The patient complained of a painful long-lasting suprapatellar mass of the right knee causing movement impairment; he was non-responder to anti-inflammatory therapy. History and physical examination were otherwise negative. Preoperative routine blood chemistry evidenced a high serum urate concentration (11.3 mg/dL; upper normal limits 8 mg/dL).
Plain radiography of the knee showed a focal osteolysis with sclerotic margins of the upper pole of the patella, swelling of the suprapatellar soft tissue and irregular calcifications at the insertion of the femoral quadriceps tendon (Fig. 1a, b). On MRI the femoral quadriceps tendon was thickened, inhomogeneous and encompassed by a fusiform tissue swelling eroding the upper pole of the patella. The peritendon swelling showed an intermediate signal on T1W-MR (Fig. 2a). On T2W-FS-MRI the lesion was hypointense but inhomogeneous because of small hyperintense foci more conspicuous in the lytic lesion of the patella (Fig. 2b, c). No other remarkable findings were evident. Surgical toilette and tenorrhaphy of the femoral quadriceps tendon were performed.
Tophaceous gout is the chronic manifestation of sodium urate microcrystals precipitation in the articular cartilage, subchondral bone, synovial membrane, capsular tissues and tendons; clinical presentations include monarticular or polyarticular arthritis [1, 2]. Cutaneous and subcutaneous tophi are typical of an advanced disease and are usually associated with an articular involvement [2]. Recurrent gout attacks, hyperuricaemia, evidence of subcutaneous tophi, periarticular or intra-articular masses with marginal osteolysis make the diagnosis easy to achieve, but lack of these criteria can cause a radiological misdiagnosis [2]. Pathogenesis of bone erosion remains unclear; possible explanations comprise a mechanical pressure from tophi, developing of erosive synovitis caused by urate crystals, production of enzymes digesting cartilage and osseous matrix, osteoclast activation within the tophus [2-7].
Radiologically findings of chronic gout are appreciable after many years from the onset of the disease and include soft-tissue or intraosseous mass, non-demineralising erosive arthropathy with sclerotic or overhanging margins [2, 3, 8]. Advanced features are articular space involvement, periosteal new bone formation, extra-articular erosions, intraosseous calcification, joint space widening and subchondral collapse.
CT can easily depict tophi, which a typical density of 160-170 HU, and their relationship with bone erosion. Preliminary reports suggest high sensitivity of dual energy CT in detecting of tophi, also in the preclinical phase [9, 10].
Because its intrinsic capability in tissue characterisation MRI is the procedure of choice to identify bone, articular and soft tissue tophi. However these findings are not specific since tophi show a broad spectrum of signal intensity depending on arrangement and variable contribution of protinaceous material, fibrous tissue, microcrystals and haemosiderin [2, 11, 12]. Tophi are usually isointense to muscles on T1W-MRI and a heterogeneous intermediate to low signal on T2W-MRI, relying on inflammatory and/or oedematous components [2, 7, 11]; a peripheral or inhomogeneous enhancement of tophi can be appreciated after gadolinium administration [2, 11, 12]. Tophaceous gout can involve all the components of the knee but with some preferential localisations: the Hoffa pad, the anterior articular recess, the intercondylar notch, the intercondylar roof and the popliteal groove [2, 7, 11]; in most of cases erosion of adjacent bones coexists [2].
MRI is not indicated in patients with a clinically and radiologically typical disease; nevertheless, because of a possible atypical presentation, the radiologist should consider the diagnosis of tophaceous gout if a heterogeneous low or intermediate periarticular mass eroding the adjacent bone is detected on T2W-MRI [13].
Patellar tophaceous gout involving the quadriceps femoris tendon
Based on the provided X-ray (anteroposterior and lateral views) and MRI images of the knee joint, the following key features are observed:
Overall imaging characteristics indicate a chronic process with involvement of the local soft tissue or joint structures.
Combining imaging findings with the patient’s hyperuricemia (serum uric acid: 11.3 mg/dL) and medical history, the following possibilities should be considered:
Based on the patient's clinically evident hyperuricemia (serum uric acid: 11.3 mg/dL), a history of chronic joint swelling and pain, and imaging findings of a soft tissue mass in the suprapatellar bursa with local bone erosion and heterogeneous signals on MRI, the most probable diagnosis is:
Gouty Arthritis with Tophi (Gouty Tophi).
If there is a need to rule out other rare conditions, further laboratory or histopathological examinations can be performed. However, given the current evidence, gouty tophi is the most likely diagnosis.
For gouty arthritis with tophi, the following measures can be considered:
Disclaimer: This report is a reference analysis based on the provided information and does not replace in-person consultation or professional medical advice. Patients should seek further diagnosis and management under the guidance of a specialist, taking into account their specific clinical circumstances.
Patellar tophaceous gout involving the quadriceps femoris tendon
