Singapore Med J 2014; 55(1): 5-11 doi:10.11622/smedj.2014003
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CMEArticle
Lipoma arborescens Sarat Kumar Sanamandra1,
MBBS, FRCR,
Keh Oon Ong1,
MRCS, FRCR
ABSTRACT Lipoma arborescens is a chronic, slowly progressive intra-articular lesion characterised by villous lipomatous proliferation of the synovium, usually involving the suprapatellar pouch of the knee joint. It is an uncommon cause of intra-articular masses that presents as slowly progressive painless swelling of the joint, which persists for many years and is accompanied by intermittent effusions. We highlight this condition to raise awareness of its clinical spectrum and imaging features, so that early diagnosis and appropriate treatment can be given, and misinterpretation of this condition as other more complex intra-articular masses is avoided. This pictorial essay aims to provide a brief yet comprehensive review of the clinical features, distribution, morphological types and imaging characteristics of lipoma arborescens, including its common differential diagnoses and management. Keywords: benign, joint masses, lipoma arborescens, magnetic resonance imaging, synovial membrane
INTRODUCTION
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Lipoma arborescens (diffuse articular lipomatosis) is a rare articular lesion consisting of subsynovial villous proliferation of mature fat cells,(1) usually involving the suprapatellar pouch of the knee joint. Albert Hoffa, a German surgeon, made the first description of lipoma arborescens in 1904; the Latin term ‘arborescens’ means ‘tree-like appearance’, describing the characteristic villous and frond-like morphology of this condition.(2) While the exact cause of lipoma arborescens is unknown, it has been hypothesised to be a nonspecific reactive synovial fatty proliferation in response to chronic traumatic or inflammatory stimuli rather than a neoplastic process.(1,3,4) We discuss the clinical features, morphological types and imaging findings of lipoma arborescens, with particular attention on the role of magnetic resonance (MR) imaging, which is considered the best,(3,4) and often the first, imaging modality in the evaluation of articular masses in the current era. The value of imaging in lipoma arborescens lies in its early diagnosis, delineating the exact anatomical extent, identifying associated abnormalities and differentiating it from other closely mimicking intra-articular masses.(5-17)
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Fig . 1 P h o t o m i c r o g r a p h s o f e x t r a - a r t i c u l a r l i p o m a a r b o r e s c e n s s h ow s e v e r a l p a p i ll a r o i d s t r u c tu r e s (a r r ow s) w i t h s y n o v i a l li n i n g c e lls (a r rowh e a ds), whic h c o nt a in a s t ro ma t hat ex hib it s in c re a s e d mature adipose tissue (stars) [H aematox ylin & eosin (a) × 2; (b) × 4].
C L I N I C A L FE AT U R E S Clinical presentation usually consists of an insidious onset of painless swelling of the affected joint, usually persisting for many years, followed by progressive pain accompanied by intermittent episodes of joint effusion.(1,4) Intermittent worsening pain and swelling of the involved joint may be related to the trapping of hypertrophied fatty villi between the moving joint surfaces. Although the knee joint is the most common site of involvement, lipoma arborescens has also been reported in several other joints, including the shoulder, hip, elbow, ankle and wrist, as well as in periarticular bursae and tendon sheaths.(6-8,11) Although
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it is usually monoarticular, polyarticular and bilateral involvements are not uncommon; for example, involvement of both knees have been reported in up to 20% of affected patients in some studies.(2,9,16) Lipoma arborescens has been observed in patients aged between 9 and 68 years, with equal predominance in men and women. There are two aetiological types of lipoma arborescens, primary and secondary, depending on the age of onset and underlying precipitating condition.(4,10,12) The more common secondary type is defined as synovial lipomatosis associated
Department of Diagnostic and Interventional Radiology, Singapore General Hospital, Singapore
Correspondence: Dr Sarat Kumar Sanamandra, Registrar, Department of Diagnostic and Interventional Radiology, Singapore General Hospital, Outram Road, Singapore 169608. [email protected]
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with an underlying chronic irritation, such as degenerative disease, trauma, meniscal injury or synovitis, and is usually seen in elderly patients.(4,9 -11) The less common primary type is idiopathic and occurs in patients of a younger age group, between the second and third decades of life.(10,11) Laboratory tests such as erythrocyte sedimentation rate, rheumatoid factor and uric acid levels are normal in this condition. Similarly, the joint aspirate is usually negative for crystals, cells and bacterial growth. Gross specimen of lipoma arborescens typically shows a characteristic frond-like pattern, which on microscopy demonstrates papillary proliferation of synovial villi with substitution of subsynovial tissue by mature adipocytes (Fig. 1).(6,12)
I M AG I N G FE AT U R E S Although radiography is of limited value in the diagnosis of lipoma arborescens, lucent areas may infrequently be evident in the periarticular soft tissue mass-like opacity (Figs. 2a & b), which may represent underlying prominent fatty projections of the lipoma arborescens.(6) More often, in most cases of knee joint involvement with associated background degenerative changes in the secondary form, radiographs demonstrate nonspecific fullness and soft tissue opacity in the suprapatellar pouch (Figs. 3a & b). In addition, a few negative radiographic findings such as the absence of sclerosis and articular surface erosions may be an important clue in the differential diagnosis with pigmented villonodular synovitis (PVNS) and gouty arthropathy. On ultrasonography, the villous fatty projections of lipoma arborescens typically display a high echopattern, similar to that of adjacent subcutaneous fat, and may undulate in real time within the surrounding effusion. The mass is usually soft in consistency and compressible (Fig. 2c & d), as opposed to the firm and noncompressible masses of PVNS. Moreover, as ultrasonography is inexpensive, easily available and fairly accurate in determining the location and extent of lipoma arborescens in various synovial surfaces, it can be used in the initial diagnostic step before the application of more expensive cross-sectional imaging.(6,13) Computed tomography, although rarely used in the evaluation of lipoma arborescens, may demonstrate characteristic fat density villous or frond-like projections that are interspersed in the surrounding soft tissue density synovial thickening and effusion of the involved joint (Figs. 2e & f).(6) The absence of high attenuation soft tissue and ossified loose bodies in the articular mass differentiate lipoma arborescens from closely mimicking PVNS and synovial osteochondromatosis, respectively. In addition, similar to that of radiographic findings, the absence of articular or juxta-articular bony erosions (Fig. 2g) may be an important negative feature that helps exclude the possibility of PVNS or gouty arthropathy. Lipoma arborescens, like any other fat-containing lesion, generally displays a specific signal on MR imaging that distinguishes it from other intra-articular mass-like lesions. Therefore, MR imaging is considered the diagnostic modality of
choice in the evaluation of this condition.(3-5) Lipoma arborescens shows high signal intensity villous or nodular foci on both T1and T2-weighted images (Figs. 2h & i) that are suppressed on short tau inversion recovery (STIR) or fat saturation sequences (Fig. 2j), similar to that of any subcutaneous fat. The remaining non-fatty component of hypertrophied synovium in the condition displays heterogeneous high signal intensity on T2 or STIR sequences and intermediate-to-low signal intensity on T1-weighted sequences (Fig. 2h–j). The absence of susceptibility artefact related to haemorrhagic products on gradient imaging (Fig. 2k) differentiates this condition from PVNS, an otherwise closely mimicking entity. (5,6,12,14) On contrast administration, although the hypertrophied subsynovial fatty tissue does not enhance, the overlying thickened synovium often displays diffuse enhancement (Figs. 2l; 4d & e; 6d). MR imaging may also demonstrate associated abnormalities such as joint effusion (Figs. 4, 5 & 6), which is seen in almost all cases at the time of presentation.(4,9) In addition, in the secondary type of lipoma arborescens, there may be background degenerative changes and meniscal tears (Fig. 3); this is not observed in the primary type (Fig. 4). The subsynovial fatty proliferation in lipoma arborescens usually attains one of three morphological patterns: (a) diffuse villous form, involving the entire hypertrophied synovium (Fig. 5); (b) focal nodular frond-like form (Fig. 6); or (c) a mixed form of the two aforementioned patterns (Fig. 7).(15) Although lipoma arborescens is usually monoarticular, involvement of bilateral and multiple joints is not uncommon, particularly in the secondary forms (Figs. 3c & e).(2,9,16) Very rarely, lipoma arborescens may occur in extra-articular tissues of synovial origin, such as in the bursae and tendon sheaths (Figs. 2 & 8). These are usually idiopathic primary forms and have been previously reported in the subacromial bursa of the shoulder, in the bicipito-radial bursa, and along the ankle and radiocarpal periarticular tendon sheaths.(6,8,11,17)
D I FFE R E N T I A L D I AG N O S I S Clinically, any of the five classified subgroups of articular masses (i.e. noninfectious synovial proliferative lesions, infectious granulomatous conditions, depositional joint diseases or articular masses of neoplastic and vascular origin) can mimic lipoma arborescens.(5) Among these, synovial osteochondromatosis, PVNS, rheumatoid arthritis, tuberculous arthritis and gouty arthropathy are the most common differential diagnoses for this condition, which may pose a diagnostic dilemma, even on imaging. However, many of these conditions have specific imaging features, particularly on MR imaging, which allows for a confident diagnosis when interpreted together with their presenting clinical features. The absence of bony erosion and sclerosis on radiographs, firm consistency of any periarticular soft tissue mass on sonography, high-density internal contents, the lack of articular surface erosion on computed tomography, and 6
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Fig. 2 A 4 3 - y e a r- o l d w o m a n w i t h s l o w l y p r o g r e s s i v e p a i n l e s s s w e l l i n g a l o n g t h e l a t e r a l a s p e c t o f t h e r i g h t a n k l e j o i n t . (a ) Fr o n t a l a n d (b) l a t e r a l v i e w r a d i o g r a p h s o f t h e a n k l e t y p i c a l l y s h o w l o b u l a t e d s o f t t i s s u e o p a c i t y o n t h e l a t e r a l a s p e c t t h a t c o n t a i n s a f e w i n te r n a l f a i n t l u c e n t f o c i (a r r o w s), c o m p a t i b l e w i t h u n d e r l y i n g p r o m i n e nt f a t t y p r o j e c t i o n s . U S i m a g e s o f t h e l e s i o n (c) w i t h o u t a n d (d ) w i t h p r o b e c o m p r e s s i o n d e m o n s t r a t e s o f t c o n s i s t e n c y o f t h e l e s i o n (a s o p p o s e d t o t h e f i r m a n d n o n c o m p r e s s i b l e n a t u r e o f pig m e nte d v ill o n o dula r s y n ov itis [P V N S]), c o nt a ining h ete roge n e o us hy p e re c h oic n o dula r a re a s (a r rows) t hat a re simila r in e c h otex ture w it h a d ja c e nt s ub c ut a n e o u s f at (s t a r). C o ro n a l r e fo r m at te d C T im a g e s in (e & f ) s o f t t i s s u e a n d ( g ) b o n e w in d ows r e ve a l a l o b ulate d ma s s at t h e late r a l a sp e c t of t h e a nkl e , c o nt a ining s eve r a l f at d e nsit y n o dula r fo ci (a r rows) inte r sp e r s e d in f lui d o r s of t t is su e d e nsit y c o m p o n e n t (a r r o w h e a d s). N o t e t h a t t h e r e i s a n a b s e n c e o f b o n y o r a r t i c u l a r s u r f a c e e r o s i o n s i n F i g . 2 g , a n i m p o r t a n t c l u e i n t h e dif ferential diagnosis for PV NS or gouty ar thropathy. Coronal (h) T1, (i) T2 and ( j) T2 fat-saturated M R images of the ankle display several nodular a n d v illo us f at t y ma s s e s (a r rows) wit hin t h e l e sio n t hat a re sur ro un d e d by a t r a c e of ef f usio n a n d hy p e r t ro p hie d s y n ov ium (a r rowh e a ds). (k) C o ro na l g r a die nt ima g e sh ows a n a b s e n c e of sus c e pt ib ilit y a r t if a c t , whic h wo ul d in dic ate t h e la c k of ha e m o r r ha gic p ro du c t s in t h e l e s i o n . (l) P o s t- c o nt r a s t T 1 f a t- s a tu r a te d M R i m a g e o f t h e l e s i o n s h ow s s e v e r a l n o n - e n h a n c i n g f a t t y m a s s e s (a r r ow s) w i t h e n h a n c i n g sur ro un ding s of t tis su e/t hic ke n e d s y n ov ium (a r rowh e a ds).
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Fig. 3 A 6 5 - ye a r- ol d wo ma n with lo ng s t a n ding inte r mit te nt bilate r a l kne e joint swe lling a n d disc omfor t , which is more severe on the r ight side. (a) Frontal and (b) later al view r adiogr aphs of the r ight knee joint sh ow n o nsp e cif ic sup r a p ate lla r f ulln e s s a n d a s of t t is su e , ma s s - like o p a cit y (a r rows) a s s o ciate d wit h ma r ke d b a c k g ro un d d e g e n e r at i ve c ha nge s (a r rowh e a ds). C o ro na l (c) T1 a n d (d) T 2 f at- s atur ate d M R ima ge s of the r ight kne e joint show the t ypic al feature s of lipoma ar bore scens (ar rows) with back ground degener ative change s in the for m of marginal osteophy te s (ar rowheads), compatible with the secondar y fo r m of lip o ma a r b o re s c e ns . (e) C o ro na l T1 M R ima ge of t h e l ef t kn e e joint a ls o sh ows s eve r a l int r a a r t i c u l a r f a t t y v i l l o u s p r o j e c t i o n s , a l t h o u g h to a l e s s e r e x te nt , i n t h e s u p r a p a te l l a r b u r s a (a r r o w s) a s s o c i a te d w i t h b a c k g r o u n d d e g e n e r a t i v e c h a n g e s (a r r o w h e a d), w h i c h i s a n e x a m p l e o f m u l t i f o c a l s e c o n da r y lip o ma a r b o re s c e ns .
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Fig. 4 A 13 - ye a r- o l d b oy wit h insidio us o ns et of p a inl e s s p rog re s si ve l ef t kn e e swe lling t hat p e r sis te d fo r t hre e ye a r s . (a) S a git t a l T1 a n d (b & c) a x ia l T 2 f at- s atur ate d M R ima g e s of t h e l ef t k n e e sh ow dif f us e v illo us a n d fro n d - like int r a -a r t ic ula r f at t y ma s s e s (a r rows) in t h e supr apatellar bursa with a s so ciate d mar ke d joint ef fusion (star). (d & e) C oronal p ost- contr a st T1 fat- satur ate d ima ge s show non - enhancing f at t y m a s s e s (a r row) a n d a n e nha n cing ove r l y ing t hic ke n e d s y n ov ium (a r rowh e a ds). T h e yo ung e r a g e of o ns et , a b s e n c e of b a c k g ro un d dege n e r ative cha nge o r m e nis c a l/lig a m e nt te a r s in this p atie nt a re in dic ative of a p r ima r y for m of lip oma a r b ore s c e ns . (f & g) A r thros c opic ima ge s of t h e a f fe c te d k n e e joint d e m o ns t r ate s eve r a l v illo us a n d fro n d - like f at t y p roje c t io ns (a r rows) a r ising fro m t h e s y n ov ia l lining .
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Fig. 5 A 6 3 - ye a r- o l d wo ma n wit h a lo ng s t a n ding his to r y of inte r mit te nt swe lling a n d p a i n i n t h e r i g h t k n e e . C o r o n a l a n d a x i a l (a & c) T 1 a n d ( b & d ) T 2 f a tsaturated M R ima ges of the right knee show several tiny intra-ar ticular fat t y villous projections (arrows) scattered throughout the suprapatellar bursa , compatible with dif f u s e lip o m a a r b o r e s c e n s a s s o c i ate d w it h b a c k g ro un d d e g e n e r at i v e c h a ng e s a n d la r g e ef f usio n (s t a r).
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Fig. 6 A 61- ye a r- o l d ma n wit h c hro nic l ef t k n e e p a in a n d inte r mit te nt swe lling . (a & b) C o ro na l a n d a x ia l T1 a n d (c) a x ia l T 2 f at- s atur ate d M R ima ge s of t h e l ef t k n e e sh ow a n o dula r fro n d - like int r a -a r t ic ula r f at t y ma s s (a r row) at t h e late r a l a sp e c t of t h e sup r a p ate lla r b ur s a w it h s ur ro un din g e f f u si o n (s t a r), c o n si s te nt w it h n o d ul a r lip o m a a r b o r e s c e n s . (d) A x i a l p o s t- c o nt r a s t T1 f at- s atur ate d M R im a g e a ls o sh ows a t y pic a l n o n - e nha n cing f at t y ma s s (a r row) a n d a n e nha n cing ove r l y ing t hic ke n e d s y n ov ium (a r rowh e a d), whic h we re c o nf ir m e d o n sub s e qu e nt a r t hros c o py (e).
the absence of haemorrhagic products or susceptibility signal on MR imaging exclude the possibility of PVNS. On the other hand, the absence of typical juxta-articular bony erosions and calcified soft tissue masses, together with negative laboratory findings such as a lack of crystals in the joint aspirate, would be helpful in ruling out gouty arthropathy. Similarly, the absence of typical cartilaginous or ossified loose bodies in the swollen joint excludes the possibility of synovial osteochondromatosis. Infective lesions such as tuberculous arthritis can be ruled out based on clinical presentation, specific imaging and laboratory findings. In cases of doubt, an arthroscopic examination could be performed to directly visualise the lesion (Figs. 4f & g; 6e; 7e & f), obtain tissue sampling for histopathological correlation and treat any underlying precipitating condition.
T R E AT M E N T A N D PR O G N O S I S Lipoma arborescens is a benign indolent condition, which does not require aggressive surgical treatment unless symptomatic despite conservative management. The primary treatment of lipoma arborescens should be directed toward mitigating any underlying precipitating condition, so as to reduce further 9
progression of secondary lipoma arborescens and its associated symptoms. However, in advanced primary cases and difficult cases of secondary lipoma arborescens, surgery can be considered. The surgical treatment of choice for lipoma arborescens is either open or arthroscopic synovectomy. The latter is minimally invasive and able to facilitate early postoperative recovery. Recurrence after synovectomy is uncommon. Rare extra-articular lipoma arborescence can be treated by open surgical excision, similar to that for any other benign periarticular mass.(17)
C O N C LU S I O N Lipoma arborescens is a benign indolent synovial proliferative disease and an uncommon cause of articular masses. It primarily involves the knee joint, with rare occurrences in the periarticular bursae and tendon sheaths. This entity can be confidently diagnosed by its characteristic features on various imaging modalities, particularly MR imaging. Awareness of its clinical and imaging findings and possible differential diagnoses is essential for early diagnosis and treatment, as well as to avoid misinterpretation of this condition as other aggressive articular masses.
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Fig. 7 A 5 9 - ye a r- o l d wo ma n wit h l o ng s t a n ding inte r mit te nt joint swe lling a n d low - g r a d e p a in in t h e l ef t k n e e . (a , b & d) S a g it t a l , a x ia l a n d c o r o n a l T 1 ; a n d (c) a x i a l T 2 f a t- s a t u r a t e d M R i m a g e s o f t h e l e f t k n e e s h o w a n i n t r a - a r t i c u l a r f r o n d - l i k e f a t t y m a s s (a r r o w) i n a d d i t i o n to s e v e r a l s c a t te r e d f a t t y v i l l o u s p r o j e c t i o n s (a r r ow h e a d s) i n t h e s u p r a p a te ll a r b u r s a , c o n s i s te nt w i t h a m i xe d m o r p h o l o g i c p at te r n o f lip o m a a r b o r e s c e n s . A r t h r o s c o py o f t h e c o n c e r n e d k n e e j o int c o r r e s p o n d in g l y d e m o n s t r ate s (e) a n o d ul a r f at t y m a s s , a n d (f ) a few s c at te re d v ill o us p roje c t io ns in t h e sup r a p ate lla r fos s a .
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Fig. 8 A 4 5 -year- old woman with longstanding right shoulder joint discomfor t. (a) A xial T1 and (b) coronal T2 fat-saturated MR images of the shoulder joint show a lobulated fatty mass in the subacromial -subdeltoid bursa (ar rows) a ssociated with a small ef fusion (star), consistent wit h ex t r a -a r tic ula r lip o ma a r b o re s c e ns .
R E FE R E N C E S
1. Hallel T, Lew S. Bansal M. Villous lipomatous proliferation of the synovial membrane (lipoma arborescens). J Bone Joint Surg Am 1988; 70:264-70. 2. Yan CH, Wong JWK, Yip DK. Bilateral knee lipoma arborescens: a case report. J Orthop Surg (Hong Kong) 2008; 16:107-10. 3. Ryu KN, Jaovishida S, Schweitzer M, Motta AO, Resnick D. MR imaging of lipoma arborescens of the knee joint. Am J Roentgenol 1996; 167:1229-32. 4. Vilanova JC, Barceló J, Villalón M, et al. MR Imaging of Lipoma arborescens and the associated lesions. Skeletal Radiol 2003; 32:504-9. 5. Sheldon PJ, Forrester DM, Learch TJ. Imaging of intraarticular masses. Radiographics 2005; 25:105-19. 6. Senocak E, Gurel K, Gurel S, et al. Lipoma arborescens of the suprapatellar bursa and extensor digitorum longus tendon sheath: report of 2 cases. J Ultrasound Med 2007; 26:1427-33. 7. Laorr A, Peterfy CG, Tirman PFJ, Rabassa AE. Lipoma arborescens of the shoulder: MRI findings. Can Assoc Radiol J 1995; 46:311-3. 8. Dawson JS, Dowling F, Preston BJ, Neumann L. Case report: lipoma arborescens of the sub-deltoid bursa. Br J Radiol 1995; 68:197-9. 9. Al-Ismail K, Torreggiani WC, Al-Sheikh F, Keogh C, Munk PL. Bilateral
lipoma arborescens associated with early osteoarthritis. Eur Radiol 2002; 12:2799-802. 10. Plotkin BE, Varma R. Lipoma arborescens of the knee in a 17-year-old man. Radiology Case Reports 2008; 3:164. 11. Huang GS, Lee HS, Hsu YC, et al. Tenosynovial lipoma arborescens of the ankle in a child. Skeletal Radiol 2006; 35:244-7. 12. Patil PB, Kamalapur MG, Joshi SK, Dasar SK, Rao RV. Lipoma arborescens of knee joint: role of imaging. J Radiol Case Rep 2011; 5:17-25. 13. Learch TJ, Braaton M. Lipoma arborescens: high-resolution ultrasonographic findings. J Ultrasound Med 2000; 19:385-9. 14. Martín S, Hernández L, Romero J, et al. Diagnostic imaging of lipoma arborescens. Skeletal Radiol 1998; 27:325-9. 15. Soler T, Rodríguez E, Bargiela A, Da Riba M. Lipoma arborescens of the knee: MR characteristics in 13 joints. J Comput Assist Tomogr 1998; 22:605-9. 16. Bejia I, Younes M, Moussa A, et al. Lipoma arborescens affecting multiple joints. Skeletal Radiol 2005; 34:536-8. 17. Hill GN, Phyo N. Extra-articular lipoma arborescens of the hand: an unusual case report. J Hand Surg Eur Vol 2011; 36:422-3.
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SINGAPORE MEDICAL COUNCIL CATEGORY 3B CME PROGRAMME (Code SMJ 201401A) True
False
Question 1. Regarding the clinical features of lipoma arborescens: (a) It can be bilateral or multifocal. (b) The knee joint is the most common site of involvement. (c) Extra-articular involvement excludes the diagnosis of lipoma arborescens. (d) The primary form is the most common. Question 2. Regarding magnetic resonance (MR) imaging of lipoma arborescens: (a) MR imaging is considered the diagnostic modality of choice. (b) The lesion typically shows high signal intensity villous foci on both T1 and T2 sequences that are suppressed on fat saturation sequences. (c) It demonstrates susceptibility artifact on gradient imaging. (d) Contrast administration shows non-enhancing fatty foci with enhancing overlying thickened synovium. Question 3. Regarding imaging of lipoma arborescens: (a) The lesion is soft in consistency and compressible with an ultrasonography probe. (b) Underlying fatty projections of the lesion are frequently demonstrated on radiographs as faint lucent foci. (c) Bony or articular erosion is an established feature of lipoma arborescens. (d) Joint effusion is seen in almost all patients at the time of presentation. Question 4. Regarding imaging of lipoma arborescens: (a) Ultrasonography can be used in the initial diagnostic step before the use of more expensive cross-sectional imaging. (b) The villous fatty projections of lipoma arborescens typically display a high echopattern and may undulate in real time ultrasonography. (c) Computed tomography shows high attenuation soft tissue and ossified loose bodies. (d) Radiography and computed tomography have limited role in its diagnosis. Question 5. Regarding the management of lipoma arborescens: (a) Surgical excision is the treatment of choice in most cases. (b) Advanced or difficult cases can be treated by arthroscopic synovectomy. (c) The primary treatment should be directed toward mitigating any underlying precipitating condition. (d) Rare extra-articular lesions are usually idiopathic primary forms and can be treated by open surgical excision.
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CMEArticle
Lipoma arborescens Sarat Kumar Sanamandra1,
MBBS, FRCR,
Keh Oon Ong1,
MRCS, FRCR
ABSTRACT Lipoma arborescens is a chronic, slowly progressive intra-articular lesion characterised by villous lipomatous proliferation of the synovium, usually involving the suprapatellar pouch of the knee joint. It is an uncommon cause of intra-articular masses that presents as slowly progressive painless swelling of the joint, which persists for many years and is accompanied by intermittent effusions. We highlight this condition to raise awareness of its clinical spectrum and imaging features, so that early diagnosis and appropriate treatment can be given, and misinterpretation of this condition as other more complex intra-articular masses is avoided. This pictorial essay aims to provide a brief yet comprehensive review of the clinical features, distribution, morphological types and imaging characteristics of lipoma arborescens, including its common differential diagnoses and management. Keywords: benign, joint masses, lipoma arborescens, magnetic resonance imaging, synovial membrane
INTRODUCTION
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Lipoma arborescens (diffuse articular lipomatosis) is a rare articular lesion consisting of subsynovial villous proliferation of mature fat cells,(1) usually involving the suprapatellar pouch of the knee joint. Albert Hoffa, a German surgeon, made the first description of lipoma arborescens in 1904; the Latin term ‘arborescens’ means ‘tree-like appearance’, describing the characteristic villous and frond-like morphology of this condition.(2) While the exact cause of lipoma arborescens is unknown, it has been hypothesised to be a nonspecific reactive synovial fatty proliferation in response to chronic traumatic or inflammatory stimuli rather than a neoplastic process.(1,3,4) We discuss the clinical features, morphological types and imaging findings of lipoma arborescens, with particular attention on the role of magnetic resonance (MR) imaging, which is considered the best,(3,4) and often the first, imaging modality in the evaluation of articular masses in the current era. The value of imaging in lipoma arborescens lies in its early diagnosis, delineating the exact anatomical extent, identifying associated abnormalities and differentiating it from other closely mimicking intra-articular masses.(5-17)
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Fig . 1 P h o t o m i c r o g r a p h s o f e x t r a - a r t i c u l a r l i p o m a a r b o r e s c e n s s h ow s e v e r a l p a p i ll a r o i d s t r u c tu r e s (a r r ow s) w i t h s y n o v i a l li n i n g c e lls (a r rowh e a ds), whic h c o nt a in a s t ro ma t hat ex hib it s in c re a s e d mature adipose tissue (stars) [H aematox ylin & eosin (a) × 2; (b) × 4].
C L I N I C A L FE AT U R E S Clinical presentation usually consists of an insidious onset of painless swelling of the affected joint, usually persisting for many years, followed by progressive pain accompanied by intermittent episodes of joint effusion.(1,4) Intermittent worsening pain and swelling of the involved joint may be related to the trapping of hypertrophied fatty villi between the moving joint surfaces. Although the knee joint is the most common site of involvement, lipoma arborescens has also been reported in several other joints, including the shoulder, hip, elbow, ankle and wrist, as well as in periarticular bursae and tendon sheaths.(6-8,11) Although
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it is usually monoarticular, polyarticular and bilateral involvements are not uncommon; for example, involvement of both knees have been reported in up to 20% of affected patients in some studies.(2,9,16) Lipoma arborescens has been observed in patients aged between 9 and 68 years, with equal predominance in men and women. There are two aetiological types of lipoma arborescens, primary and secondary, depending on the age of onset and underlying precipitating condition.(4,10,12) The more common secondary type is defined as synovial lipomatosis associated
Department of Diagnostic and Interventional Radiology, Singapore General Hospital, Singapore
Correspondence: Dr Sarat Kumar Sanamandra, Registrar, Department of Diagnostic and Interventional Radiology, Singapore General Hospital, Outram Road, Singapore 169608. [email protected]
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with an underlying chronic irritation, such as degenerative disease, trauma, meniscal injury or synovitis, and is usually seen in elderly patients.(4,9 -11) The less common primary type is idiopathic and occurs in patients of a younger age group, between the second and third decades of life.(10,11) Laboratory tests such as erythrocyte sedimentation rate, rheumatoid factor and uric acid levels are normal in this condition. Similarly, the joint aspirate is usually negative for crystals, cells and bacterial growth. Gross specimen of lipoma arborescens typically shows a characteristic frond-like pattern, which on microscopy demonstrates papillary proliferation of synovial villi with substitution of subsynovial tissue by mature adipocytes (Fig. 1).(6,12)
I M AG I N G FE AT U R E S Although radiography is of limited value in the diagnosis of lipoma arborescens, lucent areas may infrequently be evident in the periarticular soft tissue mass-like opacity (Figs. 2a & b), which may represent underlying prominent fatty projections of the lipoma arborescens.(6) More often, in most cases of knee joint involvement with associated background degenerative changes in the secondary form, radiographs demonstrate nonspecific fullness and soft tissue opacity in the suprapatellar pouch (Figs. 3a & b). In addition, a few negative radiographic findings such as the absence of sclerosis and articular surface erosions may be an important clue in the differential diagnosis with pigmented villonodular synovitis (PVNS) and gouty arthropathy. On ultrasonography, the villous fatty projections of lipoma arborescens typically display a high echopattern, similar to that of adjacent subcutaneous fat, and may undulate in real time within the surrounding effusion. The mass is usually soft in consistency and compressible (Fig. 2c & d), as opposed to the firm and noncompressible masses of PVNS. Moreover, as ultrasonography is inexpensive, easily available and fairly accurate in determining the location and extent of lipoma arborescens in various synovial surfaces, it can be used in the initial diagnostic step before the application of more expensive cross-sectional imaging.(6,13) Computed tomography, although rarely used in the evaluation of lipoma arborescens, may demonstrate characteristic fat density villous or frond-like projections that are interspersed in the surrounding soft tissue density synovial thickening and effusion of the involved joint (Figs. 2e & f).(6) The absence of high attenuation soft tissue and ossified loose bodies in the articular mass differentiate lipoma arborescens from closely mimicking PVNS and synovial osteochondromatosis, respectively. In addition, similar to that of radiographic findings, the absence of articular or juxta-articular bony erosions (Fig. 2g) may be an important negative feature that helps exclude the possibility of PVNS or gouty arthropathy. Lipoma arborescens, like any other fat-containing lesion, generally displays a specific signal on MR imaging that distinguishes it from other intra-articular mass-like lesions. Therefore, MR imaging is considered the diagnostic modality of
choice in the evaluation of this condition.(3-5) Lipoma arborescens shows high signal intensity villous or nodular foci on both T1and T2-weighted images (Figs. 2h & i) that are suppressed on short tau inversion recovery (STIR) or fat saturation sequences (Fig. 2j), similar to that of any subcutaneous fat. The remaining non-fatty component of hypertrophied synovium in the condition displays heterogeneous high signal intensity on T2 or STIR sequences and intermediate-to-low signal intensity on T1-weighted sequences (Fig. 2h–j). The absence of susceptibility artefact related to haemorrhagic products on gradient imaging (Fig. 2k) differentiates this condition from PVNS, an otherwise closely mimicking entity. (5,6,12,14) On contrast administration, although the hypertrophied subsynovial fatty tissue does not enhance, the overlying thickened synovium often displays diffuse enhancement (Figs. 2l; 4d & e; 6d). MR imaging may also demonstrate associated abnormalities such as joint effusion (Figs. 4, 5 & 6), which is seen in almost all cases at the time of presentation.(4,9) In addition, in the secondary type of lipoma arborescens, there may be background degenerative changes and meniscal tears (Fig. 3); this is not observed in the primary type (Fig. 4). The subsynovial fatty proliferation in lipoma arborescens usually attains one of three morphological patterns: (a) diffuse villous form, involving the entire hypertrophied synovium (Fig. 5); (b) focal nodular frond-like form (Fig. 6); or (c) a mixed form of the two aforementioned patterns (Fig. 7).(15) Although lipoma arborescens is usually monoarticular, involvement of bilateral and multiple joints is not uncommon, particularly in the secondary forms (Figs. 3c & e).(2,9,16) Very rarely, lipoma arborescens may occur in extra-articular tissues of synovial origin, such as in the bursae and tendon sheaths (Figs. 2 & 8). These are usually idiopathic primary forms and have been previously reported in the subacromial bursa of the shoulder, in the bicipito-radial bursa, and along the ankle and radiocarpal periarticular tendon sheaths.(6,8,11,17)
D I FFE R E N T I A L D I AG N O S I S Clinically, any of the five classified subgroups of articular masses (i.e. noninfectious synovial proliferative lesions, infectious granulomatous conditions, depositional joint diseases or articular masses of neoplastic and vascular origin) can mimic lipoma arborescens.(5) Among these, synovial osteochondromatosis, PVNS, rheumatoid arthritis, tuberculous arthritis and gouty arthropathy are the most common differential diagnoses for this condition, which may pose a diagnostic dilemma, even on imaging. However, many of these conditions have specific imaging features, particularly on MR imaging, which allows for a confident diagnosis when interpreted together with their presenting clinical features. The absence of bony erosion and sclerosis on radiographs, firm consistency of any periarticular soft tissue mass on sonography, high-density internal contents, the lack of articular surface erosion on computed tomography, and 6
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Fig. 2 A 4 3 - y e a r- o l d w o m a n w i t h s l o w l y p r o g r e s s i v e p a i n l e s s s w e l l i n g a l o n g t h e l a t e r a l a s p e c t o f t h e r i g h t a n k l e j o i n t . (a ) Fr o n t a l a n d (b) l a t e r a l v i e w r a d i o g r a p h s o f t h e a n k l e t y p i c a l l y s h o w l o b u l a t e d s o f t t i s s u e o p a c i t y o n t h e l a t e r a l a s p e c t t h a t c o n t a i n s a f e w i n te r n a l f a i n t l u c e n t f o c i (a r r o w s), c o m p a t i b l e w i t h u n d e r l y i n g p r o m i n e nt f a t t y p r o j e c t i o n s . U S i m a g e s o f t h e l e s i o n (c) w i t h o u t a n d (d ) w i t h p r o b e c o m p r e s s i o n d e m o n s t r a t e s o f t c o n s i s t e n c y o f t h e l e s i o n (a s o p p o s e d t o t h e f i r m a n d n o n c o m p r e s s i b l e n a t u r e o f pig m e nte d v ill o n o dula r s y n ov itis [P V N S]), c o nt a ining h ete roge n e o us hy p e re c h oic n o dula r a re a s (a r rows) t hat a re simila r in e c h otex ture w it h a d ja c e nt s ub c ut a n e o u s f at (s t a r). C o ro n a l r e fo r m at te d C T im a g e s in (e & f ) s o f t t i s s u e a n d ( g ) b o n e w in d ows r e ve a l a l o b ulate d ma s s at t h e late r a l a sp e c t of t h e a nkl e , c o nt a ining s eve r a l f at d e nsit y n o dula r fo ci (a r rows) inte r sp e r s e d in f lui d o r s of t t is su e d e nsit y c o m p o n e n t (a r r o w h e a d s). N o t e t h a t t h e r e i s a n a b s e n c e o f b o n y o r a r t i c u l a r s u r f a c e e r o s i o n s i n F i g . 2 g , a n i m p o r t a n t c l u e i n t h e dif ferential diagnosis for PV NS or gouty ar thropathy. Coronal (h) T1, (i) T2 and ( j) T2 fat-saturated M R images of the ankle display several nodular a n d v illo us f at t y ma s s e s (a r rows) wit hin t h e l e sio n t hat a re sur ro un d e d by a t r a c e of ef f usio n a n d hy p e r t ro p hie d s y n ov ium (a r rowh e a ds). (k) C o ro na l g r a die nt ima g e sh ows a n a b s e n c e of sus c e pt ib ilit y a r t if a c t , whic h wo ul d in dic ate t h e la c k of ha e m o r r ha gic p ro du c t s in t h e l e s i o n . (l) P o s t- c o nt r a s t T 1 f a t- s a tu r a te d M R i m a g e o f t h e l e s i o n s h ow s s e v e r a l n o n - e n h a n c i n g f a t t y m a s s e s (a r r ow s) w i t h e n h a n c i n g sur ro un ding s of t tis su e/t hic ke n e d s y n ov ium (a r rowh e a ds).
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Fig. 3 A 6 5 - ye a r- ol d wo ma n with lo ng s t a n ding inte r mit te nt bilate r a l kne e joint swe lling a n d disc omfor t , which is more severe on the r ight side. (a) Frontal and (b) later al view r adiogr aphs of the r ight knee joint sh ow n o nsp e cif ic sup r a p ate lla r f ulln e s s a n d a s of t t is su e , ma s s - like o p a cit y (a r rows) a s s o ciate d wit h ma r ke d b a c k g ro un d d e g e n e r at i ve c ha nge s (a r rowh e a ds). C o ro na l (c) T1 a n d (d) T 2 f at- s atur ate d M R ima ge s of the r ight kne e joint show the t ypic al feature s of lipoma ar bore scens (ar rows) with back ground degener ative change s in the for m of marginal osteophy te s (ar rowheads), compatible with the secondar y fo r m of lip o ma a r b o re s c e ns . (e) C o ro na l T1 M R ima ge of t h e l ef t kn e e joint a ls o sh ows s eve r a l int r a a r t i c u l a r f a t t y v i l l o u s p r o j e c t i o n s , a l t h o u g h to a l e s s e r e x te nt , i n t h e s u p r a p a te l l a r b u r s a (a r r o w s) a s s o c i a te d w i t h b a c k g r o u n d d e g e n e r a t i v e c h a n g e s (a r r o w h e a d), w h i c h i s a n e x a m p l e o f m u l t i f o c a l s e c o n da r y lip o ma a r b o re s c e ns .
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Fig. 4 A 13 - ye a r- o l d b oy wit h insidio us o ns et of p a inl e s s p rog re s si ve l ef t kn e e swe lling t hat p e r sis te d fo r t hre e ye a r s . (a) S a git t a l T1 a n d (b & c) a x ia l T 2 f at- s atur ate d M R ima g e s of t h e l ef t k n e e sh ow dif f us e v illo us a n d fro n d - like int r a -a r t ic ula r f at t y ma s s e s (a r rows) in t h e supr apatellar bursa with a s so ciate d mar ke d joint ef fusion (star). (d & e) C oronal p ost- contr a st T1 fat- satur ate d ima ge s show non - enhancing f at t y m a s s e s (a r row) a n d a n e nha n cing ove r l y ing t hic ke n e d s y n ov ium (a r rowh e a ds). T h e yo ung e r a g e of o ns et , a b s e n c e of b a c k g ro un d dege n e r ative cha nge o r m e nis c a l/lig a m e nt te a r s in this p atie nt a re in dic ative of a p r ima r y for m of lip oma a r b ore s c e ns . (f & g) A r thros c opic ima ge s of t h e a f fe c te d k n e e joint d e m o ns t r ate s eve r a l v illo us a n d fro n d - like f at t y p roje c t io ns (a r rows) a r ising fro m t h e s y n ov ia l lining .
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Fig. 5 A 6 3 - ye a r- o l d wo ma n wit h a lo ng s t a n ding his to r y of inte r mit te nt swe lling a n d p a i n i n t h e r i g h t k n e e . C o r o n a l a n d a x i a l (a & c) T 1 a n d ( b & d ) T 2 f a tsaturated M R ima ges of the right knee show several tiny intra-ar ticular fat t y villous projections (arrows) scattered throughout the suprapatellar bursa , compatible with dif f u s e lip o m a a r b o r e s c e n s a s s o c i ate d w it h b a c k g ro un d d e g e n e r at i v e c h a ng e s a n d la r g e ef f usio n (s t a r).
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Fig. 6 A 61- ye a r- o l d ma n wit h c hro nic l ef t k n e e p a in a n d inte r mit te nt swe lling . (a & b) C o ro na l a n d a x ia l T1 a n d (c) a x ia l T 2 f at- s atur ate d M R ima ge s of t h e l ef t k n e e sh ow a n o dula r fro n d - like int r a -a r t ic ula r f at t y ma s s (a r row) at t h e late r a l a sp e c t of t h e sup r a p ate lla r b ur s a w it h s ur ro un din g e f f u si o n (s t a r), c o n si s te nt w it h n o d ul a r lip o m a a r b o r e s c e n s . (d) A x i a l p o s t- c o nt r a s t T1 f at- s atur ate d M R im a g e a ls o sh ows a t y pic a l n o n - e nha n cing f at t y ma s s (a r row) a n d a n e nha n cing ove r l y ing t hic ke n e d s y n ov ium (a r rowh e a d), whic h we re c o nf ir m e d o n sub s e qu e nt a r t hros c o py (e).
the absence of haemorrhagic products or susceptibility signal on MR imaging exclude the possibility of PVNS. On the other hand, the absence of typical juxta-articular bony erosions and calcified soft tissue masses, together with negative laboratory findings such as a lack of crystals in the joint aspirate, would be helpful in ruling out gouty arthropathy. Similarly, the absence of typical cartilaginous or ossified loose bodies in the swollen joint excludes the possibility of synovial osteochondromatosis. Infective lesions such as tuberculous arthritis can be ruled out based on clinical presentation, specific imaging and laboratory findings. In cases of doubt, an arthroscopic examination could be performed to directly visualise the lesion (Figs. 4f & g; 6e; 7e & f), obtain tissue sampling for histopathological correlation and treat any underlying precipitating condition.
T R E AT M E N T A N D PR O G N O S I S Lipoma arborescens is a benign indolent condition, which does not require aggressive surgical treatment unless symptomatic despite conservative management. The primary treatment of lipoma arborescens should be directed toward mitigating any underlying precipitating condition, so as to reduce further 9
progression of secondary lipoma arborescens and its associated symptoms. However, in advanced primary cases and difficult cases of secondary lipoma arborescens, surgery can be considered. The surgical treatment of choice for lipoma arborescens is either open or arthroscopic synovectomy. The latter is minimally invasive and able to facilitate early postoperative recovery. Recurrence after synovectomy is uncommon. Rare extra-articular lipoma arborescence can be treated by open surgical excision, similar to that for any other benign periarticular mass.(17)
C O N C LU S I O N Lipoma arborescens is a benign indolent synovial proliferative disease and an uncommon cause of articular masses. It primarily involves the knee joint, with rare occurrences in the periarticular bursae and tendon sheaths. This entity can be confidently diagnosed by its characteristic features on various imaging modalities, particularly MR imaging. Awareness of its clinical and imaging findings and possible differential diagnoses is essential for early diagnosis and treatment, as well as to avoid misinterpretation of this condition as other aggressive articular masses.
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Fig. 7 A 5 9 - ye a r- o l d wo ma n wit h l o ng s t a n ding inte r mit te nt joint swe lling a n d low - g r a d e p a in in t h e l ef t k n e e . (a , b & d) S a g it t a l , a x ia l a n d c o r o n a l T 1 ; a n d (c) a x i a l T 2 f a t- s a t u r a t e d M R i m a g e s o f t h e l e f t k n e e s h o w a n i n t r a - a r t i c u l a r f r o n d - l i k e f a t t y m a s s (a r r o w) i n a d d i t i o n to s e v e r a l s c a t te r e d f a t t y v i l l o u s p r o j e c t i o n s (a r r ow h e a d s) i n t h e s u p r a p a te ll a r b u r s a , c o n s i s te nt w i t h a m i xe d m o r p h o l o g i c p at te r n o f lip o m a a r b o r e s c e n s . A r t h r o s c o py o f t h e c o n c e r n e d k n e e j o int c o r r e s p o n d in g l y d e m o n s t r ate s (e) a n o d ul a r f at t y m a s s , a n d (f ) a few s c at te re d v ill o us p roje c t io ns in t h e sup r a p ate lla r fos s a .
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Fig. 8 A 4 5 -year- old woman with longstanding right shoulder joint discomfor t. (a) A xial T1 and (b) coronal T2 fat-saturated MR images of the shoulder joint show a lobulated fatty mass in the subacromial -subdeltoid bursa (ar rows) a ssociated with a small ef fusion (star), consistent wit h ex t r a -a r tic ula r lip o ma a r b o re s c e ns .
R E FE R E N C E S
1. Hallel T, Lew S. Bansal M. Villous lipomatous proliferation of the synovial membrane (lipoma arborescens). J Bone Joint Surg Am 1988; 70:264-70. 2. Yan CH, Wong JWK, Yip DK. Bilateral knee lipoma arborescens: a case report. J Orthop Surg (Hong Kong) 2008; 16:107-10. 3. Ryu KN, Jaovishida S, Schweitzer M, Motta AO, Resnick D. MR imaging of lipoma arborescens of the knee joint. Am J Roentgenol 1996; 167:1229-32. 4. Vilanova JC, Barceló J, Villalón M, et al. MR Imaging of Lipoma arborescens and the associated lesions. Skeletal Radiol 2003; 32:504-9. 5. Sheldon PJ, Forrester DM, Learch TJ. Imaging of intraarticular masses. Radiographics 2005; 25:105-19. 6. Senocak E, Gurel K, Gurel S, et al. Lipoma arborescens of the suprapatellar bursa and extensor digitorum longus tendon sheath: report of 2 cases. J Ultrasound Med 2007; 26:1427-33. 7. Laorr A, Peterfy CG, Tirman PFJ, Rabassa AE. Lipoma arborescens of the shoulder: MRI findings. Can Assoc Radiol J 1995; 46:311-3. 8. Dawson JS, Dowling F, Preston BJ, Neumann L. Case report: lipoma arborescens of the sub-deltoid bursa. Br J Radiol 1995; 68:197-9. 9. Al-Ismail K, Torreggiani WC, Al-Sheikh F, Keogh C, Munk PL. Bilateral
lipoma arborescens associated with early osteoarthritis. Eur Radiol 2002; 12:2799-802. 10. Plotkin BE, Varma R. Lipoma arborescens of the knee in a 17-year-old man. Radiology Case Reports 2008; 3:164. 11. Huang GS, Lee HS, Hsu YC, et al. Tenosynovial lipoma arborescens of the ankle in a child. Skeletal Radiol 2006; 35:244-7. 12. Patil PB, Kamalapur MG, Joshi SK, Dasar SK, Rao RV. Lipoma arborescens of knee joint: role of imaging. J Radiol Case Rep 2011; 5:17-25. 13. Learch TJ, Braaton M. Lipoma arborescens: high-resolution ultrasonographic findings. J Ultrasound Med 2000; 19:385-9. 14. Martín S, Hernández L, Romero J, et al. Diagnostic imaging of lipoma arborescens. Skeletal Radiol 1998; 27:325-9. 15. Soler T, Rodríguez E, Bargiela A, Da Riba M. Lipoma arborescens of the knee: MR characteristics in 13 joints. J Comput Assist Tomogr 1998; 22:605-9. 16. Bejia I, Younes M, Moussa A, et al. Lipoma arborescens affecting multiple joints. Skeletal Radiol 2005; 34:536-8. 17. Hill GN, Phyo N. Extra-articular lipoma arborescens of the hand: an unusual case report. J Hand Surg Eur Vol 2011; 36:422-3.
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SINGAPORE MEDICAL COUNCIL CATEGORY 3B CME PROGRAMME (Code SMJ 201401A) True
False
Question 1. Regarding the clinical features of lipoma arborescens: (a) It can be bilateral or multifocal. (b) The knee joint is the most common site of involvement. (c) Extra-articular involvement excludes the diagnosis of lipoma arborescens. (d) The primary form is the most common. Question 2. Regarding magnetic resonance (MR) imaging of lipoma arborescens: (a) MR imaging is considered the diagnostic modality of choice. (b) The lesion typically shows high signal intensity villous foci on both T1 and T2 sequences that are suppressed on fat saturation sequences. (c) It demonstrates susceptibility artifact on gradient imaging. (d) Contrast administration shows non-enhancing fatty foci with enhancing overlying thickened synovium. Question 3. Regarding imaging of lipoma arborescens: (a) The lesion is soft in consistency and compressible with an ultrasonography probe. (b) Underlying fatty projections of the lesion are frequently demonstrated on radiographs as faint lucent foci. (c) Bony or articular erosion is an established feature of lipoma arborescens. (d) Joint effusion is seen in almost all patients at the time of presentation. Question 4. Regarding imaging of lipoma arborescens: (a) Ultrasonography can be used in the initial diagnostic step before the use of more expensive cross-sectional imaging. (b) The villous fatty projections of lipoma arborescens typically display a high echopattern and may undulate in real time ultrasonography. (c) Computed tomography shows high attenuation soft tissue and ossified loose bodies. (d) Radiography and computed tomography have limited role in its diagnosis. Question 5. Regarding the management of lipoma arborescens: (a) Surgical excision is the treatment of choice in most cases. (b) Advanced or difficult cases can be treated by arthroscopic synovectomy. (c) The primary treatment should be directed toward mitigating any underlying precipitating condition. (d) Rare extra-articular lesions are usually idiopathic primary forms and can be treated by open surgical excision.
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