APT, RP designs equivalent in early results - Academy News at the 2007 AAOS Annual Meeting

APT, RP designs equivalent in early results

By Sally Chapralis

In the first prospective randomized clinical trial comparing mobile-bearing and fixed-bearing all-poly tibia (APT) total knee arthroplasty (TKA) designs, researchers found no significant clinical advantages to either design after more than two years (mean: 32 months).

The results, which were presented in scientific paper 011 yesterday, revealed no significant differences in mean postoperative range of motion (ROM) or in mean Knee Society (KSS) clinical or pain scores, although both the mobile- and fixed-bearing groups showed significant improvement.

Advantages?

As the authors pointed out, the mobile-bearing knee with a rotating platform was designed to address the problems of contact stress and polyethelyene wear. “By allowing motion at the polyethylene-tibial tray interface,” they noted, “greater conformity between the femoral and tibial components can be accommodated without sacrificing range of motion.” They also noted the following additional proposed or theoretical advantages to a rotating platform (RP) design:

  • The ability to compensate for small errors in the rotational alignment of the femoral and tibial components
  • Increased knee ROM due to the tibia’s ability to rotate during higher degrees of flexion
  • The possibility of reducing nonarticular surface wear by polishing a forged cobalt-chromium baseplate, rather than locking an insert to cast titanium

“An alternative method to reduce polyethelyene wear is to eliminate nonarticular surface wear with a monoblock, APT design,” said the authors. Advantages of this approach include reduced costs and the potential to decrease osteolysis by reducing nonarticular surface wear.

Trial participants

Among the 209 participants, there were 203 men and 6 women. Patients ranged in age from 60 years to 85 years old (mean age: 72.7 years). Nearly all (96 percent) of the participants had been diagnosed with osteoarthritis, 3 percent had a diagnosis of inflammatory arthritis, and 1 percent were diagnosed with posttraumatic arthritis. Patients had a mean body mass index (BMI) of 31 (BMI range: 19.7 to 57.3). The average American Society of Anesthesiology (ASA) score, reflecting concurrent medical comorbidities, was 3.

Patients were prospectively randomized to receive either a cemented APT cruciate-substituting tibial component or a cemented RP cruciate-substituting tibial component with identical cemented femoral and all-polyethylene patellar components.

From October 2001 through September 2004, 222 TKAs (102 APT and 120 RP) were performed under the direction of the principal investigator. There were no significant demographic differences (in age, BMI or ASA score) between the two groups of patients. At last follow-up, 12 of the original patients have died, 7 have had revision surgery, and 1 has been lost to follow-up.

Results

According to the authors, “neither design exhibited superior postoperative ROM” nor was there any clear clinical advantage. “Pain relief and stability as assessed by the KSS clinical score did not differ significantly between the groups, although both pain and clinical scores improved markedly” from preoperative scores. Radiographic measurements (femoral and tibial component coronal/sagittal alignment, radiolucencies) did not vary significantly.

There have been seven revisions in this patient group: five for deepinfection (three in the RP group and two in the AP group), one for acute patellar fracture following a fall (AP group), and one for symptomatic instability (RP group). Ten patients (five wgi received RP and five in the AP group) needed additional surgery without component removal or exchange.

The researchers noted that “no TKAs in either group were revised for mechanical failure or aseptic loosening of either component and there are no pending revisions at this time.”

However, as the researchers conceded, “measuring wear in vivo in TKA remains problematic. Given the lack of revisions for aseptic loosening, wear, or osteolysis, we are unable to comment on the wear characteristics of either design in this time period.”

While pointing out that a randomized prospective clinical comparison is the best way to address questions about whether an RP design is “better” than an AP design, the presenters also acknowledged that their study had some limitations, including the early follow-up period, the predominance of males in the study group and the age and comorbidities of the study group.

“Long-term results of randomized trials such as the present study will still be required to definitively answer the question of wear in RP versus fixed-bearing modular or monoblock designs,” they wrote. “Longer follow-up of this population may be able to address the wear issue, and similar randomized prospective studies in different demand populations would be helpful to determine if either design confers a clinical advantage over time.”

Based on their results, the authors conclude that although the implant costs were substantially ($1,875) less for the APT design, continued use of both designs in their study population would be justified.

The research team includes Terence J. Gioe, MD; Jonathan Sembrano, MD; Jason Glynn, MD; and Edward R.G. Santos, MD, all of Minneapolis (University of Minnesota), as well as Kathleen Suthers, MS, of Warsaw, Ind. Dr. Gioe received research support from Depuy, Inc. and the Office of Research and Development in the Department of Veterans Affairs.


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