A novel finite element model of the ovine lumbar intervertebral disc with anisotropic hyperelastic material properties

The Ovine spine is an accepted model to investigate the biomechanical behaviour of the human lumbar one. Indeed, the use of animal models for in vitro studies is necessary to investigate the mechanical behaviour of biological tissue, but needs to be reduced for ethical and social reasons. The aim of this study was to create a finite element model of the lumbar intervertebral disc of the sheep that may help to refine the understanding of parallel in vitro experiments and that can be used to predict when mechanical failure occurs. Anisotropic hyperelastic material properties were assigned to the annulus fibrosus and factorial optimization analyses were performed to find out the optimal parameters of the ground substance and of the collagen fibers. For the ground substance of the annulus fibrosus the investigation was based on experimental data taken from the literature, while for the collagen fibers tensile tests on annulus specimens were conducted. Flexibility analysis in flexion-extension, lateral bending and axial rotation were conducted. Different material properties for the anterior, lateral and posterior regions of the annulus were found. The posterior part resulted the stiffest region in compression whereas the anterior one the stiffest region in tension. Since the flexibility outcomes were in a good agreement with the literature data, we considered this model suitable to be used in conjunction with in vitro and in vivo tests to investigate the mechanical behaviour of the ovine lumbar disc

Numerical prediction of the mechanical failure of the intervertebral disc under complex loading conditions

Finite element modeling has been widely used to simulate the mechanical behavior of the intervertebral disc. Previous models have been generally limited to the prediction of the disc behavior under simple loading conditions, thus neglecting its response to complex loads, which may induce its failure. The aim of this study was to generate a finite element model of the ovine lumbar intervertebral disc, in which the annulus was characterized by an anisotropic hyperelastic formulation, and to use it to define which mechanical condition was unsafe for the disc. Based on published in vitro results, numerical analyses under combined flexion, lateral bending, and axial rotation with a magnitude double that of the physiological ones were performed. The simulations showed that flexion was the most unsafe load and an axial tensile stress greater than 10 MPa can cause disc failure. The numerical model here presented can be used to predict the failure of the disc under all loading conditions, which may support indications about the degree of safety of specific motions and daily activities, such as weight lifting

A comparative analysis of a disposable and a reusable pedicle screw instrument kit for lumbar arthrodesis: integrating HTA and MCDA

Objective: Lumbar arthrodesis is a common surgical technique that consists of the fixation of one or more motion segments with pedicle screws and rods. However, spinal surgery using these techniques is expensive and has a significant impact on the budgets of hospitals and Healthcare Systems. While reusable and disposable instruments for laparoscopic interventions have been studied in literature, no specific information exists regarding instrument kits for lumbar arthrodesis. The aim of the present study was to perform a complete health technology assessment comparing a disposable instrument kit for lumbar arthrodesis (innovative device) with the standard reusable instrument. Methods: A prospective and observational study was implemented, by means of investigation of administrative records of patients undergoing a lumbar arthrodesis surgical procedure. The evaluation was conducted in 2013, over a 12- month time horizon, considering all the procedures carried out using the two technologies. A complete health technology assessment and a multi-criteria decision analysis approach were implemented in order to compare the two alternative technologies. Economic impact (with the implementation of an activity based costing approach), social, ethical, organisational, and technology-related aspects were taken into account. Results: Although the cost analysis produced similar results in the comparison of the two technologies (total cost equal to € 4,279.1 and € 4,242.6 for reusable instrument kit and the disposable one respectively), a significant difference between the two instrument kits was noted, in particular concerning the organisational impact and the patient safety. Conclusions: The replacement of a reusable instrument kit for lumbar arthrodesis, with a disposable one, could improve the management of this kind of devices in hospital settings

Mechanical properties of breast periprosthetic capsules and the correlation to capsule contracture

reserved3V. Quaglini; S. Mantero; T. VillaQuaglini, Virginio; Mantero, Sara; Villa, TOMASO MARIA TOBI

Pre-clinical Evaluation of the Biomechanical Behavior of ImplantableDevices for Orthopedic and Spinal Surgery

The assessment of the suitability of the biomechanical performances of a medical device intended to replace either a function or tissue, is a primary issue in the pre-clinical evaluation of a new device. Such evaluation is usually performed by means of either experimental facilities or computational simulation or, better, by the interaction of both the methodologies. Difficulties arise when trying to combine the need of simulating the complexity of the biological response to the implant with the necessity of maintaining a reproducible and simple experimental procedure. At LaBS, devices for the treatment of pathologies of lower limbs and spine are subjected to purposely designed experimental protocols and numerical simulations in order to take into account requests regarding their anatomical, functional and surgical compatibility, as well as their mechanical reliability in time. Two examples of such preclinical studies are here given: in particular the impact of different surgical techniques used in the implant of an interspinous device on its functional compatibility has been investigated by means of an experimental animal model; the fatigue resistances of the tibial tray of a polymethyl methacrylate (PMMA) knee spacer have been predicted and validated by means of a combined computational and experimental procedure, using advanced stress criterion based on stress invariant

The Simulation of Muscles Forces Increases the Stresses in Lumbar Fixation Implants with Respect to Pure Moment Loading

Simplified loading conditions such as pure moments are frequently used to compare different instrumentation techniques to treat spine disorders. The purpose of this study was to determine if the use of realistic loading conditions such as muscle forces can alter the stresses in the implants with respect to pure moment loading. A musculoskeletal model and a finite element model sharing the same anatomy were built and validated against in vitro data, and coupled in order to drive the finite element model with muscle forces calculated by the musculoskeletal one for a prescribed motion. Intact conditions as well as a L1-L5 posterior fixation with pedicle screws and rods were simulated in flexion-extension and lateral bending. The hardware stresses calculated with the finite element model with instrumentation under simplified and realistic loading conditions were compared. The ROM under simplified loading conditions showed good agreement with in vitro data. As expected, the ROMs between the two types of loading conditions showed relatively small differences. Realistic loading conditions increased the stresses in the pedicle screws and in the posterior rods with respect to simplified loading conditions; an increase of hardware stresses up to 40 MPa in extension for the posterior rods and 57 MPa in flexion for the pedicle screws were observed with respect to simplified loading conditions. This conclusion can be critical for the literature since it means that previous models which used pure moments may have underestimated the stresses in the implants in flexion-extension and in lateral bending

ASTM F1717 standard for the preclinical evaluation of posterior spinal fixators: can we improve it?

Preclinical evaluation of spinal implants is a necessary step to ensure their reliability and safety before implantation. The American Society for Testing and Materials reapproved F1717 standard for the assessment of mechanical properties of posterior spinal fixators, which simulates a vertebrectomy model and recommends mimicking vertebral bodies using polyethylene blocks. This set-up should represent the clinical use, but available data in the literature are few. Anatomical parameters depending on the spinal level were compared to published data or measurements on biplanar stereoradiography on 13 patients. Other mechanical variables, describing implant design were considered, and all parameters were investigated using a numerical parametric finite element model. Stress values were calculated by considering either the combination of the average values for each parameter or their worst-case combination depending on the spinal level. The standard set-up represents quite well the anatomy of an instrumented average thoracolumbar segment. The stress on the pedicular screw is significantly influenced by the lever arm of the applied load, the unsupported screw length, the position of the centre of rotation of the functional spine unit and the pedicular inclination with respect to the sagittal plane. The worst-case combination of parameters demonstrates that devices implanted below T5 could potentially undergo higher stresses than those described in the standard suggestions (maximum increase of 22.2% at L1). We propose to revise F1717 in order to describe the anatomical worst case condition we found at L1 level: this will guarantee higher safety of the implant for a wider population of patients

Evaluation on the effectiveness of standards for preclinical mechanical characterization of spinal fixators

Introduction: Posterior spinal fixators are subjected to many load cycles after implantation due to walking, and failure events are continuously reported. To avoid this issue and evaluate preclinically the mechanical reliability of fixators a vertebrectomy (ASTM F1717 standard) and a physiological anterior support (ISO 12189) models are available. The aim of the study is to assess the international standards for the preclinical evaluation of posterior spinal fixators and to propose improvements. Materials and methods: Several anatomical/biomechanical parameters useful to describe the anatomy of the functional spine units were considered. Their value depending on the spinal level was obtained from literature or from direct measurements on biplanar stereoradiographies. Numerical models describing experimental setups were used to study the contribution of each parameter on the stress on the implant. The worst case condition was also determined. Results: Vertebrectomy condition may guarantee high safety of the implant once implanted in an average patient from a physiological population. The worst-case combination of parameters demonstrates higher loads than those reached using the current standard (screw: +15 %; rod: +9 % at L1). The physiological condition may not be safe enough: despite the anterior support characteristics lay within the literature range of values, it may lead to a stress increase even beyond 350 %. Discussion and conclusions: The study investigates the influence of biomechanical parameters on the stress on the fixator. Standards revision according to the anatomical worst-case condition (L1 level) would guarantee a higher safety for a greater range of patient population. Ongoing experimental testing partially corroborates numerical results