Material World

Advances in polymers and ceramics have improved the durability of implants.

Michael Barbella

“The operation is not about achieving an immediate and spectacular result, but about the permanence and the lifetime of the initial result.” — Sir John Charnley

Few in the orthopedic industry can argue with the doctrine that defined the career of Sir John Charnley.

Ceramic femoral and acetabular components used in hip arthroplasty. Photo courtesy of C5 Medical Werks.

As an inventor, Charnley rarely focused on immediate and spectacular results, prefering instead to focus on the long-term resiliance of his particular creations. He spent years developing and perfecting the artificial hip that still bears his name and the surgical procedure that is still used by orthopedic surgeons around the world.

“Sir John Charn-ley was the one who investigated the concept of low friction arthroplasty,” said Joseph Spiro, manager of the Lifesciences Group at Westlake Plastics, a Lenni, Penn.-based manufacturer specializing in extrusion and compression molding of engineered/high-performance plastics. The company, which also has an office in Anaheim, Calif., has developed more than 25 types of medical grade plastics.

“In his initial series [of artificial hips] the Teflon failed miserably because biomechanically, it was not the right material. Sir John pursued the matter further and found that a high molecular weight polyethylene could be the answer. The rest is history.”

According to history, Charnley began using polyetrafluoroethylene (PTFE) in his artificial hips in 1956. Often referred to as Teflon (incorrectly, according to some industry experts), PTFE is composed of carbon and flourine but has a chemical structure similar to ultra high molecular weight polyethylene (UHMWPE), a tough material that has is considered the “gold standard” among implant materials.

Charnley’s high hopes for PTFE were dashed when he discovered after 300 hip replacement surgeries that the material wore down quickly in the body. In addition, the PTFE implants produced wear debris that caused an “intense foreign body reaction,” a fact Charnley knew all too well from having implanted PTFE material into his own thigh.

Discouraged by the results but unwilling to totally abandon PTFE for implants, Charnley experimented with a glass-filled PTFE that ultimately performed just as poorly as the original. By this time, UHMWPE had become available for use, but Charnley, still brooding over the failures of PTFE, rejected the material. According to an article in the Iowa Orthopedic Journal, a “resilient” assistant of Charnley’s tested the UHMWPE and discovered its superior wear properties to PTFE. Further testing showed that UHMWPE was a better plastic for the construction of artificial joints, and Charnley (won over by the material’s strong qualities) implanted the first UHMWPE socket—labeled RCH 1000—in November 1962.

“There are still Charnley hips out there today with polyethylene acetabular components in them that are still functioning beautifully,” Spiro said. “They’ve withstood the test of time.”

Perks of Polyethylene

Part of the reason Charnley’s RCH 1000 hip implants have withstood the test of time is because of the material used to make the device. The success of a total hip replacement is based largely on the design and processing of the materials used in the implant. To replicate the action of a ball-and-socket hip joint, implants are composed of three parts: the stem, which fits into the leg bone and usually is made of metal; the ball (or head), which replaces the sphere-shaped head of the leg bone and can be made of metal or ceramic; and the shell (or liner), which replaces the worn-out hip socket. The shell is usually made of metal and the liner is most always plastic, though the liner also can be made of ceramic or metal.

The durability, strength and biocompatibility of polyethylene make the material a more popular choice for the liner of the implant over metals such as cobalt chromium and titanium, which are not as durable, industry experts told Orthopedic Design & Technology. Implant liners made of metal also produce tiny debris particles that are not absorbed by the body, which can cause an adverse reaction in surrounding tissue and bone. Concerns by both surgeons and patients about such adverse reactions have led more people to undergo hip replacement or partial hip replacement procedures using polyethylene in the implant liner.

“In the early days, no one really knew the effect cobalt chromium would have on the surrounding tissue. But metal-on-metal [implants] have come a long way since then. Today’s metal-on-metal [implants] have tight bearings, they are lapped and there is no klanking,” Spiro noted. “Polyethylene is really considered the gold standard when it comes to material. When it is machined properly, polyethylene has the lowest coefficient of friction than any material other than ceramic. You want a low coefficient of friction because friction leads to wear. And wear, over time, determines the life of the implant.”

Studies have shown that implants with polyethylene liners do not produce as much debris as their metal counterparts. Orthopedic surgeons also have found polyethylene to be more effective when used against a metal shell because plastic tends to deform slightly if it is not mounted to a hard surface, experts said.

“Polyethylene tends to give a little bit, so UHMWPE against a hard surface will be more forgiving than hard against hard bearings,” said Mark Evans, global business manager for Meditech Medical Polymers, a Fort Wayne, Ind.-based manufacturer and supplier of UHMWPE to the orthopedic implant industry. “In my opinion, the ideal is metal-polyethylene or ceramic-polyethylene. Those combinations have always made up the majority of implant components and in recent years, much progress has been made on improving the wear characteristics.”

One type of material that has become a popular choice for orthopedic implant applications is implantable grade polyetheretherketone (PEEK). This type of polymer isstrong, stiff, and maintains its mechanical properties when repeatedly sterilized. The material also is hypo-allergenic and non-magnetic, enabling patients who have received PEEK implants to undergo procedures such as magnetic resonance imaging.

PEEK-OPTIMA, a brand of PEEK developed by West Conshohocken, Pa.-based Invibio Biomaterial Solutions, is one of the more popular polymer choices among orthopedic surgeons. According to the company, more than 2 million devices containing PEEK-OPTIMA material have been implanted in patients since 1999. (Editor’s note: For more information on PEEK polymers, see Invibio’s white paper on page 51).

Two years ago, Invibio released to market a carbon fiber-reinforced grade of PEEK-OPTIMA that improves the wear performance of implants. The material is composed of short carbon fibers dispersed in a PEEK-OPTIMA polymer matrix, according to the company. The resulting polymer has enhanced mechanical and physical properties for more demanding, load-bearing applications requiring blood, bone or tissue contact for more than 30 days.

Other companies have fine-tuned polyethylene in different ways over the last several years in an effort to improve wearability. One of the ways companies have improved the durability of implants is through a process called cross-linking, which creates stronger bonds between the elements that comprise the polyethylene.

Zimmer, a Warsaw, Ind.-based manufacturer of orthopedic products and instruments, cross-links polyethylene using high-dose electron beam radiation, which the company claims further links together the molecular structure of the plastic. The cross-linked product, called Longevity, was developed specifically to address the issue of wear in total hip replacements.

Some companies, such as Biomet Inc., are using vitamin E to improve the longevity of implants. The Warsaw, Ind.-based orthopedic manufacturer produces acetabular hip liners made of polyethylene cross-linked with vitamin E. Laboratory testing showed that the hip liners cross-linked with vitamin E (called E-Poly liners) experienced 95 percent to 99 percent less wear than other polyethylene liners. The material also provides greater strength when compared to first-generation remelted highly cross-linked polyethylenes, according to the company.

Ticona, a global supplier of engineering resins, unveiled two new grades of UHMWPE with vitamin E in February. The Florence, Ky.-based firm is adding ahomogeneous content of 1,000 parts per million of vitamin E as a stabilizer to two new grades of polymers, called GUR 1020-E and 1050-E. The vitamin E addition, a company executive claimed in a statement, will “allow sufficient irradiation behavior as well as effective oxidation resistance for improved implant wear.”
Ceramics Comes of Age. Most advances in the orthopedic industry result from patient need. And patient need is one of the major factors that has driven the growth of ceramics over the last two decades.

Like polyethylene, the use of ceramic material in total hip replacements was born out of a need to reduce debris from metal implants. The first all-ceramic hip, composed of 32 millimeter alumina heads and alumina cups, was developed in 1970 by French surgeon Pierre Boutin. The implant lasted 17 years (outliving the patient) and showed few signs of wear

Despite the success of that first ceramic hip replacement, however, the U.S. Food and Drug Administration did not approve ceramic-polymer combination implants until 1990. The first implants were comprised of polyethylene and either alumina or zirconia, two types of ceramics that are scratch-resistant and significantly harder than metal. These two ceramics also can be used on both the ball and socket components of an implant.

“The reason ceramics came of age is because people realized the hip joint bearing combination of metal and polyethylene parts used in the body [for implants] was a major cause of osteolysis,” said Steven Hughes, sales manager for Orthopedic and Medical Devices at C5 Medical Werks, a ceramic implant manufacturer based in Grand Junction, Colo. “The full benefit of ceramic [implants] is its low wear in combination with excellent biocombatibility. When you combine ceramic alumina and ployethylene, it generates far fewer particles of osteolysis-inducing debris.”

Implants that combine a ceramic cup with a ceramic head produce almost no debris and are much more durable than implants that combine metal and polyethylene, industry experts said. These implants also minimize trauma and recovery time for patients.

Reduced wear and minimal patient trauma have helped drive recent growth in ceramic-on-ceramic implants, and will most likely influence future growth, particularly among younger, more active recipients, industry experts said. Orthopedic surgeons are bracing for an influx of patients younger than 65 over the next two decades, and there is good reason to believe that ceramic implants will maintain their popularity with this age group during that time period.

More than 50 percent of the demand for total hip replacement is expected to come from patients younger than 65 by 2011, according to research from the American Academy of Orthopaedic Surgeons (AAOS). Patients younger than 65 also will comprise more than 50 percent of the candidate population for total knee replacement that same year, the research concluded.

By 2016, more than half of primary knee replacement patients will be younger than 65. Demand for the procedure is expected to grow the fastest among patients in the 45-54 age category, the data showed. Researchers expect a 17-fold increase in the number of total knee replacements in this age group, from 59,077 procedures in 2006 to 994,104 procedures by 2030. The demand for total hip replacements in the same age category (45-54) is projected to grow nearly six-fold in 2030.

“Improving implant reliability and survivorship will be critical in light of the current and anticipated demand for total joint replacement procedures from patients younger than age 65,” said Steven M. Kurtz, Ph.D., lead investigator of the research.

Indeed, improving the reliability of implants will be crucial as demand grows. For ceramics to be on the forefront of this growth, however, manufacturers of the material must address two issues that could trigger the same kind of hesitancy patients and surgeons once had about metal implants.

The first issue involves the material’s properties. Ceramic hip replacement materials such as alumina arehard, bitter polycrystalline substances with little flexibility; as a result, there are design limitations associated with the material that must be overcome to reduce the chance of fractures. “If loaded incorrectly, the stresses within the ceramic [part] can build up and that part can suffer a brittle fracture. It’s like a porcelin plate or cup—if you hit it with a hammer there’s a good chance it is going to break,” Hughes said.

The second issue ceramic manufacturers must address is more of a mystery. Over the last several years, a phenomenon known as the “squeaky hip syndrome” has emerged among pat-ients with ceramic ball and socket hip implants. Doctors have been unable to find a reason for the noise, leading the AAOS to call the syndrome an “underreported medical trend that has stumped the medical community.”

The noise complaints have come almost exclusively from patients with ceramic-on-ceramic hip implants. A 2006 study in the Journal of Arthroplasty found that 10 out of 143 patients, or 7 percent, who underwent total ceramic hip replacements from 2003 to 2005 developed squeaking when they walked or bent over. Another 31 patients reported other noises eminating from their hips, such as popping and grinding.

A similar study from the Rothman Institute, an orthopedics group in Philadelphia, Pa., found the squeaking condition in 3 percent of the1,500 cases it reviewed.
No noises, however, have been reported among patients who have received hip implants made of ceramic and plastic or metal combination parts.

Hughes said he believes the noise issue will be resolved once medical experts determine its cause. “There’s probably a multitude of factors that are causing the squeaking in hips. Once it’s better understood, we can find a solution,” he noted.

The success of total joint replacements is based largely on the materials used in the implant.

Over the last three decades, polymers and ceramics have evolved to create implants that are more sturdy and durable, and produce minimal amounts of debris. These factors also have helped reduce the chance of patients undergoing future replacement procedures.

An array of implant designs, bearing surface materials, and surgical approaches have been used by physicians in an attempt to improve the quality of life for patients with hip arthritis. While arguments can be made for or against each design or material, the implants and components developed over the last 30 years share one common trait: They work best in tandem. Each type of material is dependent upon another to function as a viable replacement to deteriorating joints. This co-dependency is not expected to change either, regardless of the advances made with any one particular material. As C5 Medical Werks’ Hughes noted: “There are always going to be material advances, and that’s a great thing. But until we find the wonder drug that cures all and solves all the problems, there’s always going to be combinations of all three materials involved.”

WEB EXCLUSIVE: Rubber, Glass and Ivory: A Look at Early Implantable Materials

While Sir John Charnley was not the first person to experiment with total hip arthroplasty (THA), he was indisputably the most successful. Early attempts to treat patients suffering from hip arthritis failed mostly because the treatments did not contain the proper materials or surgeons did not target the proper area of the hip.

In the early days of operative orthopedics, arthritic hips and other deteriorating joints were excised rather than replaced. The practice of excising aching joints was an attempt by physicians to provide patients with an alternative to the more drastic practice of amputations, which was being carried out with “appalling frequency” in Europe and America in the late eighteenth century, according to an Iowa Orthopedic Journal article.

Czech surgeon Vitezlav Chlumsky experimented with joint replacement surgery in the late nineteenth century, using a slew of different materials in his arthroplasty procedures, including muscle, celluloid, silver plates, rubber struts, magnesium, zinc, glass, pyres, decalcified bones and wax.

The earliest recorded attempt at a hip replacement procedure was carried out in Germany in 1891 by professor Themistocles Gluck. He created an ivory ball and socket joint that he fixed to bone with nickel-plated screws. He later experimented with a mixture of plaster of Paris and powdered pumice with resin to affix the socket joint to the bone.

In the early 1900s, orthopedic surgeon Sir Robert Jones used a strip of gold foil to cover reconstructed femoral heads. While this procedure was effective (Jones reported that his patient still retained effective motion at the joint 21 years after the surgery), the procedure was never widely used.

Over the next several decades, surgeons tried various other materials to create an artificial hip joint that would fuse to bone and last at least as long as the one designed by Jones. One surgeon experimented with glass, Bakelite and Pyrex before incorporating Vitallium in his arthroplasty procedure on the suggestion of his dentist. An alloy composed of 60 percent cobalt, 20 percent chromium, 5 percent molybdenum and traces of other materials, Vitallium is used primarily in dentistry for its light weight and resistance to corrosion.

A rubber femoral prosthesis was developed in 1919, while an ivory model came out in 1927. Shortly after World War II, two brothers from Paris, France, developed an acrylic prosthesis that eventually became susceptible to wear; around the same time, two men from England were experimenting with dental acrylic cement to affix the hip prosthesis to the bone.

By the time Charnley wrote his 1950 book, The Closed Treatment of Common Fractures, he had all the ingredients he needed to drive the evolution of modern THA.