Table 2 Secondary outcomes

Return to work

The time (i.e. days, months) between the surgery and the first day back at work, both partly and fulltime

Knee Functioning, measured with The Knee Injury and Osteoarthritis Outcome Score (KOOS) [43]

The KOOS consists of 42 items that assesses five outcomes: pain (9), symptoms (7), activities of daily living (17), sport and recreation function (5), and knee-related quality of life (4) on a 5-point Likert scale (0, no problems – 5, extreme problems). Scores are transformed to a scale of 0 – 100, with 0 representing extreme knee problems and 100 representing no knee problems

Pain intensity, measured with the von Korff questionnaire visual analog scale (VAS) [44]

The pain VAS is a unidimensional measure of pain intensity and is widely used in postsurgical patient populations. This questionnaire measures the severity of pain on a scale of 0 (no pain) to 10 (extreme pain) and the associated impediment with daily life activities

Health-related quality of life, measured with the 5-level EuroQol-5d (EQ-5D-5L) [45]

The EQ-5D-5L contains five questions that represent the following health dimensions: mobility, self-care, usual activities, pain/discomfort and anxiety/depression. For each dimension, patients have to indicate the severity of their health complaints. There are five severity levels: normal/no problems, slight problems, moderate problems, severe problems, and extreme problems, all scored from 1 (no problems) to 5 (extreme problems). Hence, answers to these questions will represent a patient’s EQ-5D-5L health state, ranging from 11,111 (no problems on all dimensions) to 55,555 (extreme problems on all dimensions)

Physical difficulty experienced at work, measured with the Work, Osteoarthritis or joint-Replacement Questionnaire (WORQ) [46]

The WORQ is a valid and reliable questionnaire that can be used to assess the impact of the knee replacement. It contains 13 questions, each representing a physical activity (e.g. kneeling, crouching or standing). Patients have to grade how difficult this activity is to perform on a five-point scale: none, mild, moderate, sever of extreme – corresponding with scores of 4 to 0, respectively. The sum of scores is then converted into a score of 0 (worst score) to 100 (best score)

Fatigue, measured with the Multidimensional Fatique Inventory (MFI-20) [47]

The MFI-20 is a valid and reliable self-report instrument that has been designed to measure fatigue in several dimensions: general fatigue, physical fatigue, reduced activity, reduced motivation and mental fatigue. Each dimension has five questions rated on a five-point Likert scale. Scores on each subscale range from 4 to 20, with higher scores indicating greater fatigue. The sum of the subscale scores represents the total fatigue score (range 20 – 100), with a higher score indicating a higher level of fatigue