Signalment:  

9-year-old, gelding, Swedish Warmblood riding horse (SWH), EquineThe horse had been lame in different limbs for more than one year. The clinical examination this day showed mild right front limb lameness. Some problems with the left hind limb were also noticed, with pain from the superficial digital flexor tendon. Images including plain radiographs and scintigraphic examination showed intense focal activity of the distal right humerus (Fig. 4-1). Radiographic examination revealed suspected radiodensity in the mid distal right humerus in close proximity to a vascular channel. The lesions were considered as enostosis-like. The horse had previous problems with gait asymmetries for more than a year and also showed evidence of tendonitis of the superficial digital flexor tendon of the left hind limb, hence the owner elected euthanasia.

Gross Description:  

A chronic tendonitis of the superficial digital flexor tendon of the right hind leg was found. Lesions were also seen in right distal humerus macroscopically after the bone had been sawed (Figs. 4-2, 4-3). Bone tissue in trabecular and dense structures could be seen in the medullary cavity of the distal third (metadiaphyseal) humerus

Histopathologic Description:

Sections from the distal area of humerus show a normal compacta with lamellar bone. In the medullary cavity intermingeled with adipose tissue, bone growth of trabecular bone tissue is found (Fig. 4-4). The bone trabeculae are lined by several osteoblasts and are composed of woven bone but often lined by lamellar bone structure. The larger trabecular are predominately of a lamellar pattern. A few osteoclasts can be seen adjacent to bone trabeculae. This type of idiopathic nonneoplastic bone growth in the medullary cavity would suggest enostosis-like lesions. The numerous osteoblasts explain the increased radiopharmaceutical uptake seen on scintigraphic examination.

Morphologic Diagnosis:  

Intramedullary trabecular bone growth with osteoblastic activity compatible with enostosis-like lesions, right distal humerus, equine

Condition:  

Enostosis

Contributor Comment:  

In man (2), enostosis is described as a relatively common lesion (14% in cadaveric material). Enostosis has been defined as a mass of proliferating bone within a bone. The lesions are often referred to bone islands, calcified medullary defects, cortical defects, enosteomas. Enostoses of man are mostly found in the pelvis, spine, ribs and metaphyseal and epiphyseal parts of femur. Scintigraphy often shows absence of radiopharmaceutical uptake, indicating chronic lesions without osteoblast activity.

Canine panosteitis is described as an idiopathic disease, characterized by bone sclerosis of the diaphyses and metaphyses of long bones of large breeds. The dogs may be showing signs of illness with fever. Lameness is often presented as intermittent involving more than one limb. Multifocal areas of radio densities, often in association with nutrient foramen can be seen on radiographs. The bone changes will disappear and the dog recover completely.

In equine, enostosis-like lesions were first described as multifocal sclerotic areas within the medullary cavity of long bones and were reported in six Thoroughbreds, three Standardbreds and one Appoloosa(1). Most of the horses had a history of chronic lameness. The bone sclerotic areas seen on radiographs and as abnormal increased radioisotope uptake at scintigraphic examination were localized to the diaphyseal areas of the long bones. The lesions were endosteal and/or medullary. These lesions were named enostosis-like in order to differentiate them from enostosis (bone islands seen in man) and panosteitis seen in dog. Later, Ramzan (4) also described enostosis-like lesions in 12 Thoroughbreds. Some of these lesions appeared related to lameness, but in many horses no association could be made. The author concluded that enostosis-like lesions are transient phenomena, with scintigraphic and radiographic resolution occurring over months. Multiple enostosis-like lesions has been reported in a racing Thoroughbred.(3) As treatment of the disease, box rest and controlled exercise during 1-6 months have been suggested. Most of the horses recover and return to working or racing.

The etiology and pathogenesis of theses lesions are not known. Often the bone lesions have been found in the diaphyseal area close to the nutrient foramen, hence bone infarcts has been discussed as a cause. Stress fractures are also mentioned, but never proven to be associated with the radiodense areas. Microscopic examinations of the enostosis-like lesions in horses are not presented in the literature, hence a comparison of the present case and the reported enostosis-like lesions can not be made; however, the radiographic and scintigraphic findings are similar.

The present microscopic picture is that of a non-neoplastic non-specific bone growth with osteoblast activity, within the medullary cavities, very similar to changes seen microscopically in canine panosteitis. 
This horse had had a long history of gait asymmetries also involving tendonitis and the owner elected euthanasia. The enostosis-like lesions probably would have resolved and was not the reason for euthanasia.

JPC Diagnosis:  

Bone: Intramedullary woven bone formation (enostosis), focal, moderate

Conference Comment:  

The contributor briefly mentioned canine panosteitis, a disease of unknown cause often seen in young large and giant breed dogs. Clinically, shifting leg lameness is often seen in affected animals. Thickening of the bone can occur on the periosteal and endosteal surfaces.(5)

Clinically, an increase in radiodensity in the marrow is initially seen and is caused by rapidly expanding areas of fibrovascular tissue that is quickly remodeled and converted to woven bone. Because of this rapid cyclical process of formation and resorption of bone, both osteoblasts and osteoclasts are often observed in the same microscopic field. Resting and reversal lines can also be present in relatively close proximity within medullary trabecular bone.(5)

Inflammation is usually not a feature of this disease, and the lesions resolve spontaneously over time. The cause is currently unknown, but canine distemper virus has been suggested as a potential suspect.(5)

References:

1. Bassage LH, Ross MW: Enostosis-like lesions in the long bones of 10 horses: scintigraphic and radiographic features. Equine Vet J 30:35-42, 1998
2. Gould CF, Ly JQ, Lattin Jr GE, Beall DP, Sutcliffe JB: Bone tumor mimics: avoiding misdiagnosis. Curr Probl Diagn Radiol 36:124-141, 2007
3. Jones E, McDiarmid A: Case report. Multiple enostosis-like lesions in a racing Thoroughbred. Equine Vet Education. 17:92-95, 2005
4. Ramzan PHL: Equine enostosis-like lesions: 12 cases. Equine Vet J. 14:143-148, 2002
5. Thompson K: Diseases of bones and joints. In: Pathology of Domestic Animals, ed. Maxie G, 5th ed., pp.104-105, vol 1. WB Saunders, Edinburgh, Scotland, 2007

Click the slide to view.

---

4-1. Bone, horse.

4-2. Bone, horse.

4-3. Bone, horse

4-4. Bone, horse.

---