Development of calcium phosphate cement for the augmentation of traumatically fractured porcine specimens using vertebroplasty

Optimizing computational methods of modeling vertebroplasty in experimentally augmented human lumbar vertebrae

Vertebroplasty has been widely used for the treatment of osteoporotic compression fractures but the efficacy of the technique has been questioned by the outcomes of randomized clinical trials. Finite-element (FE) models allow an investigation into the structural and geometric variation that affect the response to augmentation. However, current specimen-specific FE models are limited due to their poor reproduction of cement augmentation behavior. The aims of this study were to develop new methods of modeling the vertebral body in both a nonaugmented and augmented state. Experimental tests were conducted using human lumbar spine vertebral specimens. These tests included micro-computed tomography imaging, mechanical testing, augmentation with cement, reimaging, and retesting. Specimen-specific FE models of the vertebrae were made comparing different approaches to capturing the bone material properties and to modeling the cement augmentation region. These methods significantly improved the modeling accuracy of nonaugmented vertebrae. Methods that used the registration of multiple images (pre- and post-augmentation) of a vertebra achieved good agreement between augmented models and their experimental counterparts in terms of predictions of stiffness. Such models allow for further investigation into how vertebral variation influences the mechanical outcomes of vertebroplasty.

Interfacial characterization of poly (vinyl alcohol) fibers embedded in a calcium phosphate cement matrix: An experimental and numerical investigation

Because of their chemical similarity to the mineral phase of bone and teeth, calcium phosphate cements (CPCs) are extensively investigated for applications in biomedicine. Nevertheless, their applicability in load-bearing anatomical sites is restricted by their brittleness. Reinforcement of calcium phosphate cements with polymeric fibers can overcome this mechanical limitation provided that the affinity between these fibers and the surrounding matrix is optimal. To date, the effects of the fiber-matrix affinity on the mechanical properties of fiber-reinforced calcium phosphate cements are still poorly understood. The goal of this study is therefore to investigate the interfacial properties and bond-slip response between the CPC matrix and polymeric fibers. To this end, we selected poly (vinyl alcohol) (PVA) fibers as reinforcing agents because of their high strength and stiffness and their effective reinforcement of cementitious matrices. Micromechanical pull-out experiments were combined with numerical simulations based on an dedicated constitutive interfacial law to characterize the interfacial properties of PVA fibers embedded in a CPC matrix at the single fiber pull-out level. The computational model developed herein is able to predict all three main phases of pull-out response, i.e. the elastic, debonding and frictional pull-out phases. The resulting interfacial constitutive law is validated experimentally and predicts the pull-out response of fibers with different diameters and embedded lengths. STATEMENTS OF SIGNIFICANCE: To date, the effects of the fiber-matrix affinity on the mechanical properties of fiber-reinforced calcium phosphate cements are still poorly understood. In this study, we present a novel experimental protocol to investigate the affinity between poly (vinyl alcohol) PVA fibers and the calcium phosphate cement (CPC) matrix by means of single-fiber pull out tests. We determine the critical embedded length for PVA fibers with two different diameters; and we design a numerical FE model including a distinct representation of fiber, matrix and interface with a predictive interfacial constitutive law which is capable of capturing all three main phases of single-fiber pull-out, i.e. elastic, debonding and frictional stages. The resulting interfacial constitutive law is validated experimentally and predicts the pull-out response of fibers with different diameters and embedded lengths.

BIOMECHANICAL EVALUATION OF LOW-MODULUS BONE CEMENT FOR ENHANCING APPLICABILITY IN VERTEBROPLASTY — AN EXPERIMENTAL STUDY IN PORCINE MODEL

Polymethylmethacrylate (PMMA) bone cement has been widely used in vertebroplasty to treat osteoporotic vertebral compression fracture. However, the high compression stiffness of PMMA is suspected to induce adjacent vertebral fracture following vertebroplasty. In the current study, modified low-modulus cement was prepared by combining PMMA with castor oil to solve this problem. The percentage of height recovery and compression stiffness of vertebral bodies was compared after injection of standard PMMA or low-modulus cement. This study aims to investigate whether low-modulus cement is as effective as standard PMMA for storing the initial vertebral height; while lowering the compression stiffness in treatment of osteoporotic vertebral compression fractures. A total of 20 fresh porcine lumbar vertebrae were assigned into two groups (10 per group): standard and low-modulus. All specimens received a four-week decalcification to mimic human osteoporotic vertebrae. The standard and low-modulus groups received a simulated compression fracture followed by treatment of standard and low-modulus cement augmentation, respectively. The low-modulus cement was prepared by combining standard PMMA with 15% weight fractions of castor oil. For all the 20 specimens, vertebral compression fracture was created by reducing the vertebral height of 25% using a material testing machine. The compression stiffness determined from the creation of compression fracture was defined as the intact group (20 specimens). The fractured vertebrae were then treated with standard and low-modulus cement augmentation. The vertebral height was measured pre- and post-treatment, and the percentage of vertebral height recovery was compared between two cementing groups. Following cement augmentation, axial compression test was conducted to compare compression stiffness among three groups. The results indicated that there is no significant difference in percentage of vertebral height between standard (83.42[Formula: see text][Formula: see text][Formula: see text]11.60%) and low-modulus (88.50[Formula: see text][Formula: see text][Formula: see text]6.15%) groups ([Formula: see text]). Moreover, the compression stiffnesses were 1166.49[Formula: see text][Formula: see text][Formula: see text]392.91 N/mm, 1795.85[Formula: see text][Formula: see text][Formula: see text]247.45[Formula: see text]N/mm and 1362.57[Formula: see text][Formula: see text][Formula: see text]236.92[Formula: see text]N/mm for intact, standard and low-modulus groups, respectively. There is significant difference among three groups ([Formula: see text]). We concluded that the modified low-modulus cement is as effective as standard PMMA for storing the initial vertebral height while lowering the compression stiffness in treatment of osteoporotic vertebral compression fractures. These reduce the risks of adjacent vertebral body fracture following vertebroplasty.

Cement pulmonary embolism as a complication of percutaneous vertebroplasty in cancer patients

Background

Vertebroplasty is a minimally invasive procedure commonly performed for vertebral compression fractures secondary to osteoporosis or malignancy. Leakage of bone cement into the paravertebral venous system and cement pulmonary embolism (cPE) are well described, mostly in patients with osteoporosis. Little is known about the clinical sequelae and outcomes in cancer patients. In this study, we report our experience with cPE following vertebroplasty performed in cancer patients.

Methods

Records of all consecutive cancer patients who underwent vertebroplasty at our institution were retrospectively reviewed. The procedure was performed via percutaneous injection of barium-opacified polymethyl-methacrylate cement.

Results

A total of 102 cancer patients with a median age of 53 (19–83) years were included. Seventy-eight (76.5%) patients had malignant vertebral fractures, and 24 (23.5%) patients had osteoporotic fractures. Cement PE was detected in 13 (12.7%) patients; 10 (76.9%) patients had malignant fractures, and the remaining three had osteoporotic fractures. Cement PE was mostly asymptomatic; however, 5 (38.5%) patients had respiratory symptoms that led to the diagnosis. Only the five symptomatic patients were anticoagulated.

Cement PE was more common with multiple myeloma (MM); it occurred in 7 (18.9%) of the 37 patients with MM compared with only three (7.3%) of the 41 patients with other malignancies. No difference in incidence was observed between patients with osteoporotic or malignant vertebral fractures.

Conclusions

Cement PE is a relatively common complication following vertebroplasty and is mostly asymptomatic. Multiple myeloma is associated with the highest risk. Large-scale prospective studies can help identify risk factors and clinical outcomes and could lead to better prevention and therapeutic strategies.

Bone grafts and biomaterials substitutes for bone defect repair: A review

Bone grafts have been predominated used to treat bone defects, delayed union or non-union, and spinal fusion in orthopaedic clinically for a period of time, despite the emergency of synthetic bone graft substitutes. Nevertheless, the integration of allogeneic grafts and synthetic substitutes with host bone was found jeopardized in long-term follow-up studies. Hence, the enhancement of osteointegration of these grafts and substitutes with host bone is considerably important. To address this problem, addition of various growth factors, such as bone morphogenetic proteins (BMPs), parathyroid hormone (PTH) and platelet rich plasma (PRP), into structural allografts and synthetic substitutes have been considered. Although clinical applications of these factors have exhibited good bone formation, their further application was limited due to high cost and potential adverse side effects. Alternatively, bioinorganic ions such as magnesium, strontium and zinc are considered as alternative of osteogenic biological factors. Hence, this paper aims to review the currently available bone grafts and bone substitutes as well as the biological and bio-inorganic factors for the treatments of bone defect.

Methodology to Produce Specimen-Specific Models of Vertebrae: Application to Different Species

Image-based continuum-level finite element models have been used for bones to evaluate fracture risk and the biomechanical effects of diseases and therapies, capturing both the geometry and tissue mechanical properties. Although models of vertebrae of various species have been developed, an inter-species comparison has not yet been investigated. The purpose of this study was to derive species-specific modelling methods and compare the accuracy of image-based finite element models of vertebrae across species. Vertebral specimens were harvested from porcine (N = 12), ovine (N = 13) and bovine (N = 14) spines. The specimens were experimentally loaded to failure and apparent stiffness values were derived. Image-based finite element models were generated reproducing the experimental protocol. A linear relationship between the element grayscale and elastic modulus was calibrated for each species matching in vitro and in silico stiffness values, and validated on independent sets of models. The accuracy of these relationships were compared across species. Experimental stiffness values were significantly different across species and specimen-specific models required species-specific linear relationship between image grayscale and elastic modulus. A good agreement between in vitro and in silico values was achieved for all species, reinforcing the generality of the developed methodology.

Influence of polymeric additives on the cohesion and mechanical properties of calcium phosphate cements

To expand the clinical applicability of calcium phosphate cements (CPCs) to load-bearing anatomical sites, the mechanical and setting properties of CPCs need to be improved. Specifically, organic additives need to be developed that can overcome the disintegration and brittleness of CPCs. Hence, we compared two conventional polymeric additives (i.e. carboxylmethylcellulose (CMC) and hyaluronan (HA)) with a novel organic additive that was designed to bind to calcium phosphate, i.e. hyaluronan-bisphosphonate (HABP). The unmodified cement used in this study consisted of a powder phase of α-tricalcium phosphate (α-TCP) and liquid phase of 4% NaH2PO4·2H2O, while the modified cements were fabricated by adding 0.75 or 1.5 wt% of the polymeric additive to the cement. The cohesion of α-TCP was improved considerably by the addition of CMC and HABP. None of the additives improved the compression and bending strength of the cements, but the addition of 0.75% HABP resulted into a significantly increased cement toughness as compared to the other experimental groups. The stimulatory effects of HABP on the cohesion and toughness of the cements is hypothesized to derive from the strong affinity between the polymer-grafted bisphosphonate ligands and the calcium ions in the cement matrix.

Nanomaterials: the next step in injectable bone cements

Injectable bone cements (IBCs) are biocompatible materials that can be used as bone defect fillers in maxillofacial surgeries and in orthopedic fracture treatment in order to augment weakened bone due to osteoporosis. Current clinically available IBCs, such as polymethylmethacrylate and calcium phosphate cement, have certain advantages; however, they possess several drawbacks that prevent them from gaining universal acceptance. New gel-based injectable materials have also been developed, but these are too mechanically weak for load-bearing applications. Recent research has focused on improving various injectable materials using nanomaterials in order to render them suitable for bone tissue regeneration. This article outlines the requirements of IBCs, the advantages and limitations of currently available IBCs and the state-of-the-art developments that have demonstrated the effects of nanomaterials within injectable systems.

Reinforcement Strategies for Load-Bearing Calcium Phosphate Biocements

Calcium phosphate biocements based on calcium phosphate chemistry are well-established biomaterials for the repair of non-load bearing bone defects due to the brittle nature and low flexural strength of such cements. This article features reinforcement strategies of biocements based on various intrinsic or extrinsic material modifications to improve their strength and toughness. Altering particle size distribution in conjunction with using liquefiers reduces the amount of cement liquid necessary for cement paste preparation. This in turn decreases cement porosity and increases the mechanical performance, but does not change the brittle nature of the cements. The use of fibers may lead to a reinforcement of the matrix with a toughness increase of up to two orders of magnitude, but restricts at the same time cement injection for minimal invasive application techniques. A novel promising approach is the concept of dual-setting cements, in which a second hydrogel phase is simultaneously formed during setting, leading to more ductile cement–hydrogel composites with largely unaffected application properties.

Mechanical properties of α-tricalcium phosphate-based bone cements incorporating regenerative biomaterials for filling bone defects exposed to low mechanical loads

Calcium phosphate-based cements with enhanced regenerative potential are promising biomaterials for the healing of bone defects in procedures such as percutaneous vertebroplasty. With a view to the use of such cements for low load bearing applications such as sinus augmentation or filling extraction sites. However, the inclusion of certain species into bone cement formulations has the potential to diminish the mechanical properties of the formulations and thereby reduce their prospects for clinical translation. Consequently, we have prepared α-tricalcium phosphate (α-TCP)-based bone cements including materials that we would expect to improve their regenerative potential, and describe the mechanical properties of the resulting formulations herein. Formulations incorporated α-TCP, hydroxyapatite, biopolymer-thickened wetting agents, sutures, and platelet poor plasma. The mechanical properties of the composites were composition dependent, and optimized formulations had clinically relevant mechanical properties. Such calcium phosphate-based cements have potential as replacements for cements such as those based on polymethylmethacrylate.

Vertebroplasty using bisphosphonate-loaded calcium phosphate cement in a standardized vertebral body bone defect in an osteoporotic sheep model

In the context of bone regeneration in an osteoporotic environment, the present study describes the development of an approach based on the use of calcium phosphate (CaP) bone substitutes that can promote new bone formation and locally deliver in situ bisphosphonate (BP) directly at the implantation site. The formulation of a CaP material has been optimized by designing an injectable apatitic cement that (i) hardens in situ despite the presence of BP and (ii) provides immediate mechanical properties adapted to clinical applications in an osteoporotic environment. We developed a large animal model for simulating lumbar vertebroplasty through a two-level lateral corpectomy on L3 and L4 vertebrae presenting a standardized osteopenic bone defect that was filled with cements. Both 2-D and 3-D analysis of microarchitectural parameters demonstrated that implantation of BP-loaded cement in such vertebral defects positively influenced the microarchitecture of the adjacent trabecular bone. This biological effect was dependent on the distance from the implant, emphasizing the in situ effect of the BP and its release from the cement. As a drug device combination, this BP-containing apatitic cement shows good promise as a local approach for the prevention of osteoporotic vertebral fractures through percutaneous vertebroplasty procedures.

Dynamics of interpedicular widening in spinal burst fractures: an in vitro investigation

BACKGROUND CONTEXT

Spinal burst fractures are a significant cause of spinal instability and neurologic impairment. Although evidence suggests that the neurologic trauma arises during the dynamic phase of fracture, the biomechanics underpinning the phenomenon has yet to be fully explained. Interpedicular widening (IPW) is a distinctive feature of the fracture but, despite the association with the occurrence of neurologic deficit, little is known about its biomechanics.

PURPOSE

To provide a comprehensive in vitro study on spinal burst fracture, with special attention on the dynamics of IPW.

STUDY DESIGN

Experimental measurements in combination with computed tomography scanning were used to quantitatively investigate the biomechanics of burst fracture in a cadaveric model.

METHODS

Twelve human three-adjacent-vertebra segments were tested to induce burst fracture. Impact was delivered through a drop-weight tower, whereas IPW was continuously recorded by two displacement transducers. Computed tomography scanning aided quantifying canal occlusion (CO) and evaluating sample anatomy and fracture appearance. Two levels of energy were delivered to two groups: high energy (HE) and low energy (LE).

RESULTS

No difference was found between HE and LE in terms of the residual IPW (ie, post-fracture), maximum IPW, or CO (median 20.2%). Whereas IPW was not found to be correlated with CO, a moderate correlation was found between the maximum and the residual IPW. At the fracture onset, IPW reached a maximum median value of 15.8% in approximately 20 to 25 milliseconds. After the transient phase, the pedicles were recoiled to a median residual IPW of 4.9%.

CONCLUSIONS

Our study provides for the first time insight on how IPW actually evolves during the fracture onset. In addition, our results may help shedding more light on the mechanical initiation of the fracture.

In situ osteoblast mineralization mediates post-injection mechanical properties of osteoconductive material

The objective of this study was to understand the temporal relationship between in situ generated calcium content (mineralization) and the mechanical properties of an injectable orthobiologic bone-filler material. Murine derived osteoblast progenitor cells were differentiated using osteogenic factors and encapsulated within an injectable polycaprolactone nanofiber-collagen composite scaffold (PN-COL +osteo) to evaluate the effect of mineralization on the mechanical properties of the PN-COL scaffold. A comprehensive study was conducted using both an experimental and a predictive analytical mechanical analysis for mechanical property assessment as well as an extensive in vitro biological analysis for in situ mineralization. Cell proliferation was evaluated using a PicoGreen dsDNA quantification assay and in situ mineralization was analyzed using both an alkaline phosphatase (ALP) assay and an Alizarin Red stain-based assay. Mineralized matrix formation was further evaluated using energy dispersive x-ray spectroscopy (EDS) and visualized using SEM and histological analyses. Compressive mechanical properties of the PN-COL scaffolds were determined using a confined compression stress-relaxation protocol and the obtained data was fit to the standard linear solid viscoelastic material mathematical model to demonstrate a relationship between increased in situ mineralization and the mechanical properties of the PN-COL scaffold. Cell proliferation was constant over the 21 day period. ALP activity and calcium concentration significantly increased at day 14 and 21 as compared to PN-COL -osteo with undifferentiated osteoblast progenitor cells. Furthermore, at day 21 EDS, SEM and von Kossa histological staining confirmed mineralized matrix formation within the PN-COL scaffolds. After 21 days, compressive modulus, peak stress, and equilibrium stress demonstrate significant increases of 3.4-fold, 3.3-fold, and 4.0-fold respectively due to in situ mineralization. Viscoelastic parameters calculated through the standard linear solid mathematical model fit to the stress-relaxation data also indicate improved mechanical properties after in situ mineralization. This investigation clearly demonstrates that in situ mineralization can increase the mechanical properties of an injectable orthobiologic scaffold and can possibly guide the design of an effective osteoconductive injectable material.

Comparative finite-element analysis: a single computational modelling method can estimate the mechanical properties of porcine and human vertebrae

Significant advances in the functional analysis of musculoskeletal systems require the development of modelling techniques with improved focus, accuracy and validity. This need is particularly visible in the fields, such as palaeontology, where unobservable parameters may lie at the heart of the most interesting research questions, and where models and simulations may provide some of the most innovative solutions. Here, we report on the development of a computational modelling method to generate estimates of the mechanical properties of vertebral bone across two living species, using elderly human and juvenile porcine specimens as cases with very different levels of bone volume fraction and mineralization. This study is presented in two parts; part I presents the computational model development and validation, and part II the virtual loading regime and results. This work paves the way for the future estimation of mechanical properties in fossil mammalian bone.

Experimental and computational approach investigating burst fracture augmentation using PMMA and calcium phosphate cements

The aim of the study was to use a computational and experimental approach to evaluate, compare and predict the ability of calcium phosphate (CaP) and poly (methyl methacrylate) (PMMA) augmentation cements to restore mechanical stability to traumatically fractured vertebrae, following a vertebroplasty procedure. Traumatic fractures (n = 17) were generated in a series of porcine vertebrae using a drop-weight method. The fractured vertebrae were imaged using μCT and tested under axial compression. Twelve of the fractured vertebrae were randomly selected to undergo a vertebroplasty procedure using either a PMMA (n = 6) or a CaP cement variation (n = 6). The specimens were imaged using μCT and re-tested. Finite element models of the fractured and augmented vertebrae were generated from the μCT data and used to compare the effect of fracture void fill with augmented specimen stiffness. Significant increases (p < 0.05) in failure load were found for both of the augmented specimen groups compared to the fractured group. The experimental and computational results indicated that neither the CaP cement nor PMMA cement could completely restore the vertebral mechanical behavior to the intact level. The effectiveness of the procedure appeared to be more influenced by the volume of fracture filled rather than by the mechanical properties of the cement itself.