Setting
This study is a part of the NACOX study that is a prospective, multicentre cohort study.18 Patients or the public were not involved in the design, or conduct, or reporting, or dissemination plans of our research. Participants were consecutively recruited over approximately 20 months, from six different sites in Sweden. Participants provided informed consent to participate.
Participants
People seeking medical care, between October 2016 and October 2018, for an acute knee injury sustained no more than 6 weeks prior to presentation, and aged between 15 and 40 years at time of injury, were eligible. Some clinics included all participants with knee injuries and other clinics only included participants with ACL injury. Participants were excluded if they met any of following criteria: previous ACL injury/reconstruction to the same knee, fractures that required separate treatment, inability to understand written and spoken Swedish language, cognitive impairments, other illness or injury that impaired function (eg, fibromyalgia, rheumatic diseases and other diagnoses associated with chronic pain).
Procedure
Participants were recruited between October 2016 and October 2018. Participants received a clinical diagnosis from an orthopaedic surgeon and MRI when required to confirm the diagnosis and were treated according to usual care. For ACL injuries, participants usually undergo rehabilitation before a decision for ACL reconstruction (ACLR). Other surgeries, for example, meniscectomies, are performed when needed according to patients’ symptoms (for more details, see the NACOX protocol18). In the NACOX study, questionnaires are sent to the participants via short message at smartphone or email at various time points after injury. For the current analysis, data collected using the baseline questionnaire, and 3-month and 6-month follow-up questionnaires were analysed. In addition, medical charts with 6 months follow-up after the injury were reviewed.
Outcomes
Demographic and baseline characteristics including age, sex, body mass index, occupation, preinjury activity and level of participation, medical and injury history were collected. Specific questions were asked about recovery expectations (‘when do you think your knee will be recovered to the same level as before the injury?’, answer options: within 1 week, within 1 month, within 6 months, within1 year, more than 1 year, the knee will never recover) and recovery of knee function, (‘how do you rate your knee function compared with before the injury’, graded in a six-point scale from ‘fully recovered’ to ‘much worse’).
For self-reported knee function, participants completed the Swedish version of the International Knee Documentation Committee subjective knee form (IKDC-SKF) and the Single Assessment Numeric Evaluation (SANE-score).19 The IKDC-SKF is a knee-specific self-reported outcome measure for symptoms, function and participation in sports. The IKDC-SKF has good measurement properties.20 21 Possible scores range from 0 to 100; a higher score indicates superior self-reported knee function. Minimal clinical important difference early after ACLR is 14 points.20 Muller et al22 defined 75.9 points and over were acceptable symptoms for patients who had undergone ACLR. The SANE is a 1-question measure where participants answer the question ‘If I had to give my knee a grade from 1 to 100, with 100 being the best, I would give my knee a …’. The SANE-score has good validity and is positively correlated with IKDC-SKF score.19 23
Self-reported frequency of participation in vigorous physical activity was collected, according to the recommendations from Swedish National Board of Welfare, with a separate question, ‘how much time did you spend last week on exercising at a level that makes you short winded, for example, running, fitness class or ball games’.24 Participants reported the type of physical activity they participated in (eg, football, strength training) and the level of participation during the previous week. Participation in up to three activities could be recorded at every questionnaire. The most strenuous activity was analysed.
Activity level was classified according to Tegner Activity Scale25 and modified IKDC activity level.26 27 In the Tegner Activity Scale, sports activities and work are graded on an 11-level scale, according to functional demands on the knee. Level 0 is no participation in physical activity due to knee problems; level X is participation in elite football. In the modified IKDC, activity is classified in four levels: level I is pivoting and contact sports, level II is pivoting non-contact sports and III is neither pivoting nor contact sport. Participants who were not participating in physical activity, or who are completing rehabilitation were classified in level IV.
Information about any knee surgery during the follow-up period was collected by a direct question to the participants and reviewing medical charts.
Data analysis
For the current analyses, participants were grouped according to injury in two main groups, that is, ACL-injury and other knee injuries. Specific diagnoses are reported in online supplemental file. Participants with two or more knee injuries were grouped based on the most serious injury.
Injury groups are described with descriptive statistics. Missing data and loss to follow-up is reported under results and in tables.
Comparisons between groups were made with χ2 tests or Mann-Whitney U tests, and within groups with repeated analysis of variance (ANOVA) or Friedman with post-hoc tests, as appropriate.