Purpose or Case Report: Underlying causes for loss of knee flexion in a child are myriad. Once an intrinsic cause has been excluded and a physical block to flexion has been confirmed, pathology within the extensor component must be sought. Idiopathic contractures, congenital contractures, and fibrosis of multiple components of the quadriceps muscle as causes of limitation to flexion of the knee have been reported in the orthopedic literature.

More recently, additional structures within the quadriceps muscle have been described, leading to the new designation of a ‘quinticeps femoris’ muscle. This has been termed both an accessory quadriceps femoris and a tensor vastus intermedius (TVI). The latter has been described as a previously unrecognised common variant of anatomy, whereas the former presents as a pathological entity, resulting in progressive fixed flexion of the knee.

Two such cases of 'quinticeps femoris' have presented to our institution. Both patients were reviewed by multiple health professionals and had a significant delay in diagnosis (of two and five years respectively), with marked progression of fixed knee flexion during this period. Both diagnoses were ultimately made on MRI, with the causative abnormality appearing as a fusiform structure of low signal intensity arising from the anterolateral proximal femur and blending with the common quadriceps tendon distally. This highlights the importance of radiologists being aware of this anatomical entity as they will often be first to suggest the diagnosis. Both patients underwent surgical release of the anomalous quadriceps band with significant functional improvement.

Here we discuss the presentation, underlying pathology, and treatment of this uncommon cause of restricted knee flexion, to our knowledge unreported in the pediatric imaging literature, as well as the need for its recognition and inclusion in the differential diagnosis of progressive loss of knee flexion.
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Conclusions: In cases of progressive fixed flexion of the knee where no intrinsic pathology is evident, an accessory quadriceps muscle should be included in the differential of the extrinsic cause. Our experience demonstrated this to be readily identifiable on MRI, necessitating its inclusion in the radiologists’ checklist. Early recognition of this rare condition is desirable to prevent unnecessary intervention such as repeat knee arthroscopy, and debilitating loss of flexion due to delayed diagnosis.