_Applied_  
_sciences_  
_Review_  
**Why Determining the Native Length Change Pattern of Medial Patellofemoral Ligament Is Still a Challenge: State-of-the-Art Review of Potential Sources of Heterogeneity within Studies Evaluating Isometry of MPFL**  
**[PERSON] \({}^{1,\star}\)\({}^{\text{(c)}}\), [PERSON] \({}^{1}\), [PERSON] \({}^{\text{(c)}}\), [PERSON] \({}^{1}\), [PERSON] \({}^{3}\), [PERSON] \({}^{4}\), [PERSON] \({}^{5}\) and [PERSON] \({}^{1}\)**  
\({}^{1}\) Orthopedic and Trauma Department, Veter's Memorial Teaching Hospital in Lodz, Medical University of Lodz, 90-549 Lodz, Poland; [EMAIL_ADDRESS] (K.K.); [EMAIL_ADDRESS] (M.K.); [EMAIL_ADDRESS] (M.E.D.)  
\({}^{2}\) Department of Normal and Clinical Anatomy, Chair of Anatomy and Histology, Medical University of Lodz, 90-752 Lodz, Poland; [EMAIL_ADDRESS] \({}^{3}\) Clinic of Orthopedics and Pediatric Orthopedics, Medical University of Lodz, 92-213 Lodz, Poland; [EMAIL_ADDRESS] \({}^{4}\) Department of Physical Therapy, College of Rehabilitation Sciences, University of Manitoba, Winnipeg, MB R3T 2N2, Canada; [EMAIL_ADDRESS] \({}^{5}\) Artromedical Orthopaedic Clinic, 97-400 Belchatovo, Poland; [EMAIL_ADDRESS] \({}^{*}\) Correspondence: [EMAIL_ADDRESS]; Tel.: +48-605-197-981  
**Abstract: Background**: In the literature there are divergent results as to the native MPFL length change pattern. The reason for such divergent results may be the heterogeneity of design of studies analyzing MPFL isometry. The hypothesis of this review was that studies assessing MPFL length change pattern are highly heterogenous. The aim was to present a state-of-the-art review of sources of this heterogeneity. **Materials and Methods**: A total of 816 records were identified through the initial search of MEDLINE and Scopus databases. After eligibility assessment, 10 original articles and five reviews were included. In the included studies, the following 15 potential sources of heterogeneity were assessed: number of patients/cadavers, age, males to females ratio (demographics), identification of measured fibers, measurement method, measurement precision, quadriceps muscle activity, lithoitial band activity, hamstrings activity (study design), patellar height, trochlear or patellar dysplasia, femoral anstevers, mechanical axis of the limb, tibial tuberlec-trochlear groove distance, and condydlar anteroposterior dimensions (morphology). Each variable was graded in every included article with 1 point if reported precisely and not introducing bias; or with 0 points if reported not precisely, introducing bias, or not reported at all. **Results**: Within original articles, the highest achieved score was 10 out of 15 possible points with mean score of 6.7, SD = 2.37, and minimum score of just 3 out of 15 points. In the demographics section, mean score was 2.4, SD = 0.8 (80% of maximum possible score of 3); in the study design section it was 3.1, SD = 1.87 (52% of maximum possible score of 6); and in the morphology section it was 1.5, SD = 1.43 (25% of maximum possible score of 6). **Conclusions**: There is high heterogeneity and incomplete reporting of potential sources of bias in studies assessing native MPFL length change pattern. Future investigators should be aware of the presented factors and their potential impact on MPFL isometry. All methodologic factors should be meticulously reported. Detailed description of demographic data is already a standard; however, authors should more extensively report variables concerning study design and morphology of patients' patellofemoral joint. Furthermore, future studies should try to meticulously simulate the real-life working environment of MPFL and ensure usage of proper measurement methods.  
**Keywords:** medical patellofemoral ligament; length change pattern; isometry; heterogeneity; studies design; anisometry