## 8 Considerations for Future Research  
We hope that the \"to-report list\" of potential sources of bias presented in this review will be of use to the future investigators interested in MPFL length change pattern. Controlling each of these factors (and potentially some more that were not identified) may be hardly feasible. However, it may not be possible to determine what the native length change pattern of MPFL is without controlling this heterogeneity in the future research. Therefore, below we provide our conclusions about how to design an experiment to properly evaluate the MPFL isometry.  
Up to date, it is not clear which sources of heterogeneity may influence the measurements the most; however, trying to simulate the real-life working environment of the MPFL seems to be the first crucial issue. We lobby for patients-based studies with quadriceps, ITB, and hamstrings contraction, preferably in real-life situations such as squatting or lungs. The second crucial issue, in our opinion, is proper identification of MPFL femoral and patellar insertions. Ideally, this identification should be made two times by two or more independent researchers with experience in MPFL dissection and imaging. Afterwards, intra- and interrater agreement in identification of insertions should be calculated. The third crucial issue, in our opinion, is measurement methods and their precision. 3D measurements taking into account the convexity of MPFL with high and meticulously reported precision (i.e., 3D CT or MRI models) seem mandatory for proper assessment of MPFL isometry. The fourth consideration is that \"apples should not be compared to oranges\"--patients of different genders and from different age groups should be assessed separately. The fifth point is to exclude participants with variations in patellofemoral joint anatomy (i.e., patella alta, highly valgus knee, dysplastic knees) or to perform a subgroup analysis. Some anatomical variations (patellar height, trochlear or patellar dysplasia, TT-TG distance and femoral conyday dimensions) are relatively easy to assess on knee CT or MRI which are performed either way to evaluate MPFL isometry. However, precise assessment of mechanical axis of the limb or femoral anteversion requires performing additional imaging studies. We acknowledge this as a practical problem. Therefore, properly powered and designed study analyzing whether mechanical axis of the limb and femoral anteversion has an impact on MPFL isometry is needed. The sixth issue is that ideally MPFL length should be measured and reported in 10-degree or, even better, 5-degree intervals. Such a high \"resolution\" of available MPFL lengths in given knee flexion angles would prevent the inability to compare the results with other studies. However, we are aware that achieving such a detailed \"map\" of MPFL length changes according to the flexion angle can only be achieved with the use of a 3D model. We are also aware that 3D modelling is always another potential source of bias. Therefore, as an alternative approach, we propose that before designing the study one should review what flexion angles were analyzed in the studies that one wants to compare the results with. The last advice from the authors of this review is that future researchers should meticulously report methodology of their studies, namely demographics, study design, and morphology of participants' patellofemoral joint. As stated in the limitations section, raw datasheets containing the data should be made available online by the authors. Such availability would potentially allow future authors to perform a meta-analysis with subgroup analysis of the impact of given variables, i.e., by the means of linear regression model or other statistical tools.