Enostosis  is defined as 'a mass of proliferating bone within a bone' or 'a morbid bony growth developed within the cavity of a bone or on the  internal surface of the bone cortex' (Stedman 1990; Dorland 1994).  Enostosis is a general term used synonymously with bone islands in man (Greenspan et all. 1991) and for panosteitis in the dog (Lenehan et al. 1985).

"Equine enostosis-like lesions are characterized radiographically by focal or multifocal, intramedullary sclerosis located in the diaphyseal region of long bones near the nutrient foramen.  Apparent bone formation in horses with the enostosis-like condition was seen most consistently along the endosteal surface of the affected bones.  The lack of cortical or periosteal involvement is used to  differentiate enostosis-like lesions from cortical stress fracture.....sometimes associated with lameness, and exhibit consistent scintigraphic and radiographic progression and resolution over a period ranging  from several weeks to a few months." (Bassage and Ross 1998)

Enostosis of the humerus:   Although it may appear that the lesion is on the caudal cortex of the humerus in the lateral image the cranial caudal view may be equivocal as to the location, cortical or medullary.  Radiographs confirmed poorly marginated  increased opacity to the medullary cavity in the mid to distal diaphysis of the humerus, most likely enostosis.