A multicenter cross-sectional study was performed on all patients with tibial plateau fractures who were treated between January 2003 and December 2019 at the University Medical Center Groningen (The Netherlands), Isala hospitals (The Netherlands), Gelre hospitals (The Netherlands), Martini hospital (The Netherlands), Medical Spectrum Twente (The Netherlands) and University Hospitals Leuven (Belgium). These include four Level 1 trauma centers and two Level 2 trauma centers. The study procedure was approved in all participating centers by the institutional review board (research number: 201800411) and was performed in accordance with the relevant guidelines and regulations. This study is reported in accordance with the STROBE (Strengthening the Reporting of Observational Studies in Epidemiology) guideline [18].

All patients of 18 years and older, treated for tibial plateau fractures, who had a diagnostic CT-scan, that were still alive and had at least 1 year of follow-up were considered eligible for inclusion. The exclusion criteria included, preexisting comorbidities of the injured leg, unknown trauma mechanism, patients with no knowledge of the Dutch language and patients with an unknown residential address.

All eligible patients were asked to complete the standardized Knee Injury and Osteoarthritis Outcome Score (KOOS) questionnaire [19]. In addition, patients were asked whether they underwent conversion to a total knee arthroplasty (TKA). KOOS is a validated questionnaire consisting of five subscales: symptoms, pain, activities of daily living (ADL), function in sports and recreation, and knee-related quality of life (QoL) [19]. A normalized score (100 indicating no symptoms and 0 indicating extreme symptoms) was calculated for each subscale. Patients who had conversion to TKA were assigned an average KOOS score that was reported in a previous cohort of patients with KOOS scores before conversion to TKA [20]. These scores represent the situation as it was before conversion to TKA. The assumed KOOS sub-scores were 52 for symptoms, 45 for pain, 55 for ADL, 16 for sport and 27 for QoL. The minimal clinically important difference of each KOOS subscale is 9 for symptoms, 12 for pain, 10 for ADL, 9 for sports, and 14 for QoL [21].

Patients were divided into two groups based on the energy of the trauma mechanism: high-energy (HET) and low-energy trauma (LET). The trauma mechanisms of all patients were reassessed through consensus by three observers (TPV, TWK, FFAIJ). The energy of the trauma mechanism was defined according to the criteria of the Advanced Trauma Life Support (ATLS) guidelines (appendix 1) [22]. Patients who met at least one of these criteria were classified as high-energy trauma.

Baseline characteristics, treatment (surgical or nonsurgical), trauma mechanism, injury severity scores (ISS), and associated injuries were retrieved from the patients’ electronic records. All knee radiographs and CT images were reassessed through consensus by two observers (TPV, FFAIJ) with experience in tibial plateau fracture management according to the Schatzker classification systems [23]. Gap and step-off measurements were performed on CT images to describe the fracture displacement. Gap was defined as separation of fracture fragments along the articular surface and step-off as separation of fracture fragments perpendicular to the articular surface [16]. The maximum gap and step-off were measured by going through axial, coronal and sagittal CT slices. The scores from each subscale of the KOOS questionnaire were calculated from the patient survey of the responding patients.

The primary study goal was to determine whether there were differences in patient and fracture characteristics between patients with a tibial plateau fracture after high- or low-energy trauma. The secondary goal was to assess whether there were differences in mid-term clinical outcomes after high- versus low-energy trauma in terms of patient-reported functional outcome, complications (e.g. reoperations, revision surgery) and conversion to TKA during follow-up.

SPSS software (version 28, IBM Corp) was used for statistical analyses. Continuous variables are presented as the mean and standard deviation (SD) for normally distributed data and median and IQR for nonnormally distributed data. First, the study population was divided into two groups based on trauma mechanism (high- or low-energy). Descriptive statistics were used to describe the study population, and we used independent- samples t-test for continuous variables and chi-square test for noncontinuous variables to assess differences between the groups in terms patient and fracture characteristics. For sub-analysis, the study population after high- or low-energy trauma was divided in groups based on the patients age (≤ 50 and ≥ 51), because from the age of 50 the prevalence of osteoporosis is increasing [24, 25]. We used independent- samples t-test for continuous variables and chi-square test for noncontinuous variables to assess differences in fracture characteristics between young and older aged patients. Secondly, we used linear regression to analyze the relationship between the trauma mechanism and KOOS score. The model was adjusted for five potential confounders, including age, sex, BMI, smoking and diabetes. A Fisher’s exact test was used for the noncontinuous variables to assess differences in complications and conversion to TKA between patients after high- and low-energy trauma. Significance level was set at p < 0.05.

For the nonresponse analysis, we used an independent- samples t-test for continuous variables and a chi-square test for noncontinuous variables. The nonresponse analysis showed that responders were slightly older than nonresponders (53 ± 15 years versus 51 ± 17 years; p =  < 0.001). There was not much difference in sex between responders and nonresponders (female 68% [725 of 1066] versus female 58% [518 of 890]; p =  < 0.001). Due to data protection and privacy legislation as well as medical ethical reasons, knee radiographs and CT images of the nonresponders were only available in the initiating center and one affiliated center (n = 545). There was no difference in fracture classification between responders and nonresponders (Schatzker I [11% versus 12%], Schatzker II [31% versus 27%], Schatzker III [17% versus 18%], Schatzker IV [13% versus 16%], Schatzker V [8% versus 6%], Schatzker VI [20% versus 21%] p = 0.778).