Suprapatellar Fat-Pad Mass Effect: MRI Findings and Correlation With Anterior Knee Pain

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1 Musculoskeletal Imaging Original Research Tsavalas and Karantanas MRI of Suprapatellar Fat-Pad Mass Effect Musculoskeletal Imaging Original Research Nikolaos Tsavalas 1 Apostolos H. Karantanas 1,2 Tsavalas N, Karantanas AH Keywords: adipose tissue edema, anterior knee joint pain, knee joint, MRI DOI: /AJR Received February 29, 2012; accepted after revision June 5, Department of Medical Imaging, University Hospital, Stavrakia-Voutes, Heraklion 71110, Crete, Greece. Address correspondence to A. H. Karantanas (akarantanas@gmail.com; karantanas@med.uoc.gr). 2 Department of Radiology, University of Crete, Heraklion, Crete, Greece. WEB This is a Web exclusive article. AJR 2013; 200:W291 W X/13/2003 W291 American Roentgen Ray Society Suprapatellar Fat-Pad Mass Effect: MRI Findings and Correlation With Anterior Knee Pain OBJECTIVE. The purpose of this study was to evaluate the MRI characteristics of the suprapatellar fat-pad, determine the prevalence and pattern of suprapatellar fat-pad edema, and correlate the findings with the presence of anterior knee pain, patellofemoral malalignment, and patellofemoral joint osteoarthritis. MATERIALS AND METHODS. We retrospectively reviewed 879 consecutive knee MRI examinations of 843 patients for the presence of a suprapatellar fat-pad mass effect on the suprapatellar joint recess. The relative signal intensity and the maximum anteroposterior, craniocaudal, and oblique diameters of the suprapatellar fat-pad on sagittal fat-suppressed intermediate-weighted turbo spin-echo images were measured. Findings of anterior knee pain, patellofemoral malalignment, and patellofemoral joint osteoarthritis were also recorded. The Fisher exact, Mann-Whitney, and independent samples Student t tests and Spearman rank correlation coefficient were used for statistical analysis. RESULTS. The prevalence of suprapatellar fat-pad mass effect on the suprapatellar joint recess in our study population was 13.8%. The relative signal intensity (p < ) and maximum anteroposterior (p < ), craniocaudal (p = ), and oblique (p < ) diameters of the pad were significantly greater in patients with a mass effect. Significant correlation was found between the relative signal intensity and the maximum anteroposterior (ρ = , p = ), craniocaudal (ρ = , p = ), and oblique (ρ = 0.123, p = ) diameters. Mass effect was not significantly associated with anterior knee pain, patellofemoral malalignment, or patellofemoral joint osteoarthritis. Six patients with suprapatellar fat-pad edema had anterior knee pain. CONCLUSION. Suprapatellar fat-pad edema with a mass effect on the suprapatellar joint recess is a common finding at MRI examinations of the knee that is rarely associated with anterior knee pain. T hree distinct fat-pads are located in the anterior knee compartment: the suprapatellar or quadriceps fatpad, the prefemoral or supratrochlear fat-pad, and the infrapatellar or Hoffa fat-pad. Each is interposed between the joint capsule externally and the synovium-lined joint cavity and its extensions internally, making it intracapsular but extrasynovial. A normal suprapatellar fat-pad lies on the patellar base and is rather triangular. It fills the gap between the posterior aspect of the quadriceps tendon insertion and the superior aspect of the retropatellar cartilage, increasing the congruency of the extensor mechanism. Posteriorly, the suprapatellar joint recess, an upward extension of the knee joint cavity, separates the suprapatellar fat-pad from the prefemoral fat-pad, which sits immediately anterior to the femur [1 5] (Fig. 1). Acute or repetitive trauma to the infrapatellar fat-pad can cause hemorrhage and inflammation of the pad and subsequent hypertrophy and predisposition to impingement between the femur and the tibia, a condition known as Hoffa disease. An increase in signal intensity and size of the infrapatellar fat-pad with accompanying mass effect on the patellar tendon represents a common pattern on fat-suppressed MR images in the acute phase of the disease [1, 6 8]. We have noticed similar MRI findings regarding the suprapatellar fat-pad, namely, edema with a mass effect on the suprapatellar joint recess (suprapatellar fat-pad swelling). To our knowledge, limited MRI-based studies have focused on the suprapatellar fat-pad AJR:200, March 2013 W291

2 Tsavalas and Karantanas Fig year-old man with normal fat-pads. Midsagittal fat-suppressed turbo spin-echo intermediate-weighted MR image of anterior knee compartment shows normal suprapatellar (SFP) and prefemoral (PFP) fat-pads (arrows). Mild effusion enables direct visualization of suprapatellar joint recess (asterisk). and its clinical significance [4, 5]. The purpose of this investigation was to evaluate the MRI characteristics of the suprapatellar fat-pad, determine the prevalence and pattern of suprapatellar fat-pad edema, and correlate the findings with anterior knee pain, patellofemoral malalignment, patellofemoral joint osteoarthritis, and body fat content. Materials and Methods Patients We retrospectively reviewed the records of 879 consecutive knee MRI examinations of a total of 843 patients referred to our MRI department because of various knee-related clinical conditions between November 2007 and December No ethical committee approval was required owing to the retrospective nature of our study. Patients with a history of arthroscopic surgery, severe knee trauma, or knee joint tumor were excluded from the study. Patients with MRI evidence of suprapatellar joint recess synovial thickening and associated effusion were similarly excluded. A total of 831 knee MRI examinations of 795 patients were eventually included in the study. Our study population consisted of 415 male patients (mean age, 37.1 ± 16.1 [SD] years; range, 4 85 years) and 380 female patients (mean age, 50.1 ± 18.5 years; range, 7 84 years). Imaging All MRI examinations were performed with a 1.5-T system (Vision Hybrid, Siemens Healthcare) with a phased-array knee coil. Each patient was positioned supine with the affected knee in flexion and 15 external rotation. The standard knee MRI protocol at our institution includes the following sequences: axial fat-suppressed turbo spin-echo (TSE) proton density weighted images (TR/TE, 3500/14; echo-train length, 9; slice thickness, 4 mm; interslice gap, 5 mm; matrix, ; FOV, 15 cm); sagittal fat-suppressed TSE intermediate-weighted images (TR/TE, 2000/41; echo-train length, 5; slice thickness, 4 mm; interslice gap, 5 mm; matrix, ; FOV, 15 cm); sagittal T2*-weighted multiecho data image combination (MEDIC) images (TR/TE, 530/26; echotrain length, 1; slice thickness, 3 mm; interslice gap, 4 mm; matrix, ; FOV, 18 cm); and coronal T1-weighted spin-echo images (TR/TE, /14; echo-train length, 1; slice thickness, 4 mm; interslice gap, 5 mm; matrix, ; FOV, 16 cm). The sagittal fat-suppressed intermediate-weighted TSE MR images were used for our study quantitative analysis. Images were reviewed and analyzed with a radiology information system PACS (Evorad Research PACS, Evorad). Study Groups According to suprapatellar fat-pad imaging configuration, our study population was divided into control and mass effect groups. The control group included patients with normal morphologic features of the suprapatellar fat-pad (triangular shape), and the mass effect group consisted of patients with suprapatellar fat-pad expansion with a mass A effect on the suprapatellar joint recess, defined by the presence of a convex posterior fat-pad border on the sagittal intermediate-weighted images (Fig. 2). Group classification was conducted separately by two observers, one fellowship-trained (2 years of experience) and one senior musculoskeletal radiologist (27 years of experience), who rendered their opinion by consensus. The researchers were blinded to each other s results. Measurements The anteroposterior, craniocaudal, and oblique diameters of the suprapatellar fat-pad were measured with the aid of electronic calipers on consecutive sagittal slices. The maximal measurement of each diameter was recorded in millimeters. The maximum anteroposterior diameter was defined as the distance between the most posterior point of the pad and the dorsal contour of the distal quadriceps tendon along a line parallel to the patient s axial plane. The maximum craniocaudal diameter was defined as the distance between the most superior and inferior points of the pad along a line vertical to the patient s axial plane. The maximum oblique diameter of the suprapatellar fat-pad was defined as the distance between the most posterior and anterior points of the pad along an obliquely oriented measurement tangent running parallel to the superior aspect of the base of the osseous contour of the patella [3] (Fig. 3). Relative signal intensity was measured on the midsagittal image. For this purpose, the fellowshiptrained musculoskeletal radiologist manually chose three 4-mm-diameter circular regions of interest (ROIs). The first ROI was centered inside the suprapatellar fat-pad, the second inside the prefemoral Fig. 2 Magnified midsagittal fat-suppressed intermediate-weighted turbo spin-echo MR images show differences between control and mass effect groups. A, 29-year-old man with normal triangular shape of suprapatellar fat-pad (same patient as in Fig. 1). B, 27-year-old man with suprapatellar fat-pad mass effect on posterior joint recess and anterior scalloping of prefemoral pad (arrow). B W292 AJR:200, March 2013

3 MRI of Suprapatellar Fat-Pad Mass Effect Clinical Data Clinical data based on the referring physician s notes and confirmed by thorough history and a basic knee joint examination conducted on site by radiology residents were retrospectively evaluated by the senior musculoskeletal radiologist for clear evidence of anterior knee pain. Similarly, available information regarding occupation and physical activity was recorded for the patients with abnormal suprapatellar fat-pads who had anterior knee pain. The fellowship-trained musculoskeletal radiologist was blinded to the patients histories and clinical diagnoses. Fig year-old man with normal pad shape (same patient as in Figs. 1 and 2A). Reference MR image of Fig. 2A shows suprapatellar fat-pad. Anteroposterior (A-P, orange), craniocaudal (C-C, yellow), and oblique (OBL, red) diameters of pad are clearly delineated. Orange and dashed green lines run parallel to patient s axial plane. fat-pad at the same level, and the third in the background noise at the bottom of the image (Fig. 4). Each ROI generated a mean value and the SD of the signal intensity of the pixels enclosed. We then subtracted the mean signal intensity of the prefemoral fat-pad from the respective value of the suprapatellar fat-pad and calculated their difference, recorded the corresponding SD of the background noise, and determined the ratio of mean signal intensity difference to background noise SD, which was used as a suprapatellar fat-pad relative signal intensity index (relative signal intensity = mean signal intensity difference / background noise SD). All measurements Diameter (mm) Male population Female population Fig year-old woman with normal fat-pads. Midsagittal fat-suppressed intermediate-weighted turbo spin-echo MR image shows placement of circular regions of interest inside suprapatellar fat-pad (1), prefemoral fat-pad (2), and background noise (3). were performed by the fellowship-trained musculoskeletal radiologist. With a minimal interval of 1 month, all measurements were repeated by the same observer on 50 randomly chosen knee MRI datasets. Diameter (mm) Registered MRI Data Most of our study population had been recruited for a study on patellofemoral malalignment and patellofemoral joint osteoarthritis. Data from that study were correlated with the results of the current investigation. Various patellofemoral alignment parameters, including femoral sulcus angle and depth, lateral patellar displacement, lateral patellofemoral angle, tibial tubercle trochlear groove distance, and Insall-Salvati index, and MRI evidence of patellofemoral joint osteoarthritis had been registered for 510 of our patients (531 knee MRI examinations). Similarly, fat thickness in the medial compartment of the knee had been recorded for the same group of patients and used as an indirect body fat content index [9]. Statistical Analysis Statistical analysis was performed with Med- Calc statistical software (version , MedCalc Software). The D Agostino-Pearson test was chosen to determine distribution for all studied parameters. We used the independent samples Student t test and Mann-Whitney test to compare the acquired imaging data between the control and the mass effect groups. A possible association between suprapatellar fat-pad imaging configuration and anterior knee pain or patellofemoral joint osteoarthritis was examined with the Fisher exact test. Spearman rank correlation coefficient (ρ) Control group Mass effect group 0 MAPD MCCD MOD 0 MAPD MCCD MOD A Fig. 5 Graphs show comparison of suprapatellar fat-pad diameter measurements between male and female populations (A) and control and mass effect groups (B). Values are means; error bars indicate SD. MAPD = maximum anteroposterior diameter, MCCD = maximum craniocaudal diameter, MOD = maximum oblique diameter. B AJR:200, March 2013 W293

4 Tsavalas and Karantanas TABLE 1: Results of Mann-Whitney and Student t Tests Comparing Studied Measurements Between the Control and Mass Effect Groups Measurement p Suprapatellar fat-pad Maximum anteroposterior diameter < Maximum craniocaudal diameter Maximum oblique diameter < Relative signal intensity < Patellofemoral alignment parameters Femoral sulcus angle Femoral sulcus depth Lateral patellar displacement Lateral patellofemoral angle Tibial tubercle trochlear groove distance Insall-Salvati index Fat thickness in medial compartment of knee Note Bold type indicates a statistically significant difference. was used to evaluate correlation between suprapatellar fat-pad diameter measurements, suprapatellar fat-pad relative signal intensity measurements, and age. Statistically significant differences and associations were defined by p < Interrater and intrarater reliability (regarding group classification and studied measurements) was assessed with Cohen kappa coefficient. Results Normal suprapatellar fat-pad morphologic features were found in 685 subjects (344 male patients, 341 female patients; mean age, 44.5 ± 18.8 years; range, 4 85 years). The mass effect group consisted of 110 patients (71 male patients, 39 female patients; mean age, 35.9 ± 13.9 years; range, 9 68 years). Accordingly, the prevalence of suprapatellar fat-pad expansion with a mass effect on the suprapatellar joint recess in our study population was 13.8% (17.1% in male and 10.2% in female patients). Bilateral suprapatellar fat-pad swelling was found in one female patient. Excellent classification agreement (κ = 0.857) was found between the two raters. A statistically significant difference was found between the control and mass effect groups regarding sex (p = ) and age (p < ). Significantly younger patients with a male predominance composed the mass effect group. The mean relative signal intensity (male patients, 2.9 ± 5.2; female patients, 1.5 ± 5.7; p = ) and the maximum anteroposterior, craniocaudal, and oblique diameters (p < ) of the suprapatellar fat-pad were significantly greater in men (Fig. 5A). The mean suprapatellar fat-pad relative signal intensity (control group, 1.7 ± 5.2; mass effect group, 5.3 ± 6.2) and diameter measurements were also significantly greater in patients with suprapatellar fat-pad swelling (Fig. 5B and Table 1). The intrarater agreement for each of these measurements was excellent (κ = ). Significant positive correlation was found between suprapatellar fat-pad relative signal intensity and each of the measured suprapatellar fat-pad diameters, indicating that a larger fat-pad has higher signal intensity. However, advanced age was significantly associated with decreasing suprapatellar fatpad maximum anteroposterior and oblique diameters, whereas no correlation was found with maximum craniocaudal diameter and relative signal intensity (Table 2). The prevalence of anterior knee pain was significantly higher in the control group (Table 3). No statistically significant association was established between anterior knee pain and suprapatellar fat-pad swelling. Only six (5.4%) of the patients with suprapatellar fat-pad swelling had anterior knee pain according to history and physical examination findings. Three of these patients were young male ball game players, including a 27-year-old volleyball player, a 25-year-old basketball player (Fig. 6), and a 28-year-old tennis player. Similarly, one 60-year-old male physician (Fig. 7), one 51-year-old female nurse, and one 23-year-old male student, all with a high level of activity on a daily basis, had anterior knee pain and an edematous-like suprapatellar fat-pad as the only notable imaging findings. The nurse underwent a single sonographically guided intra suprapatellar fat-pad corticosteroid injection with subsequent complete resolution of anterior knee pain. Registered MRI data regarding patellofemoral malalignment, patellofemoral joint osteoarthritis, and fat thickness in the medial compartment of the knee were available for 510 of the patients: 439 (200 male, 239 female) in the control group and 71 (43 male, 28 female) in the mass effect group. There was no statistically significant difference between the two groups in regard to any of the measured patellofemoral alignment parameters or fat thickness in the medial compartment of the knee (Table 1). The relative prevalence of patellofemoral joint osteoarthritis was significantly higher in the control group, but no statistically significant relation was established between patellofemoral joint osteoarthritis and suprapatellar fatpad imaging configuration after adjustment for sex (Table 4). Our findings indicate lack of a significant association between suprapatellar fat-pad swelling and patellofemoral malalignment, patellofemoral joint osteoarthritis, or fat thickness in the medial compartment of the knee. TABLE 2: Spearman Rank Correlation Coefficient Results Regarding Correlation Between Age, Suprapatellar Fat-Pad Diameter, and Suprapatellar Fat-Pad Signal Intensity Measurements Age Relative Signal Intensity Measurement p ρ p ρ Maximum anteroposterior diameter < Maximum craniocaudal diameter Maximum oblique diameter Relative signal intensity Note Bold type indicates a statistically significant association. Positive and negative correlations are indicated by positive and negative ρ values. W294 AJR:200, March 2013

5 MRI of Suprapatellar Fat-Pad Mass Effect TABLE 3: Distribution of Anterior Knee Pain Among the Control and Mass Effect Groups Control Group Mass Effect Group Group n No. Affected Prevalence (%) n No. Affected Prevalence (%) p a Overall < Male patients Female patients a Fisher exact test of correlation between anterior knee pain and suprapatellar fat-pad imaging configuration. Fig year-old male basketball player with anterior knee pain. Sagittal fat-suppressed intermediate-weighted turbo spin-echo MR image shows suprapatellar fat-pad swelling indicated by convex posterior border (arrow). High signal intensity of suprapatellar fat-pad is evident. No other internal derangement was identified. Discussion Articular fat-pads are space-occupying structures that promote joint lubrication and stability [10 12]. The infrapatellar fat-pad contains a framework of connective tissue septa interspersed among fat lobules [8, 13, 14]. It is considered richly vascularized and innervated and appears to play an additional role in biomechanical support and neurovascular supply to adjacent structures. Substance P immunoreactive nerve fibers have been found in the infrapatellar fat-pad, rendering it a potential source of nociceptive output that can therefore result in subjective perception of anterior knee pain [12, 14 16]. The suprapatellar and prefemoral fat-pads may have similar properties. Staeubli et al. [3] reported the constant presence of the suprapatellar fat-pad on cryosections and MR arthrograms in the midline sagittal plane. Suprapatellar fat-pad enlargement has been studied by Roth et al. [4] and Shabshin et al. [5] with equivocal conclusions in regard to its clinical significance and pathogenesis. In the current study, we sought to evaluate the relation between suprapatellar fat-pad swelling and anterior knee pain, patellofemoral malalignment, patellofemoral joint osteoarthritis, and fat thickness in the medial compartment of the knee and to consider a plausible pathogenetic mechanism for this finding. The performance of a multivariate analysis with a large study population is a substantial advantage of our study. The additional measurements of maximum craniocaudal and oblique diameters, evaluation of consecutive sagittal images, and A Fig year-old male physician with anterior knee pain. A and B, Magnified sagittal T2*-weighted multiecho data image combination (MEDIC) (A) and axial fatsuppressed proton density weighted turbo spin-echo (B) MR images depict bulging edematous suprapatellar fat-pad with mass effect on suprapatellar joint recess and severe anterior scalloping of prefemoral fat-pad (arrows). quantification of relative signal intensity add to the value of our observations. Suprapatellar fat-pad swelling was present in 13.8% of our study population. Our results indicate significant correlation between suprapatellar fat-pad swelling and an underlying edematous process affecting the suprapatellar fat-pad. Therefore, the convex appearance of the posterior border of the suprapatellar fatpad can be used as an effective criterion for its edematous enlargement. Our findings are TABLE 4: Distribution of Patellofemoral Joint Osteoarthritis Among the Control and Mass Effect Groups Control Group Mass Effect Group No. No. Group n Affected Prevalence (%) n Affected Prevalence (%) p a Overall Male patients Female patients b a Fisher exact test of relation between patellofemoral joint osteoarthritis and suprapatellar fat-pad imaging configuration. b Three female patients in the control group had bilateral patellofemoral joint osteoarthritis. B AJR:200, March 2013 W295

6 Tsavalas and Karantanas in accordance with those of Roth et al. [4]. Shabshin et al. [5], however, applied stricter imaging criteria for suprapatellar fat-pad edema identification and found a significantly lower prevalence. Infrapatellar fat-pad impingement can be a source of anterior knee pain. The superolateral form has been associated with chondromalacia patellae and patellofemoral malalignment [1, 6 8, 17 23]. In the current study, we did not find a significant association between suprapatellar fat-pad edema and anterior knee pain, patellofemoral malalignment, patellofemoral joint osteoarthritis, or fat thickness in the medial compartment of the knee. Roth et al. [4] and Shabshin et al. [5] also did not observe a relation between suprapatellar fat-pad edema and patellofemoral cartilage lesions. However, they postulated a possible correlation with anterior knee pain, which, in our view, was not established because of the limited study population [4] and lack of adequate clinical data and statistical analysis [5]. We suggest that suprapatellar fat-pad swelling might have resulted from repetitive microtrauma or overuse injury with ensuing intrapad hemorrhage and inflammation. The imaging similarities to Hoffa disease; young male predominance; lack of significant association with patellofemoral malalignment, patellofemoral joint osteoarthritis, or fat thickness in the medial compartment of the knee; and symptomatic involvement in a few highly active patients favor the aforementioned pathogenetic mechanism. Suprapatellar fat-pad edema was not associated with anterior knee pain in most of our patients. This finding may be attributed to the presence of a pain-free threshold that suprapatellar fat-pad repetitive microtrauma and overuse injury fail to exceed [15]. In the six patients with anterior knee pain and suprapatellar fat-pad swelling as the sole imaging finding, we assume that an abnormal overload might have exceeded this threshold and caused nociceptive nerve-ending irritation. Further prospective studies are needed to establish the clinical significance of suprapatellar fat-pad edema. Finally, no significant correlation was found between suprapatellar fat-pad edema and fat thickness in the medial compartment of the knee. Although similar in structure to subcutaneous fat, the suprapatellar fatpad probably constitutes a distinct functional and metabolic unit. The observed age-related thinning of the suprapatellar fat-pad may be attributed, at least in part, to an underlying degenerative process. Lack of histopathologic correlation and limited clinical and imaging follow-up data regarding conservative treatment, corticosteroid injection, or surgical resection of the suprapatellar fat-pad are the limitations of the current study, which was also retrospective. Conclusion Suprapatellar fat-pad edema with a mass effect on the suprapatellar joint recess is a common finding at MRI examinations of the knee and is rarely associated with anterior knee pain. References 1. Jacobson JA, Lenchik L, Ruhoy MK, Schweitzer ME, Resnick D. MR imaging of the infrapatellar fat pad of Hoffa. RadioGraphics 1997; 17: Schweitzer ME, Falk A, Pathria M, Brahme S, Hodler J, Resnick D. MR imaging of the knee: can changes in the intracapsular fat pads be used as a sign of synovial proliferation in the presence of an effusion? AJR 1993; 160: Staeubli HU, Bollmann C, Kreutz R, Becker W, Rauschning W. 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