Thresholds for the Knee Injury and Osteoarthritis Outcome Score After TKA
Key findings
- In an international, prospective study of a modern knee implant system, all patient-reported outcome measures (PROMs) used were highly predictive of patient satisfaction at one and three years
- The key PROM studied was the Knee Injury and Osteoarthritis Outcome Score (KOOS), for which no patient-acceptable symptom state (PASS) thresholds have been established in patients who have undergone total knee arthroplasty (TKA)
- At one year, the PASS thresholds on the KOOS that divided satisfied from dissatisfied patients were 84.5 points for pain, 80.5 points for symptoms, 83.0 points for activities of daily living and 66.0 points for quality of life
- At three years, the thresholds were 87.5 points for pain, 84.0 points for symptoms, 87.5 points for activities of daily living and 66.0 points for quality of life
- Between the one- and three-year evaluations there were no significantly or clinically important changes in PASS thresholds for KOOS or other PROMs, so one-year follow-up with PROMs may be sufficiently predictive of the results of TKA
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Patient-reported outcome measures (PROMs) are now considered important in both orthopedics research and for the allocation of resources within a health care system. However, statistically significant differences between study groups in these outcomes can be clinically irrelevant.
To facilitate interpretation of PROMs, two concepts have been developed:
- Minimal Clinically Important Difference (MCID) — the smallest difference between two scores that is perceived as consequential to patients
- Patient-acceptable Symptom State (PASS) — the value on a numerical PROM scale beyond which patients consider themselves well or in a satisfactory state. Unlike MCID, PASS can be used to evaluate a single score in isolation, as long as a second, dichotomous outcome measure is used as an anchor
James W. Connelly, clinical research coordinator, Henrik Malchau, MD, PhD, director emeritus of the Harris Orthopaedics Laboratory at Massachusetts General Hospital, and colleagues recently became the first to define PASS thresholds for the Knee Injury and Osteoarthritis Outcome Score (KOOS) after total knee arthroplasty (TKA). Their report is published in The Journal of Bone and Joint Surgery.
Study Details
The researchers retrospectively analyzed data gathered on 499 patients as part of a prospective trial of the Vanguard Knee System, which included patients at 11 centers in the U.S., Australia, Denmark, Spain, South Korea and Sweden, who underwent TKA with that implant system between 2011 and 2014.
One of the PROMs used in the trial was, "How satisfied are you with the result of your most recent knee treatment?" Patients answered on an 11-point numeric rating scale (NRS) ranging from 0 (very satisfied) to 10 (dissatisfied). In the current analysis, that scale was used as the anchor outcome measure. Based on the distribution of responses, patients who marked ≤2.5 were considered satisfied.
Other PROMs used in the trial were the KOOS, an NRS for knee-related pain and the EuroQol 5-dimension 3-level (EQ-5D-3L) and EuroQol visual analog scale (EQ-VAS) measures of general health.
Correlation Between PROMs and the Anchor
Of 383 patients who answered the anchor question at one year, 79% indicated satisfaction with their treatment, and of 301 patients who answered at three years, 80% were satisfied. Answers to the anchor question correlated well with all other PROMs. However, at one year the EQ-VAS was not highly correlated.
PASS Thresholds at One Year
The cut-points dividing satisfied from dissatisfied patients were:
- KOOS subscales — 84.5 points for pain, 80.5 points for symptoms, 83.0 points for activities of daily living and 66.0 points for quality of life
- EQ-5D-3L — 0.80 point
- EQ-VAS — 83.0 points
- NRS for pain — 1.8 points
PASS Thresholds at Three Years
- KOOS subscales — 87.5 points for pain, 84.0 points for symptoms, 87.5 points for activities of daily living and 66.0 points for quality of life
- EQ-5D-3L — 0.80 point
- EQ-VAS — 90.5 points
- NRS pain for pain — 1.8 points
Changes in Thresholds Over Time
For each PROM, changes between PASS thresholds between one and three years were smaller than the MCID, and 95% confidence intervals for the two time points overlapped. Therefore, there were no significant or clinically important changes in PASS thresholds over time.
Given that, and considering the lack of change in the proportions of satisfied patients (79% at one year and 80% at three years), a one-year follow-up with PROMs may be sufficient to evaluate the treatment result. The PASS thresholds identified in this study can serve as benchmarks for surgeons to assess recovery after TKA and counsel patients.
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