When Pads of Fat are a Welcome Sight: Fat Pads in Acute Musculoskeletal Imaging Poster No.: C-2444 Congress: ECR 2013 Type: Authors: Keywords: DOI: Educational Exhibit M. Zakhary 1, M. Adix 2, C. Yablon 2, S. Chong 2 ; 1 Philadelphia, PA/ US, 2 Ann Arbor, MI/US Trauma, Inflammation, Infection, Education, MR, Plain radiographic studies, Musculoskeletal system, Musculoskeletal soft tissue, Musculoskeletal bone 10.1594/ecr2013/C-2444 Any information contained in this pdf file is automatically generated from digital material submitted to EPOS by third parties in the form of scientific presentations. References to any names, marks, products, or services of third parties or hypertext links to thirdparty sites or information are provided solely as a convenience to you and do not in any way constitute or imply ECR's endorsement, sponsorship or recommendation of the third party, information, product or service. ECR is not responsible for the content of these pages and does not make any representations regarding the content or accuracy of material in this file. As per copyright regulations, any unauthorised use of the material or parts thereof as well as commercial reproduction or multiple distribution by any traditional or electronically based reproduction/publication method ist strictly prohibited. You agree to defend, indemnify, and hold ECR harmless from and against any and all claims, damages, costs, and expenses, including attorneys' fees, arising from or related to your use of these pages. Please note: Links to movies, ppt slideshows and any other multimedia files are not available in the pdf version of presentations. www.myesr.org Page 1 of 18
Learning objectives This exhibit will showcase the most common fat pads in musculoskeletal imaging using a case-based format to illustrate their anatomic locations and highlight a spectrum of acute musculoskeletal abnormalities using a multimodality approach. Background Fat pads are normal anatomic structures that may become visible only when an abnormality is present, making them useful indicators for acute musculoskeletal processes. Imaging findings OR Procedure details Hoffa's Fat Pad Normal Anatomy (Fig. 1) The infrapatellar fat pad is located at the inferior aspect of the patella in the anterior knee joint and interposed between the joint capsule and the synovial lining of the joint cavity. It is bounded superiorly by the inferior patella, anteriorly by the knee joint capsule and patellar tendon, inferiorly by the proximal tibia and the deep infrapatellar bursa, and posteriorly by the synovial lining at the anterior aspect of the knee joint cavity. Hoffa's fat pad is anchored superiorly to the infrapatellar synovial fold and inferiorly to the anterior horns of the medial and lateral menisci. Pathology Increased density or distortion of Hoffa's fat pad can be caused by a number of diseases. Intrinsic pathologies result in increased density within the fat pad and include traumatic injuries, Hoffa disease, intracapsular chondroma, localized nodular synovitis and postsurgical fibrosis. Extrinsic pathologies cause distortion of Hoffa's fat pad and include articular and synovial abnormalities such as infection (Fig. 2a & 2b), inflammatory arthropathies, intra-articular bodies, pigmented villonodular synovitis, hemophilia, primary synovial chondromatosis and lipoma arborescens. QUIZ Page 2 of 18
What is the diagnosis in Fig. 3a & b? Kager's Fat Pad Normal Anatomy (Fig. 4) The pre-achilles fat pad is located in the posterior ankle joint. It is bounded anteriorly by the flexor hallucis muscle and tendon, inferiorly by the superior cortex of the calcaneus, and posteriorly by the Achilles tendon. A normal Kager's fat pad is seen as a sharply demarcated, lucent triangle on the lateral knee radiograph. Pathology Intrinsic pathologies typically cause increased density within the fat pad and are most commonly due to traumatic injury with ankle joint effusions. Extrinsic pathologies can result in distortion of the fat pad and includes Achilles (Fig 5a & 5b) and flexor hallucis longus tendon abnormalities, calcaneal fractures, infection, malignancy, os trigonum syndrome and retrocalcaneal bursitits. QUIZ What is the diagnosis in Fig. 6a, b & c? Anterior and Posterior Fat Pads of the Elbow Normal Anatomy (Fig 7) The fat pads of the elbow are intracapsular extrasynovial structures enveloped by the deep joint capsule. The anterior fat pad is compressed by the brachialis muscle and normally appears as a lucent triangle abutting the distal humerus on the lateral radiograph. The posterior fat pad is compressed by the triceps tendon and anconeus muscle, and is never visible when normal. Pathology The anterior fat pad is considered abnormal when it is displaces anteriorly and superiorly ('sail sign'). Bulging of the fat pads indicated an effusion, which can be secondary to trauma (Fig. 8a & 8b), hemarthrosis, infection (Fig. 9a & 9b) or inflammation. When abnormal fat pads are present following trauma, but a fracture is radiographically inapparent, an occult fracture must be suspected (supracondylar in children; radial head in adults, Fig 8a & b). Page 3 of 18
Imaging Pitfalls While often associated with fractures, elevation of the anterior fat pad can be a normal variant (Fig. 10). The fat pad's morphology helps distinguish between normal and pathologic elevation as distention of the joint capsule causes a 'ship's sail' appearance of the fat pad. Imaging Pearl The anterior and posterior elbow fat pads are best evaluated when the elbow is in the true lateral position. Rotation of the elbow causes the radial head to partially obscure the anterior fat pad (compare Fig. 11a with Fig. 11b). Pronator Quadratus Fat Pad Normal Anatomy (Fig. 12) The pronator quadrates fat pad overlies the thin fascia covering the pronator quadrates muscle. This fascia creates a forearm space that allows for the accumulation of fluid. A normal pronator quadrates fat pad is noted when the fat pad is seen in its entirety as a thin, continuous and lucent line parallel to the distal radius, about 3-5 mm anterior to the volar aspect of the radius. Pathology The pronator fat pad sign is of moderate reliability. When abnormal (elevated, disrupted, or bulging) in a trauma patient, a fracture (Fig. 13a & 13b) or soft tissue injury should be suspected. A normal appearing pronator fat pad does not preclude a fracture as a distal radial fracture or soft tissue contusion may be present even with a negative fat pad sign. QUIZ What disease process is present in Fig. 14a, b, c & d? Images for this section: Page 4 of 18
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Fig. 1: Normal: Lateral knee radiograph. Hoffa's fat pad is outlined. Fig. 2: Lateral knee radiograph demonstrates intraarticular gas in Hoffa's fat pad (solid arrows) as well as other intraarticular gas (open arrows). Page 6 of 18
Fig. 3: ANSWER: MRI demonstrates patellar tendon rupture with edema in Hoffa's fat pad and retraction of the patella and quadriceps tendon resulting in patella alta. Page 7 of 18
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Fig. 4: Normal: Lateral ankle radiograph. Kager's fat pad is outlined. Fig. 5: Ankle radiograph (A) shows irregularity of the Achilles tendon with increased density within Kager's fat pad suggesting Achilles tendon rupture. MRI (B) confirms complete Achilles tendon rupture. Fig. 6: ANSWER: The accessory soleus muscle is the most common accessory muscle in the ankle that appears as a soft tissue mass in Kager's fat pad. This is best characterized by MRI. Page 9 of 18
Fig. 7: Normal: Lateral elbow radiograph. The anterior fat pad is noted (arrows). The posterior fat pad is inapparent. Page 10 of 18
Fig. 8: Lateral elbow radiograph (A) demonstrates abnormal anterior (closed head arrows) and posterior (open head arrows) fat pads. Oblique elbow radiograph (B) confirms a radial head fracture. Page 11 of 18
Fig. 9: Lateral (A) and oblique (B) elbow radiographs demonstrates displacement of the fat pads (closed and open arrows), as well as joint space narrowing, sclerosis and erosive changes along the distal humerus and radial head, due to septic arthritis. Page 12 of 18
Fig. 10: Normal variant elevation of anterior fat pad. Page 13 of 18
Fig. 11: A. True lateral view, optimizing evaluation of the normal fat pads. B. Rotated lateral view, obscuring the anterior fat pad, and limiting evaluation for joint effusions. Page 14 of 18
Fig. 12: Normal: Lateral wrist radiograph. The pronator quadratus fat pad is indicated (arrows). Page 15 of 18
Fig. 13: Oblique (A) and lateral (B) radiographs of the wrist showing convex bulging of the quadratus fat pad (arrows) due to a distal radius fracture. Page 16 of 18
Fig. 14: ANSWER: Obliteration of the pronator quadratus fat pad with thickening of the anterior compartment muscles. MRI reveals T2 hyperintensity within the anterior compartment muscles, representing edema from myositis and cellulitis. Page 17 of 18
Conclusion Imaging findings involving the periarticular fat pads play an important role in acute musculoskeletal imaging. The radiologist should be familiar with the locations of the most important fat pads in the body and recognize a spectrum of acute musculoskeletal processes that can result in their abnormal appearance. References 1. Fallahi F, Jafari H, et al. Explorative study of the sensitivity and specificity of the pronator quadrates fat pad sign as a predictor of subtle wrist fractures. Skeletal Radiol. 2013;42:249-53. 2. Zammit-Maempel I, Bisset RA, et al. The value of soft tissue signs in wrist trauma. Clin Radiol. 1988;39:664-8. 3. Jacobson JA, Lenchik L, et al. MR imaging of the infrapatellar fat pad of Hoffa. RadioGraphics. 1997;17:675-91. 4. O'Dwyer H, O'Sullivan P, et al. The fat pad sign following elbow trauma in adults. J Comput Assist Tomogr. 2004;28:562-5. 5. Blumberg SM, Kunkov S, et al. The predictive value of a normal radiographic anterior fat pad sign following elbow trauma in children. Pediatr Emerg Care. 2001;27:596-600. 6. Ly JQ, Bui-Mansfield LT, et al. Anatomy of and abnormalities associated with Kager's fat pad. AJR. 2004;182:147-54. 7. De Maesenneeer M, Jacobson JA, et al. Elbow effusions: distribution of joint fluid with flexion and extension and imaging implications. Invest Radiolo. 1998;33:117-25. Personal Information Page 18 of 18