Lipoma arborescens (tree like) is a benign intra-articular tumor characterized by villous synovial hypertrophy and lipomatous infiltration of the subsynovial tissue.1-7 Lipoma arborescens has an unknown etiology, most commonly occurs in the knee, and is usually unilateral.1-6,8-10 Patients typically present with an insidious onset of joint effusion and pain. It most commonly presents in adults and is very rare in children.1-5,7-9,11 Although it is rare, it should be considered in the differential diagnosis for a patient with atraumatic chronic knee pain or effusion. We present a case of lipoma arborescens in the knee of a 10-year-old boy and discuss the clinical workup and treatment. The patient and parents gave their consent for submitting this case and associated imaging for publication.
A 10-year-old boy presented to the clinic with a greater than a 6-month history of atraumatic left knee pain and swelling. He described his pain as achy, located primarily in the anteromedial and lateral knee. His symptoms worsened with activity, and his mother noticed a limp with running. He denied any hip or groin pain, and he did not have any previous injuries to his left lower extremity. He did not have any night pain, fevers, or recent weight loss. He was otherwise healthy and enjoyed playing baseball.
On physical examination, he was afebrile and generally well appearing, with a body mass index of 26.2 kg/m2. He had full painless range of motion of his bilateral hips. His left knee had a mild effusion without overlying erythema. He had a slight flexion contracture compared with his contralateral knee, with range of motion of 5° to 140° on the left and −7° to 140° on the right. He was distally neurovascularly intact and had a stable knee ligamentous examination.
Radiographs of the left knee, pelvis, and bilateral hips were obtained. Radiographs of the pelvis and hips demonstrated normal bony morphology for his age. Knee radiographs demonstrated open physes, normal bony morphology and alignment, normal joint spacing, and no fractures, dislocations, or bony lesions (Figure 1).
Laboratory analysis demonstrated a normal complete blood count with WBC 7.8 × 103/mL, hemoglobin 12.4 g/dL, hematocrit 37.8%, and platelet count 278 × 103/mL. C-reactive protein was normal at <0.5 mg/dL with an elevated erythrocyte sedimentation rate of 18 mm/hr (nl 1 to 10 mm/hr). Antinuclear antibody screen by ELISA was positive with a titer of 1:80 with a speckled appearance. Lyme IgM and IgG screens were negative.
MRI with and without contrast demonstrated a knee effusion with frond-like synovial hypertrophy, most notable in the suprapatellar pouch. This hypertrophic material had similar signal intensity as fat on all sequences and was contrast enhancing (Figure 2). Other structures in the knee were of normal appearance.
The patient was diagnosed with lipoma arborescens after the workup and consented for left knee arthroscopic biopsy and débridement. On diagnostic arthroscopy, synovial villous hypertrophy was present diffusely throughout the knee, with increased burden of disease in the suprapatellar pouch and lateral gutter (Figure 3). A near-total arthroscopic synovectomy was required to obtain adequate débridement of the lesions. Microscopic evaluation of the specimen revealed synovium with fatty infiltration, chronic inflammation, fibrosis, and neovascularization, with a final diagnosis of lipoma arborescens (Figure 4). The patient was allowed to fully bear weight immediately after the operation and began physical therapy to improve his range of motion once his incisions had healed.
At 6 months post-op, his knee pain and swelling had resolved. Physical examination at this point was notable for a continued 7° flexion contracture compared with the contralateral side, bilateral knee flexion to 140°, and bilateral 5/5 quadriceps strength. He was able to return to competitive baseball including running, fielding, and hitting.
Lipoma arborescens is a rare benign intra-articular tumor. It is characterized by the replacement of subsynovial tissue by mature adiposites and fibrous tissue, with resultant villous and lipomatous proliferation of synovial tissue in a branching or arbor-like pattern.1-7 The etiology of this disease is unknown.2,4-7 Lipoma arborescens has been described most commonly in the knee but has also been reported in the shoulder, elbow, wrist, hip, and ankle.1-6,8-10 It is commonly unilateral, but cases of bilateral disease have also been described.1,3,6-10 It most often presents in adults in the third to fifth decade.1-3,6,7 Cases in children are exceedingly rare, with less than 15 cases identified in the literature.1,3-5,7-11
Patients typically present with several months to years of pain without a history of trauma. An effusion may be present, and mechanical symptoms are rare. Radiographs are typically normal but can demonstrate features of degenerative joint disease, especially in older patients. Laboratory analysis is usually normal but can be used to exclude other possible diagnoses. MRI findings include a villous lipomatous proliferation of synovium in either a diffuse or dominant mass-like lesion, with characteristic hypertrophic adipose proliferation of the subsynovial tissue. These characteristic findings allow for a specific and precise diagnosis on MRI, especially using fat-suppressed or STIR sequences.6 Pediatric and adult cases of lipoma arborescens have similar presenting symptoms, imaging features, and histologic findings. However, adults with lipoma arborescens are more likely to have incidental associated findings on advanced imaging.6
The differential diagnosis for pediatric atraumatic knee chronic effusion includes juvenile rheumatoid arthritis, Lyme disease, acute rheumatic fever, tuberculosis, Behçet syndrome, meniscal malformation, hemophilia, and other synovial disorders such as pigmented villonodular synovitis, synovial hemangioma, synovial chondromatosis, and lipoma arborescens.
Clinical workup should include a detailed history including timing and nature of joint symptoms along with any history of pharyngitis, rashes, possible exposure to tuberculosis, travel to Lyme endemic areas, and any personal or familial history of bleeding disorders. Physical examination should include detailed examination of the affected extremity, including neighboring regions that may present with referred pain, such as the hip in this instance of knee pain.
Laboratory analysis to rule out conditions on the differential diagnosis should include complete blood count, C-reactive protein, erythrocyte sedimentation rate, antinuclear antibody screening, and screening for Lyme disease.
Lipoma arborescens has a characteristic and specific appearance on MRI that allows for diagnosis from imaging alone. Vilanova and colleagues described the typical features of lipoma arborescens on MRI. They found that lipoma arborescens presents as diffuse villous proliferation in 79% of cases and as focal pseudomasses in 21% and demonstrates hypertrophic adipose proliferation of the subsynovial tissue. The lesion follows fat on all sequences, with high signal intensity on T1 and T2 sequences and low signal intensity on fat-suppressed and STIR sequences. The magnetic resonance appearance of fatty synovial proliferation without other signal intensities within the lesion distinguishes lipoma arborescens from other diffuse pathologies of the synovium including pigmented villonodular synovitis, synovial chondromatosis, synovial hemangioma, and rheumatoid arthritis. Lipoma arborescens commonly enhances with intravenous contrast administration, but the absence of enhancement has been described. Characteristics of lipoma arborescens are similar between adults and pediatric patients, although adults have been reported to more commonly present with associated conditions in the knee. Common associated findings on MRI in adults include joint effusion (100%), degenerative changes (87%), meniscal tear (72%), synovial cysts (38%), bone erosions (25%), chondromatosis (13%), patellar subluxation (6%), and discoid meniscus (3%).6
A systematic review done by Wang et al investigated the role of knee arthroscopy for the treatment of lipoma arborescens and identified 28 studies including 71 knees in 65 patients. Patients included in the studies were aged 13 to 78 years and were treated with arthroscopic synovectomy with a follow-up from 3 weeks to 7 years. Arthroscopic synovectomy was found to have an overall success rate of >95%. Recurrence occurred in two patients (2.8%) who were both treated successfully with repeat arthroscopic synovectomy. The cause of the recurrences was unknown; however, it may have been secondary to incomplete synovectomy because repeat synovectomy was reported to be curative. Intraoperative conversion to open synovectomy was required in two patients (2.8%), persistent pain was noted in one patient, and one patient required an unplanned return to the operating room for evacuation of a postoperative hematoma.7 Because this condition is often chronic in nature and most patients experience favorable outcomes, it does not seem that delayed excision has a negative patient impact. However, individual patient factors may warrant earlier surgical treatment, such as the presence of a knee flexion contracture in this case.1 Therefore, we elected to proceed with arthroscopic synovectomy for the treatment of our patient.
Lipoma arborescens is a rare intra-articular benign tumor that should be considered in the differential diagnosis for patients with chronic joint pain and effusion. Patient history, physical examination, and laboratory analysis can help to narrow the differential diagnosis. MRI is a valuable tool because lipoma arborescens has a characteristic and diagnostic MRI appearance. Lipoma arborescens of the knee can be safely and effectively treated with arthroscopic synovectomy.
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