Getting your bearings in TKA: Comparing fixed-bearing to mobile-bearing designs

Polyethylene and implant design improvements may negate need for mobile-bearing knees.

When they were introduced, mobile-bearing knee designs were hailed as a solution to poor polyethylene that resulted in excessive wear and osteolysis. Unfortunately, the concepts and benefits of mobile bearings have not been borne out in the literature.

Bryan D. Springer

“There have been improvements in polyethylene and knee design that may negate the need for a mobile bearing,” said Bryan D. Springer, MD, an adult reconstructive surgeon at OrthoCarolina Hip and Knee Center. “I think that newer, fixed-bearing designs have shown promising early results.”

The philosophy of mobile bearings for total knee arthroplasty (TKA) is appealing, said Springer, who for the first 5 years of his practice was a mobile-bearing user. He still uses them occasionally.

The mobile-bearing implant has a more conforming design, so, in theory, patients should have better kinematics and function. The self-centering polyethylene should permit better motion and patellofemoral kinematics. Long-term, there should be less polyethylene wear and less osteolysis, making the implant especially appealing to young, active patients.

“I think that newer, fixed-bearing designs have shown promising early results.”

— Bryan D. Springer, MD

Two issues drove mobile-bearing use

Surgeons turned to the mobile bearing because of two issues: poor polyethylene and a poor locking mechanism.

Polyethylene wear and osteolysis are related to the issue of topside wear and backside wear. With noncongruent surfaces or round-on-flat designs, the polyethylene demonstrates high-contact stresses. When that is combined with poor polyethylene, the result is pitting, delamination and large particles.

“Backside wear is different. The matching radii of a highly conforming topside design impart stress on the backside of the implant,” Springer said. Stress transfer is increased to the backside of the polyethylene and locking mechanism of the implant.

“The problem with the backside wear is that it tends to generate smaller particles, which make the bone more susceptible to osteolysis. The literature shows that nonarticular polyethylene wear [backside wear] correlates with radiographic osteolysis,”¹ Springer said. “What a mobile bearing has tried to do by not locking the polyethylene in place, is basically make that backside of the polyethylene a flat-on-flat congruent surface to reduce wear.”²

Previous polyethylene that was gamma sterilized-in-air combined with poorly conforming surfaces, had resulted in polyethylene bearing failure due to wear and oxidation. “We know that oxidized poly is weak poly, that it is brittle poly and that it has a higher amount of wear,”³ Springer said. At just 1 million cycles — about a year’s worth of use in a high-demand patient — there is evidence of delamination and polishing wear with this gamma sterilized-in-air polyethylene.

Better polyethylene

Improvements in polyethylene may negate the use of mobile bearings. Highly crosslinking the polyethylene minimizes oxidation risk and makes the polyethylene more wear resistant,^4,5 he said. “We see orders of magnitude in the reduction of the amount of wear when you look at conventional and crosslinked polyethylene.”

Unfortunately, mechanical strength can be compromised during crosslinking,⁶ Springer said. Although that has been an issue in hip replacement, thus far, it has not been a problem in knees.

Historically, the locking mechanism on implants has been susceptible to degeneration over time, mainly due to transfer of stress to the locking mechanism and polyethylene wear. If the locking mechanism broke down or there was excessive wear, surgeons saw unintended motion inside the tibial tray,⁷ Springer said.

“All of these knees ended up moving with time on the backside of the polyethylene, particularly some of the earlier generations of fixed-bearing knees,” he said.

Promise not shown in literature

Mobile bearing designs were developed in theory to improve function and longevity of total knee arthroplasty and to try to reduce polyethylene wear. “To date, if you look at most of the comparative studies that have been presented on rotating platform knees, they have not shown any clinical advantage over a fixed-bearing knee,”^8-10 Springer said.

A Mayo Clinic study⁷ looked at the rotating platform and patella tracking with the hypothesis that a self-centering device should improve patellofemoral kinematics. The researchers found no difference in the prevalence of lateral release rates, no difference in patellar tilt or subluxation, no difference in maximum flexion at 3 months and 1 year, no change in stair-climbing scores and no difference in Knee Society Scores for pain and function.

“With improvements in the polyethylene with crosslinking and with improvements in locking mechanisms and design, I think people can feel very comfortable with fixed-bearing designs.”

— Bryan D. Springer, MD

A randomized, controlled trial¹¹ compared the Sigma RP (DePuy) and an all-polyethylene tibia. The results demonstrated no difference in knee scores, WOMAC scores, flexion extension or SF-36 scores.

Another study¹² found differences between fixed and mobile bearings for wear, pitting, scratching and burnishing on the top and bottom side of the implant.

Osteolysis is another concern with mobile bearings. A study out of Taiwan¹³ found that the mobile-bearing group had smaller and more granular particulate debris, which may increase the potential for osteolysis compared with the fixed-bearing group.

In another Mayo Clinic study¹⁴ presented at this year’s annual meeting of the American Association of Hip and Knee Surgeons (AAHKS), the researchers prospectively randomized 240 patients to receive either a fixed- or a mobile-bearing knee. Five-year results showed no difference in motion, function or implant durability, Springer said.

“At this point in time, I do not think the literature supports the advantage of a mobile-bearing knee over a fixed-bearing knee, at least from a clinical standpoint,” Springer said. “With improvements in the polyethylene with crosslinking and with improvements in locking mechanisms and design, I think people can feel very comfortable with fixed-bearing designs.”

Accommodates knee kinematics

“Fixed-bearing implant design has changed in a way that may negate the need for mobile bearings. For example, the Triathlon (Stryker) fixed-bearing knee has a rotary arc on the polyethylene that allows it to accommodate 20· of internal and external rotation,” Springer said.

“The design does not drive or force the kinematics of the knee, it accommodates the kinematics on the knee,” he said.

Range of motion is easier with this rotary arc and single radius design, allowing smooth and consistent articulation, he said. “It is an unconstrained design and allows what we all want, which is soft tissues rather than the implant driving the motion and kinematics of the knee.”

The locking mechanism is better. It has a full peripheral locking rim, a locking wire and a rotational island. Data indicate that backside wear is reduced and that the locking mechanism is more stable.¹⁵ Overall, early results seem promising with this device.

References

1. Garcia RM, et al. Analysis of Retrieved Ultra-High Molecular Weight Polyethylene Tibial Components From Rotating Platform Total Knee Arthroplasty. J Arthro. 2009;24(1).
2. Kurtz SM. The UHMWPE Handbook: Ultra-High Molecular Weight Polyethylene in Total Joint Replacement. San Diego, CA: Elsevier Academic Press; 2004.
3. Sutula LC, et al. The Otto Aufranc Award: impact of gamma sterilization on clinical performance of polyethylene in the hip. Clin Orthop Relat Res. 1995; (319):28-40.
4. Hermida JC, et al. The effect of oxidative aging on the wear performance of highly crosslinked polyethylene knee inserts under conditions of severe malalignment. J Orthop Res. 2008; 26(12):1585-90.
5. Wang A, et al. A highly crosslinked UHMWPE for CR and PS total knee arthroplasties. J Arthroplasty. 2008; 23(4):559-566.
6. Collier JP. Comparison of cross-linked polyethylene materials for orthopaedic applications. Clin Orthop Relat Res. 2003; (414):289-304.
7. Galvin A, et al. The influence of tibial tray design on the wear of fixed-bearing total knee replacements. Proc Inst Mech Eng H. 2008; 222(8):1289-1293.
8. Pagnano MW, et al. Rotating platform knees did not improve patellar tracking: a prospective, randomized study of 240 primary total knee arthroplasties. Clin Orthop Relat Res. 2004; (428):221-227.
9. Haas B, et al. Range of motion of posterior-cruciate-substituting total knee replacements: the effect of bearing mobility. J Bone Joint Surg (Am). 2001; 83-A (suppl 2[Pt 1]):51-55.
10. Gioe TJ, et al. Mobile and fixed-bearing (all-polyethylene tibial component) total knee arthroplasty designs. A prospective randomized trial. J Bone Joint Surg (Am). 2009; 91(9):2104-2112.
11. Muller SD, et al. Should we reconsider all-polyethylene tibial implants in total knee replacement? J Bone Joint Surg (Br). 2006; 88(12):1596-1602.
12. Engh GA, et al. Analysis of wear in retrieved mobile and fixed bearing knee inserts. J Arthroplasty. 2009; 24(6 supp):28-32.
13. Huang C, et al. Osteolysis in Failed TKA: A Comparison of MBK and FBK. J Bone Joint Surg (Am). 2002; 84:2224-2229.
14. Kalisvaart M, et al. A randomized controlled trial of 240 rotating platform and fixed bearing TKA shows no difference in function, range of motion, or durability at minimum 5 year follow up. Presented at: AAHKS November 2010. Paper #1. Dallas, Texas.
15. Harman MK, et al. Backside damage corresponding to articular damage in retrieved tibial polyethylene inserts. Clin Orthop Relat Res. 2007;458; 137-144.

Bryan D. Springer, MD, can be reached at OrthoCarolina, 2001 N. Vail Ave., Suite 200A, Charlotte, NC, 28207; (704) 323-2564. Springer is a consultant for Stryker and Convatec.