The aetiology of the non-ossifying fibroma of the distal femur and its relationship to the surrounding soft tissues

PURPOSE We aim to retrospectively evaluate patients with non-ossifying fibroma (NOF) of the distal femur by radiographs, CT and MRI, and to provide a theory describing the reasoning for the distal femur NOF's location and aetiology. METHODS Charts of patients with NOFs between 2003 and 2014 were retrospectively reviewed. Inclusion criteria encompassed a diagnosis of NOF of the distal femur by imaging, and histologically, if available. Radiographs, CT and MRI were used to characterise the relationship of the NOF lesions with the surrounding soft tissues. RESULTS The 68 NOFs from 60 patients were included. By radiograph, 41 (60.3%) of the 68 lesions appeared at the medial and 25 (36.7%) at the lateral aspect of the distal femur. In total, 41 lesions had CT scans, showing 22 NOFs (53.7%) attached to the origin of the medial gastrocnemius, 12 (29.3%) to the origin of the lateral gastrocnemius and four (9.8%) at the attachment of the adductor magnus. Of the CT scans, 93% identified the NOF's relationship with an adjoining tendon of the distal femur. Six had MRIs, all of which showed attachment at the medial gastrocnemius. CONCLUSION The study reveals a relationship between tendinous structures and NOFs. NOFs of the distal femur occur most commonly at the origin of the medial and lateral gastrocnemius. They may originate from the physis/metaphysis but they do not always attach to the physis, as we observe them 'migrating' as patients grow. More research is required to understand the exact aetiology of NOFs.