Bone Implant Material
The prototypical bioceramic is hydroxyapatite, a hydrated calcium phosphate similar in crystalline structure to the mineral component of bone. The material is produced by treating a common form of marine coral with a a patented chemical process that converts the coral to hydroxyapatite. The porous, interconnected structure of the coral remains intact, providing an ideal matrix through which new bone tissue can grow. One coral head weighing 150-200 pounds provides enough material for hundreds of bone grafts.
The interconnected, porous structure is similar to the porosity of human bone. When placed in contact with viable bone the implant provides a strong natural foundation for new bone in-growth and offers structural support during the healing process. Upon healing, the composite of bone is comparable in strength to the surrounding bone.
Hydroxyapatite is a widely used material for orthopedic, dental, and related biological applications. Recently it has been developed in a droplet-based microfluidic technology to formulate consistent, dense hydroxyapatite nanoparticles that can effectively mimic the functions of bone material in their natural biological systems, providing unique microstructures with very high surface areas and porosity.
It also has better form, structure and configuration for improved bonding than other synthetic bone materials. This material can be produced at very high rates, having a simpler production processes than bones synthetic materials.
A synthetic bone filler offered by Berkeley Products is tailored for slow
resorbtion; it is based on Tricalcium Phosphate (TCP) and Hydroxyapatite
(HAP).The product is formulated to enhance bone regeneration and provide
complete bone in-growth. The compound is available either in granular form
or as blocks. The spongy bioceramic granules resemble cancellous bone chips
and the implant is radio-opaque and biocompatible.
Synthetic biodegradable polymers
Synthetic polymers have some advantages over natural materials. They can be designed and made to have desired physical properties (within limits). Because they can be manufactured in controlled processes, they have predictable lot-to-lot uniformity and have fewer concerns regarding immunogenicity. As polymers made from fundamental chemical building blocks, they have simple and well known chemical structures and properties and are a reliable source of raw material.
Saturated aliphatic polymer materials are one of the oldest and most frequently used group of materials in bone tissue engineering .This class includes poly(lactic acid), poly(glycolic acid), poly(lactic-coglycolide) as well as derived copolymers. The mechanical characteristics and the degradation can be affected by the type of processing technique. As poly(glycolic acid) is highly sensitive to the degradation; it requires precise control on processing conditions. The solvent casting and particulate leaching method and compression molding are used to fabricate the poly(glycolic acid) based porous scaffolds. Its strength and modulus are very high, and that is why the fiber of polyglycolic acid is used in as sutures. Polylactic acid is prepared by the cyclic dimer of lactic acid that exists as two optical isomers: D & L- lactate is the naturally occurring isomer, and DL-lactide is the synthetic blend of D-lactide and L-lactide. The homopolymer L-lactide is a semi-crystalline polymer. This material has high tensile strength, elongation and modulus that make it more suitable for the load bearing applications such as sutures and orthopedic fixation. The homopolymers and the related copolymers have multiple uses due to their good mechanical strength, degradation, biocompatibility such as in sutures, scaffolds for tissue engineering, mesh, and drug delivery.
Polypropylene fumarate (PPF) is valuable because it can be injected into the body. Before cross-linking the pre-polymer is in liquid form, which makes the derived polymer easy to handle. It can also easily produce asymmetrical formed implants by injection molding. The characteristic of injectability makes it appropriate for the orthopedic implant procedures.
Photo cross-linkable, polyanhydrides have also been developed for the orthopedic application predominantly focusing on achieving good mechanical strength. The polymer is synthesized starting from dimethacrylated anhydrides. The curing of the macro monomer is achieved by ultraviolet and visible light. The monomer choice affects the material mechanical properties and degradation.
Injectable photo-crosslinked polyanhydrides can be used to renew irregularly shaped bone imperfection or soft tissue repairs. The degradation occurred by means of hydrolysis of anhydride bonds, subsequently the hydrolysis of imide bonds of these copolymers. The hydrolytic degradation of polyanhydride is nontoxic and composed of the diacid molecules and water soluble linear methacrylic acid molecules. Thus, the main advantages of such scaffolds are non-toxic, injectability, low degradation and high compatibility. The various properties can be modified during the scaffolds fabrication.
Table 1 below summarizes the mechanical characteristics, applications and
processing methods for biodegradable polymers vs. hydroxyapatites and blends.
Table 1 -
|HAP and blends||20-43||40%|
|HAP and blends||bulk||Implants,adhesion barriers||SC,SF,IM|
|Polyglycolide||6-12||Glycolic acid||Suture anchors,menicus,drug delivery||SC,SFF,CM|
|Polylactide||12-18||Lactic acid||Fracture fixation,suture anchors,meniscus||SC,SFF|
|5-6||Lactic acid||Suture anchors,screws||SC,SFF,CM|
|Polylactide-coglycolide 50/50||1-2||Lactic and glicolic acid||Plates,mesh,screws||SC,SFF,CM|
|Polycaprolactone||> 24||Caproic acid||Suture coating,implants||SC,SFF,CM|
|PPF||>24||Fumaric acid, propylene glycol||Orthopedic implants, foam coatings,
|Inject able PP further cross-linked via free radicals initiation|
|Polyanhydride||1.4-14||Bone replacement,medical devices|
Explanations of processing methods: SC - solution casting, CM - compression molding, SFF - solid free forming, IM - injection molding, EFD - emulsion freeze drying
Natural polymer materials
Another area of research is materials from nature as the bases for synthetic bone. Natural polymers can serve as templates - a scaffolding - for cell attachment and growth. (Unfortunately some of these materials can stimulate an immune response and cause the implant to be rejected.) Due to their excellent biocompatibility collagen, fibrin, agarose (polysaccharides derived from agar), chitosan, and alginate materials are used in the bone and cartilage tissue engineering application. The microstructures of these materials are highly patterned and suitable for cell growth.
However there are some disadvantages to these materials. such as (1) high cost, (2) unreliability in obtaining the materials in bulk quantity, and (3) difficulties in the processing the materials for suitable use in a body.
The degradation rate of natural polymer materials may vary from patient to patient.
In 2013 British researchers announced they had developed scaffolding of a combination of plastics that could be used to replace bones. The blood vessels would grow inside the scaffolding and bone cells would form there. The plastic would degrade over time according to press reports. Technical details were not included in the popular press.
Scientists have experimented with diamond implants as a method to improve bone strength. Spherical diameters measuring 5 nanometers or so in diameter can carry proteins into bone and tooth tissue, and deliver treatment. This is early in development but experts hope the technology could lead to new methods to address osteonecrosis and even osteoporosis.
Doctors use cements to anchor artificial implants and to build up the spine in vertebroplasty and kyphoplasty procedures.
Biomimetic Polymer Composite Materials for Bone Repair - National Research Council of Canada
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