Surgeons look to hip tribology to provide better clinical results with THA, new bearing surfaces

With the fixation of total hip arthroplasty prostheses no longer the major issue it once was, the focus among orthopaedic surgeons has largely shifted to total hip arthroplasty bearing surfaces, including which head/cup combinations are safest and most durable to use in different patients and any solutions on the horizon that may further improve hip tribology.

“With many of the short-term complications of total hip arthroplasty solved, wear problems appear,” Enrique Gómez-Barrena, MD, of Fundación Jiménez Diaz, in Madrid, said.

Now that total hip arthroplasty (THA) implants remain well-fixed and in service for a longer time than ever before, the extent of wear of hip articulating surfaces has become more evident. Because it can be viewed on radiographs and may also be reflected in such complications as osteolysis or component loosening, THA bearing wear has evolved into one of the biggest issues in orthopaedics today.

According to Karl Knahr, MD, of Orthopaedic Hospital Vienna-Speising, in Vienna, “It is difficult to know what the magnitude is of implant wear as an isolated factor because all the papers always talk about aseptic loosening, wear and osteolysis. This is a more complex situation because aseptic loosening may occur due to the failure of the contact of the implant to bone … but also it can occur because of the wear.”

Karl Knahr, MD, of Vienna, said wear of THA bearings is difficult to single out as a tribology factor because it has always been reported in the literature in relation to aseptic loosening or osteolysis. Image: Orthopädisches Spital Speising GmbH

Knahr, Gómez-Barrena and the other orthopaedic surgeons who spoke with Orthopaedics Today Europe about hip tribology said they are all familiar with the usual wear issues and complications associated with metal-on-polyethylene, ceramic-on-polyethylene, ceramic-on-ceramic and metal-on-metal articulations — the most commonly used bearing materials worldwide. Yet, they all cautioned against taking unnecessary risks by adopting prostheses that involve major changes to those materials or combinations of them or include modified bearing designs in an attempt to further reduce wear. Such efforts may not provide optimal solutions to the problem either and it could take years before surgeons, researchers and patients see clinically proven results and benefits from their use, they said.

Liner exchanges

Among the well-established THA designs favored in Europe is ceramic-on-polyethylene, first used with standard and later with highly crosslinked polyethylene.

Knahr, a member of the Orthopaedics Today Europe Editorial Board, and colleagues studied their 15-year results using ceramic heads on conventional polyethylene in approximately 100 cementless THAs. They found a cup loosening rate of 2.5%. Liners were exchanged in 10% of cases for wear.

“It was clear the problem was more than wear of the polyethylene. If you just exchange it early, before the particulate leads to osteolysis, then you can save the implant,” Knahr said.

An analysis of the 20-year results revealed most liner exchanges occurred around the 12th postoperative year and that liner exchange before the 10th year was usually not necessary. Therefore, Knahr expects his patients who underwent THA within the last decade with ceramic heads on highly crosslinked polyethylene to enjoy a well-functioning hip for the rest of their lives.

Ceramic usage

Developed in the early 1970s, the alumina-on-alumina sliding THA system — a ceramic-on-ceramic articulation — is the articulation of choice for Laurent Sedel, MD, of Paris, past president of the International Hip Society.

He began using alumina-on-alumina THA in 1977 in his young active patients and today that group includes individuals up to 75 years old if they are still active.

Sedel noted members of his department have implanted this style hip articulation in approximately 5,000 patients. “We try to avoid reoperation at a very old age … and we expect that this is a material that, at the moment, is the best choice with this goal,” he told Orthopaedics Today Europe.

Low fracture rate

Sedel’s published ceramic-on-ceramic THA results, with 14 ceramic fractures over 25 years, show fractures are exceptional. His soon-to-be-published study will point to acetabular component aseptic loosening, not fractures, as the main reason patients have needed ceramic-on-ceramic THA revisions.

Friction coefficient and the amount of debris an articulation generates are the most important aspects of hip tribology to Sedel.

“The friction coefficient with ceramic-on-ceramic is very good due to a special lubrication system called boundary lubrication, which involves a very thin layer of water or liquid lubrication,” Sedel said.

Ceramic-on-ceramic generates very little debris so it minimizes the chance of foreign body, macrophagic or lymphocytic reactions. The alumina-on-alumina system Sedel uses generates 100 times less debris than all-metal articulations, Sedel added.

“The main issues of tribology at the moment are a little too focused on the problems of metal-on-metal.” — Klaus-Peter Günther, MD

Metal-on-metal THA has received some bad press lately, drawing attention to its possible tissue and bone damage and local and systemic problems related to the implants’ release of chromium or cobalt ions and particulate.

With the increasingly better performance of highly crosslinked polyethylenes and newer ceramics, the use of all-metal articulations has declined substantially in Germany, Klaus-Peter Günther, MD, of Universitätsklinik Carl-Gustav Carus, in Dresden, said, although it is still used there for surface replacement surgery and some conventional THAs.

Günther, an Orthopaedics Today Europe Editorial Board member, defined tribology as the wear, as well as the corrosion characteristics of bearing couples and mechanical strength of implant surfaces.

“The main issues of tribology at the moment are a little too focused on the problems of metal-on-metal. Every type of bearing couple has specific advantages and disadvantages and the individual value that one particular type of implant or bearing has depends on these factors and not only on the material itself,” he said.

Metal combating wear

David J. Langton, MRCS, a full-time researcher focused on metal-on-metal THA agrees the advantages of metal-on-metal THA have been lost in all the hype. In fact, he describes metal-on-metal articulations as a possible solution to the well-known wear issues with polyethylene.

“We are still trying to solve the problem [of wear] for the young active patients. There is a strong argument that metal-on-metal hip resurfacing does extremely well if the surgeon performs a technically satisfactory operation and also selects the right patient for the operation,” Langton said.

He noted the Birmingham Hip Resurfacing System (Smith & Nephew) has had nearly excellent long-term results. And although Langton has frequently seen high levels of cobalt chromium ions in some patients’ blood following metal-on-metal THA, those are mostly outliers. During resurfacing arthroplasty they either received smaller femoral heads, a problematic brand of implant or a poorly oriented one, he explained.

Therefore, Langton recommended not abandoning use of these prostheses but informing patients of the known problems and regularly testing their postoperative metal ion levels. Under proposed British Hip Society guidelines, he noted such testing may be required in the United Kingdom at 9 months postoperative, after the bedding-in period.

“[In] any joint that is suspected of wearing at an accelerated rate, the surgeon should not wait beyond 9 months,” Langton stated.

Teaching tribology

It is important to educate orthopaedic surgeons about hip tribology, the factors influencing the wear of current styles of bearings and their indications and contraindications, said Günther.

Conveying this kind of information is one of the purposes of Tribology Day that EFORT has decided to include at their upcoming congresses, Günther noted.

“You need a good teaching environment with someone showing you how to do the surgery perfectly.” Knahr said.

What’s in store

Sedel believes that hip tribology will become less of a materials issue than it is now, with more extensive use of hip navigation and advances in biomechanics research possibly solving many current tribology problems. “With these navigation systems and a hard-on-hard bearing like ceramic, you can have better fixation of components, better orientation and fewer revisions,” he said.

Gómez-Barrena said any solutions aimed at reducing bearing wear, such as the new ceramic-on-metal articulations or mobile bearing-design hip implants, suffer from one critical problem: they are based on multiple variables that add even more unknowns to the equation. “I would recommend caution in incorporating new solutions in every patient without appropriate control mechanisms in the follow-up,” he said.

At the moment, Gómez-Barrena remains confident in his preferred articulations of 28-mm or 32-mm cobalt chromium heads on highly crosslinked polyethylene in patients older than 70 years; 32-mm ceramic heads on polyethylene in patients between 45 and 60 years old, and alumina-on-alumina articulations in anyone younger than 45 years. — by Susan M. Rapp

References:

• Bozic KJ, Kurtz S, Lau E, et al. The epidemiology of bearing surface usage in total hip arthroplasty in the United States. J Bone Joint Surg (Am). 2009;91(7):1614-1620.
• Geerdink CH, Grimm B, Vencken W, et al. Cross-linked compared with historical polyethylene in THA: an 8-year clinical study. Clin Orthop Relat Res. 2009;467(4):979-984. Epub 2008 Nov 22.
• Gómez-Barrena E, Medel F, Puértolas JA. Polyethylene oxidation in total hip arthroplasty: Evolution and new advances. Open Orthop J. 2009;3:115-120.
• Isaac GH, Brockett C, Breckon A, et al. Ceramic-on-metal bearings in total hip replacement: Whole blood metal ion levels and analysis of retrieved components. J Bone Joint Surg (Br). 2009;91:1134-1141.
• Kretzer JP, Kleinhans JA, Jakubowitz E, et al. A meta-analysis of design- and manufacturing-related parameters influencing the wear behavior of metal-on-metal hip joint replacements. J Orthop Res. 2009;27(11)1473-1480.
• Sedel L. Total hip replacement: current concepts and basic issues. Bull Acad Natl Med. 2009;193(1):8-86.
• Yoshitomi H, Shikata S, Ito H, et al. Manufacturers affect clinical results of THA with zirconia heads: a systematic review. Clin Orthop Relat Res. 2009;467(9):2349-2355.

• Enrique Gómez-Barrena, MD, can be reached at the Orthopaedic Surgery and Traumatology Service, Fundación Jiménez Diaz, Universidad Autónoma de Madrid, Avda Reyes Católicos 2, Madrid 28040 Spain; +34-9-155-04827; e-mail: enrique.gomezbarrena@uam.es.
• Klaus-Peter Günther, MD, can be reached at University Carl-Gustav Carus, Fetscherstrasse 74, 01307 Dresden, Germany; +49-351-4-58-31-37; e-mail: klaus-peter.guenther@uniklinikum-dresden.de.
• Karl Knahr, MD, can be reached at Orthopaedic Hospital Vienna-Speising, Speisingerstr. 109, A-1130 Vienna, Austria; +43-1-80-182-242; e-mail: karl.knahr@oss.at.
• David J. Langton, MRCS, can be reached at the Joint Replacement Unit, University Hospital of North Tees, Hardwick, Stockton-on-Tees, TS19 8PE England; e-mail: djlangton22@doctors.org.uk.
• Laurent Sedel, MD, can be reached at Hõpital Lariboisiere, University Denis Diderot Paris, Paris, France; e-mail: laurent.sedel@lrb.aphp.fr.

Is the generation of metal ions a reason to not use metal-on-metal hip articulations?

Contraindicated for women

Soren Overgaard

The most important problem with metal ions is that some patients develop pseudotumors. Some studies show the risk for developing pseudotumors is up to 8%. This basically occurs in women and especially with resurfacing total hips. The more ions, the higher the risk might be for developing pseudotumors. Whether an allergic type IV hypersensitivity reaction plays a role is not fully understood, in addition to general allergic reactions caused by the metal ions.

Also, there is the systemic reaction, where there might be a risk for developing cancer. It is only on the “might be” level, but there are some studies of the Finnish Arthroplasty Register that showed a possible increased risk for cancer.

Metal ions may be transferred through the placenta to an unborn child for childbearing-aged women. The metal ions might cause a chromosomal aberration which might be transferred to the next generation. I haven’t seen any studies that focus on the influence these particles or metal ions have on men and their reproductive capacity, so we just do not know.

There are a lot of concerns with metal particles, so using metal-on-metal articulations in women younger than 40 years of age is contraindicated. Gender is probably the best known risk factor for pseudotumors following resurfacing hip replacement. In my clinic we actually have said, more or less, it is contraindicated to do a resurfacing in women.

Soren Overgaard, MD, is a professor of orthopaedic surgery in the Department of Orthopaedics and Traumatology at Odense University Hospital, Denmark.

Will not use metal-on-metal

Edward R. Dunstan

In the right patient, resurfacing arthroplasty done well functions very effectively, but it is a technically demanding procedure. Component position is absolutely critical. I am gravely concerned about the procedure being performed on women and I am concerned about the potential effects of exposure to raised metal ions over a prolonged period of time, as long-term scientific studies in this area have not been conducted.

Regarding my particular interest in chromosomal aberrations and the potential long-term side effects of exposure to elevated metal ion levels, several epidemiological studies were inconclusive regarding whether long-term metal ion exposure is detrimental. Chromium and cobalt ions are released from the wear debris of metal-on-metal articulations. In the laboratory chromium can cause genetic damage and be carcinogenic. Cobalt can cause DNA damage and interfere with DNA repair mechanisms. While we don’t yet fully understand the effects of these metal ions in vivo, we do know a huge number of particles (10¹⁴) are generated by metal-on-metal articulations and these particles are not inert.

Metal wear debris can also cause an inflammatory reaction, fibrosis, allergy, type IV hypersensitivity reaction and aseptic lymphocyte-dominated vasculitis-associated lesions (ALVAL) — a condition that can cause implants to become painful and catastrophically fail.

Therefore, I do not use a metal-on-metal articulation in anyone.

Edward R. Dunstan, FRCS, an orthopaedic consultant at Queen Margaret Hospital in Dunfermline, Scotland, has studied chronic exposure to metal ions.