Current Concept Review Fat Pads as a Cause of Adolescent Anterior Knee Pain Mitchell G. Foster, BS1; Jerry Dwek, MD2; James D. Bomar, MPH2; Andrew T. Pennock, MD2 1University of California, San Diego, Department of Orthopedic Surgery, San Diego, CA; 2Rady Children’s Hospital, San Diego, Department of Orthopedic Surgery, San Diego, CA Abstract: Anterior knee pain is one of the most frequently encountered symptoms in pediatric sports medicine. The fat pad is a structure with mounting evidence supporting its dynamic involvement in many pathological states in the anterior knee. There are three peripatellar fat pads that occupy much of the extrasynovial space of the knee. This review explores the anatomy, innervation, vasculature, function, imaging, and pathology of these fat pads. Fat pad pathology is likely underestimated given the limited literature on such disease in the pediatric population. In particular, the prefemoral fat pad is the least described of the fat pads with only a few reports detailing chronic pathological pro- cesses. To highlight the relevance of the fat pad, particularly in the pediatric population, we describe an atypical case of a self-limiting acute prefemoral fat pad impingement due to a hyperextension injury in a young athlete. Key Concepts: • The three peripatellar fat pads of the anterior knee, although often overlooked, are important nociceptive struc- tures with robust vasculature that undergo dynamic changes in many pathological states. • Fat pad impingement is often described as a chronic process predominantly involving the infrapatellar fat pad; however, acute impingement is also clinically significant and all three of the fat pads may be implicated in disease. • Acute fat pad impingement injury and chronic fat pad pathology should be included in the differential diagnosis in adult and pediatric patients with anterior knee pain. The Anterior Knee Fat Pads Structure and Anatomy stem cells capable of differentiating into chondrocytes, Articular fat pads are intracapsular, synovial lined adi- osteoblasts, or adipocytes.3,4 Embryologically, adipo- pose structures that make up much of the extrasynovial cytes are first present in the joint by the 17th week of space of the knee by occupying irregularities within the gestation and lobules begin to form in the infrapatellar joint cavity. Histologically, the fat pads are ultrastructur- region by the 18th week with increasing size and colla- ally distinct from other types of adipose tissue by their genous density thereafter.4,5 In children, the fat pad vol- small adipocytes ensheathed in a dense network of colla- ume increases linearly with age until maturity, while genic fibers which provide resistance to mechanical maintaining a constant ratio to body weight.86 The exact stress.1,2,42 The fat pad is also composed of vasculature, developmental process in the genesis of the fat pad is not nociceptive nerve fibers, immune cells, and multipotent yet understood. Copyright @ 2021 JPOSNA 1 www.jposna.org JPOSNA Volume 3, Number 2, May 2021 Figure 2. Sagittal fluid sensitive MRI image demonstrat- ing the superior synovial recess (short arrow) as well as the inferior synovial recess (long arrow) within the infrapatellar fat pad. Figure 1. Sagittal T1 MRI image (1a) and schematic di- agram (1b) demonstrating the infrapatellar fat pad (ar- row), the suprapatellar fat pad (arrowhead), and the prefemoral fat pad (asterisk). Three peripatellar fat pads exist within the anterior reduce hemarthrosis and fibrosis should the fat pad be knee capsule, the infrapatellar fat pad (IFP) or Hoffa’s incised.14 The IFP shares anastomoses with local struc- fat pad, the suprapatellar fat pad (SFP) or quadriceps tures in the knee including the patellar tendon, menisci, fat pad, and the prefemoral fat pad (PFP) or posterior and tibial periosteum. Small branches have also been suprapatellar fat pad (Figure 1). The IFP is the most found to supply the cruciate ligaments from the synovial well described and often studied of the fat pads. Its at- network supplying the fat pad. In less common variants tachments are to the anterior meniscus, the patellar ten- where the middle genicular artery persists in its vestigial don, and the patella7. Structurally, there is a central association with the ligamentum mucosum, a large por- body with two protrusions from the central body tion of the blood supply to the ACL is shared with the termed the superior tag superiorly and the ligamentum IFP.14,15 This rich vascular supply with local connections mucosum inferiorly which is also known as the in- supports the hypothesis that the fat pad aids with healing frapatellar plica. Two additional anatomical variants of of local structures but also that it may be a secondary lo- note are the synovial recesses, a vertically oriented su- cus of pain and pathology in the knee. However, multi- perior recess and a horizontally oriented inferior recess, ple studies have attempted to elucidate the direct correla- which occasionally collect fluid that may be mistakenly tion between IFP blood supply and local pathology with interpreted on imaging as a pathological area of effu- inconclusive findings.16-19 sion7 (Figure 2).11-13 These recesses can also harbor loose bodies during arthroscopy making it difficult for There is also rich innervation of the IFP originating pre- identification and retrieval.12 dominately from the posterior articular nerve, a branch of the posterior tibial nerve that also courses through the me- The IFP is vascularized by a rich anastomotic network nisci, synovium, and cruciate ligaments.21 Dye et al. de- supplied peripherally by vertical contributions from the scribe conscious pain perception through arthroscopic superomedial and superolateral genicular arteries anasto- probing of intraarticular structures without anesthesia; this mosing with the inferior genicular arteries. Two to three procedure identified the IFP to be one of the most sensi- horizontal anastomoses connect these vertical arteries. tive structures in the knee, along with the capsule and syn- The central body of the fat pad remains the least vascu- ovium, in triggering and localizing pain.26 In addition, larized of the fat pad and is the ideal access site to healthy subjects with hypertonic saline injected into the Copyright @ 2021 JPOSNA 2 www.jposna.org JPOSNA Volume 3, Number 2, May 2021 IFP experienced significant knee pain equivalent to the Figure 3. Sagittal quality and location of anterior knee pain.34 Similar to the fluid sensitive MRI synovium, there is evidence of a high density of Type IVa image demonstrat- ing scarring within free nerve endings and substance-P immunoreactive nerve the infrapatellar fat fibers in the fat pad which increase in density in patients pad (arrow) after with chronic knee pain, indicating substantial nociceptive she underwent a potential that is hypothesized be involved in anterior knee plica excision as a pain and inflammation in the setting of local mechanical teenager. or chemical irritation.20, 22-25,42 There is limited literature regarding the SFP and PFP, with the PFP being the least studied structure of the ante- rior knee fat pads. The SFP is a triangular structure found on the base of the superior pole of the patella be- tween the quadriceps tendon insertion anteriorly and the 27-30 suprapatellar recess posteriorly. The suprapatellar re- provide a similar fortification and congruency of the ex- cess is continuous with the joint space and as such the tensor mechanism, while enhancing gliding of the quad- SFP is lined with synovium posteriorly. The PFP is riceps tendon and trochlear articulation during flexion.80 found directly posterior to the suprapatellar recess with Cadaveric and animal model studies suggest an in- synovium lining its anterior surface. Directly posterior to creased joint stability and decreased contact pressure of the PFP is the cortex of the distal femoral metaphysis, articular structures with the fat pads in place, while sig- 31-33 extending just superior to the trochlea. nificant changes in kinematics result following resec- tion35-37,39 (Figure 3). These mechanical properties are of Function particular relevance in pathological states, such as osteo- In the appendicular skeleton, fat serves a mechanical role arthritis, as several features, including composition and beyond its widely recognized biochemical properties. In stiffness, may be altered. fact, the fat pads of the knee are only metabolized in se- vere malnutrition and do not expand with increasing Furthermore, there is increased inflammatory cells, BMI.38,50 Although the exact evolutionary purpose of edema, fat necrosis, and fibrosis in the IFP in subjects these structures has not been elucidated, many studies, with osteoarthritis, rheumatoid arthritis, and induced ar- primarily focusing on the IFP, have hypothesized on the thritides. In particular, the IFP has been found to be a normal function of the fat pad in the knee. From a me- source of adipokines (i.e., leptin, adiponectin, chemerin), chanical perspective, the fat pad may serve to absorb interleukins (IL‐6 and IL‐8), TNF‐α, as well as growth compressive stress by accommodating the dynamic factors (fibroblast growth factor, vascular endothelial shape and volume of the joint space in flexion-extension, growth factor). These molecules infiltrate the synovial thus also allowing expansion of the synovial mem- fluid and alter joint homeostasis which is thought to play brane.23,35-37,39,42 This pressure modulating activity has a role in the development and progression of knee osteo- also been attributed to the PFP and SFP. During flexion- arthritis.43-49 extension of the knee, bone movement results in defor- mation and compression of the PFP between the distal Pathology femoral cortex and deep aspect of the quadricep tendon There are multiple pathologic processes that can affect and patella, thereby protecting the surfaces from bone- the anterior knee peripatellar fat pads. The first descrip- bone and bone-tendon friction.40-41 The SFP is thought to tion of pathology of the fat pad was in 1904 by Albert Copyright @ 2021 JPOSNA 3 www.jposna.org JPOSNA Volume 3, Number 2, May 2021 Figure 5.