This patient presents to your clinic for right pelvic limb lameness beginning 3 weeks ago and moderate distension in the right tarsus.
You order right tarsal radiographs.
Findings
Associated with the medial trochlear ridge of the talus, there is a 1.6 cm concave defect (yellow arrows) with underlying subchondral sclerosis.
A moderate amount of increased soft tissue opacity is present within the tibiotarsal joint.
Diagnosis
Osteochondrosis of the medial trochlear ridge of the talus with moderate joint effusion and/or synovitis.
Osteochondrosis
Osteochondrosis (OC) is the most common developmental orthopedic disorder. It is defined as a focal disturbance in the process of endochondral ossification resulting from a failure of the blood supply to the growth cartilage. It leads to necrosis of chondrocytes; the area of ischemic chondronecrosis later resists replacement by bone and causes the focal delay in endochondral ossification.
Osteoscondrosis can appear as a flattening or a cleft defect in the subchondral bone. Bone cysts have been observed in association with osteochondrosis lesions in central, weight bearing regions of the epiphyses.
The cartilage superficial to an osteochondrosis lesion in epiphyseal growth cartilage can fracture or fissure and give rise to an intra-articular hinged flap or free-floating fragment, known as osteochondritis dissecans (OCD).
Osteochondrosis in the equine tarsus
The most frequently affected breeds are Standardbred, Thoroughbred and Warmblood.
The incidence of tarsal osteochondrosis has been estimated to be 10-19%.
It occurs more frequently in stallions and geldings.
6-10% of horses have bilateral lesions.
The incidence of OC in the tarsocrural joint is higher in horses born in the later part of the foaling season.
OC lesions in the tarsus are most often found in the following locations (from more to less commonly affected):
The distal intermediate ridge of the tibia (illustrated)
The lateral trochlear ridge of the talus (illustrated)
The medial malleolus of the tibia
The medial trochlear ridge of the talus (this case)
Schematic drawing from a DMPLO view of the tarsocrural area. Depiction of the more common placement of osteochondral bone defect at the dorsal part of the intermediate ridge of the tibia, and on the lateral trochlea of the talus. (Sandgren et al, 1993)
The evaluation should include four views: Dorsoplantar, Lateromedial, DMPLO and DLPMO
Lesions on the distal intermediate ridge of the tibia, medial and lateral trochlear ridge of the talus are usually better detected on either DMPLO or LM views
Lesions in the medial malleolus of the tibia seems to be better to detected in the DL30PMO view
Ultrasound has a high sensitivity (96%) to detect osteochondral lesion in the tarsocrural joint, specially for lesions in the medial malleolus of the tibia that may be difficult to detect in radiographs.
References
Olstad, K. Ostevik, L. Carlson, C. Ekman, S. Osteochondrosis can lead to formation of pseudocyst and true cyst in the subchondral bone of hoses. Veterinary Patology. 2015. vol 52(5) 862-872.
Sandgren, B. Dalin, G. Carlsten J. Osteochndrosis in the tarsocrural joint and osteochondral fragm,ents in the fetlock joints in Standardbred trotters. I. Epidemiology. Equine Veterinary Journal. Suppl.16 (1993) 31-37.
Shelley, J. Interpreting radiographs 5: Radiology of the equine hock. Equine Veterinary Journal (1984) 1696), 488-495.
Relave, F. Meulyzer, M. Alexander, K. Beauchamp, G. Marcoux, M. Comparison of radiography and ultrasonography to detect osteochondrosis lesions in the tarsocrural joint: aprospective study. Equine Veterinary Journal. (2009) 41 (1) 34-40.
