Professor of Radiology, Paediatric Radiologist. Department of Radiology, Maastricht University Medical Center, Maastricht, The Netherlands
The paediatric musculoskeletal system differs from the adult musculoskeletal system in many ways. These differences account for many unique features in paediatric musculoskeletal imaging. Awareness of these differences and good knowledge of normal variants is important to prevent false diagnoses. Many normal variants may simulate disease, especially in paediatric radiology. Some growth plates have an undulating or angular course that does not run completely parallel to the x-ray beam. At these sites some parts of the growth plates project distant from the growth plate and simulate fractures e.g. distal tibia en proximal humerus. But also “simple” growth plates can produce confusing radiographs when they are not optimally projected. Another confusing bony entity is the irregular mineralization that may occur in some epiphyses that simulates fragmentation, e.g. the distal femoral epiphysis and trochlea of the elbow. Also the mineralization centres of apophyses may appear as avulsion fractures especially when the radiograph is taken after a trauma. Well known examples are the epiphyses and apophyses around the elbow (figure) and the apophysis at the base of the fifth metatarsal. The small physiological offset of the distal fibular epiphysis might be mistaken for a Salter-Harris type I fracture, especially in spoke wheel bicycle accidents. Because of all these variants, the book of Keats (“Normal variants that may simulate disease”) is indispensable in a radiology department. However, a sound knowledge of these variants and good clinical judgement are necessary because one should always realize there is another book, with the same number of pages and even the same images with the title: “Diseases that may simulate normal variants”. MRI is useful to discriminate between normal variants and pathology, e.g. is a fragmented epiphysis a normal variant or pathologic (fractured, infarcted, infected), especially in patients who experience pain at the site of an apparent normal variant. In these questionable cases marrow edema , soft tissue edema and joint effusion favour pathology.