¹Beckett D, ²Cornejo Ochoa C, ³Davidson M
¹Western University , Lebanon, OR, United states; ²Western University, Lebenon, OR, United States; ³Advantage Orthopedics and Sports Medicine, Gresham, OR, United States

Introduction
The infrapatellar fat pad (IFP) is one of three extra synovial fat pads in the anterior knee that can cause pain when impinged due to its dense nerve supply (1). Diagnosing IFP impingement can be difficult, and a few testing modalities may be utilized to make a clinical diagnosis. In our patient’s case, the diagnosis of IFP impingement was made with infrapatellar edema shown in the MRI, a positive Hoffa’s test, and pain relief after intra-articular lidocaine injection. Lack of familiarity and low prevalence may result in cases remaining under-detected.

This case report presents bilateral IFP impingement diagnosis for a former baseball player who presented with bilateral knee pain for five years.

Diagnosis
After reviewing knee MRIs, a therapeutic injection was performed to confirm the diagnosis. A mixture of 4mL 1% lidocaine, 4mL 0.25% Marcaine, and 40 mg Depo-Medrol was used and injected superolaterally. Post injection, the patient experienced complete pain relief with no tenderness to palpation and a negative Hoffa’s test. This helped localize the etiology of the patients’ pain and led to the clinical decision to arthroscopically resect the IFP.

Discussion
The primary purpose of this case study was to report the effectiveness of using intra-articular joint injections as a diagnostic tool for IFP impingement and to guide the decision-making process of surgical intervention. The patient’s presentation consisting of a normal knee examination, except for a positive Hoffa’s test, with complete resolution of pain following intra-articular injection with local anesthetic ultimately led to the decision to proceed with resection of the IFP.

One study showed that while MRIs can be used to aid diagnose IFP impingement, they are not sensitive nor specific for IFP impingement (2). Patients in the study above also presented with sharp anterior knee pain, positive Hoffa’s tests, and pain relief with lidocaine injections. These findings suggest that clinicians consider IFP impingement as a differential when patients present with anterior knee pain, a positive Hoffa’s test, and relief after local anesthetic injection.

Conclusion
Diagnosing IFP impingement can be challenging due to similar clinical presentation to other conditions. Our patient’s bilateral chronic IFP impingement was diagnosed by physical exam, MRI, and intra- articular joint injection. Medial to lateral arthroscopic fat pad excision was ultimately chosen as the optimal treatment because of the chronicity of the patient’s condition and the failure of non-operative modalities to provide lasting relief. Intra-articular local anesthetic injections and Hoffa’s test are valuable tools that can be useful in diagnosing IFP impingement.

1. Gallagher, J., Tierney, P., Murray, P. et al. The infrapatellar fat pad: anatomy and clinical correlations. Knee Surg Sports Traumatol Arthrosc 13, 268–272 (2005). https://doi.org/10.1007/s00167-004-0592-7
2. Kim YM, Joo YB. Arthroscopic treatment of infrapatellar fat pad impingement between the patella and femoral trochlea: Comparison of the clinical outcomes of partial and subtotal resection. Knee Surg Relat Res. 2019;31(1):54-60. doi:10.5792/ksrr.18.026