This was a prospective study, conducted according to the REporting of studies Conducted using Observational Routinely-collected health Data (RECORD) statement.30 Data for the present study were extracted on 22 November 2022 from a rehabilitation-specific register: Project ACL, which started in 2014 and is located in Gothenburg, Sweden. Project ACL contains data on patients with an ACL injury and has previously been described in detail.31,32 Patients registered in Project ACL are evaluated according to a predefined follow-up schedule at 10 weeks, four, eight, 12 and 18 months, two years and every five years after baseline, i.e, ACL injury or reconstruction. Patients are evaluated with both validated muscle function tests and PROs. Informed consent was obtained from patients at time for registration in Project ACL. All methods were carried out in accordance with the declaration of Helsinki. Ethical approval was obtained from the Regional Ethical Review Board in Gothenburg, Sweden (registration numbers: 265–13, T023–17).
Muscle function tests
The tests of muscle function included strength tests for the quadriceps and hamstring muscle groups, as well as hop performance tests. Muscle strength tests for knee extension and flexion were performed in an isokinetic dynamometer; Biodex System 4 (Biodex Medical Systems, Shirley, New York, USA)33 at an angular velocity of 90°/second. The Biodex dynamometer has good instrumental validity34 (intraclass correlation (ICC) = 0.99-1.00) and test-retest reliability35 (ICC = 0.95) when measuring strength in knee extension and knee flexion reflecting quadriceps and hamstring strength. Prior to the tests of muscle function, patients performed a standardised warm-up of 10 minutes on a stationary bike. Following the standardised warm-up, the Beighton Score5 was assessed. Knee extension was measured from 90 − 0° of knee flexion and knee flexion was measured from 0–90° of knee flexion. Patients were tested in a seated position with straps around the torso and the leg that was being tested. The injured leg was tested first. Prior to a maximum muscle strength test, patients familiarised themselves with the Biodex, starting with 10 repetitions at 50% of maximum effort (ME), 10 repetitions at 75% of ME and one repetition at 90% of ME. The maximum test was then performed through three separate one-ME repetition trials with a 30-second rest in between. The highest measured peak torque (Nm) was registered in the database and used for the analysis of knee extension and flexion respectively.
Following the strength tests, hop tests were performed; they consisted of a vertical hop, hop for distance and the 30-second side-hop test. All the hop tests are valid and reliable for measuring hop performance in patients with an ACL injury.36 Hop tests were first performed on the injured leg and patients were instructed to hold their hands behind their back throughout the tests. For the vertical hop, flight time was converted into centimetres (cm) using the Muscle lab, Ergotest Technology, Oslo, Norway. In the hop for distance, the distance in cm from the toe at take-off to the heel at landing was recorded. For the vertical hop and the hop for distance, two to three warm-up hops were allowed before maximum performance. Three maximum hops were performed after warm-up and the best recorded result in cm was registered in Project ACL’s database and used for analysis. During the 30-second side-hop test, patients performed as many hops as possible for 30 seconds over two lines 40 cm apart. The total number of hops was counted, with a one-hop deduction for every hop not completely over one of the lines, and was then registered in Project ACL’s database and used for analysis. Ten warm-up hops were allowed before the maximum effort test.
The results of muscle function tests were reported and analysed with the LSI, which is calculated as followed: \(\left(\frac{Result for injured leg}{Result for non-injured leg}\right)*100\), presented as a percentage.
Patient-reported outcomes
The PROs used in this study comprised the Tegner Activity Scale (Tegner), the Knee Self-Efficacy Scale (K-SES), the Knee injury and Osteoarthritis Outcome Score (KOOS) and, from the eight-month Project ACL follow-up onwards, the ACL Return to Sport after Injury (ACL-RSI) scale. All the PROs used in this study are self-reported questionnaires.
Alongside the standardised PROs, as of March 2018, Project ACL implemented an additional question with regard to whether the patients had returned to their pre-injury activity and, if the answer was yes, how long ago, measured in months. The answer to this specific question was used to calculate the time to return to sport measured in months.
The Tegner aims to classify the patient’s level of knee-demanding activity from 0 (lowest knee-demanding activity) to 10 (highest knee-demanding activity).37 The Tegner used in this study was a modified version,31 in which the value “0”, representing “sick leave or disability pension because of knee problems”, was removed. In this study, RTS was defined as a return to level ≥ 6 on the Tegner,37 equalling activities like badminton, tennis, skiing or floorball, as well as active participation in sports such as baseball, snowboarding and hurdling. At Tegner level 6, no work activities are listed and the patients answering ≥ 6 are therefore expected to perform sports. Additionally, RTP was defined as reporting Tegner equal to or above pre-injury. Test-retest reliability: ICC = 0.8 for patients with an ACL injury and ACL reconstruction.38
The K-SES aims to evaluate knee-related self-efficacy in patients with an ACL injury and consists of 18 items. The scale comprises two subscales, present and future,39 and the present was used in this study. Each item is scored on a 11-point Likert scale from 0 (poor self-efficacy) to 10 (strong self-efficacy). Only the K-SES present subscale was analysed in this study. The K-SES has sufficient construct validity tested as structural validity, hypothesis testing and cross-cultural adaptation.39 Test-retest reliability: ICC = 0.92 and internal consistency: Cronbach’s α = 0.81–0.96.39
The KOOS aims to evaluate subjective knee function divided into five subscales: pain, symptoms, activity of daily living, function in sports and recreation and quality of life (QoL).40 Patients answer the questions with respect to the previous week. Each question is scored on a five-point Likert scale from 0 (maximum negative response) to 4 (maximum positive response). The answers are recalculated to produce a normalised score ranging from 0 (severe knee-related symptoms/QoL) to 100 (no knee-related symptoms/QoL).40 The KOOS has test-retest reliability: ICC = 0.78–0.9740 and internal consistency: Cronbach’s α = 0.70-0.9541 and only the function on the sports and recreation subscale exhibits acceptable construct validity tested as > 75% confirmation of a predefined hypothesis.42 The KOOS has not been validated for evaluation after an ACL injury, but it is often used for evaluation after ACL reconstruction43 and was therefore included in this study. As the items on the KOOS subscale activity of daily living are not aimed at sports participation, the subscale was not analysed in this present study.
The ACL-RSI aims to assess emotions, confidence and risk appraisal in relation to RTS.44 The 12-item version was used in this study.45 The items are scored from 1 (lowest emotion, confidence and risk appraisal in relation to RTS) to 10 (highest emotion, confidence and risk appraisal in relation to RTS).44,46 The score is presented as a normalised score from 10 (lowest emotion, confidence and risk appraisal in relation to RTS) to 100 (highest emotion, confidence and risk appraisal in relation to RTS). The ACL-RSI has internal consistency: Cronbach’s α = 0.95, relevant face validity tested in a discussion with experts and patients, construct validity tested as hypothesis testing44 and the ability to predict RTS is fair to good.44,47
Patients
Patients registered in Project ACL who were between 16–50 years of age at the time of ACL reconstruction, who had suffered an ACL injury treated with reconstruction and had follow-up data on one of Project ACL’s follow-ups from four months to two years were eligible for inclusion. Patients who did not have data registered for GJH, Tegner, or had suffered more than one ACL injury were excluded from this study.
Data from all the follow-ups in Project ACL between four months after ACL reconstruction and the two-year follow-up were extracted for analysis, containing demographics such as sex, age, height, weight, days between injury and surgery, time to RTS, details of surgical treatment (graft choice), presence of knee hyperextension (HE), GJH, tests of muscle function and patient-reported outcomes.
Definition of study groups
In the present study, patients were divided into two groups: patients with a Beighton score6 of ≥ 5 comprised the GJH group, while patients with a Beighton score6 of ≤ 4 comprised the non-GJH group (Table 1).
Outcomes
The primary outcome of this study was the proportion of patients who RTS (defined as Tegner ≥6), the comparison of the muscle function tests and the K-SES,39 KOOS40 and ACL-RSI44,45 at the time of RTS and the time to RTS in each of the study groups: the GJH group and the non-GJH group.
The secondary outcome was the proportion of patients who RTP (defined as Tegner equal to or above pre-injury), the comparison of the muscle function tests and the K-SES,39 KOOS40 and ACL-RSI44,45 at the time of RTP and the time to RTP in each of the study groups: the GJH group and the non-GJH group.
Statistical analyses
Statistical analyses were performed with the Statistical Product and Service Solutions (IBM Corp. Released 2017. IBM SPSS Statistics for Windows, Version 25.0. Armonk, NY: IBM Corp.). The results were presented stratified by group. The difference in the proportion of patients who achieved RTS and RTP was analysed using Fisher’s exact test across all follow-ups from four months to two years. Muscle function tests, K-SES, KOOS and ACL-RSI were analysed using the independent t-test, while Tegner was analysed using the Mann-Whitney U test at the time of RTS and RTP respectively. The group difference in terms of time to RTS and time to RTP, measured in months, was analysed using the independent t-test. Demographic data were analysed using an independent t-test for continuous variables and Fisher’s exact test for categorical variables for each of the primary and secondary outcomes. To evaluate the significance of differences, Cohen’s d was calculated and the following reference values were used: 0.20 = small, 0.50 = medium and 0.80 = large.48 The significance level was set at 95%. In the event of an analysis showing a statistically significant result, confidence intervals (CI) and Cohen’s d were also reported.