Introduction

The knee is a complex joint with movements in multiple planes and along multiple axes. Many anatomical structures cause complex motion in the knee. The peripatellar fat pads are one of the most significant soft tissues of the knee. The synovium is separated from the joint capsule by fat pads (1). Fat pads consist of deformable fat and fibrous tissues. Fat pads adapt during movement in the knee and protect the joint from stress by displacing it. It increases the synovial surface and acts as a lubricant (2,3). The suprapatellar fat pad, perifemoral fat pad, and infrapatellar (Hoffa fat pad) fat pads are all parts of the peripatellar fat pad (2). The suprapatellar fat pad is triangular in the anterior knee. It is the smallest of the fat pads on the front knee. On average, the suprapatellar fat pad is 7-8 mm thick (4). It occupies the anatomical space between the quadriceps tendon insertion and the posterior superior portion of the quadriceps tendon.

Trauma, inflammation, tumors, infection, and congenital anatomical abnormalities can cause soft tissue injuries. Impingement syndrome occurs due to 2 main reasons in lower extremity pathologies. The first are bone deformities, the second are soft tissue abnormalities, or combinations thereof. Magnetic resonance imaging (MRI) is a suitable imaging method for evaluating bone and soft tissue components in the evaluation of impingement syndrome. This study aimed to characterize the MRI findings of suprapatellar fat-pad impingement syndrome, define its prevalence and pattern, and investigate the correlation between its clinical findings and MRI findings.

Methods

Between December 2010 and December 2015, 5,700 patients’ knee MRIs were reviewed retrospectively. All knee MRIs were performed with 1.5 Tesla and 3 Tesla systems (Magnetom Symphony and Skyra; Siemens Healthineers, Germany) equipped with a knee coil with 15 channels. Supine position according to standard knee MRI protocol 10-15º flexion and 15º external rotation were also applied. The radiology information system was evaluated with PACS (Evorad Research PACS). The knee MRI protocol included PD-weighted sequences (sagittal, coronal, and axial) with fat-saturation and T1-weighted coronal and T2-weighted fat-sat sagittal sequences.

Suprapatellar fat pad impingement was detected in 304 patients. The age, gender, physical activity history (whether they participated in sports regularly or not), trauma, tumor, and arthroscopic surgery history were all investigated. The clinical symptoms were divided into three categories: anterior knee pain, meniscus pain (pressing and sticking sensations), and non-specific pain. Chondromalacia (types, if any), patellofemoral joint relationship (normal, lateral subluxation, lateral tilt), and other associated findings [intra-articular fluid, synovitis, medial collateral ligament (MCL) tear, quadriceps femoris tendinitis, patellar tendinitis, medial or lateral meniscus tear, anterior cruciate ligament (ACL) tear, medial meniscus degeneration, Hoffa’s edema, Baker’s cyst, soft-tissue edema] were defined.

The patients were evaluated together by two radiologists (8 years and 20 years of musculoskeletal MRI experience). The study was approved by the Acıbadem University Institutional Ethics Committee (approval number: 2022-11/38). In patients with suprapatellar impingement syndrome, Pearson’s correlations were used to assess the age-osteoarthritis-chondromalacia-patellofemoral joint relationship. This study was performed in accordance with the ethical standards as laid down in the 1964 Declaration of Helsinki.

Statistical Analysis

Statistical analysis was performed using the IBM SPSS Statistics 23.0 package program (SPSS Corp; Armonk, NY, USA). The correlation between age, osteoarthritis, chondromalacia, and the patellofemoral joint were analyzed using Pearson’s correlations. P<0.05 was considered statistically significant.

Results

Suprapatellar impingement was identified on MRI in 304 patients (Table 1). Two hundred ten (69%) patients were visualized in a 1.5 Tesla and 94 (31%) patients in a 3 Tesla MRI device. In patients with suprapatellar fat pad impingement, there was a positive correlation between age and the presence of osteoarthritis (r=0.4660, p<0.05), between the presence of chondromalacia and its grade of chondromalacia (r=0.5198, p<0.05), and between the presence of lateral subluxation and lateral tilt instead of the normal patellofemoral relationship (r=0.3171, p<0.05). No significant difference was observed between 1.5 Tesla and 3 Tesla devices.

The prevalence of suprapatellar fat pad impingement in our study group was 5.3%. One hundred fifty-eight (52%) patients were female, and 146 (48%) patients were male (Table 1). Anterior knee pain was present in 44 (14.5%) of the patients, while pain from meniscus degeneration and ACL rupture were reported in 16 (5.2%) of the patients (difficulty pressing, stuck feeling). Two hundred fourty four (80.3%) patients were admitted to the clinic with the complaint of non-specific pain (Table 2).

While 296 (97.3%) of 304 patients with suprapatellar impingement did not participate in sports regularly, 8 (2.7%) did. There were 23 (7.56%) patients with a history of trauma, 2 (0.66%) patients with a history of malignancy, and 10 (3.3%) patients with a history of arthroscopic surgery (Table 1). Osteoarthritis was observed in 65 (21.3%) patients (Table 2).

Chondromalacia patella (CP) was not found in 138 (45.4%) patients: grade 1 CP in 28 (9.2%) patients, grade 2 CP in 24 (7.9%) patients, grade 3 CP in 44 (14.5%) patients, and grade 4 CP in 70 (23%) patients. The patellofemoral joint relationship was observed as lateral subluxation in 60 (19.7%) patients, lateral tilt in 10 (3.3%) patients, and normal in 234 (77%) patients (Table 3). A contrast-enhanced knee MRI was performed in 6 patients. In all of the patients, contrast enhancement was observed.

Isolated suprapatellar impingement syndrome was observed in 27 (8.9%) of the patients, intra-articular fluid increase in 185 (60.9%), synovitis findings in 4 (1.3%), MCL tears in 17 (5.6%), quadriceps femoris tendinitis in 107 (35.2%), patellar tendinitis in 8 (2.6%), medial meniscus tear in 80 (26.3%), Baker’s cyst in 23 (7.6%), and soft-tissue edema in 30 (9.9%). Medial meniscus degeneration was detected in 51 (16.8%), Hoffa edema in 31 (10.2%), and ACL tear in 3 (1%) patients (Table 2).

Discussion

In this study, we aimed to evaluate and characterize the MRI findings of suprapatellar fat pad impingement syndrome, define its prevalence and pattern, and evaluate the correlation between its clinical findings and MRI findings. Non-specific knee pain is the most prevalent clinical finding in suprapatellar fat pad impingement syndrome, but anterior knee pain is also a significant clinical finding. In addition, no significant relationship was found between suprapatellar impingement syndrome and gender, sports activities, or trauma. The most common accompanying MRI findings in this study were increased intra-articular fluid and quadriceps femoris tendinitis.

In the study of Tsavalas and Karantanas (5), no significant relationship was established between anterior knee pain and suprapatellar impingement syndrome. This finding suggests that injury to the suprapatellar fat pad after repetitive microtrauma and overuse does not exceed the threshold for pain. Edema of the suprapatellar fat pad and anterior knee pain are controversial issues. Studies show that 12-14% of patients undergoing knee MRI have suprapatellar fat pad edema (5-7). Some studies found a relationship between suprapatellar fat pad volume and knee pain (7). However, some studies did not find a relationship between suprapatellar fat pad with pain (8). Suprapatellar impingement syndrome was found in 304 (5.3 %) of 5700 knee MRIs of patients in our study. The most common clinical finding with suprapatellar impingement syndrome was non-specific knee pain in 244 patients (80.3%). The second most common symptom was anterior knee pain in 44 (14.5%) patients. Anterior knee pain was found to be similar to previous studies. The association between suprapatellar impingement syndrome and knee pain requires more research.

In the study of Lapègue et al. (3) and Nouri et al. (9), repetitive compression, trauma, and instability of the patellofemoral joint are all known to cause inflammation and metaplasia of the fat pad. After metaplasia, fibrotic tissue is formed. This causes anterior knee pain (3,9). Publications are showing that edema-like changes in the suprapatellar fat pad seen on knee MRIs are rarely associated with anterior knee pain (10). In our study, 23 (7.56%) patients had a history of trauma, 2 (0.66%) patients had a history of tumors, and 10 (3.3%) patients had a history of arthroscopic surgery.

Suprapatellar fat pad edema is thought to result from repetitive friction against the trochlea during knee flexion (7,11). The terms suprapatellar fat pad impingement and suprapatellar fat pad edema impingement are interchangeable (12). However, there is no evidence to support it. In retrospective studies, no relationship was found between suprapatellar fat pad edema and patellar maltracking (5-7). In our study group, patella lateral subluxation was found in 60 (19.7%) patients, patella lateral tilt in 10 (3.3%) patients, and patellofemoral joint pathology in 224 (77%) patients. Suprapatellar fat pad edema and patellofemoral degeneration have been linked in recent publications (13). The relationship between suprapatellar fat pad size and osteoarthritis is unclear. Wang et al. (14) found a positive association with femorotibial osteoarthritis in their study. Shabshin et al. (1) found no association between patellofemoral osteoarthritis and suprapatellar fat pad size. Osteoarthritis was found in 65 (21.3%) of the participants in our study.

Quadriceps tendon edema has been observed in patients with suprapatellar fat pad edema in the literature (15). In our study, isolated suprapatellar impingement syndrome was observed in 27 (8.9%) patients, an intra-articular fluid increase was observed in 185 (60.9%) patients, and synovitis findings were observed in 4 (1.3%) patients. Quadriceps femoris tendinitis was found in 35.2% of the patients, patellar tendonitis in 8 (2.6%), and soft tissue edema in 30 (9.9%). In the study of Roth et al. (7), no significant relationship was found between chondromalacia and suprapatellar fat pad. In our study, CP was found in 138 (45.4%) patients, grade 1 CP in 28 (9.2%) patients, grade 2 CP in 24 (7.9%) patients, grade 3 CP in 44 (14.5%) patients, and grade 4 CP in 70 (23%) patients.

Study Limitations

There were certain limitations to our research. To begin with, our study was a retrospective, single-center study. Multicenter prospective studies should back up our findings. Since our study included a large patient population, all patients might not be adequately and objectively screened for clinical symptom evaluation. Our paper included a heterogeneous patient group. The study’s strengths included a large-scale analysis of comprehensive MRI data with a large patient cohort.

Conclusion

Non-specific pain was the most common symptom in the suprapatellar impingement, it was one of the most significant causes of anterior knee pain. No significant relationship was found between suprapatellar impingement and gender, sports, and trauma. Increased intra-articular fluid and quadriceps femoris tendinitis were the most prevalent additional MRI findings.

Ethics

Ethics Committee Approval: The study was approved by the Acıbadem University Institutional Ethics Committee (approval number: 2022-11/38).

Informed Consent: Retrospective study.

Peer-review: Externally peer-reviewed.

Authorship Contributions

Surgical and Medical Practices: B.B., A.A., H.M.Ö., Ö.B., Concept: B.B., A.A., H.M.Ö., Ö.B., Design: B.B., A.A., H.M.Ö., Ö.B., Data Collection or Processing: B.B., A.A., H.M.Ö., Ö.B., Analysis or Interpretation: B.B., A.A., O.Ö., H.M.Ö., Ö.B., Literature Search: B.B., A.A., H.M.Ö., Ö.B., Writing: B.B., A.A., H.M.Ö., Ö.B.

Conflict of Interest: No conflict of interest was declared by the authors.

Financial Disclosure: The authors declared that this study received no financial support.