Table 1.
Dependent and independent variables
| Variables | Time of measurement | Clarification |
|---|---|---|
| Dependent | ||
| RTW (return to work) | By questionnaire at follow-up | RTW was defined as the number of days between the date of surgery and the date the patient (fully) RTW. RTW was dichotomized into ‘rapid RTW’ and ‘prolonged RTW’ using the median of the number of days between surgery and (full) RTW as cut-off point |
| Independent: patient characteristics | ||
| Gender | Extracted from patient files | Male–female |
| Age | Extracted from patient files | Age at time of surgery in years |
| BMI | Extracted from patient files | Weight/Length2, BMI was dichotomized into ≤24 and > 24 using the median of the BMI |
| IKDC | By questionnaire at follow-up | The IKDC contains 18 items that measure symptoms, function, and sports activities in patients with a variety of knee problems. The score can vary between 0 and 100. A higher score means a higher level of functioning and a lower level of symptoms |
| EQ-5D | By questionnaire at follow-up | The questionnaire measures five dimensions: mobility, self-care, daily activities, pain/discomfort and anxiety/depression, and a visual analog scale for overall health. Each dimension can be assessed at three levels: no problems (1), some problems (2), and extreme problems (3). These health profiles can be converted into a weighted health index for each patient. Dutch tariffs were used to estimate the weighted health index. The weighted health index gives a number between 0 (death) and 1 (complete healthy) |
| Independent: injury-related characteristics | ||
| Side | Extracted from patient files | Right-left |
| Knee extension | Extracted from patient files | Dichotomized into ‘full knee extension’ and ‘no full knee extension’ |
| Time injury to surgery | Extracted from patient files | Time between injury and surgery, dichotomized into ≤3 months and > 3 months |
| Independent: characteristics of surgery | ||
| Medial menisectomy | Extracted from patient files | Menisectomy done by surgeon. Yes–No |
| Lateral menisectomy | Extracted from patient files | Menisectomy done by surgeon. Yes–No |
| Cartilage damage | Extracted from patient files | Assessed by surgeon using the guidelines of the International Cartilage Repair Society, grade zero (normal) up to grade four (severe abnormal) |
| Independent: sports activity level | ||
| Knee-taxing sport | Extracted from patient files | Practicing knee-taxing sports before ACL injury? Yes–No |
| Contact sports | Extracted from patient files | Practicing contact sports before ACL injury? Yes–No |
| Pivoting sports | Extracted from patient files | Practicing pivoting sports before ACL injury? Yes–No |
| Tegner score before injury | Extracted from patient files | The patients had to rate their level of activity with an integer between 0 (‘sick leave or disability pension because of knee problems’) and 10 (‘competitive sports, soccer—national and international elite’) |
| Tegner score after injury before surgery | Extracted from patient files | The patients had to rate their level of activity with an integer between 0 (‘sick leave or disability pension because of knee problems’) and 10 (‘competitive sports, soccer—national and international elite’) |
| Contact during injury | By questionnaire at follow-up | Was there physical contact during the ACL injury? Yes–No |
| Independent: work-related characteristics | ||
| Knee-demanding work | By questionnaire at follow-up | Knee-demanding work is measured with the WORQ. Knee-demanding work is defined as often or nearly always performing crouching, kneeling, clambering, lifting or carrying or taking the stairs during work |
| WORQ experience | By questionnaire at follow-up | The trouble the patient experiences during work measured with the WORQ. It generates a score between 0 and 100. A lower score indicates that the patient experiences more trouble during knee-related activities |
| Independent: characteristics of rehabilitation | ||
| Physiotherapy up to 6 weeks (frequency per week) | By questionnaire at follow-up | Frequency of physiotherapy, dichotomized into ≤2 times a week and >2 times a week |
| Physiotherapy 6 weeks up to 3 months (frequency per week) | By questionnaire at follow-up | Frequency of physiotherapy, dichotomized into ≤2 times a week and >2 times a week |
| Gym | By questionnaire at follow-up | Perform exercises in the gym. Yes–no |
| Homework exercise | By questionnaire at follow-up | Perform homework exercises. Yes–no |
| Crutch walking | By questionnaire at follow-up | Number of weeks crutch walking |