The CT shows this lesion to be mostly sclerotic, although it has a small lucent component in its superiormost extent (upper left image). It is only slightly expansile (best seen on digital AP radiograph). Normal trabeculae are noted within this lesion, but they appear a bit denser than the other trabeculae outside of the lesion. It has a sharp zone of transition, no periosteal reaction, and lies eccentrically in the distal tibia adjacent to the lateral cortex.
This radiographic appearance strongly suggests a benign lesion. The most likely lesion in this age group would be a healing fibrous cortical defect. Fibrous dysplasia would be a less likely possibility.
If one were to radiograph the legs of kids between the ages of 4 and 5, one would find fibrous cortical defects in about half of them, especially around the knees, the most rapidly growing part of the skeleton. They represent small rests of fibrous tissue left behind as the bone grows in length. However, if you look at radiographs of adult legs, you only see fibrous cortical defects in a small fraction of cases. This fact tells us that the usual natural history of fibrous cortical defects is that they involute on their own as the child grows up. However, some of them can linger on into adulthood, where they usually ossify or "heal". The main significance to these common benign lesions is that they are not mistaken for something more ominous and biopsied. The roentgenographic appearance is so characteristic that few if any of these lesions should ever undergo biopsy. Once in a rare while, a patient may develop a stress fracture through one of these.
In some patients, these lesions may actually grow a bit and present as an expansile lesion. They are then known as non-ossifying fibromas. If left alone, they will usually involute on their own and ossify, whereupon they become known as ossifying, non-ossifying fibromas. It is names like this that sometimes give medical nomenclaturists a bad name -- as it should.