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Study highlights important factors that explain why partial knee arthroplasties fail
Most patients were revised due to aseptic loosening, other factors included lateral progression and wear.
CHICAGO — A study presented at the Knee Society Specialty Day highlighted aseptic loosening as the cause of failure in half of unicompartmental knee arthroplasties and that they are more likely to fail due to pain than total knees.
“Unicompartmentals are revised earlier than total knees,”
Bias toward early revision
Berend pointed out there is a bias toward early revision of unicompartmental knee arthroplasty (UKA), but not for total knee arthroplasty (TKA). He said many studies do not consider other factors for failure such as loosening, progressive arthritis, infection, issues with bearings, patient dissatisfaction or pain.
“We have been lulled into thinking all of our total knee replacements do well,” Berend said. “Increasing information from London, Ontario, [Canada] and other places show that it is as high as one in five patients who receive a total knee replacement are not satisfied with their arthroplasty. We need to look at the archer and the arrow when we evaluate our results.”
Michael E. Berend
Causes of failure
To examine the cause for failure of UKAs, Berend and colleagues conducted a retrospective multicenter study that included 813 revision knee arthroplasties performed during a 2-year period. Ten percent of the cohort was UKAs converted to TKAs. The average patient age was 62 years, and the average body mass index was 33. The average time to revision was 4.1 years. The researchers did not determine the number of unrevised UKAs.
The researchers discovered 57% of the patients were revised for aseptic loosening, 11% were revised for pain, 9% involved lateral progression, 6% were revised for instability, 5% for wear, 3% for malalignment and 2% for infection or fracture. The researchers found the rate of revision for pain was 13% for TKA vs. 2% for UKA. Patients who underwent TKAs required revisions at an average of 5.8 years compared to 4.1 years for UKAs. No patients needed revisions for isolated patellofemoral progression.
Limitations
Berend said the study was limited by its retrospective nature and lack of clear definitions for aseptic loosening and pain levels that necessitated revision.
“The effect of surgical volume of UKA and TKA for surgeons performing partial knee replacement needs to be significantly considered in future studies,” Berend said. “The registry data concludes that there is a higher ‘failure’ rate. The is an apples to oranges comparison between total knees and partial knee replacements. It only accounts for revision and not really failure. Dissatisfied TKA patients are counted as a success in registries that do not get revised. Other authors have reported a five-fold higher revision rate for partial knee replacement compared to TKAs with equal outcome scores. This highlights the difference in thresholds for revision for UKA compared to TKA.” – by Renee Blisard Buddle
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Disclosure: Berend receives research support and intellectual property royalties from Biomet for partial and total knee replacement products.
Perspective
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Loosening, pain and progression of lateral compartment disease were the most common reasons for revision unicompartmental knee arthroplasty (UKA). Tibial loosening can result from several factors. If an all polyethylene component is used, the undersurface macrostructure can be the causative factor resulting in failure. When tested in vitro, components that do not use a dovetail undersurface configuration tend to fail with less load. If too much cancellous bone has been removed prior to implantation of the tibial component, the cancellous bone is predominantly air when resected 10 mm below the articular surface and therefore, implant stability is compromised and early subsidence may occur.
Preoperative determination of appropriate patient selection is still the most difficult decision-making [aspect] in patients with unicompartmental disease. Failure to localize preoperative pain has been implicated in postoperative failure, although this is controversial. Lateral compartment pain progression can be secondary to resultant valgus deformity as a result of the procedure, although there are some patients who have findings of degeneration upon biopsy of the lateral compartment at the time of the initial UKA, despite a pristine appearance of the lateral compartment. Despite better functional results in UKA patients, the decision of who to operate on and use of precise surgical technique with a biomechanically sound component are still important factors affecting the survivorship of UKA.