Long-term data on total knee replacement surgery is largely limited to revision surgeries, leaving clinicians and patients in the dark about outcomes such as residual pain and disability, researchers said.
Currently the best data come from national procedure registries, but the traditional outcome measure is subsequent revision surgery, which "can underestimate problems [because] patients can remain with pain or poor function without necessarily undergoing revision," according to Andrew J. Carr of the University of Oxford in England, and colleagues.
More than 600,000 knee replacement procedures are performed in the United States annually, according to the Agency for Healthcare Research and Quality.
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Writing online in The Lancet, Carr and colleagues outlined four directions for the future of knee replacement surgery:
More consistent patient selection for knee replacement;
Long-term monitoring with patient-oriented outcomes, as well as revision, as endpoints;
Approval of new designs only after large randomized trials that demonstrate cost-effectiveness as well as clinical efficacy;
Better management of young people with early arthritis to avoid need for replacement surgery.
Each point addresses a problem Carr and colleagues saw in existing research and practice.
With regard to patient selection, they cited numerous studies that found a wide range of patients were receiving total knee replacements -- not only those with severe pain and disability but also, sometimes, in people with only mild symptoms.
They noted that an international panel reported last year that surgeons' recommendations for knee replacement were not significantly correlated with pain, disability, or radiographic severity.
Ideally, Carr and colleagues suggested, patients should have longstanding pain at night or when bearing weight, which has not responded to six months of medical and physical therapy -- although they acknowledged that "there is no consensus on what the exact severity of symptoms should be to indicate surgery."
When it is performed, the researchers argued, incorporating patient-centered outcomes into clinical follow-up and registry data is vital to ensuring that knee replacement is genuinely serving patients' needs. These could include global measures of satisfaction as well as specific outcomes such as residual pain and functional impairment.
But such measures have their own issues that remain to be fully resolved. "Patient satisfaction itself is challenging to interpret and understand without psychological profiling," Carr and colleagues indicated.
But the researchers stressed that revision remains a good objective measure of long-term performance of knee implants, and many of the national registries (especially Sweden's, which was the first) have been extremely valuable in uncovering problems with specific products.
When problems are detected, surgeons typically stop using the offending product and the manufacturer may withdraw it from the market.
But when they introduce new designs, clinicians may expect better performance than from older models, Carr and colleagues suggested. "There is evidence to suggest that this belief is incorrect," they wrote, citing several studies suggesting that new joint implants may, in fact, be more prone to revision rather than less.
Nevertheless, such products "are often introduced with little or no clinical evidence," the researchers wrote.
The growing number of younger people undergoing knee replacements -- which, incidentally, is known only because of registry data -- is something of a mystery, Carr and colleagues noted.
Several explanations have been proposed but remain unconfirmed: obesity or other factors are bringing down the age of onset for severe knee osteoarthritis; clinicians and patients are increasingly familiar with the procedure and confident in positive outcomes; or younger patients, intolerant of pain, are demanding that something be done.
Whatever the reason, Carr and colleagues suggested, new treatment strategies for osteoarthritis that avoid the need for surgical joint replacement should be a "major emphasis" for research.