Low-Modulus PMMA Has the Potential to Reduce Stresses on Endplates after Cement Discoplasty

Cement discoplasty has been developed to treat patients with advanced intervertebral disc degeneration. In discoplasty, poly(methylmethacrylate) (PMMA) bone cement is injected into the disc, leading to reduced pain and certain spinal alignment correction. Standard PMMA-cements have much higher elastic modulus than the surrounding vertebral bone, which may lead to a propensity for adjacent fractures. A PMMA-cement with lower modulus might be biomechanically beneficial. In this study, PMMA-cements with lower modulus were obtained using previously established methods. A commercial PMMA-cement (V-steady^®, G21 srl) was used as control, and as base cement. The low-modulus PMMA-cements were modified by 12 vol% (LA12), 16 vol% (LA16) and 20 vol% (LA20) linoleic acid (LA). After storage in 37 °C PBS from 24 h up to 8 weeks, specimens were tested in compression to obtain the material properties. A lower E-modulus was obtained with increasing amount of LA. However, with storage time, the E-modulus increased. Standard and low-modulus PMMA discoplasty were compared in a previously developed and validated computational lumbar spine model. All discoplasty models showed the same trend, namely a substantial reduction in range of motion (ROM), compared to the healthy model. The V-steady model had the largest ROM-reduction (77%), and the LA20 model had the smallest (45%). The average stress at the endplate was higher for all discoplasty models than for the healthy model, but the stresses were reduced for cements with higher amounts of LA. The study indicates that low-modulus PMMA is promising for discoplasty from a mechanical viewpoint. However, validation experiments are needed, and the clinical setting needs to be further considered.

Changes in microgaps, micromotion, and trabecular strain from interlocked cement-trabecular bone interfaces in total knee replacements with in vivo service

The initial fixation of cemented Total Knee Replacements (TKRs) relies on mechanical interlock between cement and bone, but loss of interlock occurs with in vivo service. In this study, cement-trabeculae gap morphology and micromechanics were measured for lab prepared (representing post-operative state) and postmortem retrieval (with in vivo remodeling) TKRs to determine how changes in fixation affect local micromechanics. Small specimens taken from beneath the tibial tray were loaded with 1 MPa axial compression and the local micromechanics of the trabeculae-cement interface was quantified using digital image correlation. Lab prepared trabeculae that initially interlock with cement had small gaps (ave:14 μm) and limited micromotion (ave:1 μm) which were larger near the cement border. Trabecular resorption was prevalent following in vivo service; interface gaps became larger (ave:40 μm) and micromotion increased (ave:6 μm), particularly near the cement border. Interlocked trabeculae from lab prepared specimens exhibited strains that were 20% of the supporting bone strain, indicating the trabeculae were initially strain shielded. The spatial and temporal progression of gaps, micromotion, and bone strain was complex and much more variable for post-mortem retrievals compared to the lab prepared specimens. From a clinical perspective, attaining more initial interlock results in cement-bone interfaces that are better fixed with less micromotion. © 2015 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:1019-1025, 2016.

In silico investigation of vertebroplasty as a stand-alone treatment for vertebral burst fractures

BACKGROUND

The use of percutaneous vertebroplasty as a stand-alone treatment for stable vertebral burst fractures has been investigated in vitro and in clinical studies. These studies present inconsistent results on the mechanical response of vertebroplasty-treated burst fractures. In addition, observations of the loss of sagittal alignment after vertebroplasty raise questions on the applicability of vertebroplasty for burst fractures. Therefore, the aim of this study was to investigate the mechanical stability of burst fractures after stand-alone treatment by vertebroplasty.

METHODS

Finite element simulations were performed with models generated from two laboratory-induced burst fractures in human thoracolumbar specimens. The burst fracture models were virtually injected with various cement volumes using a unipedicular or bipedicular approach. The models were subjected to four individual loads (compression, lateral bending, extension and torsion) and a multi-axial load case in the physiological range.

FINDINGS

All treated burst fractures showed improvements in stiffness and a reduction in inter-fragmentary displacements, thus potentially providing a suitable mechanical environment for fracture healing. However, large volumes of the trabecular bone (<43%), cement (<53%) and bone-cement composite (<58%) were predicted to experience strain levels exceeding the yield point. While damage was not specifically modeled, this implies a potential collapse of the treated vertebra due to local failure.

INTERPRETATION

To improve the primary stability and to prevent the collapse of treated burst fractures, the use of posterior instrumentation is suggested as an adjunct to vertebroplasty.

An experimentally validated finite element method for augmented vertebral bodies

BACKGROUND

Finite element models of augmented vertebral bodies require a realistic modelling of the cement infiltrated region. Most methods published so far used idealized cement shapes or oversimplified material models for the augmented region. In this study, an improved, anatomy-specific, homogenized finite element method was developed and validated to predict the apparent as well as the local mechanical behavior of augmented vertebral bodies.

METHODS

Forty-nine human vertebral body sections were prepared by removing the cortical endplates and scanned with high-resolution peripheral quantitative CT before and after injection of a standard and a low-modulus bone cement. Forty-one specimens were tested in compression to measure stiffness, strength and contact pressure distributions between specimens and loading-plates. From the remaining eight, fourteen cylindrical specimens were extracted from the augmented region and tested in compression to obtain material properties. Anatomy-specific finite element models were generated from the CT data. The models featured element-specific, density-fabric-based material properties, damage accumulation, real cement distributions and experimentally determined material properties for the augmented region. Apparent stiffness and strength as well as contact pressure distributions at the loading plates were compared between simulations and experiments.

FINDINGS

The finite element models were able to predict apparent stiffness (R(2)>0.86) and apparent strength (R(2)>0.92) very well. Also, the numerically obtained pressure distributions were in reasonable quantitative (R(2)>0.48) and qualitative agreement with the experiments.

INTERPRETATION

The proposed finite element models have proven to be an accurate tool for studying the apparent as well as the local mechanical behavior of augmented vertebral bodies.

Trabecular level analysis of bone cement augmentation: a comparative experimental and finite element study

The representation of cement–augmented bone in finite element (FE) models of vertebrae following vertebroplasty remains a challenge, and the methods of the model validation are limited. The aim of this study was to create specimen-specific FE models of cement–augmented synthetic bone at the microscopic level, and to develop a new methodology to validate these models. An open cell polyurethane foam was used reduce drying effects and because of its similar structure to osteoporotic trabecular bone. Cylindrical specimens of the foam were augmented with PMMA cement. Each specimen was loaded to three levels of compression inside a micro-computed tomography (μCT) scanner and imaged both before compression and in each of the loaded states. Micro-FE models were generated from the unloaded μCT images and displacements applied to match measurements taken from the images. A morphological comparison between the FE-predicted trabecular deformations and the corresponding experimental measurements was developed to validate the accuracy of the FE model. The predicted deformation was found to be accurate (less than 12% error) in the elastic region. This method can now be used to evaluate real bone and different types of bone cements for different clinical situations.