Introduction: Hoffa’s fat pad (HFP) of the knee is affected by a variety of tumours and tumour-like conditions. HFP can be affected by diffuse or solitary, focal disease. Solitary tumours are relatively uncommon but with widespread uptake of Magnetic Resonance Imaging Scans (MRI) an increasing number of Hoffa’s fat pad tumours (HFP) are being recognized.

Methods: This paper reports a consecutive series of 20 cases of solitary symptomatic HFP tumours referred to Oxford bone and soft tissue tumour service between 1999 and 2008. The commonest presenting symptom was anterior knee pain. All patients underwent open excision after diagnostic magnetic resonance imaging (MRI).

Results: Histology revealed varied diagnoses with the commonest being pigmented villonodular synovitis (PVNS) and ganglia. American Knee Society scores improved from 76 pre-operatively to 96 post-operatively with an improvement in functional scores from 92 to 100. In one patient, MRI identified the cause of hypo-phosphataemic osteomalacia as an HFP phosphaturic mesenchyma tumour despite the lack of local symptoms.

Discussion In conclusion the majority of solitary HFP tumours are benign and may be either cystic or solid. MRI and plain radiographs are the imaging of choice. The definitive treatment of both cystic and solid tumours should be selective arthrotomy and excision biopsy. Arthroscopic resection is not advised, as complete excision is not always possible. None of the 20 patients in this series had a malignant tumour but this has been reported in the literature. Calcification on plain radiographs may indicate a malignant lesion. All patients in our series reported substantial improvement in symptoms following open tumour resection.