFEA than aBMD (R2 = 0.67, R2 = 0.58, resp.). hFEA outperformed aBMD for predictions of yield stress (R2 = 0.63 vs R2 = 0.34 for female and R2 = 0.55 for male) and strength (R2 = 0.48 vs R2 = 0.33 for female and R2 = 0.15 for male). The inclusion of age did not improve the multiple linear models for apparent modulus, yield stress, and strength. The resolution of the μFE meshes seems to account for most morphological changes induced by aging. The errors between the simulation and the experiment for apparent modulus, yield stress, and strength were age-independent, suggesting no rationale for correcting tissue material parameters in the current FE analysis of the aging femoral neck

Homogenized finite element analysis of distal tibia sections: Achievements and limitations.

High-resolution peripheral quantitative computed tomography (HR-pQCT) based micro-finite element (μFE) analysis allows accurate prediction of stiffness and ultimate load of standardised (∼1 cm) distal radius and tibia sections. An alternative homogenized finite element method (hFE) was recently validated to compute the ultimate load of larger (∼2 cm) distal radius sections that include Colles' fracture sites. Since the mechanical integrity of the weight-bearing distal tibia is gaining clinical interest, it has been shown that the same properties can be used to predict the strength of both distal segments of the radius and the tibia. Despite the capacity of hFE to predict structural properties of distal segments of the radius and the tibia, the limitations of such homogenization scheme remain unclear. Therefore, the objective of this study is to build a complete mechanical data set of the compressive behavior of distal segments of the tibia and to compare quantitatively the structural properties with the hFE predictions. As a further aim, it is intended to verify whether hFE is also able to capture the post-yield strain localisation or fracture zones in such a bone section, despite the absence of strain softening in the constitutive model. Twenty-five fresh-frozen distal parts of tibias of human donors were used in this study. Sections were cut corresponding to an in-house triple-stack protocol HR-pQCT scan, lapped, and scanned using micro computed tomography (μCT). The sections were tested in compression until failure, unloaded and scanned again in μCT. Volumetric bone mineral density (vBMD) and bone mineral content (BMC) were correlated to compression test results. hFE analysis was performed in order to compare computational predictions (stiffness, yield load and plastic deformation field pattern) with the compressive experiment. Namely, strain localization was assessed based on digital volume correlation (DVC) results and qualitatively compared to hFE predictions by comparing mid-slices patterns. Bone mineral content (BMC) showed a good correlation with stiffness (R2 = 0.92) and yield (R2 = 0.88). Structural parameters also showed good agreement between the experiment and hFE for both stiffness (R2 = 0.96, slope = 1.05 with 95 % CI [0.97, 1.14]) and yield (R2 = 0.95, slope = 1.04 [0.94, 1.13]). The qualitative comparison between hFE and DVC strain localization patterns allowed the classification of the samples into 3 categories: bad (15 sections), semi (8), and good agreement (2). The good correlations between BMC or hFE and experiment for structural parameters were similar to those obtained previously for the distal part of the radius. The failure zones determined by hFE corresponded to registration only in 8 % of the cases. We attribute these discrepancies to local elastic/plastic buckling effects that are not captured by the continuum-based FE approach exempt from strain softening. A way to improve strain localization hFE prediction would be to use longer distal segments with intact cortical shells, as done for the radius. To conclude, the used hFE scheme captures the elastic and yield response of the tibia sections reliably but not the subsequent failure process

Textile Design of an Intervertebral Disc Replacement Device from Silk Yarn

Low back pain is often due to degeneration of the intervertebral discs (IVD). It is one of the most common age- and work-related problems in today’s society. Current treatments are not able to efficiently restore the full function of the IVD. Therefore, the aim of the present work was to reconstruct the two parts of the intervertebral disc—the annulus fibrosus (AF) and the nucleus pulposus (NP)—in such a way that the natural structural features were mimicked by a textile design. Silk was selected as the biomaterial for realization of a textile IVD because of its cytocompatibility, biodegradability, high strength, stiffness, and toughness, both in tension and compression. Therefore, an embroidered structure made of silk yarn was developed that reproduces the alternating fiber structure of +30° and −30° fiber orientation found in the AF and mimics its lamellar structure. The developed embroidered ribbons showed a tensile strength that corresponded to that of the natural AF. Fiber additive manufacturing with 1 mm silk staple fibers was used to replicate the fiber network of the NP and generate an open porous textile 3D structure that may serve as a reinforcement structure for the gel-like NP

Adjustable loop ACL suspension devices demonstrate less reliability in terms of reproducibility and irreversible displacement.

PURPOSE The aim of this study was to perform a comprehensive biomechanical examination of frequently applied femoral cortical suspension devices, comparing the properties of both fixed and adjustable fixation mechanisms. It was hypothesized that adjustable loop devices demonstrate less consistent fixation properties with increased variability compared to fixed loop devices. METHODS Nine frequently applied fixation button types were tested, six adjustable and three rigid loop devices. Six samples of each device type were purchased. Each device was installed in a servo-hydraulic mechanical testing machine, running a 2000 cycle loading protocol at force increments between 50 and 500 N. Irreversible displacement in mm was measured for all of the tested samples of each implant. Ultimately, maximum load to failure was applied and measured in Nm. An irreversible displacement of 3 mm was considered failure of the implant. RESULTS Three of the six adjustable devices (GraftMax™, TightRope® ToggleLoc™) demonstrated a median displacement above the threshold of clinical failure before completion of the cycles. All adjustable loop devices showed a wide intragroup variation in terms of irreversible displacement, compared to fixed-loop devices. Fixed-loop devices provided consistent reproducible results with narrow ranges and significantly lower irreversible displacement (p < 0.05), the maximum being 1.4 mm. All devices withstood an ultimate force of more than 500 N. CONCLUSION Adjustable loop devices still show biomechanical inferiority and demonstrate heterogeneity of fixation properties with wide- and less-reproducible displacement ranges resultant to the mechanism of adjustment, denoting less reliability. However, three adjustable devices (RIGIDLOOP™ Adjustable, Ultrabutton ◊, ProCinch™) demonstrate fixation capacities within the margins of clinical acceptance. RIGIDLOOP™ Adjustable provides the most comparable fixation properties to fixed loop devices

A nonlinear homogenized finite element analysis of the primary stability of the bone–implant interface

Stability of an implant is defined by its ability to undergo physiological loading–unloading cycles without showing excessive tissue damage and micromotions at the interface. Distinction is usually made between the immediate primary stability and the long-term, secondary stability resulting from the biological healing process. The aim of this research is to numerically investigate the effect of initial implantation press-fit, bone yielding, densification and friction at the interface on the primary stability of a simple bone–implant system subjected to loading–unloading cycles. In order to achieve this goal, human trabecular bone was modeled as a continuous, elasto-plastic tissue with damage and densification, which material constants depend on bone volume fraction and fabric. Implantation press-fit related damage in the bone was simulated by expanding the drilled hole to the outer contour of the implant. The bone–implant interface was then modeled with unilateral contact with friction. The implant was modeled as a rigid body and was subjected to increasing off-axis loading cycles. This modeling approach is able to capture the experimentally observed primary stability in terms of initial stiffness, ultimate force and progression of damage. In addition, it is able to quantify the micromotions around the implant relevant for bone healing and osseointegration. In conclusion, the computationally efficient modeling approach used in this study provides a realistic structural response of the bone–implant interface and represents a powerful tool to explore implant design, implantation press-fit and the resulting risk of implant failure under physiological loading

Comparison of the biomechanical performance of a customized unilateral locking compression plate with and without an intervertebral spacer applied to the first and second lumbar vertebrae after intervertebral diskectomy in canine cadaveric specimens

Objective: To determine whether a customized unilateral intervertebral anchored fusion device combined with (vs without) an intervertebral spacer would increase the stability of the L1-L2 motion segment following complete intervertebral diskectomy in canine cadaveric specimens. Sample: Vertebral columns from T13 through L3 harvested from 16 skeletally mature Beagles without thoracolumbar disease. Procedures: Complete diskectomy of the L1-2 disk was performed in each specimen. Unilateral stabilization of the L1-L2 motion segment was performed with the first of 2 implants: a unilateral intervertebral anchored fusion device that consisted of a locking compression plate with or without an intervertebral spacer. The resulting construct was biomechanically tested; then, the first implant was removed, and the second implant was applied to the contralateral side and tested. Range of motion in flexion and extension, lateral bending, and torsion was compared among intact specimens (prior to diskectomy) and constructs. Results: Compared with intact specimens, constructs stabilized with either implant were as stable in flexion and extension, significantly more stable in lateral bending, and significantly less stable in axial rotation. Constructs stabilized with the fusion device plus intervertebral spacer were significantly stiffer in lateral bending than those stabilized with the fusion device alone. No significant differences in flexion and extension and rotation were noted between implants. Conclusions and clinical relevance: Findings did not support the use of this customized unilateral intervertebral anchored fusion device with an intervertebral spacer to improve unilateral stabilization of the L1-L2 motion segment after complete L1-2 diskectomy in dogs

Bone collagen tensile properties of the aging human proximal femur.

Despite the dominant role of bone mass in osteoporotic fractures, aging bone tissue properties must be thoroughly understood to improve osteoporosis management. In this context, collagen content and integrity are considered important factors, although limited research has been conducted on the tensile behavior of demineralized compact bone in relation to its porosity and elastic properties in the native mineralized state. Therefore, this study aims (i) at examining the age-dependency of mineralized bone and collagen micromechanical properties; (ii) to test whether, and if so to which extent, collagen properties contribute to mineralized bone mechanical properties. Two cylindrical cortical bone samples from fresh frozen human anatomic donor material were extracted from 80 proximal diaphyseal sections from a cohort of 24 female and 19 male donors (57 to 96 years at death). One sample per section was tested in uniaxial tension under hydrated conditions. First, the native sample was tested elastically (0.25 % strain), and after demineralization, up to failure. Morphology and composition of the second specimen was assessed using micro-computed tomography, Raman spectroscopy, and gravimetric methods. Simple and multiple linear regression were employed to relate morphological, compositional, and mechanical variables with age and sex. Macro-tensile properties revealed that only elastic modulus of native samples was age dependent whereas apparent elastic modulus was sex dependent (p < 0.01). Compositional and morphological analysis detected a weak but significant age and sex dependency of relative mineral weight (r = -0.24, p < 0.05) and collagen disorder ratio (I∼1670/I∼1640, r = 0.25, p < 0.05) and a strong sex dependency of bone volume fraction while generally showing consistent results in mineral content assessment. Young's modulus of demineralized bone was significantly related to tissue mineral density and Young's modulus of native bone. The results indicate that mechanical properties of the organic phase, that include collagen and non-collagenous proteins, are independent of donor age. The observed reduction in relative mineral weight and corresponding overall stiffer response of the collagen network may be caused by a reduced number of mineral-collagen connections and a lack of extrafibrillar and intrafibrillar mineralization that induces a loss of waviness and a collagen fiber pre-stretch

Dataset of angular and compressive responses of biomimetic artificial spinal discs fabricated using multi-material additive manufacturing

To explore the influence of different biomimetic designs and multi-material additive manufacturing on the performance of a multi-material artificial spinal disc (ASD) in terms of restoring natural mechanics, four biomimetic ASD designs together with a control design are first fabricated using a Stratasys Connex3 Objet500 inkjet-based, multi-material 3D printer and their mechanical responses are measured using in-vitro mechanical testing. The mechanical tests include an angular test and a compression test to measure the ASD's behavior in the seven most frequent loading scenarios of a spine: flexion, extension, left/right lateral bending, left/right axial rotation, and compression. The angular test is performed using a custom six degrees of freedom, computer-controlled spine testing system together with an optoelectronic motion analysis system, while the compression test is performed using an Instron testing machine. The presented dataset includes 3D models of the five ASD designs, and raw data of the angular and compressive responses at different strain rates of the five ASD designs. This dataset is related to the article “Exploration of the influence of different biomimetic designs of 3D printed multi-material artificial spinal disc on the natural mechanics restoration” where the detailed designs and load responses of the five multi-material ASDs are presented (Yu et al., 2021). This dataset helps to gain insights into the influence of different biomimetic design concepts on the mechanics of a multi-material ASD and serves as a reference for the future design of multi-material ASDs.ISSN:2352-340

Biomechanical evaluation and comparison of two dorsal and two ventral stabilization techniques for atlantoaxial instability in dogs: a canine cadaveric study.

Objective: To compare the biomechanical properties of atlantoaxial joints (AAJs) in canine vertebral column specimens stabilized with 4 techniques (dorsal wire, modified dorsal clamp, ventral transarticular pin, and augmented ventral transarticular pin fixation) after transection of the AAJ ligaments. Sample: 13 skull and cranial vertebral column segments from 13 cadaveric toy-breed dogs. Procedures: Vertebral column segments from the middle aspect of the skull to C5 were harvested and prepared; AAJ ligament and joint capsule integrity was preserved. The atlantooccipital joint and C2 to C5 vertebral column segments were fixed with 2 transarticular Kirschner wires each. The occipital bone and caudalmost aspect of each specimen were embedded in polymethylmethacrylate. Range of motion of the AAJ under shear loading conditions up to 15 N was determined for each specimen during the third of 3 loading cycles with intact ligaments, after ligament transection, and after stabilization with each technique in random order. For each specimen, a load-to-failure test was performed with the fixation type tested last. Results: All stabilization techniques except for dorsal clamp fixation were associated with significantly decreased AAJ range of motion, compared with results when ligaments were intact or transected. The AAJs with dorsal wire, ventral transarticular pin, and augmented ventral transarticular pin fixations had similar biomechanical properties. Conclusions and clinical relevance: Dorsal wire, ventral transarticular pin, and augmented ventral transarticular pin fixation increased rigidity, compared with results for AAJs with intact ligaments and for AAJs with experimentally created instability. Additional studies are needed to assess long-term stability of AAJs stabilized with these techniques