Bone Plates/Materials

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Metals Hydroxyapatite Poly(lactic acid) Polycaprolactone



Bone Plates
Bone tissue, unlike most of the body's tissues, has the remarkable ability to regenerate itself. If a fractured bone can be held together it can regenerate the tissue and regain most of its original strength. For severe fractures, bone plates are surgically implanted to hold the bone in place. (Illustration1)
When designing bone plates design, material selection, and biocompatibility are the three important considerations. The bone plate must be strong enough to support the load normally placed on the bone while the bone heals. The plate must also have a stiffness similar to that of the bone to which it is attached. The implant must be non-toxic and cannot cause an inflammatory response in the body.
The stiffness of the bone plate is important because the stress shielding will increase with the difference in stiffness. Stress shielding is the phenomenon in which the implant bears most of the load normally placed on the bone. Although this is favorable while the bone is weak, as the bone heals and regains strength, if the bone plate does not allow the bone to carry an increasing load, there will be a reduction of bone mass and final regained strength. From the beginning of their use, material selection was the limiting factor to their success. As technology advanced so did the materials...

Metals
Iron and steel were the most widely employed materials in the 1920s. It was their tensile strength that made iron and steel attractive. They dissolved rapidly and provoked erosion of adjacent bone. From the standpoint of erosion, biocompatability, and fatigue life, however, stainless steel is inferior to other super alloys. However, it still may have applications in elderly patients, in whom physical demands and life expectancy are limited, especially when cost is a major determinant. Stainless steels are suitable to use only in temporary implant devices.
Copper and nickel discolored bone in which they were embedded. Gold, silver, and pure aluminum do not produce discoloration but are too weak and soft for this application. A chromium nickel stainless steel was more corrosion resistant in body fluids than other metals.
By the 1930s, titanium was being used. Its lightness and good mechano chemical properties are important features for this application. Aluminum is one of the alloying elements. The strength of the titanium alloys is lower to equal to that of stainless steel, but its specific strength (strength per density) is far greater than other alloys. Nevertheless, titanium has poor shear strength, making it less desirable for bone screws, plates and similar applications (www.corrosion-doctors.org2).
One disadvantage of metallic implants is that a second surgery is required to remove the implant. This additional surgery can itself damage the bone and is an obvious inconvenience. Therefore it would be desirable to make an implanting a material that would degrade or be resorbed in vivo. Certain ceramic and polymeric materials are bioresorbably and some even help to promote bone growth. The main challenge with this group of materials is to engineer the required mechanical properties, as they are generally less strong than metals.

Hydroxyapatite
Hydroxyapatite, Ca10(PO4)6(OH)2, is a form of calcium phosphate. This ceramic material is used for bone implants and drug delivery system. In each application geometry, dimension, density, pore size, mechanical strength, purity, and chemical phase are important parameters to consider. (www.hydroxyapatite.com3). Hydroxyapatite is also the mineral component of natural hard tissues. When it is added to or coated on any candidate implant material, hydroxyapatite forms a composite which is inherently biocompatible and stimulates bone growth at the interface between the hydroxyapatite and the bone. When added to a polymer matrix in particular, hydroxyapatite strengthens the material as a whole, often raising the tensile strength to within the range of cortical bone.
Many polymers have been researched for biomedical applications. The advantage of using a polymer is that it's possible to achieve bioresorbability by controlling polymer chemistry so that the polymer is water-soluble (i.e. undergoes hydrolysis). Today there are approximately 40 known bioresorbable polymers and of these the following have been studied for use in implants: polylactides (PLA), polyglycolides, polydioxanone, trimethylene carbonate, polyorthoester, and polycaprolactone (PCL).

Poly(lactic acid) - (PLA)
Polylactic acid (PLA), a stereoisomer, which is considered to have the best overall properties of the known bioresorbable polymers. Table14 compares PLA with other materials. Its degradation time is the longest, ensuring that a PLA implant will support a bone for the duration of healing. Most degradation tests have been performed in vitro, and exact degradation times vary greatly, from 6 weeks to several years. It is only possible the say that the degradation time of PLA is longer relative to other polymers. PLA is also the only polymeric material that allows osseous ingrowth (i.e. new bone formation) to occur while the implant is degrading. This feature is advantageous for the proper regrowth and strengthening of bone around the damaged site. And unlike polyglycolic acid, PGA, which is otherwise very similar to PLA, PLA does not excite the body's inflammatory response. Finally, PLA and especially self-reinforced PLA (i.e. composed of ordered polymer chains), can match the mechanical properties of cortical bone better than its metallic counterparts. These properties have made PLA the dominant bioresorbable polymer on the market.

Polycaprolactone - (PCL)
Like PLA and PGA, polycaprolactone (PCL) is a bioresorbable polymer belonging to the group of aliphatic polyesters. It is semicrystalline and has a low melting point of approximately 60C. When synthesized by a ring-opening polymerization, PCL tends to be high-molecular-weight; direct polycondensation produces low-molecular-weight PCL. The PCL we have purchased from Polysciences has a molecular weight of approximately 120000 g/mol. In general, lower molecular weight results in higher degradation rate, but higher molecular weight increases strength. The disadvantage of this trade-off between degradation and strength are especially evident in PCL, which has one of the longest degradation times of any bioresorble polymer (Table 3) and is also one of the softest (Table14).
The challenge for our team will be to process as-received PCL in order to make it suitable for use in bone plates, that is, with higher mechanical properties and an acceptable degradation rate. The availability and cost of PCL make it a favorable alternative to other polymers--like PLA and PGA--that already have more desirable properties. PCL has been used in the past as a bioresorable drug delivery system, but there is no reference in the literature to PCL having been used as a structural implant. Our team's application of PCL to such a device will be a novel and experimental undertaking.

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Composite Materials

Composites are made from two or more different substances, each with its own characteristics, are combined to create a new substance whose properties are superior to those of the original components in a specific application. The term composite more specifically refers to a structural material (such as plastic) within which a fibrous material (such as silicon carbide) is embedded.

The remarkable properties of composites are achieved by embedding fibres of one substance in a host matrix of another. While the structural value of a bundle of fibres is low, the strength of individual fibres can be harnessed if they are embedded in a matrix that acts as an adhesive, binding the fibres together and lending solidity to the material. The rigid fibres impart structural strength to the composite, while the matrix protects the fibres from environmental stress and physical damage and imparts thermal stability to them. The fibre-matrix combination also reduces the potential for a complete fracture; if one fibre fails the crack may not extend to other fibres, whereas a crack that starts in a monolithic (or single) material generally continues to propagate until that material fails.

Most conventional composites resemble plywood in that they are built in thin layers, each of which is reinforced by long fibres laid down in a single direction. Such materials exhibit enhanced strength only along the direction of the fibres. To produce composites that are strong in all directions, the fibres are woven into a three-dimensional structure in which they lie along three mutually perpendicular axes.

The structural component of a composite may consist of fibres made of glass or carbon-graphite, shorter “whiskers” made of silicon carbide or aluminum oxide, or longer tungsten-boron filaments. The matrix material may be an epoxy resin or other high-temperature plastic, aluminum or some other metal, or a ceramic such as silicon nitride. Fibreglass-reinforced plastic is the best-known composite and has found wide application in both household goods and industrial products. Composites are of greatest use in the aerospace industry, however, where their stiffness, lightness, and heat resistance make them the materials of choice in reinforcing the engine cowls, wings, doors, and flaps of aircraft. Composite materials are also used in rackets and other sports equipment, in cutting tools, and in certain parts of automotive engines.




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Last Updated
May 12, 2001