Revision rates may only tell part of the story

Commonly accepted rates of successful outcomes after total knee arthroplasty (TKA) in younger patients may be excessively optimistic, and revision rates provide only part of the full outcome picture, according to Andrew J. Price, PhD, FRCS, in a paper he presented at the 2009 AAOS Annual Meeting.

Revision is often considered the endpoint for survival for TKA. In his paper, “Fifteen-year survival and functional outcome of TKA in patients under the age of 60,” Dr. Price pointed out that because the primary indication for TKA is severe pain, pain may be a more appropriate outcome measure for evaluating treatment success following TKA.

“Pin has the highest correlation with patient satisfaction of all variables studied following other forms of arthroplasty,” he said. “Function, performance, and activity levels are also accepted as valid markers of treatment success and may be more appropriate for the younger population.”

Study design
The study had two goals: to determine the long-term survival results of cemented TKA among younger patients, and to investigate whether the results and interpretation changed if patient-reported outcomes and pain were used to assess the success of the treatment.

To examine the issue, Dr. Price and his team selected all patients aged 60 years or younger (mean age = 55.4 years) at the time of surgery from a consecutive series of 1,429 patients who underwent TKA from 1987 to 1993 at Nuffield Orthopaedic Centre in Oxford, UK.

Within the original cohort of 53 patients (60 knees), 8 patients (9 knees) required revision. Four of the revision surgeries were due to infection (mean time = 6.0 years, range 2.1–11.8 years), and five were due to aseptic loosening (mean time = 7.3 years, range 2.5–12.1 years). Using revision as the endpoint, therefore, the 15-year cumulative survival rate was 82.2 percent.

Choosing the endpoint
“Although the implant survival data paints an encouraging picture for young TKA patients,” wrote the authors, “some notice should be given to functional outcome.” Clinical review data (history, physical examination, radiographs, and clinician-based knee and functional scores) at a mean follow-up of 15.5 years was available for 32 patients (37 knees).

Researchers also used the Oxford Knee Score (OKS), a self-reported pain measure that goes from 0 (no pain) to 48 (maximum pain). The mean OKS for the 32 patients was 30.9 (range 4 to 48), with 15 pa­tients (15/37 knees, or 41 per­cent) reporting severe or moderate pain and 11 patients (11/37 knees, or 30 percent) reporting no pain.

“A significant number of patients have a low OKS, with a majority of these patients reporting at least moderate pain,” wrote the researchers. “This self-reported outcome measure suggests that, despite not being revised, the outcome may still not be designated a “success” at the measured time point of 15 years.”

The researchers pointed out that the choice of variable for the endpoint in survival and assessing treatment is critical in predicting outcomes. Survival using all-cause revision was 82.2 percent, but if revision or pain was used as the endpoint, the 15-year survival fell to less than 60 percent.

Caveats to the study
Dr. Price also pointed out the following caveats:

For all patients, OKS was taken at a single follow-up point. Without sequential or yearly OKS data, it was impossible to ascertain when the patient’s function reached its current level. Therefore, OKS cannot be used as a complete metric/endpoint.

Without preoperative data, a relative change in functional score (currently considered a more valid assessment than absolute score) is impossible to determine.

Functional scores can be affected by coexisting pathology and can vary with time due to factors unrelated to the procedure. Dr. Price suggested that preoperative and yearly reports of functional outcome would be valuable in developing a better metric for understanding implant success.

Dr. Price’s co-authors for “Fifteen-Year Survival and Functional Outcome of TKA in Patients Under the Age of 60” include David Longino, MD; Rajesh Rout, MD; Hemant G. Pandit, FRCS; David J. Beard, DPhil; Christopher A.F. Dodd, FRCS; David W. Murray, MD; Kassim Javaid, FRCP; and Samantha Hynes, BSc. The authors report no conflicts of interest.

Peter Pollack is a staff writer for AAOS Now. He can be reached at ppollack@aaos.org