iliotibial band ( itb ) friction syndrome is a common overuse injury typically seen in runners , cyclists , and military recruits ( 1 ) . magnetic resonance imaging ( mri )
can show the characteristic imaging findings ( 2 ) . however , coincidental findings or other accompanying mass lesions on mri have been rarely reported . a few reports described synovial cysts , meniscal cysts , periarticular ganglia , and synovial sarcomas causing itb friction syndrome ( 3 , 4 , 5 ) .
a fibroma of tendon sheath ( fts ) is an uncommon benign , painless , slowly growing tumor that usually occurs in the tendon or tendon sheaths of the distal upper extremity ( wrist and hand ) .
an intra - articular location arising in the knee joint is extremely rare ( 6 , 7 ) .
intra - articular involvement with fts may irritate the surrounding soft tissue ( 7 ) .
we report a case of intra - articular fts in a knee joint associated with itb friction syndrome , confirmed by arthroscopy and pathology .
a 45-year - old male presented with a history of several months of right lateral knee pain .
he also complained of a creak and a movable mass at the lateral joint area during ambulation .
an mri was performed using a magnetom vision 1.5-t mr imaging unit ( siemens medical systems , erlangen , germany ) .
the mri showed a thickened itb and poorly defined high signal intensity abnormalities in a compartment - like space bounded laterally by the itb on coronal fat suppressed proton density weighted images ( repetition time [ tr ] 1700 ms , echo time [ te ] 10 ms ) ( fig .
the t1-weighted image ( tr 400 ms , te 20 ms ) showed a decreased signal intensity lesion compared to adjacent fat , deep to the itb ( fig .
a well - demarcated , ovoid nodular lesion was also observed in the compartment - like space .
the nodule showed iso - signal intensity compared to the knee muscle on a t1-weighted image , high signal intensity on t2-weighted images ( tr 3200 ms , te 100 ms ) , and a slightly higher signal intensity with thin rim on fat suppressed proton density weighted images ( fig .
1a - c ) . the initial diagnosis based on the mri findings , clinical history , and physical examination was itb friction syndrome .
we suspected the nodular lesion was a ganglion , focal synovial thickening , focal villonodular synovitis , or focal degenerative change of invaginated extraaritcular fatty tissue .
unfortunately , we did not consider the possibility of a fts . during an arthroscopic examination
, the surgeon found an inflamed lateral synovial recess , which is typically observed in itb friction syndrome ( fig .
after a sequential arthroscopic inflamed synovial recess resection with synovial shaver , a whitish intraarticular polypoid nodule was observed , attached to the lateral joint capsule ( fig .
the nodule did not contain necrosis , mitotic activity , or cellular atypia ( fig .
arthroscopic debridement of the adjacent tissues near the surface between itb and lateral femoral condyle was also performed to relieve the symptoms of itb friction syndrome .
the resected tissues showed fibrosis , marked hemorrhage , prominent capillary proliferation , and mild chronic inflammation that was consistent with itb friction syndrome ( fig .
fibroma of tendon sheath usually forms as a small , slow - growing , firm nodule in association with tendons and tendon sheath ( 6 ) .
an estimated 80 - 95% of fts occurred in the small joint of the upper extremity ( finger , hand , and wrist ) ( 8 , 9 ) . in 1979 , chung and enzinger ( 8) reported that only 7 cases from 138 cases of fts were found around the knee joint , mostly in an extra - articular location . to date , less than 30 reported cases of the disease have been related to the knee ( 9 ) . furthermore , the intra - articular location is less common . to our knowledge ,
only 7 cases of intra - articular fts in the knee joint have been reported ( 6 , 7 , 9 ) . however , among cases of intra - articular fts , the knee joint is the most common location ( 7 , 9 ) .
clinically , fts can occur at any age , with peak incidence at 20 - 50 years , and predominantly in males ( 8 , 9 ) .
fts usually presents as a solitary , painless tumor that may later irritate the surrounding tissue ( 10 ) .
intra - articular fts usually present with symptoms of fullness or mechanical symptoms ( 7 , 9 , 11 , 12 ) . in the knee joint , 71% of lesion present with pain or discomfort and 31% present with palpable masses ( 9 ) .
this case was an intra - articular fts that presented with recurrent pain and a movable mass , consistent with itb friction syndrome .
fibroma of tendon sheath typically show a smooth , well - circumscribed , lobulated architecture . fts has been described as a fibrotic neoplasm or a reactive fibrosis , but its precise origin is still unclear ( 7 , 11 ) .
microscopically , the mass is composed of scattered , spindle shaped fibroblasts or myofibroblasts within a dense fibrocollageneous stroma and frequently bears dilated or slit - like vascular channels ( 8 , 13 , 14 ) .
actually , most of the cases do occur in close association with the tendon or tendon sheath ( 8 , 14 ) .
however , various unusual locations have been reported ( 10 , 11 , 12 , 15 ) .
intra - articular fts usually originates in the joint synovium or capsule , with no apparent connection to any tendons ( 7 , 11 , 12 , 15 ) .
the major differential diagnosis of intra - articular fts includes giant cell tumor of the tendon sheath ( gctts ) and nodular fasciitis ( nf ) ( 12 , 14 , 15 ) .
fts is most often confused with gctts at clinical examination and even under gross pathology ( 14 ) .
fts is hypocellular with slit - like vascular channels within a dense collagen matrix ( 12 , 14 ) .
gctts is less hyalinized and more cellular , with multiple multinucleated giant cells , foamy histiocytes , and hemosiderin - laden macrophages ( 12 , 14 ) .
due to similarities between some forms of the two tumors , some authors have hypothesized that they may be two phenotypic extremities of a single entity ( 17 ) .
other authors have suggested that fts is the end sclerosing stage of gctts , probably consequent on progressive vascular impairment ( 18 ) .
intra - articular fts must also be distinguished from intra - articular nf ( 7 , 15 ) .
nf is a benign myofibroblastic proliferation with a prediction for the subcutaneous tissue or muscle ( 16 ) .
nf resembles fts histologically and up to one fourth of cases of fts are indistinguishable from nf ( 19 ) .
intra - articular nf is very rare , similar to intra - articular fts ( 15 , 16 ) .
but , most cases of intra - articular nf seem to have a longer clinical history and nonspecific symptoms , compared with subcutaneous or intramuscular nf ( 16 ) .
fibroma of tendon sheath usually appears as well defined , small , soft tissue mass on mri ( 9 , 14 ) .
on t1-weighted images , the tumor is generally of a low signal intensity compared to the adjacent muscle .
the mass shows heterogeneous , low to high signal intensity on t2-weighted images and a variable enhancement pattern ( 7 , 9 , 14 ) .
this variation on t2-weighted images and contrast enhancement study mostly rests on the contents of fibrocollagenous tissue and capillary vascularity near the mass ( 14 ) .
fibroma of tendon sheath can be hard to distinguish from other benign or malignant lesions , because of its infrequent occurrence in the knee joint and small , nonspecific nodular appearance on mri ( 9 , 14 ) .
the radiologic differential diagnosis of fts includes gctts , nf , pigmented villonodular synovitis , and extra - abdominal desmoid tumor .
inflammatory synovitis , juxta - articular myxoma , synovial chondromatosis , synovial sarcoma , and fibrosarcoma also could be considered ( 9 , 14 ) .
the patients with itb friction syndrome can show the characteristic mr imaging findings , including thickened iliotibial band , poorly defined signal intensity alterations in the fatty tissue deep to the itb on coronal images , or circumscribed fluid collection in compartment - like space bounded laterally by itb ( 2 ) . in chronic cases , mri studies are essential to rule out other pathologic entities causing lateral knee pain , such as lateral meniscal tear and lateral compartment degenerative joint disease ( 1 ) .
in our case , the mri showed a typical finding of itb friction syndrome with a small ovoid nodule in the compartment - like space of the lateral gutter .
the nodule was confirmed as fts based on the histopathologic examination . to the best of our knowledge , we report the first case of an intra - articular fts in a knee joint associated with itb friction syndrome in english literature .
generally proposed etiologies of itb friction syndrome include friction of the itb against the lateral femoral epicondyle during repetitive flexion and extension activities , compression of the fat and connective tissue deep to the itb , or chronic inflammation of the itb bursa ( 1 , 21 ) .
nemeth and sanders ( 4 ) found and labeled a lateral synovial recess under the itb that consists of a synovium that is a lateral extension and invagination of the knee joint capsule and is not a separate bursa
. biopsies of lateral synovial recesses in patients with itb friction syndrome showed histopathologic changes consisting of chronic inflammation , hyperplasia , fibrosis , and mucoid degeneration ( 4 , 20 ) .
we found an inflamed lateral synovial recess on arthroscopic and histologic examination of our patient .
even though we did not clearly understand the correlation between a fibroma of a tendon sheath and itb friction syndrome , the tumor can aggravate the itb friction syndrome .
the mainstay therapy of itb friction syndrome is nonsurgical management . in persistent or chronic cases
there are several different surgical techniques , percutaneous release of itb , open surgical release of itb , itb z lengthening , arthroscopic itb debridement , and open itb bursectomy ( 20 , 21 ) .
if there is a nodular lesion in a patient with itb friction syndrome , mri recognition is also important to help the physician choose the proper surgical technique .
when nodular lesions on mri are detected in patients with itb friction syndrome , a fibroma of tendon sheath could be included in the differential diagnosis .
 iliotibial band ( itb ) friction syndrome is a common overuse injury typically seen in the active athlete population . 
 a nodular lesion on the inner side of the itb as an etiology or an accompanying lesion from friction syndrome has been rarely reported . a 45-year - old male presented with recurrent pain and a movable nodule at the lateral joint area , diagnosed as itb friction syndrome . 
 the nodule was confirmed as a rare intra - articular fibroma of the tendon sheath ( fts ) on the basis of histopathologic findings . 
 we describe the mri findings , arthroscopic and pathologic features , in this case of intra - articular fts presenting with itb friction syndrome . 
INTRODUCTION
CASE REPORT
DISCUSSION