1. David Beckett OMS-II – Western University of Health Sciences
2. Cesar Cornejo-Ochoa OMS-II – Western University of Health Sciences
3. Marc Davidson MD – Advantage Orthopedics and Sports Medicine

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.

Keywords: infrapatellar fat pad, impingement, diagnosis, injection

Article

Introduction
The infrapatellar fat pad (IFP) is one of three extra synovial fat pads in the anterior knee. It functions to stabilize the patella by occupying the space underneath and providing support (1). It is positioned posterior to the patellar tendon and anterior to the intercondylar notch of the femur and proximal end of the tibia. Pain can occur when impinged due to its dense nerve supply (2).

While IFP impingement is rare, two mechanisms commonly occur as described by Mathieu et al. The first is posterior impingement which is caused by the IFP entering the femorotibial space. The second is superolateral impingement caused by the IFP dislodging in between the lateral femoral condyle and the inferior portion of the patella (3). Patients will usually present to the clinic with anterior knee pain specifically located near the inferior portion of the patella, and hyperextended knees when standing (2).

Diagnosis of IFP impingement can be difficult and a few testing modalities may be utilized to make a clinical diagnosis. No best practices for the diagnosis of IFP were found during our review. However, one study diagnosed IFP impingement if the patient presented with infrapatellar knee pain and at least two of the following: a positive Hoffa’s test, an MRI indicating possible impingement without other abnormalities, or pain relief after injecting lidocaine into the affected area1. In our patient’s case, a clinical 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 a case of a former baseball player who presented with bilateral knee pain for the last five years.

Case Presentation
Patient is a 21-year-old male who presented to the clinic with intermittent anterior knee pain bilaterally. Patient began experiencing knee pain while playing baseball, which persisted despite discontinuing the sport. Physical therapy resulted in no symptom improvement. He complained of anterior knee pain while both knees were flexed when sitting or transitioning into a standing position.

Exam
Upon physical examination, the patient had 3 degrees of genu valgum and 5 degrees of hyperextension in the lateral plane of the knees bilaterally. No obvious lesions or joint effusions were noted. Patient admitted to tenderness over the patellar tendon and had retro patellar crepitus bilaterally. There was no tenderness around the tibial tubercle or quadriceps tendon. There was no plica tenderness. Hyperflexion, internal/external rotation, or medial/lateral patellar glide bilaterally were painful. Strength and ligament exams were normal bilaterally.

Hyperextension bounce test with fat pad compression was painful bilaterally. 

Imaging
MRI without contrast of left knee (Figure 1a): Mild degeneration of meniscus accompanied by mild effusion. No evidence of masses or ligamentum damage was present. Edema was noted in the inferior aspect of the patella and the suprapatellar fat pad. All other gross structures were unremarkable.

MRI without contrast of right knee (Figure 1b): No evidence of meniscus tear, masses, or ligamentum damage. Edema was noted in the superior portion of the IFP, suprapatellar fat pad, and the pre- femoral quadriceps fat pad. Evidence of edema was also present in the anterior portion of the patella subcutaneously and the proximal patellar tendon.

Diagnosis
After retrieving the right knee MRI, a therapeutic injection was performed to confirm the diagnosis. A mixture of 4mL 1% lidocaine, 4mL 0.25% marcaine, and 40 mg depomedrol was used and injected using a superolateral approach. A small portion of air was injected to ensure an intra-articular injection. Post injection, the patient experienced complete pain relief and denied any tenderness or pain to palpation or Hoffa’s test.

The same procedure was done with the left knee using only 1% lidocaine.

In conjunction with the MRIs, the post-injection resolution of pain helped localize the etiology of the patients pain and led to the decision to arthroscopically resect the IFP bilaterally.

Surgery
Each knee was addressed with diagnostic arthroscopy to rule out any missed pathology followed by resection of the ligamentum mucosum, IFP, and small medial plica at a separate operative setting. Both knees had no structural pathology on arthroscopic exam. The right knee was operated on first at the patient’s request (Figure 2). The left knee underwent the same procedures 2 months later (Figure 3).

Post-Op
Postoperatively the patient was treated with naproxen, icing, brief narcotic, and formal physical therapy.  Although weight-bearing was not restricted, crutch use was required for pain management for the first two weeks. After 2 months the patient’s preoperative symptoms had resolved, there were no complications, and he requested that his left knee be addressed in the same fashion. The left knee had a similar postoperative course with no complications. At 4 months post-operative, the patient reported complete resolution of his preoperative symptoms and no pain or restrictions during any activity. At 1 year follow up the patient reported durability of his complete symptom relief.

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 with complete resolution of pain following intra-articular injection with local anesthetic ultimately led to the decision to proceed with resection of the IFP. This was coupled with a positive Hoffa’s test, which was conducted by applying medial and lateral pressure to the inferior pole of the patella when the knee is flexed to 45 degrees and in extension with pain being the positive indicator. 

While MRIs can be used to aid in the diagnosis of IFP impingement, they are not sensitive nor specific for IFP impingement. A study by Kim and Joo found that they were unable to identify fat pad impingement in 27.6% of patients based on MRI alone, but after an exploratory arthroscopy, all of these patients were found to have a “tongue-like protrusion” of the IFP into the joint, which was resected (4). All of the Kim and Joo study patients complained of sharp anterior knee pain, had positive Hoffa’s test, and pain relief with lidocaine injection. These findings suggest that clinicians should take a rigorous approach when diagnosing IFP impingement and consider it as a differential in cases where there is anterior knee pain with a positive Hoffa’s test and relief after local anesthetic injection.

Considerations of utilizing diagnostic injections include selecting the appropriate injection approach, considering the patient’s BMI, and being aware of the risks associated with intra-articular injections. A lateral approach with the needle passing between the patella and femoral condyle is generally preferable since there is less soft tissue and superficial, but an anterior approach targeting the trochlear notch starting medial or lateral to the patellar tendon can be used for morbidly obese patients or patients with severe patellofemoral osteoarthritis (7). The most common complications of intra-articular injections are pain and bleeding at the injection site, while infections are exceedingly rare, cited to occur in less than 1 in 10,000 cases8. Ultrasound guidance can also be used to ensure intraarticular placement of the medication.

Finally, IFP impingement has been successfully treated both non-operatively and operatively. Non-operative treatment consists of rest, cold compressions, bracing, non-steroidal anti-inflammatory drugs, physical therapy, and corticosteroid injections. Operative treatment consists of arthroscopy with partial or subtotal IFP resection. It should be noted that partial resection helps maintain the natural biomechanics of the patella, and it was found to be just as effective in relieving pain as a subtotal resection (4). A meta-analysis of 15 studies with 167 patients conducted by Genin et al. found that 118 patients ultimately underwent surgical resection of the IFP after failing non-operative treatment, but in the end, all patients had improvement or complete resolution of symptoms (9). These results show that IFP impingement is a treatable condition with a good prognosis.

Conclusion
Diagnosis of IFP impingement can be challenging because its presentation is similar to other conditions such as patellar tendinitis. In this case, the patient’s bilateral chronic fat pad 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 such as NSAIDs, physical therapy, and cortisone injections to provide lasting relief. While Intra-articular local anesthetic injections are a valuable tool that can be useful in diagnosing IFP impingement, more randomized controlled studies need to be conducted to assess its utility in practice.

References 

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. Zeng N, Yan ZP, Chen XY, Ni GX. Infrapatellar Fat Pad and Knee Osteoarthritis. Aging Dis. 2020;11(5):1317-1328. Published 2020 Oct 1. doi:10.14336/AD.2019.1116
3. Mathieu L, Chetouani M, Janku D, Vandenbussche E, Augereau B. Posttraumatic dislodgement of the infrapatellar fat pad: An unusual type of superolateral impingement. Orthopaedics & Traumatology: Surgery & Research. 2011;97(7):776-778. doi:10.1016/j.otsr.2011.05.009
4. 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
5. Sonn KA, Deckard ER, Aasar AR, Wolf LK, Meneghini RM. Utility and prognostic ability of a diagnostic injection before revision total knee arthroplasty. Journal of Arthroplasty. 2021;36(6):2116-2120. doi:10.1016/j.arth.2020.12.056
6. McFarland E, Bernard J, Dein E, Johnson A. Diagnostic injections about the shoulder. J Am Acad Orthop Surg. 2017;25(12):799-807. doi:10.5435/JAAOS-D-16-00076
7. Rastogi AK, Davis KW, Ross A, Rosas HG. Fundamentals of joint injection. American Journal of Roentgenology. 2016;207(3):484-494. doi:10.2214/AJR.16.16243
8. Testa G, Giardina SMC, Culmone A, et al. Intra-articular injections in knee osteoarthritis: A review of literature. J Funct Morphol Kinesiol. 2021;6(1). doi:10.3390/jfmk6010015
9. Genin J, Faour M, Ramkumar PN, et al. Infrapatellar fat pad impingement: A systematic review. J Knee Surg. 2017;30(7):639-646. doi:10.1055/s-0037-1604447
   

Required Disclosures and Declaration

Copyright Information: No Copyright Information Added

IRB Approval Information: Not applicable

Disclosure Information: No known conflicts of interest