A 19-year-old woman presents with a history of progressive right mid-shin pain, which is worse at night, but significantly improved with aspirin. She’s a long distance runner and otherwise entirely healthy. She’s not on any medications, nor does she have other abnormalities on physical examination. She was sent for a plain right tibia-fibula radiograph (Fig. 1), which lead to a nuclear medicine bone scan (Fig. 2) and a CT scan (Fig. 3).
The most likely diagnosis in this case is:
a) parosteal osteosarcoma
b) myositis ossificans
c) shin splints (stress fracture)
d) osteoid osteoma
The answer is d) osteoid osteoma.
Although relatively non-specific, the history of nocturnal bone pain, especially in the extremities, relieved by aspirin, is highly suggestive of an osteoid osteoma in a young patient presenting with the x-ray findings as revealed. The plain radiograph and bone scan findings on their own are relatively non-specific and based just on the x-rays. Only the diagnoses of myositis ossificans and enchondroma could be ruled out from the differential list provided. The CT appearance, though, combined with the young age and clinical presentation, essentially seals the diagnosis.
An osteoid osteoma is a benign bone lesion characterized by a small typically intracortical vascular nidus, which incites local bone osteoblastic reaction resulting in the appearance of a small lucent lesion wrapped in an envelope of cortical thickening, sclerosis and occasionally smooth non-aggressive periosteal reaction. There may be associated bone marrow edema on the MRI exam. Not all osteoid osteomas are typical and they can present great diagnostic dilemmas. They can occur in virtually any bone and when they develop within a joint can incite significant synovial reaction and joint effusion. They occur almost exclusively in patients younger than 30 years old and are treated successfully by either surgical excision (especially when the diagnosis is suspect) or percutaneous radiofrequency ablation therapy.
Parosteal osteosarcomas are low-grade malignant neoplasms that develop on the surface of the metaphyses of long bones, between the cortex and the adjacent muscle tissue. These often present as a fixed slow-growing painless mass, either in the proximal humerus or the distal femur (> 50% of cases). They appear on x-rays as dense heavily ossified broad-based fusiform masses that appear to partially encircle the involved bone. The tumour is typically separated from the underlying parent bone by a thin radiolucent zone. The appearance of the case presented is not typical for a parosteal osteosarcoma.
Post-traumatic myositis ossificans is a benign condition of heterotopic bone formation in soft tissues following local trauma and muscle tear and/or hematoma formation. It can be secondary to trauma, or even burns or surgery. It is a benign self-limited condition but can mimic soft-tissue and bone malignancies, especially in cases where the history of trauma is not prevalent or even absent. Additional directed questioning usually reveals a source for the local injury. Some cases can resolve on their own, in time. Others, particularly if they are near joints and impede range of motion, or are unusually symptomatic, may require surgical excision, with no guarantee the condition will not recur. Myositis ossificans can be confused with parosteal osteosarcoma. There’s an old rule of thumb for distinguishing the heterotopic ossification in myositis ossificans from parosteal osteosarcoma: if the bone formation is denser centrally, it’s more likely to be osteosarcoma; if it’s denser peripherally, with relative central lucency, it’s more likely to be myositis ossificans. The present case does not suggest myositis ossificans.
“Shin splints” is a layman term for a tibial stress fracture, a painful condition that develops from repetitive “microtrauma” and “overuse,” as is seen in long-distance running, and leg impact sports like long-jumping, aerobics, tennis, dance (especially ballet), volleyball and basketball. Shin splints can also be caused by training too hard, too fast or for too long without sufficient rest. Plain radiographs will often provide the diagnosis, revealing subtle diffuse lamellated periosteal reaction along the anterior surface of the proximal and/or mid-shaft region of the tibia. If x-rays are equivocal or negative, nuclear medicine bone scan will be positive, with linear increased uptake along the anterior tibial shaft. The condition is often bilateral, and if untreated could progress to full cortical fracture. It’s usually treated with cessation of the stressful activity, with or without local icing, anti-inflammatories and perhaps upgrading to improved footwear in appropriate cases. The present case could in theory represent a very early stress fracture on the x-ray, but the bone scan and CT findings aren’t in keeping with that diagnosis.
An enchondroma is a benign bone tumour characterized by a nest of cartilage cells residing in the marrow space, typically in a tubular long bone, resulting in a discrete geographic lucent lesion with or without endosteal scalloping but without additional aggressive features such as cortical breakthrough, associated soft tissue masses or periosteal reaction. Although benign, they can lead to thinning of the adjacent cortex to such a degree that the patient is at risk for a pathologic fracture with relatively minimal trauma (most often when the lesion resides in phalanges of the hands or feet, but also in the proximal humeri and femora). These lesions are typically solitary but have been linked to two clinical syndromes: Ollier’s disease describes a rare non-hereditary condition in which multiple enchondromas are present; when multiple enchondromas are discovered with cutaneous hemangiomas it’s known as Maffucci’s syndrome, and in these patients the enchondromas are associated with a much higher risk for potential malignant transformation not present in the solitary form or with Ollier’s disease. The lesion presented does not share any characteristics of an enchondroma.
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