## 4 Study Design as the Source of Heterogeneity  
### Identification of Measured Fibers and Measurement Methods  
Proper identification of measured MPFL fibers connecting femoral and patellar insertions is of great importance when measuring MPFL isometry. However, different authors adopted different measures to properly mark them. For example, femoral attachment of MPFL was reported to be identified either by using Schottle's point [25; 27] or midway between adductor tubercle and medial epicondyle [14; 23]. On the patellar attachment, authors reported identifying different insertions such as \"at 20%, 40%, and 60% of the patellar length from the proximal tip\" [22] or \"20% (point 20), 30% (point 30), 40% (point 40), 50% (point 50), and 60% (point 60) from the superior pole of the patella\" [25]. Furthermore, recent literature suggests that some of the measurement methods used in the previous studies assessing MPFL length change pattern may be inaccurate. [PERSON] et al. and [PERSON] \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & **Authors** & **Year** & **Patients/Cadavers** & _n of Subjects_ & **Age, Years** & **Sex** \\ \hline \multirow{8}{*}{**Cadavers**} & [PERSON] et al. & 2004 & Cadavers & 11 & “Elderly” & 7M; 4F \\  & [PERSON] et al. & 2009 & Cadavers & 12 & 78–87 at the time of death & 8M; 4F \\  & [PERSON] et al. & 2010 & Patients & 20 & Mean 32 (range, 26–38) & 10M; 10F \\  & [PERSON] et al. & 2013 & Cadavers & 8 & Mean 73.5 (range, 46–88) & 5M; 3F \\  & [PERSON] et al. & 2014 & Patients & 10 & Mean 35 (range, 25–42) & 10M; 0F \\  & [PERSON] et al. & 2015 & Patients & 11 & Mean 32.0 + 3.9 (range, 27–39) & 11M; 0F \\  & [PERSON] et al. & 2019 & Cadavers & 8 & Mean 85.25 & Not reported \\  & [PERSON] et al. & 2019 & Patients & 15 & Mean 25.1 + 5.2 & 9M; 6F \\  & [PERSON] et al. & 2020 & Cadavers & 8 & Below 65 & Not reported \\  & [PERSON] et al. & 2020 & Cadavers & 8 & Below 65 & Not reported \\  & [PERSON] et al. & 2003 & - & Not investigated & Not investigated \\  & [PERSON] et al. & 2006 & - & Not investigated & Not investigated \\ \hline \multirow{8}{*}{**Cadavers**} & [PERSON] et al. & 2013 & - & Not investigated & Not investigated & Not investigated \\  & [PERSON] et al. & 2019 & - & Not investigated & Not investigated & Not investigated \\ \cline{1-1}  & [PERSON] et al. & 2020 & - & Investigated & Investigated & Not investigated \\ \hline \hline \end{tabular} M—males; F—females.  
\end{table} 
Table 1: Demographics within included articles. et al. described that MPFL fibers do not run straight between insertions but, according to the degree of knee flexion, they \"travel\" through positions named A, B, to C from extension to flexion [7; 30]. The type A was present at 0\({}^{\circ}\) and 30\({}^{\circ}\) of flexion, meaning \"convex course toward the side of the patellofemoral joint surface\"; type B was present in the majority of participants at 60\({}^{\circ}\) of flexion, meaning straight fibers; type C was present in 90\({}^{\circ}\) and 120\({}^{\circ}\) of flexion, meaning \"a convex course toward the extra-articular side\" [7; 30]. If these reports are accurate, measurements made by calipers or navigation systems assessing the distance in a straight line can be biased both in initial and final degrees of flexion [21; 22]. In addition, measurements made by sutures attached to linear variable displacement transducers may be biased if they were not threaded along MPFL fibers [31; 32]. Furthermore, MPFL is a 3D structure; however, in some studies it was measured on a single 2D MRI slice [7; 30]. From 10 original articles included in this review, nine studies identified multiple fibers of MPFL for assessment of length change [7; 14; 21; 22; 23; 24; 25; 26; 27; 28] and one study assessed only a single fiber [23]. Various femoral and patellar attachments were analyzed, as specified in Table 2. Nine studies performed measurements in 3D [14; 21; 22; 23; 24; 25; 26; 27; 28]; however, only in four was MPFL measured with respect to its convexity [7; 14; 24; 25].