Tug Lesions (Metaphyseal fibrous defect)

(Cortical Irregularity Syndrome)

This is a nonneoplastic, reactive fibrous lesion that occurs at the insertion or origin of major muscles, and is characterized by focal cortical disruption, sclerosis, and thickening. Cortical desmoid and periosteal desmoid were also used to describe this lesion in the past, but since the lesion is not a neoplasm, these terms should be avoided. 

The origin is related to the forces (the "tug") exerted by the muscle on the periosteum and cortical bone in active adolescents.  Patients are often male, although the recent increase in popularity of girl's sports has made this a common lesion in both sexes.   Ages range from 3 to 20 years, with a peak in the early teen years.  The lesion may be an incidental finding, but an aching pain may occur. 

The distal femur is the most common site, either at the insertion of the adductor longus on the medial metaphyseal flare, or posteriorly where the gastrocnemius originates.  The humerus at the insertion of the pectoralis major is also a possible site. 

Radiographically, the lesions are 1 to 3 cm. in size, irregular, with areas of lucency and areas of sclerosis.  There may be some mineralization within the lesion. The margin may be latent or active.  A carefully formatted  MRI will show the relationship of the muscle insertion to the lesion, but the "severity" of the lesion may be overcalled on the MRI, and increase the anxiety levels all around.   Bone scan   will show uptake due to the presence of reactive bone, then become "cold" as the lesion involutes. CT scan will differentiate this lesion from osteoid osteoma and show the complex nature of the lesion, with areas of cortical thinning and   thickening, some possible small "cystic" araes, and the surrounding mild sclerotic bone reaction.

Biopsy is not indicated or warranted.  A lesion whose nature is in doubt should be referred by an orthopaedic oncologist.

On pathological examination the lesion consists of reactive fibrous tissue, cartilage, and bone with a nonmalignant appearance. The presence of poorly organized osteoid and hypercellularity   has lead to a misdiagnosis of osteosarcoma with tragic results. 

Treatment consists of reassurance, observation, and occasionally radiographic follow-up.  The prognosis is excellent and the lesion will spontaneously resolve in early adulthood.

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