Modularity may have caused past problems with cementless tibial components

Mayo Clinic orthopedists turn to monoblock designs to improve cementless tibial fixation results.

Issue: June 2006

ORLANDO, Fla. – Some orthopedists attribute earlier failures of cementless tibial components to several variables, and place most fault on modular designs.

The reintroduction of cementless tibial components has forced surgeons to take another look at why some components failed the first time around. In addition to modularity, they have identified other possible variables, including screw use, metal-backed patellas, poor locking mechanisms, polyethelene issues, insufficient ingrowth surfaces and inconsistent tibial fixation in some designs.

Arlen D. Hanssen, MD [photo]
Arlen D. Hanssen

At a time when surgical indications for total knee arthroplasty (TKA) are expanding to include younger and more active patients, Arlen D. Hanssen, MD, of the Mayo Clinic in Rochester, Minn., believes it is logical to use successful biologic fixation approaches. But, Hanssen added: “My opinion, quite strongly so, is that I also try to do this by eliminating the modularity.”

He and Thomas S. Thornhill, MD, of Brigham & Women’s Center in Boston, debated the cemented vs. cementless tibial components issue at the 22nd Annual Current Concepts in Joint Replacement Winter 2005 Meeting. Both agreed that modularity could be the most distinguishing variable that caused the decreased use of cementless tibial components.

A short history

Cementless tibial knee components were introduced in the 1980s to improve long-term fixation, improve longevity and ease revision. It was also around this time that modularity was introduced, according to Hanssen. Manufacturers and surgeons wanted to use the modular design for intraoperative versatility, to add stems or wedges and to accommodate screw fixation for cementless tibial components.

About five years later, surgeons noticed an increase in polyethelene wear and osteolysis. “The exact reasons for this increased prevalence of synovitis, progressive osteolysis and severe polyethelene wear remain unclear,” Hanssen wrote in his abstract. The likely factors included polyethelene manufacturing and sterilization techniques, minimum polyethelene thickness, variable articular surface geometry and a younger, more active patient population.

This 65-year-old man underwent total knee replacement with a cementless monoblock tibia. At three-year follow-up this image shows excellent fixation and no evidence of proximal tibial stress relief.

Eight years after undergoing a total knee replacement with a cementless modular tibial component, this 80-year-old man experienced severe femoral and tibial osteolysis.

Images: Hanssen AD

In the mid-1990s, surgeons also noticed design- and material-related issues industry-wide with modular designs, including micromotion, back-sided wear and osteolysis, Thornhill said.

Addressing modularity

Monoblock tibia designs offer surgeons another avenue for cementless tibia fixation. “In terms of trying to reduce back-sided wear … you could use a monoblock tibia design,” Thornhill said. “That could be an all polyethelene or a metal-poly monoblock … [Monoblock has] less micromotion, less back-sided wear and if it is all poly, lower cost.”

Hanssen and his colleagues at the Mayo Clinic have been using a cementless metal-backed non-modular tibial component for several years. They hope to avoid modularity and gain biologic fixation in the tibial component, which has already been achieved with the cementless femoral component. Early results are promising, Hanssen said.

“If you look at one recent study in the literature, the monoblock knee performed better than the modular knee, suggesting that this interface does have something to do with the osteolysis,” Hanssen said.

In this case, an 80-year-old man underwent total knee replacement with a cemented modular tibial component and experienced severe femoral and tibial osteolysis at six years postop.

The particular study by Weber evaluated 353 modular knees and 698 monoblock knees at an average 5.49-year and 6.25-year follow-up, respectively. Researchers found a 1.4% revision rate for the monoblock group, compared to a 3.1% revision rate in the modular patients (P<.05). Balloon osteolysis occurred in 0.1% of monoblock patients and 1.7% of modular knee patients (P<.05).

New approaches available

“In contrast to the first-generation designs, there are a whole number of new approaches now,” Hanssen said, including “… a number of coatings, actual three-dimensional designs of metals. And we’ve been interested in working with … trabecular metal in an effort to push the cementless technology.” [See accompanying article for more on using trabecular metal with cementless tibial components.]

Thornhill pointed out some disadvantages with a monoblock tibia: fewer intraoperative options for changing tibial thickness and tibial constraint, fewer revision options, axis of debris without a good seal and a variable tibial bone surface.

Hanssen said that revision with a cementless monoblock design is not so complicated. “You can put the saw beneath the tibial tray and cut off the pegs,” he said. “And I actually revised it by putting a stem in between the pegs and left the pegs. It is not at all hard to take out.”

Surgeons may not be able to choose a single variable for which to attribute the decreased use of cementless tibial components, but prospectively evaluating the different variables can help, Thornhill said.

Mayo Clinic investigators are currently conducting a study comparing cemented and cementless tibial fixation in a prospective, randomized clinical trial.

For more information:
Hanssen A. Cementless tibial fixation: Its future is now – Affirms. #98.
Thornhill T. Cementless tibial fixation: Its future is now – Opposes. #99. Presented at the 22nd Annual Current Concepts in Joint Replacement Winter 2005 Meeting. Dec. 14-17, 2005. Orlando, Fla.
Weber AB, Worland RL, Keenan J, et al. A study of polyethelene and modularity issues in >1000 posterior cruciate-retaining knees at five to 11 years. J Arthroplasty. 2002;17(8):987-91.