The Tarsus (Hock)



Osteochondritis Dissecans (OCD)

The most common joint in which to diagnose osteochondritis dissecans (OCD) in the horse is the tarsus. The most common location for OCD is the cranial distal aspect of the intermediate ridge of the tibia. Examples of this can be seen in Figures 64 and 65. Many horses with OCD involving the intermediate ridge of the tibia are not lame. The most common clinical finding is effusion of the tibial tarsal joint (Bog Spavin). If there are multiple or large fragments of bone present, clinical signs may also include a variable degree of lameness in addition to the effusion. Many horses with OCD of the intermediate ridge of the tibia are asymptomatic even though fragments of bone are seen in the purchase exam radiographs. In most instances, these small fragments of bone are not radiographically significant. When the fragments are associated with clinical signs, arthroscopic surgery for removal is very successful. Therefore, they are worth noting when seen, but probably do not pose a serious threat to the future soundness of the horse.

Figure 64. Figure 65.
Medial oblique radiograph of the tarsus demonstrating a well rounded bone fragment involving the cranial and distal aspect of the intermediate ridge of the tibia consistent with a radiographic diagnosis of osteochondritis dissecans. Lateral radiograph of the tarsus demonstrating a well rounded fragment of bone along the cranial and distal aspect of the intermediate ridge of the tibia consistent with a radiographic diagnosis of osteochondritis dissecans.


Figures 66 and 67 demonstrate a large OCD fragment involving the distal aspect of the lateral trochlear ridge of the tibial tarsal bone. OCD of the distal aspect of the lateral trochlear ridge of the tibial tarsal bone frequently presents with effusion of the tibial tarsal joint and lameness. The fragment of bone is usually quite large and can compromise a large portion of the lateral ridge of the tibial tarsal bone. These fragments of bone can be removed arthroscopically, but the prognosis is not always as favorable as described for the intermediate ridge of the distal end of the tibia. In as much as the fragments are quite large and frequently involve a large portion of the lateral trochlea ridge of the tibial tarsal bone; caution should be exercised when evaluating these abnormalities as part of the purchase examination.

Figure 66. Figure 67.
Lateral radiograph of the tarsus with a large fragment of bone (osteochondritis dissecans) involving the distal aspect of the lateral trochlear ridge of the tibial tarsal bone. Medial oblique radiograph of the tarsus demonstrating a large fragment of bone (osteochondritis dissecans) involving the distal aspect of the lateral trochlear ridge of the tibial tarsal bone.


Other areas in the tarsus have also been identified as locations for OCD lesions. They include the following anatomic locations: the medial trochlear ridge of the tibial tarsal bone (Figure 68), the medial and lateral malleoli of the distal end of the tibia, and the medial aspect of the fibular tarsal bone (Figures 69 and 70).

Figure 68. Figure 69.
OCD lesion located along the distal aspect of the medial trochlear ridge of the tibial tarsal bone. A radiograph demonstrating an OCD type lesion located along the medial aspect of the fibular tarsal bone. The projection utilized to demonstrate the abnormality is the tunnel or skyline projection of the tarsus.


Figure 70. A xeroradiograph demonstrating an OCD type lesion located along the medial aspect of the fibular tarsal bone. The projection utilized to demonstrate the abnormality is a tunnel or skyline projection of the tarsus.

Occasionally an OCD fragment located along the cranial and distal aspect of the intermediate ridge of the tibia will detach itself from this location and be seen along the distal aspect of the tibial tarsal bone. Figures 71 and 72 demonstrates this phenomenon. This can be seen in completely sound horses that will remain sound.
Figure 71. Figure 72.
OCD lesion originally seen along the cranial and distal aspect of the intermediate ridge of the tibia. Follow up radiographs to Fig 71 taken at a later date demonstrate that the fragment of bone is now located along the cranial and distal aspect of the tibial tarsal bone.