Reducing polyethylene wear for increased implant longevity

William P. Barrett

When assessing patient outcomes of mobile bearing and fixed bearing knees, one must differentiate between long- and short-term results and consider the patient population. Short-term clinical outcomes for total knee arthroplasty (TKA) with fixed and mobile bearing knees are similar, with no significant difference in range of motion or function.^1,2 Long-term studies evaluating survivorship of fixed and mobile bearing knees at 10 to 15 years’ follow-up produce similar results in similar populations.^3-6

Patients enrolled in a majority of the long-term studies have been older, more sedentary individuals, but the patient population is changing. The fastest growing segment of the TKA patient population comprises people 45 to 65 years old, and the number of replacements being implanted in these patients is also rapidly increasing.⁷ Patients in their late 40s require implants that will function longer than do patients in their late 60s. Research data at 10 to 15 years’ follow-up, while significant, may not be as revealing as results from longer-term studies that evaluate patients at more than 20 years’ follow-up.

Factors impacting patient outcomes

Surgeons measure patient outcomes in several ways, including pain relief, measures of function such as range of motion, strength, stair climbing, and the ability to participate in various activities of daily life. Several factors impact the outcome and survivorship of knee replacement systems in vivo, with surgical technique being one of the most important. If an operation is performed correctly, the patient will have a well-aligned, well-balanced knee. Certain implant design factors such as surface geometry, implant finish, particular material properties, articulation between various components and fixation will influence the outcome and survivorship of the implant in vivo.⁸

Design features that reduce wear

Polyethylene wear is an important part of the equation determining longevity of a knee implant. Polyethylene has a yield strength in the range of 15 to 20 megapascals and is sensitive to contact stress. As the polyethylene wears, it can cause implant particle burden in the joint, which can lead to osteolysis, loosening or mechanical failure of the joint. Decreasing polyethylene wear should decrease the rate of osteolysis, loosening and implant failure.

The main difference between a fixed and mobile bearing knee is the design of the polyethylene insert. In a fixed bearing knee, the polyethylene insert is rigidly fixed to the base-plate of the tibial component, which causes multidirectional stresses to occur on the top surface of the fixed bearing knee.

Mobile bearing knees allow mobility between the polyethylene insert and the tibial tray and decouple the stresses present in a fixed bearing knee. Primary flexion and extension stresses are placed on the upper surface of the polyethylene and rotational stresses are placed on the inferior surface of the polyethylene — both stresses are primarily unidirectional. In vitro data indicate unidirectional forces lead to less wear of the polyethylene when compared with multidirectional forces; thus mobile bearing knees may produce less polyethylene wear over time.⁹

Fixed bearing and mobile bearing knees also differ in size of the contact area. Mobile bearing knees have increased conformity, which allows for a larger contact area. Several studies on mobile bearing knees show that contact areas range from 400 to 800 mm², whereas the contact area of fixed bearing knees range from 200 to 250 mm². The stress placed on the polyethylene per unit area significantly increases in a small contact area and decreases in a large contact area.

The wear features of mobile bearing design may be particularly important in revision surgery. Highly conforming designs are often used to increase stability in revision cases. When using a highly conforming fixed bearing knee, patients may lose some rotation or transfer rotational stresses to the fixation interface, which can impair fixation. Pairing increased conformity with mobile bearing design decreases wear without imparting greater rotational stresses on the fixation interface.¹⁰

In older, more sedentary patients, most total knee designs, fixed or mobile bearing, perform well at 10- to 15-year follow-up. I use rotating platform (RP) knees in young, active patients because in vitro data show that RP knees have less wear than fixed bearing knees (Figure).¹¹ Long-term studies on knee replacements implanted in young, active individuals may show a difference between fixed and mobile bearing designs, but only clinical in vivo studies evaluating patients at prolonged follow-up, 20+ years, will prove this theory.

Figure A: Preoperative radiograph of a 47-year-old woman with osteonecrosis of the proximal tibia with extensive medial tibial bone loss. B: Postoperative radiograph showing mobile bearing TKA with medial bone graft and cemented stem extension for protection of graft. Use of rotating platform technology in this case may decrease long-term wear and decrease rotational forces at the fixation interfaces, giving potential for longer term survivorship in this young, active person. Source: William P. Barrett, MD Click here for larger version of Figure.

References

1. Post ZD, Matar WY, van de Leur T, Grossman EL, Austin MS. Mobile-bearing total knee arthroplasty better than a fixed-bearing? J Arthroplasty. 2009 Sep 22 [Epub ahead of print]
2. Oh KJ, Pandher DS, Lee SH, Sung Joon SD Jr, Lee ST. Meta-analysis comparing outcomes of fixed-bearing and mobile-bearing prostheses in total knee arthroplasty. J Arthroplasty. 2009;24:873-884.
3. Colizza WA, Insall JN, Scuderi GR. The posterior stabilized total knee prosthesis. Assessment of polyethylene damage and osteolysis after a ten-year-minimum follow-up. J Bone Joint Surg (Am). 1995;77:1713-1720.
4. Ritter MA, Berend ME, Meding JB, et al. Long-term follow-up of anatomic graduated components posterior cruciate-retaining total knee replacement. Clin Orthop Relat Res. 2001;(388):51-57.
5. Callaghan JJ, O’Rourke MR, Iossi MF, et al. Cemented rotating-platform total knee replacement. A concise follow-up, at a minimum of fifteen years, of a previous report. J Bone Joint Surg Am. 2005;87:1995-1998.
6. Buechel FF Sr, Buechel FF Jr, Pappas MJ, Dalessio J. Twenty-year evaluation of the New Jersey LCS Rotating Platform Knee Replacement. J Knee Surg. 2002;15:84-89.
7. Kurtz SM, Lau E, Ong K, Zhao K, Kelly M, Bozic KJ. Future young patient demand for primary and revision joint replacement: national projections from 2010 to 2030. Clin Orthop Relat Res. 2009;(467):2606-2612.
8. Dennis DA, Heekin RD, Clark C, Driessnack R, Sukin D, Teeny SM, Murphy J, O’Dell T. Does implant design improve postoperative flexion? A prospective, multi-center RCT in simultaneous bilateral knees. Presented at AAOS 2010, paper No. 500.
9. Pooley CM, Tabor D. Friction and molecular structure: the behavior of some thermoplastics. Proc R Soc Lond. 1972; 329:251-274.
10. Bottlang M, Erne OK, Lacatusu E, Sommers MB, Kessler O. A mobile-bearing knee prosthesis can reduce strain at the proximal tibia. Clin Orthop Relat Res. 2006;(447):105-111.
11. Fisher J, McEwen H, Tipper J, et al. Wear-simulation analysis of rotating-platform mobile-bearing knees. Orthopedics. 2006;29(9 Suppl):36.

William P. Barrett, MD, is a member of Proliance Surgeons in Seattle, Wash., and medical director of the Center for Joint Replacement at Valley Medical Center in Renton, Wash.

Perspective Kirk A. Kindsfater, MD Dr. Barrett addresses an important point about the changing population of patients who are now having total knee replacements. Although the current data do not show any significant difference in survivorship at 10 to 15 years when comparing fixed bearing with rotating platform knees, I agree with Dr. Barrett that the younger, more active population who are now receiving TKAs will be “harder” on their knees and demand more from their knees than the previous older patients in the studies that currently have 15-year follow-up with fixed bearings. I believe that longer follow-up in these active populations longer follow-up will show a “performance” or survivorship advantage that favors the rotating platform when comparing rotating platform and fixed bearing technologies. Chitranjan S. Ranawat, MD Dr. Barrett points out that the majority of published data are from patients with a mean age at surgery of 68 to 70 years. However, Swedish registry data show that TKA has a higher failure rate in younger patients.1 If reduced polyethylene wear can be demonstrated in younger patients with 15 to 20 years’ follow-up in a case control series, prospective randomized studies or registry data, this would make a strong case for mobile bearing knees. Swedish Knee Arthroplasty Register. http://www.knee.nko.se/english/online/thePages/publication.php. Accessed July 7, 2010. James E. Dowd, MD As Dr. Barrett points out, it is not just the age of the patient but the activity level that will govern wear and long-term success. We see widely disparate activity levels that may cover 1 to 6 million cycles per year. That means that today’s active patient needs six times the wear protection that yesterday’s standard patient needed. This is where the wear data on a mobile bearing knee really hits home. Dr. Barrett’s assessment of today’s patient’s expectations of activity level and range of motion is spot on. It is interesting that the design rationale of every new fixed bearing knee system on the market acknowledges a need to accommodate femoral-tibial rotation in a high-flexion knee design. Russell T. Nevins, MD Over the past several years I have personally seen an increase in the economic pressures of health care in my community. This is likely the case around the country. From a technical standpoint, I enjoy performing revision arthroplasty, especially with metaphyseal sleeves and mobile bearing constrained options. However, I would prefer to avoid performing revision surgery on 80- and 90-year-old patients. People are living longer and I respect the idea of stratifying patients and choosing fixed bearing for older patients and mobile bearing for younger patients. However, I would like to remind all surgeons to base this decision on physiology and not on chronologic age and economic pressure because a 72 year-old has a good chance of being around for his or her 20-year revision.