Background and purpose: Development of minimal invasive operation techniques has given unicompartemental knee arthroplasty (UKA) renewed interest. Indications for use of UKA are however debated, and short-term advantages of UKA over total knee arthroplasty (TKA) should be weighed against the higher risk of reoperation. More knowledge on long term results of pain and function after knee arthroplasties is therefore needed and was the purpose of this study.

Methods: Patient-reported pain and function were collected at least two years after the operations in postal questionnaires from 1643 osteoarthritis patients reported to the Norwegian Arthroplasty Register with intact primary TKA (n=1271) or UKA (n=372). The questionnaire contained instruments for calculation of the knee specific Knee Osteoarthritis Outcome Score (KOOS), and for quality of life (EQ-5D, post- and pre-operative). 5 subscales from KOOS were used as outcome. To incorporate an outcome for anterior knee pain an additional subscale based on 7 questions from KOOS, clinically accepted to be related to such discomfort, was calculated. Pain and satisfaction from a visual analogue scale (VAS) were also used as outcomes together with improvement in EQ-5D index score. The outcomes were measured on a scale from 0 (worst) to 100 (best) units with an argued minimal perceptible clinical difference of 8–10 units. Group differences were analysed with multiple linear regression, adjusted for confounding by age, gender, Charnley category, time since operation and preoperative EQ-5D index score.

Results: UKA performed better than TKA for the KOOS subscales Activity in Daily Living (difference in mean outcome score =−3.4,p=0.02) and Sport and Recreation (difference =−4.4,p=0.02) and for Anterior Pain (difference=−4.5,p< 0.01). The difference was also significant for the outcome Pain(VAS) but now in favour of TKA (difference=3.3,p=0.02). Motivated by the discrepancy in the results of the pain related outcomes, Anterior Pain (UKA best), Pain(VAS) (TKA best) and Pain(KOOS) (no difference) further investigation of the questions (0=best to 4=worst) used for calculation of Pain(KOOS) and Anterior Pain were performed. Patients that had undergone UKA had more often pain from the knee (difference=0.26,p< 0.01), while they had less pain when they were bending the knee fully (difference=−0.37,p< 0.01) and less problems when squatting (difference=−0.25,p< 0.01).

Interpretation: Estimated differences did not reach the level of minimal perceptible clinical difference. There are however indication of differences in the way the two treatment groups experience knee related discomfort. Even though UKA offers a lower level of pain and less problems in activities involving bending of the knee, these patients seem to experience pain from the knee more often.