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Ceramics: Products and Manufacturers (from our Orthopedic Biomaterials 2005 report) BY EDITOR, JULY 18, 2005

This is an excerpt from our Orthopedic Biomaterials 2005 report, which is a comprehensive listing of all the products and manufacturers in this exciting space. The report has six more sections like this one (for the other orthopedic applications of biomaterials), plus a manufacturer listing, and you can get a copy of the report here.

Ceramics Overview

Ceramics can be broadly defined as chemically inorganic non-metallic materials. Traditional ceramics include porcelain, glass, bricks and many refractory materials. Traditional ceramics were typically not substitutes for metals, but new industrial ceramics such as alumina, zirconia, silicon carbide and silicon nitride have excellent properties, which enable them to compete with and even surpass metals in some applications. The term bio-ceramics is used primarily for alumina, zirconia, calcium phosphates, glass ceramics and pyrolytic carbons.

The term ceramics applies to a very wide range of materials that exhibit markedly different properties. However, most ceramics that compete with metals are harder and have higher temperature resistance, strength, lower density and resistance to corrosion. These properties make ceramics invaluable in applications involving high temperature, corrosive or abrasive environments. Owing to its low density and resistance to abrasion/corrosion, ceramics are more suited than metal for implantation in the human body.

Applications

Ceramics' various properties make it ideal for load bearing applications (e.g. bones) or other applications that require strength, hardness (e.g. dental implants) and chemical inertness. Ceramics (primarily pyrolytic carbon) are also used in artificial heart valves.

Ceramics (especially alumina) are being extensively used in total hip replacement, knee replacement and other joint replacement surgeries. Ceramics are also used for bone bonding (e.g. hydroxyapatite), bone spacing (e.g. porous alumina) and small orthopedic joints such as fingers and spinal inserts.

The first all-ceramic hip - comprising 32 mm alumina heads and alumina cups - was developed and used in 1970 by French surgeon Pierre Boutin, M.D. The implant lasted 17 years until the patient died. Unlike the standard polyethylene cups, wear was virtually non-existent in the retrieved ceramic implant.

The FDA approval for ceramic-polymer implants was first received in 1990, when alumina and zirconia were used along with polyethylene cups for hip replacements. Since February 2002, three companies, Stryker, Wright Medical & CeramTec AG, have successfully marketed ceramic-on-ceramic (both cup and head made of ceramic) hip implants in the US.

Ceramic Bone Implants
Desired properties of bone implants:

• Biocompatibility: An implant must not degrade in the body or cause degradation of the body.


• Reasonable tensile, compressive and sheer strengths: A load bearing implant must, like the bones of the body, be very strong. However, excessively strong implants are not desirable because if they take the entire load on themselves, the bones around them would tend to lose their use and strength.


• Resistance to corrosion and abrasion: If the implant gets corroded or starts wearing out, it will become lose and cause problems. A good implant should be able to retain its integrity for a long duration. Corrosion is one of the major problems with metal implants as it leaves metallic ions into the body.


• Formability: A good implant material should be easily formable into the exact desired shape and size.

Table 4.1

Types of ceramic bone implants

Bioinert Ceramics
Bioinert ceramics, as the name implies, elicit minimal response from the host body. Bioinert ceramics do not undergo much physical or chemical alteration inside the body. All these implants exhibit high compressive strength, excellent wear resistance and bioinertness. Alumina, partially stabilized zirconia and silicon nitride are the most widely used materials in this category.

Alumina is the most popular among these and is used to make the femoral head in hip replacements and some dental implants. Alumina's most distinguishing property is its resistance to wear and chemical inertness. However, its low tensile strength and propensity to fracture makes alumina less than desirable for many applications that require high tensile strength.

Bioactive Ceramics
Bioactive ceramics make a direct bond with surrounding tissues. Most such ceramics are not directly load bearing because of their low mechanical strength and fracture toughness. These ceramics are often used as a coating on metal bone implants and as fillers in dental implants. Hydroxyapatite is a prime example of such materials. HA is porous in nature and allows bone tissue to grow inside its pores. This results in better bonding of the implant with the surrounding tissue. Bioglass is also used for such purposes.

Resorbable Ceramics

Resorbable ceramics are gradually broken down by the body and resorbed. It is imperative that such materials do not produce harmful substances in the breakdown process and that the dissolution rate is controllable and predictable. Calcium phosphate ceramics (especially tri-calcium phosphate) are prime examples of resorbable ceramics. Resorbable ceramics are normally used in bone repair.

Ceramic Dental Implants
Desired properties of dental implants

• Ease of fabrication


• Good mechanical strength and corrosion resistance


• Good aesthetic appearance

Ceramic dental implants, in general, score well on these counts, but occasional mechanical fractures and cracking have been their main drawbacks. On the other hand, ceramic dental implants are very favorably placed in terms of aesthetics because they can easily be given a natural dental color and tooth-like translucence.

Table 4.2

Types of Ceramic Dental Implants

Felspathic porcelains
Felspathic porcelains are often used as a veneer material on ceramic and metal implants because of their more natural and aesthetic appearance. Sodium oxide or some other alkaline oxide is often used to improve its translucence. Fluxes are also added to the material to lower its melting point. This allows dental laboratories to handle these materials more easily.

Leucite systems
Leucites are often used to modify the coefficient of thermal expansion. This becomes extremely important when ceramic is coated on metals. Differences in the coefficients of thermal expansion may create internal stresses, leading to the failure of the implant.

Castable glasses
Castable glass ceramics use various forms of mica to strengthen glass. The composition of the material is altered to ensure easy handling in dental laboratories.

Aluminous jacket crown
This involves use of alumina in dental ceramics. These systems have been in use since the mid-1960s. The original materials suggested had several drawbacks, especially a propensity for in vivo strength degradation when the material contained porosity. These defects were overcome by certain modifications to the original process.

• Pure alumina core-Heat cured after pressing: The Nobel Biocare Company of Sweden has introduced two systems named CeraOne and Procera, which involved pressing of alumina onto a metal die followed by sintering. This step gives an alumina core, which is further coated with felspathic veneering porcelains.


• The glass infiltrated alumina system for cores: Dr Michael Sadoun and Vita Zahnfabrik developed this system for the alumina cores. This uses fine-grained alumina and slip casting followed by sintering. The resultant is infiltrated with Lanthana-based glass to give a glass infiltrated alumina core named In-Ceram. A felspathic veneering ceramic can be used on this core for better aesthetic appeal. In-Ceram has the highest flexural strength and fracture toughness among all the dental ceramics currently used by commercial laboratories.

Ceramic-on-Ceramics are Setting a New Benchmark in the Hip Implants Market

Ceramic-on-ceramic hip implant has a huge potential to become the preferred hip implant type among patients and orthopedic surgeons. There are two major factors responsible for such a potential for ceramic-on-ceramic hip implant: 1) reduced wear and tear of implants and 2) minimizing trauma and recovery time for the patients.

The introduction of hot isostatic pressing technique to ceramics has helped to overcome low fracture toughness characteristics of ceramics, a major impediment for the growth of ceramic-on-ceramic hip implants.

Medtech Insight has estimated the overall hip replacement products market at approximately $1.4 billion in 2002. Three categories have been identified as a source of this value-primary implants, partial implants, and revision implants. Medtech Insight has forecasted sales of hip replacement products to grow at compounded annual growth rate of 10.3% and resulting into sales of approximately $3.7 billion in 2012. In 2004, US market of hip implant for all product types grew approximately 20%.

Table 4.3

In 2002, Trident Acetabular Cup system, a ceramic-on-ceramic based hip replacement system has helped Stryker Howmedica Osteonics to maintain its market leader position in hip replacement market with the companywide sales of approximately $352.3 million (a 25.3% market share).

Wright Medical Group's sales of hip implants were $99.1 million in 2004, an increase of 27% from the previous year. The company had a favorable shift in its U.S. sales mix to premium-priced hip implants segment, which is composed of ceramic-on-ceramic and metal-on-metal products. The company has increased its market share in hard-bearing procedures from 57% in 2003 to 66% in 2004.

In a significant move, Centerpulse Orthopedics has also announced its intention to enter ceramic-on-ceramic hip implant product market and started the development of its product.

Ceramics Continue to Make Inroads in Other Orthopedic Segments

Many companies are using ceramics to develop innovative orthopedic products either made of ceramics or containing ceramics. Examples include:

• Smith & Nephew has a modular shoulder system made of titanium stem with several options of humeral heads, including ceramic, cobalt chrome, and titanium nitrite.


• Wright Medical has come out with OrthoSphere, a ceramic spherical interposition implant, for replacement of the joint between the first metacarpal and the trapezium in the thumb.


• Biocomposities has brought out a resorbable polymer interference screw named Bilok screw, which is useful in cruciate ligament reconstruction. The screw has applied the company's proprietary resorbable ceramic/polymer technology--PLLA with tricalcium phosphate.


• DePuy AcroMed has developed resorbable synthetic bone substitute Conduit TCP, a ceramic graft material composed of tricalcium phosphate. Biomet is developing a ceramic-on-ceramic articulation system which is currently in clinical trials in the U.S. but available in the European market.


• Dr Besim Ben-Nissan and PhD student Warwick Payten from the UTS Faculty of Science, along with their team, have developed the first largely ceramic prosthetic knee; which by reducing effects of friction and wear has a much longer life span than existing knee replacement products. The design is primarily ceramic with a stiff titanium alloy base plate attached to the ceramic using clips.


• Nobel Biocare, a leader in aesthetic dental solutions, introduced the new NobelEsthetics offering of crowns, bridges and abutments, in August 2004. The long-awaited launch includes two-to-four unit ceramic bridges for natural teeth and implants, a new CAD/CAM-based scanner and a new dental ceramic porcelain system. This new offer is to address patient needs for a metal-free aesthetic treatment.


• Centerpulse Dental Inc. has announced the launch of PureForm ceramic components developed for cemented restorations. The new components have a similar appearance to the shapes and shade of natural teeth, providing aesthetic outcomes for dental implant patients and less preparation time and cost for the dental laboratory.

Low fracture toughness and a propensity for fracture have historically been the major technical problems in ceramic implants. Fractures in implants can be avoided by eliminating flaws that provide sites for crack initiation. A fine grain size and more uniform grain size normally translates to greater strength and lower wear. A quantum leap in this direction was made by the introduction of hot isostatic pressing (HIP) technique. HIP exposes components to extremely high temperature and pressure. This reduces porosity and increases the density and strength of the material. Scientists are also trying to use combination ceramics for better implants. For example, CeramTec in Germany is working on an alumina/zirconia composite that appears very promising for many applications. Crystaloy, a new bioceramic with reinforced fibers, has shown excellent wear resistance.

Meanwhile, the European Commission has funded a three-year BIOKER project to investigate the use of zirconia toughened alumina nanocomposites to form ceramic-ceramic implants with a potential life span of more than 30 years. The material contains approximately 2.5% zirconia, which is distributed uniformly among the alumina grains. The stress intensity threshold of this material is much higher than that of its constituents. Further, the material does not need stabilizers (which are required for pure zirconia ceramics). Since stabilizers are among the major sources of crack generation, the ability to work without stabilizers would be a major breakthrough for material scientists.

The full Orthopedic Biomaterials 2005 report, which this is excerpted from, is available here.