Diagnostic Validity Ofclinical Tests for Infrapatellar Fat Pad Impingement Syndrome

Study Protocol Description The ETICHOFFA study is a clinical research project aiming to improve the diagnosis of infrapatellar fat pad (IFP) impingement syndrome, a common but under-recognized source of anterior knee pain. Although magnetic resonance imaging (MRI) remains the diagnostic gold standard, clinical testing strategies for IFP involvement are poorly standardized and lack validated diagnostic performance.

This observational study has two main objectives: (1) to assess the diagnostic accuracy of a cluster of standardized clinical provocation tests for IFP impingement compared to MRI findings, and (2) to identify clinical and subjective characteristics associated with IFP involvement confirmed by imaging.

The sample size of 50 participants was determined by an a priori power calculation. The primary hypothesis is that a composite score based on five clinical provocation tests will yield an area under the ROC curve (AUC) of at least 0.80, a threshold commonly considered indicative of good diagnostic discrimination. The null hypothesis is set at an AUC of 0.50, representing no discriminative ability. Using a two-sided alpha level of 0.05 and a statistical power of 80% (1-β), and assuming a prevalence of positive MRI findings of approximately 33% (i.e., a 2:1 ratio of test negatives to positives), the number of positive cases required was estimated at 11, corresponding to a total of 33 participants (11 positives and 22 negatives). Accounting for a 10% anticipated loss or exclusion rate, the total sample size was increased to 37 participants. However, this calculation is based on idealized assumptions and does not fully capture the uncertainties inherent to clinical research under real-world conditions. Variability in actual IFP prevalence, incomplete or uninterpretable MRI data, and interindividual variability in clinical test responses may all affect the study's effective statistical power. Therefore, a sample size of 50 participants was ultimately planned to ensure robust analysis and compensate for such contingencies.

The study will proceed in the following steps:

Step 1: Patient Inclusion Patients will be eligible if they are aged between 18 and 70 years and experience anterior knee pain with an intensity of ≥ 3/10 on a numerical pain scale on most days over the past month.

Certain medical conditions will be exclusion criteria, including: recent corticosteroid injections (within 12 months), hip or back surgery, specific neurological conditions, and any contraindications to MRI.

Prior to enrollment, each patient will receive detailed information and have the opportunity to ask any questions. Informed written consent will be obtained.

Step 2: Clinical Evaluation Each participant will complete a secured online questionnaire (GDPR-compliant platform) to record symptoms, painful areas (via bodychart), aggravating and relieving factors, and pain intensity (via visual analogue scale [VAS]/100) at rest, during physical activity, and in everyday situations. Three validated functional questionnaires (AKPS, KOOS-PF and Tampa scale Kinesiophobia) will also be completed to assess the impact of pain on daily activities and quality of life.

The clinical examination will be performed by a team of three physiotherapists and three physical medicine and rehabilitation (PM\&R) physicians, all trained across two standardized theoretical sessions. These sessions will cover both the study procedures and the standardized execution and interpretation of the clinical tests. All examiners will be blinded to the MRI results and to participants' self-reported data in order to minimize assessment bias. A standardized clinical examination will be conducted, including measurement of infrapatellar skin temperature using an infrared thermometer, assessment of knee swelling via the stroke test, passive knee range of motion (flexion and extension), and a set of five clinical tests targeting IFP. These include provocative palpation, passive extension and flexion tests, Hoffa's test, and IFP gliding test (a mobility assessment scored on a three-level ordinal scale: hypomobile, normal, or hypermobile). All tests will be performed in randomized order to minimize examiner bias and will be considered positive if they reproduce the patient's typical pain with an intensity of at least 2/10.

Step 3: Knee MRI A knee MRI must have been performed within two weeks before or after the clinical evaluation.

The MRI will follow a standardized protocol including T1 and T2 sequences, allowing the detection of Hoffa's fat pad abnormalities (inflammation, fibrosis, hypertrophy).

Interpretations will be carried out by an independent radiologist blinded to the clinical findings to ensure objective assessment.

Step 4: Data Processing and Analysis The clinical test results will be compared to MRI findings. Following data collection, a random sample of 33 participants will be selected a posteriori for the primary diagnostic accuracy analysis. This post hoc randomization is intended to reduce selection bias, overfitting, and spectrum bias, by ensuring that the analysis is not influenced by knowledge of the final diagnostic outcome or distribution of positive and negative cases. Diagnostic performance (both individual and combined tests) will be evaluated through: Sensitivity, Specificity, Predictive values, Likelihood ratios, Overall discriminative ability and post-test probability estimates using Fagan nomograms.Recognized statistical methods (notably SPSS software) will be used, and all analyses will be conducted blinded to group assignments.

All collected information will remain strictly confidential and anonymized. Data will be coded using a unique identifier (e.g., the first three letters of the last name plus date of birth) and stored in a secured Excel file accessible only to the research team.

The study's total duration is estimated at 12 months, with a 6-month inclusion period. Participation involves a single session, during a regular consultation with a physiotherapist or physician. No particular restrictions will apply regarding participation in other studies in parallel.

The ETICHOFFA study is expected to improve the clinical diagnostic pathway for anterior knee pain by validating simple clinical tests to detect infrapatellar fat pad impingement in routine practice.