Trabecular metal tibial component hailed as viable alternative to cemented implant

Porous tantalum offers good initial stability and bone ingrowth when implanted according to the recommended surgical protocol.

Issue: June 2006

Many orthopedic surgeons use cemented tibial components to ensure fixation and enhance survivorship, but a new trabecular metal device may provide comparable stability and fixation in a cementless form.

The device, the Trabecular Metal Monoblock Tibial Component (Zimmer), is made of tantalum, a type of easily shaped trabecular metal. Friction between porous tantalum’s rough texture and host bone provides strong fixation and high stability, according to a Zimmer press release.

Some surgeons consider Zimmer’s Trabecular Metal Monoblock Tibial Component the safe and reliable cementless knee implant they have sought for 20 years. It is made of tantalum, a soft, porous and biologically inert metal.

Images: Zimmer

Porous tantalum, which mimics cancellous bone, is 75% to 80% porous, allowing two or three times more bone ingrowth than conventional porous coatings and twice the interface shear strength. It also accommodates direct compression-molded polyethylene components, and hex-shaped pegs can be formed that may be press-fit into the tibial surface, according to Zimmer literature.

“I think it’s the only tibial component in the marketplace that’s got clinical history that allows for the offsetting of stress into the bone,” said Sheryl Conley, Zimmer’s group president for the Americas and global marketing and chief marketing officer. “This is a construct that is very unique in the marketplace and can answer past problems that surgeons had with porous technologies in the tibia.”

Some surgeons cement the new device and consider trabecular metal the best surface for cementing, Conley said.

Recognizing some surgeons’ reluctance to sacrifice modularity, Zimmer is developing a modular version of the Trabecular Metal Monoblock Tibial Component, Conley noted. Modular components usually make a minimally invasive surgery a little easier, however there are a number of surgeons who are using the Trabecular Metal monoblock tibia in MIS approaches, she said.

“It’s interesting that we have a component that may match different camps or philosophies,” Conley said.

Bob Poggie, of Zimmer’s Trabecular Metal Technology operation, recalled failures of past designs and materials that greatly reduced the use of cementless tibial components in the U.S. He hailed the trabecular metal biomaterial and the monoblock design as the keys to cementless tibial components making a successful comeback in North America.

“The whole issue why this country went away from the idea of cementless technology in the knee was that there was one particular bad [implant] in the 1980s called the PCA that worked great for the first [one to three years] and then it experienced a high rate of loosening and premature revisions. There was a large number put in and then a high percentage that had to be revised relatively soon,” Poggie told Orthopedics Today.

Poggie cited some of the tibial component’s “biomechanical advantages,” including the monoblock configuration offloading stress to the bone “in a more normal physiological fashion.” Trabecular metal, combined with the monoblock design, will “change the mainstream thinking” toward cementless technology in the next five to 10 years, he predicted.

Upsides and downsides

A randomized, prospective clinical trial at the Mayo Clinic is showing positive results, Poggie said, noting that patients at three-year follow-up have had no revisions or abnormal radiographic findings.

“The five-year data are going to start rolling out in the next year or two,” he said.

Despite the promising early results, the fact that the component is nonmodular adds some challenges, said John P. Collier, PhD, of Dartmouth College, Hanover, N.H. Also, a trabecular metal component is likely to fail when placed in an area with a defect or where bone graft material was used, he said.

The porous material may be problematic in some situations, Collier added, explaining: “The porous surfaces are not particularly strong, so they have to be put in where there’s uniform support. So, if you put them in over a defect or where you have used, let’s say, bone graft material, you’re likely to have problems.”

Zimmer noted its product labeling for the monoblock tibia warns against using the device in such situations where bone quality is poor or incapable of providing good initial fixation. However, the company asserts strength is not an issue and has developed its Trabecular Metal products specifically for knee revision surgeries where poor bone quality and defects are likely to be encountered. “These products have years of clinically successful usage in conjunction with bone graft,” company officials told Orthopedics Today.

In a debate at the 22nd Annual Current Concepts in Joint Replacement Winter 2005 Meeting, Arlen D. Hanssen, MD, of the Mayo Clinic, lauded the component’s fixation and bone ingrowth. He cited the Mayo Clinic trial, in which more than 400 patients younger than 80 years were randomized to cemented and uncemented fixation.

“At this point, we’re quite confident in the fixation we can achieve in almost all patients,” Hanssen said.

(Staff writer Tina DiMarcantonio contributed to this article.)

A cross-section of Zimmer’s Trabecular Metal Monoblock Tibial Component shows the implant’s monoblock configuration, which designers claim enhances stability.

A radiograph shows Zimmer’s Trabecular Metal Monoblock Tibial Component five years after implantation.

For more information:
Hanssen A. Cementless tibial fixation: Its future is now – Affirms. #98. Presented at the 22nd Annual Current Concepts in Joint Replacement Winter 2005 Meeting. Dec. 14-17, 2005. Orlando, Fla.
Zimmer Web site: zimmer.com.