Intra-articular lipoma arborescens (LA) is an uncommon benign synovial neoplasm of unknown etiology. LA is characterized by villous transformation of the synovium due to diffuse infiltration of the subsynovial stroma by mature adipose cells. The Latin word “arbor” means tree-like morphology. LA affects mainly large joints such as hips and knees;¹ however, other joints can be involved such as elbows, shoulders, and ankles.² Clinically, it can present as intermittent joint pain and swelling with a monoarticular or polyarticular pattern.³ Most of cases can be predicted by MRI.¹ This report describes a case a 26-year-old man who presented with an intermittent left knee joint pain and effusion and was diagnosed histopathologically with lipoma arborescens. Radiological and histomorphologic features are described.

A 26-year-old man presented with a history of intermittent pain and swelling of the left knee joint for the previous 2 years. He had a history of repeated soccer trauma to the same joint, without any other underlying joints disorders. The patient was treated initially by physiotherapy and nonsteroidal anti-inflammatory drugs such as piroxicam and diclofenac with limited response. Physical examination revealed an obese individual with tenderness over the medial line of the left knee joint. Pivot shift, McMurray, and Lachman test results were negative. Blood tests revealed a normal leukocyte and differential count. Other laboratory investigations were unremarkable. Initial MRI showed only mild effusion. The patient continued to have intermittent pain and tenderness over the medial side of the knee joint. He had another MRI, which found no significant abnormality except joint effusion. The patient subsequently was referred to the orthopedic clinic where he underwent diagnostic arthroscopy, which revealed diffuse yellow soft tissue synovial papillary growth involving both medial and lateral gutters of the suprapatellar pouch. In addition, there was a mild effusion, but no signs of injury to ligaments or menisci. A synovial biopsy was taken and histopathologic examination showed a synovial lined villous proliferation, in which the villi are diffusely infiltrated by mature adipose tissue (Figure 1). After the establishment of the histopathologic diagnosis of intra-articular lipoma arborescens, the second MRI was reexamined by the radiologist in the light of discussion with the orthopedic surgeon and the pathologist, allowing a precise radiologic localization of the lesion (Figure 2A, ​,2B).2B). MRI post-arthroscopic intervention showed disappearance of the lesion (Figure 3A, ​,3B).3B). The patient elected to continue on a conservative management without open synovectomy, unless the symptoms got worse. The clinical follow up showed mild improvement.

Tumors of the synovium are rare and can involve the lining of the joints, tendons, or bursa. LA is a very rare benign primary intra-articular tumor of the synovium. It is known also as synovial lipomatosis, and is characterized by replacement of the subsynovial tissue by mature adipocytes, giving rise to prominent intra-articular villous proliferation of the synovium. Less than 60 cases have been published in the English language medical literature as shown by the database of medical search engines of Pub Med and OVID.¹ The majority are case reports and the largest series of cases included six cases.³ I believe that this is the first case ever reported in Saudi Arabia.