Many of the diseases causing intra-articular masses have characteristic MRI imaging findings that enables a confident diagnosis making the differential diagnosis for such intra-articular masses somewhat limited .
In the current study, males were affected more than females and the knee joint was the most affected joint in agreement with Patrick J. Sheldon et al.  and Daniel Peixoto et al.  who reported similar sex and site incidence.
Synovial chondromatosis is a benign monoarticular disorder of uncertain etiology; more common in male and also more common in the knee joint, usually seen in the third to fifth decades. The disease is characterized by synovial proliferation and metaplastic transformation together with multiple formations of cartilaginous nodules. In the initial active phase of the disease in which there is synovial proliferation and formation of cartilaginous nodules intra synovial while in the final inactive phase of the disease, there are persistent nodules that may breakaway into the joint space. About 25–30% of the patients do not show calcification and the treatment is synovectomy with about 25% recurrence rate. Associated marginal erosions, late secondary degenerative joint disease, and small joint effusion may be associated with synovial chondromatosis [2, 6,7,8,9,10,11].
Synovial chondromatosis can be classified into two types; primary type in which there is multiple intra-articular uniform-sized loose bodies which often calcify in its peripheral and central parts, and secondary type in which there is injury to the hyaline cartilage related to trauma, osteoarthritis, infectious arthritis, or osteochondritis dissecans and characterized by the presence of different shapes and sizes intra-articular loose bodies with various ring calcifications [10, 12, 13].
In the current study; synovial chondromatosis was found in 14 patients (43.8%) and their MRI appearance was variable depending on the proportion of proliferative synovium and formation of calcified nodules. Non-mineralized lesions show low signal at T1WI and high signal at T2 and STIR while calcified lesions display low signal at T1, low signal intensity of the periphery of the nodules, and high signal of the central bone marrow fat on T2WI in agreement with Patrick J. Sheldon et al. , Daniel Peixoto et al. , and Jie Wen et al.  who reported similar imaging features.
Synovial chondrosarcoma is remarkably uncommon and unclear whether it originates from malignant degeneration of synovial chondromatosis or de novo making the differentiation between them radiologically hard. Conventional radiographs usually show intraarticular soft-tissue mass that may contain calcified bodies. Both synovial chondrosarcoma and synovial chondromatosis may show bone erosions [2, 11].
Scott Evans et al.  reported that the true incidence of synovial chondrosarcoma is unknown and in their study included 800 patients and reported that 0.6% of chondrosarcomas were originating on top of primary synovial chondromatosis. They postulated that both conditions have similar patterns of clinical presentation with joint swelling, pain, and joint restricted movement. Many authors mentioned that found clinical and radiological criteria are unable to differentiate between both of them as there is usually noteworthy overlap with no definitive imaging features to separate both [11, 14, 15].
We encountered in the current study one patient whose MRI showed thickened low signal synovium at T1, high at T2 with multiple low signal calcified bodies and initially diagnosed as synovial chondromatosis, but arthroscopy later revealed synovial chondrosarcoma in agreement with Patrick J. Sheldon et al.  and Scott Evans et al.  who mentioned that the imaging differentiation of synovial chondrosarcoma from synovial osteochondromatosis is difficult.
Scott Evans et al.  concluded that multiple recurrences and the development of bone marrow invasion are highly suspicious of osteochondromatosis malignant transformation.
PVNS is a proliferative synovial benign disorder that may affect the joints, tendon sheaths, or bursae and usually occurs in young to middle aged adults and extremely rare to be polyarticular. It can be diffuse intra-articular or less commonly focal and the knee is the most frequently affected joint with 50% recurrence rate after synovectomy. The affected joint by PVNS may appear normal or may show periarticular soft-tissue swelling in X-ray. Bone mineralization and joint spaces are specifically preserved until late in the disease .
MRI was able to diagnose all cases of PVNS in the current study, all of which was found in the knee joint with male predominance in agreement with Patricket al . who mentioned similar site and sex incidence.
In the current series, all cases of PVNS were diffuse and MRI showed lobulated margin mass-like proliferative synovium with low signal foci at all pulse sequences due to deposition of hemosiderin in agreement with Patricket al ., A. Rodríguez Pan et al. , and AynurTuran et al. , who reported that the diffuse form PVNS is the commonest form and reported similar imaging appearance.
MRI reveals prominent diffuse villous or nodular proliferation of synovium and associated joint effusion. Synovial thickening is visualized as an intermediate to low signal intensity on T1WI. There are low signal intensity areas due to the hemosiderin on T2WI. Particularly, hemosiderin appears as blooming artifacts on gradient echo (GRE) images because of its magnetic susceptibility, which is almost pathognomonic in all forms of PVNS. Lesions may show evident contrast enhancement after administration of IV gadolinium denoting the synovial proliferation high vascularity [3, 4, 16, 17].
Lipoma arborescens is an intra-articular uncommon lesion, usually monoarticular and seen often in the knee. In lipoma arborescens, there is villous synovial proliferation resulting from substitution of the subsynovial tissue by mature fat cells. The exact reason is unidentified, but the most reasonable hypothesis suggests a nonspecific synovial response to traumatic or inflammatory stimuli more willingly than a neoplasm. Lipoma arborescens can be confidently diagnosed because of its characteristic imaging findings [1, 2, 18,19,20,21,22,23,24].
MRI was able to diagnose all the cases of lipoma arborescens in the current series, all complained of painless swelling of the knee and they appear in MRI as intra-articular frond like fat-containing masses at the supra-patellar region (high signal at T1, T2, and suppressed at STIR) and was proved by arthroscopy in agreement with Pushpender Gupta et al.  who reported similar clinical presentation, and reported its more common predilection to the supra-patellar recess of the knee. He also mentioned that the MRI findings of intra-articular frond like fat-containing masses with added joint effusion are characteristic feature for lipoma arborescens.
We encountered one case (3.1%) with synovial ganglion cyst in the current study in agreement with Sayaka Kodaira1 et al.  and Partha Saha et al.  who reported similar rare incidence.
Sayaka Kodaira1 et al.  reported that intra-articular ganglion cysts are frequently seen in the dorsal wrist, shoulder, and palm but rarely to be intra-articular in the knee joint, usually detected as incidental MRI findings. Knee ganglion cysts may arise from both menisci, cruciate ligaments, subchondral bone, or popliteal tendon. The differential diagnosis of it may include synovial chondromatosis, PVNS, synovial hemangioma, and meniscal or parameniscal cysts.
Intra-articular ganglion cysts are frequently seen in the dorsal wrist, shoulder, and palm but rarely to be intra-articular in the knee joint, usually detected as incidental MRI findings [1, 2, 7, 27].
In our study, the ganglion cyst was located at the infrapetallar region (Hofa’s fat) near the meniscus displaying low signal at T1, mixed low and high signal at T2 and STIR, and was confirmed by arthroscopic removal of the lesion in agreement with .
Synovial hematoma was observed in one patient (3.1%) of the study group patients with history of trauma; CT in this patient reveals no fracture lines and US reveals effusion; MRI reveals mass-like lesion within the effusion displaying low signal hematoma with blooming effect at T2 in agreement with Vishal Kumar Jain et al.  who mentioned that in intra-articular hemorrhage regardless the cause, iron-containing hemosiderin is kept in synovial tissue forming proliferative reaction and rusty discoloration. This hemosiderin causes typical T2WI blooming along the synovium, as seen in our patient. This blooming due to synovial hemorrhage can occur in many instances as in hereditary clotting factor deficiency diseases (e.g., hemophilia), trauma, use of anticoagulant, psoriatic or rheumatoid arthritis, collagen diseases, osteoarthritis, hemochromatosis, PVNS, sickle cell anemia, scurvy, synovial hemangioma, and myeloproliferative diseases.
The concordance of MRI in relation to the gold standard was 96.6% accuracy, 91.7% sensitivity, 99% specificity, 52.3% PPV, and 99.9% NPV in agreement with Patrick et al.  and Scott Evans et al.  who mentioned difficulty for MRI to predict malignant transformation of synovial chondromatosis.
Using weighted kappa statistic, the inter-observer agreement for evaluating the intra-articular synovial lesions was 0.86 denoting excellent agreement in agreement with .
Limitation of the study is small number of patients that hinders us from studying other synovial intra-articular lesions during the study duration; large multicenter study is of value for better understanding of other varieties not encountered in our study.