Angioleiomyoma generally presents as a solitary and painful subcutaneous lesion occurring in the extremities, most frequently in the lower limb. The first comprehensive review of this rare tumor was published by Stout in 1937 [
2]. To the best of our knowledge, to date, only two cases of intra-articular angioleiomyoma in the knee joint have been reported [
3,
4]. An angioleiomyoma can clinically resemble a glomus tumor because of episodic focal hyperesthesia, temperature sensitivity, and severe tenderness. However, our case and the previous two cases of intra-articular manifestation did not show these characteristic symptoms. Diagnosis and treatment of these tumors is frequently delayed because of their location and unusual presentation. MRI is a useful tool in diagnosing intra-articular tumors but it is not specific. The typical MRI findings of an angioleiomyoma that originates subcutaneously show an isointense lesion on T1-weighted image and high signal intensity or isointense lesion on T2-weighted image, which was compatible with our case. Previous reports have suggested a homogenous positive enhancement on a T1-weighted image with contrast media. Similarly, in a previous case report of an intra-articular angioleiomyoma with Gd enhancement, homogeneous enhancement of the nodular lesion and heterogeneous signal intensity specks within the lesion were observed. However, in our case the Gd-enhanced MRI finding was atypical, only peripheral enhancement was observed. Based on the conventional MRI findings of our case, differential diagnosis would be localized nodular-type pigmented villonodular synovitis, synovial hemangioma, angiomyoma, giant cell tumor and synovial sarcoma [
5‐
7]. However, even after Gd-enhanced MRI, it was difficult to narrow down diagnosis. The differences in MRI findings may be explained by histological variance; that is, the number of smooth muscle fibers, sinusoidal and dilated vascular spaces, and collagen fibers varied in each case [
8]. Alternatively, the intra-articular environment, where the reactive synovial coverage occurs, may have affected the enhancement. These variations were reported in chondromyxoid fibroma, where 69% of them showed peripheral enhancement and 31% homogenous or heterogenous enhancement [
9].
Gd-enhanced MRI did not appear to be useful in this specific tumor, but it can still be useful in distinguishing between benign and malignant tumors, evaluating tumor extent, regional metastasis, and identifying satellite lesions. Intra-articular malignant conditions are rare, but cases have been reported such as synovial sarcoma and chondrosarcoma [
10,
11]. MRI characteristics of synovial sarcoma have been described as follows: triple signal intensity sign (areas of hyper, iso, and hypointensity on T2-weighted sequences), multilobulated or lobulated septate cystic masses with heterogeneous septal, and/or peripheral nodular enhancement [
12]. Published studies have identified the following as characteristic MRI features of chondrosarcoma: the tumor reveals low to intermediate intensity lesions on the T1-weighted sequence, possibly with small speckled areas of high signal intensity because of entrapped areas of yellow marrow, and punctate areas of signal void because of matrix mineralization. The signal intensity of non-calcified areas of a tumor was very high on the T2-weighted sequence, with lobules of chondral tissue being separated by hypointense internal septa. Gd-enhanced MRI demonstrates septal and peripheral enhancement of the tumor [
13].
To the best of our knowledge, no other cases with SOL in the knee joint, which is compatible with our MRI findings has been reported. Although the precise diagnosis can only be confirmed by histological examination, MRI with Gd enhancement before surgery could be helpful. Excision of the tumor generally results in complete resolution of symptoms, and a recurrence case has not been reported in the literature. The patient in this case recovered completely after the surgery and showed no signs of recurrence at the 1-year follow-up in an MRI examination.