Review of bone substitutes

The effect of particle size on the in vivo degradation of poly(d,l-lactide-co-glycolide)/α-tricalcium phosphate micro- and nanocomposites

This paper reports the effect of particle size within a resorbable composite on the in vivo degradation rate and host response. Resorbable composites based on poly(d,l-lactide-co-glycolide) (PLGA) reinforced with tricalcium phosphate (TCP) have shown suitable degradation, biological and mechanical properties for bone repair. Composites with nano-sized TCP particles degrade more homogenously in vitro than equivalent composites with micro-sized particles. In this study, PLGA and PLGA/TCP composites containing micro- or nano-sized α-TCP particles were implanted into an ovine distal femoral condyle defect and harvested at 6, 12, 18 and 24weeks. An intimate interface was observed between the new bone tissue and degrading implants. Visual scoring of histological images and semi-automated segmentation of X-ray images were used to quantify implant degradation and the growth of new bone tissue in the implant site. Bone growth into the implant site occurred at a similar rate for both composites and the PLGA control. However, the in vivo degradation rate of the nanocomposite was slower than that of the microcomposite and consequently more closely matched the rate of bone growth. For the first 6weeks, the rate of in vivo degradation matched that of in vitro degradation, but lagged significantly at longer time points. These results point to the potential use of ceramic particle size in controlling composite degradation whilst maintaining good bone formation.

STATEMENT OF SIGNIFICANCE

This paper concerns degradable composites for orthopaedic application. The effect of particle size on implant degradation in vivo is not yet well characterised and these results give the first opportunity to directly compare in vitro and in vivo degradation rates for composites with micro- and nano-sized particles. This type of data is vital for the validation of models of composite degradation behaviour, which will lead to the design and manufacture of composites with a tailored, predictable degradation profile. The trainable segmentation tool can be used for future studies where X-rays of partially degraded implants (which have complicated greyscales and morphologies) need to be quantified without bias.

Hard tissue regeneration using bone substitutes: an update on innovations in materials

Bone is a unique organ composed of mineralized hard tissue, unlike any other body part. The unique manner in which bone can constantly undergo self-remodeling has created interesting clinical approaches to the healing of damaged bone. Healing of large bone defects is achieved using implant materials that gradually integrate with the body after healing is completed. Such strategies require a multidisciplinary approach by material scientists, biological scientists, and clinicians. Development of materials for bone healing and exploration of the interactions thereof with the body are active research areas. In this review, we explore ongoing developments in the creation of materials for regenerating hard tissues.

Cranioplasty of Hemispherical Defects Using Calcium Phosphate Cements Along with Titanium Mesh: Our Experience

INTRODUCTION

Cranial defects may arise due to trauma, infection, surgical ablation or errors in development. Restoration of such defects is important for esthetics, function and morale of the patient. Several materials are available. Each has its advantages and disadvantages. Search is on for an ideal material. Autogenous grafts remain the gold standard in reconstruction of such defects. However, the morbidity associated with their harvest, additional time required, the need for a second surgical site and the limited supply has led to the search for newer substitutes. Although many materials are available today including biologic and non biologic substitutes, there is still no consensus about the best material. In this article we describe our use of calcium phosphate cements for reconstruction of hemispherical cranial defects.

MATERIALS AND METHODS

Cases requiring reconstruction of hemispherical cranial defects (more than 15 cm in any dimension) were selected for study. After exposing the defect under GA, titanium mesh was adapted to the defect for support. Then the calcium phosphate cement was prepared and injected on the mesh to establish good contour. The alloplastic insert in each patient was evaluated for: (a) Immediate post-operative complications (b) Restoration of contour and soft tissue support (c) New bone formation ascertained on HRCT at the end of 2 years. Patients were examined on postoperative first week, at 3 and at 6 months. High resolution computed tomography scans were taken at 2 years postop. There were two female and three male patients.

RESULTS

There were no complications in the post operative period. The general condition of the patients improved post operatively. Even though the cements maintained their contour at 2 years, there were no signs of bone formation within the cement.

CONCLUSION

Calcium phosphate cement is a good bone substitute for use in cranioplasty. In defects requiring mechanical strength, it should be supported by a titanium mesh. It retains the contour but is not replaced by bone even after 2 years.

Bone augmentation in rabbit tibia using microfixed cobalt-chromium membranes with whole blood, tricalcium phosphate and bone marrow cells

To assess bone augmentation based on a non-critical defect, 1.5 mm deep cobalt-chromium (Co-Cr) barrier membranes were placed on seven adult California rabbits with three different grafting situations: whole blood, whole blood with tricalcium phosphate (TCP), and TCP mixed with bone marrow cells. Macroscopic assessment of the animals was performed once a week and densitometric studies were performed once a month. Three months post-surgery, after detaching the membranes, tibias were sectioned and followed the routine laboratory processing for decalcified sections, with inclusion in paraffin and staining by hematoxylin and eosin technique. Bone augmentation was observed for each animal, even sometimes over the Co-Cr membranes. Compact bone was mostly observed for every situation, with a higher cellular activity on those samples with bone grafts. This could be due to the presence of graft remains at the growth area. It could be concluded that blood supply to the site providing growth factors by the blood clot formation, and the placement of an osteoconductive non-resolvable membrane that favors osseoinduction, may be sufficient elements to achieve bone augmentation in a period of three months in rabbit tibia.

Magnesium-enriched hydroxyapatite as bone filler in an ameloblastoma mandibular defect

The aim of this study was to evaluate the clinical performance of a magnesium-enriched hydroxyapatite biomaterial used as bone substitute in a case of mandibular ameloblastoma treated with conservative surgery. A 63 year old male patient was treated for an ameloblastoma in the anterior mandibular profile. After tissue excision, the bone defect was filled with a synthetic hydroxyapatite biomaterial enriched with magnesium ions, in order to promote bone tissue regeneration and obtain a good aesthetic result. Twenty-five months after surgery, due to ameloblastoma recurrence in an area adjacent to the previously treated one, the patient underwent to a further surgery. In that occasion the surgeon performed a biopsy in the initially treated area, in order to investigate the nature of the newly-formed tissue and to evaluate the bone regenerative potential of this biomaterial by clinical, radiographic and histological analyses. The clinical, radiographic and histological evaluations showed various characteristics of bone remodeling stage with an ongoing osteogenic formation and a good osteo-integration. In conclusion, magnesium-enriched hydroxyapatite used as bone substitute in a mandibular defect due to ameloblastoma excision showed an effective bone regeneration at 25 months follow-up, demonstrating an excellent biocompatibility and a high osteo-integration property.

Development of an injectable bioactive bone filler cement with hydrogen orthophosphate incorporated calcium sulfate

Calcium sulfate cement (CSC) has emerged as a potential bone filler material mainly because of the possibility of incorporating therapeutic agents. Delivery of the cement through a needle or cannula will make it more useful in clinical applications. However, it was not possible to make CSC injectable because of the inherent lack of viscosity. The present work demonstrates the design development of a viscous and fully-injectable CSC by incorporating hydrogen orthophosphate ions, which does not hamper the biocompatibility of the material. The effect of addition of hydrogen orthophosphate on the rheological properties of the CSC paste was studied using a custom made capillary rheometer. The physicochemical changes associated with cement setting process were examined using X-ray diffraction and Fourier transform infrared spectroscopy and the thermal changes were measured through isothermal differential scanning calorimetry. Micromorphological features of different compositions were observed in environmental scanning electron microscopy and the presence of phosphate ions was identified with energy dispersive X-ray spectroscopic analysis and inductively coupled plasma-optical emission spectroscopy. The results indicated that HPO4 (2-) ions have profound effects on the rheological properties and setting of the CSC paste. Significant finding is that the HPO4 (2-) ions are getting substituted in the calcium sulfate dihydrate crystals during setting. The variations of setting time and compressive strength of the cement with the additive concentration were investigated. An optimum concentration of 2.5 % w/w gave a fully-injectable cement with clinically relevant setting time (below 20 min) and compressive strength (12 MPa). It was possible to inject the optimised cement paste from a syringe through an 18-gauge needle with thumb pressure. This cement will be useful both as bone filler and as a local drug delivery medium and it allows minimally invasive bone defect management.

Bone substitutes in orthopaedic surgery: from basic science to clinical practice

Bone substitutes are being increasingly used in surgery as over two millions bone grafting procedures are performed worldwide per year. Autografts still represent the gold standard for bone substitution, though the morbidity and the inherent limited availability are the main limitations. Allografts, i.e. banked bone, are osteoconductive and weakly osteoinductive, though there are still concerns about the residual infective risks, costs and donor availability issues. As an alternative, xenograft substitutes are cheap, but their use provided contrasting results, so far. Ceramic-based synthetic bone substitutes are alternatively based on hydroxyapatite (HA) and tricalcium phosphates, and are widely used in the clinical practice. Indeed, despite being completely resorbable and weaker than cortical bone, they have exhaustively proved to be effective. Biomimetic HAs are the evolution of traditional HA and contains ions (carbonates, Si, Sr, Fl, Mg) that mimic natural HA (biomimetic HA). Injectable cements represent another evolution, enabling mininvasive techniques. Bone morphogenetic proteins (namely BMP2 and 7) are the only bone inducing growth factors approved for human use in spine surgery and for the treatment of tibial nonunion. Demineralized bone matrix and platelet rich plasma did not prove to be effective and their use as bone substitutes remains controversial. Experimental cell-based approaches are considered the best suitable emerging strategies in several regenerative medicine application, including bone regeneration. In some cases, cells have been used as bioactive vehicles delivering osteoinductive genes locally to achieve bone regeneration. In particular, mesenchymal stem cells have been widely exploited for this purpose, being multipotent cells capable of efficient osteogenic potential. Here we intend to review and update the alternative available techniques used for bone fusion, along with some hints on the advancements achieved through the experimental research in this field.

Preparation and Characterization of Injectable Brushite Filled-Poly (Methyl Methacrylate) Bone Cement

Powder-liquid poly (methyl methacrylate) (PMMA) bone cements are widely utilized for augmentation of bone fractures and fixation of orthopedic implants. These cements typically have an abundance of beneficial qualities, however their lack of bioactivity allows for continued development. To enhance osseointegration and bioactivity, calcium phosphate cements prepared with hydroxyapatite, brushite or tricalcium phosphates have been introduced with rather unsuccessful results due to increased cement viscosity, poor handling and reduced mechanical performance. This has limited the use of such cements in applications requiring delivery through small cannulas and in load bearing. The goal of this study is to design an alternative cement system that can better accommodate calcium-phosphate additives while preserving cement rheological properties and performance. In the present work, a number of brushite-filled two-solution bone cements were prepared and characterized by studying their complex viscosity-versus-test frequency, extrusion stress, clumping tendency during injection through a syringe, extent of fill of a machined void in cortical bone analog specimens, and compressive strength. The addition of brushite into the two-solution cement formulations investigated did not affect the pseudoplastic behavior and handling properties of the materials as demonstrated by rheological experiments. Extrusion stress was observed to vary with brushite concentration with values lower or in the range of control PMMA-based cements. The materials were observed to completely fill pre-formed voids in bone analog specimens. Cement compressive strength was observed to decrease with increasing concentration of fillers; however, the materials exhibited high enough strength for consideration in load bearing applications. The results indicated that partially substituting the PMMA phase of the two-solution cement with brushite at a 40% by mass concentration provided the best combination of the properties investigated. This alternative material may find applications in systems requiring highly injectable and viscous cements such as in the treatment of spinal fractures and bone defects.