Conducting this meta-analysis, the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) provided by Moher et al. [39] were followed and adapted to the current data properties.

A systematic literature search in the databases PubMed and Scopus was conducted from their inception to December 2016 to capture all pertinent articles investigating postural stability in ACL patients. The search strategy included the key terms: (postural control OR postural balance OR vestibular OR posture OR balance) AND (“ACL” OR “anterior cruciate ligament”). Since there is no universal definition of postural control and balance, this search strategy comprised a widespread spectrum in order to cover all potentially relevant studies. Search limitations were imposed to full access articles in English language and studies investigating human species. Additionally, reference lists of articles found were inspected, and relevant review articles [20, 28, 41] were scrutinized to identify further evidence.

Inclusion Criteria

The inclusion criteria for this meta-analysis were as follows: (1) controlled trials of post-injury postural stability in patients after ACL injury, (2) static postural stability tests in single-leg stance utilizing force or pressure plates, (3) subjects of all ages and sexes without any neurological or psychological diseases or history of lower limb musculoskeletal surgery, and (4) investigations reporting at least one primary outcome measure of static postural stability based on the CoP. Due to standardization demands, the testing protocol was limited to ordinary joint loading tasks that allow for functional assessment of the ACL-injured and ACL-non-injured limbs. Therefore, any papers not meeting these criteria, solely investigating dynamic tasks, double leg stance or eyes closed, just as effect or interventional studies were not eligible for inclusion.

Study Selection

Based on the predetermined inclusion criteria, records were identified and screened through database searching. Records of both databases were then merged, and duplicates were removed using Mendeley Desktop (v.1.17, Mendeley Ltd., London, UK). Two independent reviewers (TL and LP) conducted the study selection. If the included studies did not report means, standard deviations, or F values, the corresponding authors were contacted. In two of three cases, the authors responded [18, 57] and the respective study was included, while the remainder [21] was excluded from this meta-analysis.

To assess methodological quality of the studies, a modified Quality Assessment Tool for Observational Cohort and Cross-Sectional Studies [58] was independently applied to each included article in order to assess the internal validity and risk of selection-, information-, or measurement bias. The tool is composed of 14 criteria inspecting the objectives, population, participation, exposures, and outcomes of the particular investigations. In case of this meta-analysis, three criteria (3, 10, 13) were not applicable in relation to the research objectives pursued and therefore excepted from the assessment. The remaining 11 criteria were evaluated on the scale “yes,” “no,” “not applicable,” “not reported,” or “cannot determine,” with any response other than “yes” posing a certain risk of bias. A total score was generated counting all “yes” responses for each study. On the basis of recommendations provided by Aderem and Louw [2], total scores below 50% were considered as “poor,” total scores between 50 and 75% as “fair,” and total scores above 75% as “good” methodological quality. Additionally, funnel plots of the effect size and the standard error were generated for the included trials in order to assess publication bias.

Outcome Measures

The outcome measures considered in this review correspond to basic descriptions of CoP trajectories with regard to magnitude, direction, and velocity of the displacement [45]:

1) The sway amplitude is the mean of all data points collected for one or more trials. 2) Path line length further represents the total distance traveled by the CoP over the course of a trial. 3) Area of sway describes the total area covered by the CoP in both anteroposterior (AP) and mediolateral (ML) direction. 4) CoP mean velocity is determined as the total distance traveled by the CoP divided by time. 5) The maximum speed is calculated as the peak velocity reached by CoP dislocation across trials.

Data Extraction

For each study meeting the inclusion criteria, descriptive information related to the country of origin, subject characteristics, sample size, time from injury/surgery to testing, the research protocol, and associated injuries were summarized using a customized Excel (Microsoft, Redmond Washington, USA) spreadsheet. Measures for these data were means and standard deviations. The primary outcome measures for the present meta-analysis were categorized to sway magnitude (sway amplitude, sway area, path length) and sway velocity (mean velocity, maximum speed) in total, AP, and ML direction. In two cases, the data for classified groups of functional recovery [57], just as males and females [53], were matched together by means, since no differentiation was intended for these subgroups of patients in the current meta-analysis. If repeated measures or different conditions were reported, the first or baseline measurement was considered exclusively.

Statistical Analysis

The main statistical analyses were executed for leg (injured, non-injured, matched) and direction (total, AP, ML) for the parameters sway magnitude and sway velocity. Based on sample size, means/F values, and standard deviation, the particular effect sizes were calculated as the standardized mean difference (SMD) for each CoP measure and study [36] in order to examine statistical differences between patients after ACL injury and healthy controls. The SMDs of all studies were then weighted with respect to the magnitude of their standard error (SMD wm ). Positive effect sizes indicate better postural stability in the control group or leg, while negative effect sizes favor the ACL group.

Using meta-analyses, eight hypotheses were tested for differences in ACL and healthy subjects with regard to sway magnitude, sway velocity, injured vs. matched leg, non-injured vs. matched leg, injured vs. non-injured leg, anteroposterior sway, and mediolateral sway. All comparisons were computed with a random-effects model using Review Manager (v.5.3.5, The Nordic Cochrane Centre, The Cochrane Collaboration, Copenhagen, DK) to calculate the overall standardized mean difference of the respective outcome measures. Further, the 95% confidence interval (CI) was computed for the individual and overall effect. Based on recommendations provided by Cohen [14], effect sizes were interpreted as follows: 0.00 to 0.49 indicate a small effect, 0.50 to 0.79 were considered a medium, and values greater than 0.80 indicate a large effect. Heterogeneity between trials was tested and interpreted using I 2 percentages. Hereof, the impact of potential heterogeneity on the results of the meta-analysis was estimated referring to suggestions from Higgins [26]: I 2 values from 30 to 50% indicate moderate heterogeneity, values greater than 50% display a substantial heterogeneity, and values of greater than 75% may be interpreted as considerable heterogeneity.