Knee Injury and Osteoarthritis Outcome Score for Joint Replacement Scores Continue to Improve Beyond Two Years

The Knee Injury and Osteoarthritis Outcome Score for Joint Replacement (KOOS, JR) metric is used to evaluate improvements in stiffness, pain, and function following primary knee arthroplasty. The purpose of this study was to determine when KOOS, JR scores peaked following unicompartmental knee arthroplasty (UKA) and total knee arthroplasty (TKA) at our institution.

Cross-sectional KOOS, JR responses from 1,818 UKAs and 4,342 TKAs performed between January 2011 and March 2024 were analyzed. A total of 11,057 surveys were collected preoperatively and at 4 weeks, 4 months, and at 1, 2, 5, and 10 years postoperatively. Outcomes included mean KOOS, JR scores at each interval, timing of peak scores, and attainment of clinical thresholds, including minimal clinically important difference, minimal detectable change, patient acceptable symptom state, and substantial clinical benefit.

Statistically significant improvements in KOOS, JR scores were observed through 5-year follow-up for TKA and 2-year follow-up for UKA (P < 0.05). The patient's acceptable symptom state (PASS, ≥ 63.7) was reached earlier after UKA (4 weeks) than after TKA (4 months). Substantial clinical benefits (SCB, +20 points) were achieved by 4 months for TKA (mean score, 71.8 ± 12.8) and at 1 year for UKA (78.9 ± 14.8). Pain with twisting/pivoting, stair use, and morning stiffness showed the greatest improvements from preoperative to peak scores, with favorable outcomes rising 72.2, 72.0, and 63.4% for TKA and 72.2, 69.3, and 60.8% for UKA, respectively. However, these parameters were among the slowest to recover. Morning stiffness reached only 57.4 and 63.2% of its maximum improvement for TKA and UKA, respectively, at 4 months, compared to over 80% for most other survey questions.

Although recovery is most rapid in the early postoperative period, clinically important improvements in pain and function continue to occur up to 5 years after TKA and 2 years after UKA.