The patient presented for evaluation of a right sided pelvic limb lameness. The owners reported that the patient became lame on his right pelvic limb after spending the day throwing the ball three months prior to presentation. On presentation to orthopedic surgery grade II/IV right sided pelvic limb lameness with mild muscle atrophy was noted. Pain was appreciated and repeatable on extension of the hip. Remainder of orthopedic examination was unremarkable.

A well-defined rounded lucency is noted within the medial aspect of the right femoral head with a sclerotic rim. On the extended leg VD views there are 1-2 small round osseous fragments summating with the right hip joint adjacent to the radiolucent defect. These opacities are not clearly appreciated on the frog leg VD view, but irregularity of the articular margin of the femoral head is visible on this view. Minimal remodeling is seen along the dorsal aspect of the right femoral neck and the rim of the right acetabulum. The right hip is mildly subluxated on some views. Diagnosis: Large concave defect within the right femoral head/neck with adjacent small bone fragments most consistent with osteochondritis dissecans. Minimal secondary degenerative joint disease.

A 1.3 cm high by 1.1 cm wide by 1.5 cm deep spherical soft tissue attenuation is noted within the right femoral head with surrounding subchondral sclerosis. Along the craniomedial margin of this defect, several mineral fragments are noted. A migrated mineral fragment is noted within the mid joint capsule as well. Overall the right femoral head is reduced and acetabular coverage with increased soft tissue between it and the acetabulum. Smooth periarticular osseous proliferation is noted within the acetabular rim femoral head and neck region.

Right femoral head osseous cyst-like lesion with associated regional mineral fragments. This change is likely due to failure of endochondral ossification and associated mineral fragments (osteochronditis dessicans). Response to previous trauma is unlikely. Secondary degenerative joint disease. Continued FHO with submission of the femoral head for histopathology was recommended.

Cartilage defects were appreciated on the femoral head at the level of the round ligament. The femoral head separated through the lesion appreciated on radiographs/CT scan.

The articular cartilage is variably thickened and irregular with areas of fragmentation, clefting, erosion and loss. In some regions it appears essentially normal while in others it has a proliferating, hyperplastic rim of chondrocytes along its deep aspect. These proliferating chondrocytes then intermingle with irregularly shaped foci of immature woven bone spicules. Many of these immature spicules have retained cartilage cores and portions of the marrow space also contain a myxomatous chondroid matrix. Diagnosis: Irregular articular cartilage formation with clefting and erosion, irregular chondrocyte proliferation and retained cartilage cores. Evaluation of multiple sections of femoral head reveal findings compatible with a developmental bone disorder that appears most consistent with a stage of osteochondrosis (i.e. osteochondritis dissecans). There are varying degrees of cartilage loss and erosion, but no evidence of an underlying osteomyelitis or neoplasia.

Osteochondrosis dissecans of the femoral head is not commonly reported in dogs^2,4 with only two case reports noted within the literature dating back to the late 80’s with no cross sectional imaging available in the reports. It has been described as focal subchondral defects in in Labrador retrievers, Border Collies, and Pekingese dogs1. The pathogenesis of articular osteochondritis dissecans is still not fully understood. Proposed pathogenesis⁷ suggests focal interruption of the anastomosis at the osteochondral junction of the cartilage canal vessels and subchondral vessels cause large areas of necrotic cartilage that resist vascular invasion. These regions persist during growth and are replaced by fibrous tissue undergoing intramembranous ossification, and if overlying articular cartilage fissures, an osteochondrosis dissecans lesion develops. Most animals with osteochondritis dissecans present when there are clinical signs at which point the lesion is chronic and the subchondral bone defect is well developed. Survey radiography is a sensitive modality for detection of these types of lesions³. Additional imaging options such as CT and MRI can provide further information into extent and regional subchondral bone changes as well as region of migrated cartilage fragments.

_{1. Castro, Patrícia F et al. “What is your diagnosis? Osteochondritis dissecans.” Journal of the American Veterinary Medical Association vol. 235,2 (2009): 151-2.}

_{2. Johnson AL, Pijanowski GJ, Stein LE. Osteochondritis dissecans of the femoral head of a Pekingese. J Am Vet Med Assoc 1985;187:623–625.}

_{3. Kippenes H, Johnston G: Diagnostic imaging of osteochondrosis. Vet Clin North Am Small Anim Pract 28: 137, 1998.}

_{4. McDonald M. Osteochondritis dissecans of the femoral head: a case report. J Small Anim Pract 1988;29:49–53.}

_{5. van Osch GJVM, Brittberg M, Dennis JE, et al: Cartilage repair: past and future—lessons for regenerative medicine. J Cell Mol Med 13:792, 2009.}

_{6. Ytrehus B, Carlson CS, Ekman S: Etiology and pathogenesis of osteochondrosis. Vet Pathol 44:429, 2007.}