Gout is the most prevalent form of inflammatory arthritis, affecting middle-aged men. It is characterized by the deposition of monosodium urate (MSU) crystals in and around joints and other tissues ^[1]. Clinical manifestations can present as recurrent acute attacks of severe pain, or chronic inflammation affecting peripheral, small and large joints with tophaceous intra- or extra-articular deposits ^[2]. However, it could also present atypically as intra-articular cyst without any history of inflammatory arthritis. Such presentations are very challenging to surgeons due to broad differential diagnosis ^[3]. Using conventional imaging such as magnetic resonance imaging (MRI), it is difficult to differentiate between these pathologies as they lack specific features. Herein, we report a case of posterior cruciate ligament (PCL) cyst which finally got diagnosed as gouty tophi mimicking as PCL cyst.

A 25-year-old male, formerly a manual worker, presented with pain in the posterior aspect of left knee and occasional locking episodes and stiffness for 3 weeks with no history of trauma. On local examination, tenderness present at posterior aspect of knee without any effusion and restricted terminal flexion, there was no ligament laxity, pain aggravated on work, and relieved on taking analgesics, there were no constitutional symptoms. X-ray was normal, the patient was not responding to conservative therapy; hence, MRI was advised. MRI revealed a well-defined T2/PDW intermediate signal intensity cyst of size 10.4 × 9.3 × 8.4 mm noted adjacent to upper third of PCL toward lateral aspect between anterior cruciate ligament [ACL] and PCL with normal signal intensity of PCL and the rest of the joint appearing normal, likely PCL ganglion cyst (Fig. 1), surgery was scheduled with arthroscopy and excisional biopsy of cyst, during arthroscopy a cyst was seen attached to PCL (Fig. 2) with the rest of the joint appearing normal. On puncturing the cyst with 18G needle, white chalky paste-like calcified tissue extruded out (Fig. 3), the cyst was resected and sent for histopathology and the rest of the tissue debrided without injuring PCL (Fig. 4). histopathological analysis revealed inflammatory cell collections and tophi composed of gout crystals and giant cells features suggestive of gouty arthritis (Fig. 5). Synovial lavage fluid showed elevated uric acid and needle-shaped MSU crystals. Postoperatively, serum uric acid levels were 9.4 mg/dL. The patient had drastic relief of pain postoperatively and was kept on febuxostat and advised dietary modifications. Follow-up after 2 months revealed good range of motion and complete cessation of pain and serum uric acid 7.2 mg/dL. The patient was followed till 18 months during which there was no recurrence of symptoms.

Tophi are the pathognomonic lesions of gout appearing in subcutaneous tissue with a predilection for peripheral joints of lower extremities [4, 5]. Tophi occur as late clinical presentation of gout. However, tophi can occur atypically in and around knee as tophi of patellar tendon, within synovium, patella, intercondylar notch, and cruciate ligaments. Such acute presentation in patients without prior inflammatory arthritis may mimic neoplasm [6, 7]. They may also occur at precarious positions which can lead to fatal problems, such as spinal cord, carpal tunnel, and heart valves [8, 9]. Gouty tophus presenting as acute intra-articular cyst is very difficult to differentiate from other intra-articular pathologies through conventional MRI. Common pathologies of intra-articular mass include ganglion cysts, pigmented villonodular synovitis, meniscal cysts, synovial cysts, synovial chondromatosis, Lipoma arborescens, synovial sarcoma, pannus, and gouty tophi ^[10]. Most of these lesions including gout show intermediate signal intensity on T1-weighted images of MRI. MR features of gouty tophi have not been studied in detail, and MRI appearance of tophi is non-specific ^[11]. On review of literature, few authors have reported atypical presentation of gouty tophi and their respective modes of management, most of these groups had difficulty in diagnosing tophi preoperatively (Table 1).

Gout could present atypically as intra-articular cyst without any prior symptoms and elevated serum uric acid levels; hence, it should also be considered in differential diagnosis of intra-articular cyst. Treatment of PCL cyst is uniform throughout literature, some groups did arthroscopic debridement till base and some groups excised through open arthrotomy; here we performed an arthroscopic debridement and biopsy of specimen revealed it as gout. To accurately diagnose a gouty tophus, a dual approach using MRI and histological examination is necessary to differentiate it from other intra-articular cysts. No such case has been reported in literature till now, and findings of this report add to the growing knowledge base surrounding atypical presentations of tophi and their management.

Intra articular cysts in knee are difficult to diagnose, gouty tophi could atypically present in cruciate ligaments of knee without prior signs of inflammation and could be misdiagnoses with various knee pathologies, biopsy is the only diagnostic modality for such presentation; hence gout should also be considered in differential diagnosis of intra-articular cysts in knee.