It is important to localise the source of lameness before referring a case.  In particular, the abaxial sesamoid block alone is not sufficient. Horses referred for a foot scan should have a positive response to a palmar digital or coffin joint block.  Additional scans of the pastern and fetlock should be considered for horses that are positive to abaxial sesamoid but negative to more distal blocks.  You should ideally consider an MRI referral in the following cases:

• Lameness localised by nerve blocks to a specific region the of limb, where radiographs are negative or unclear and access by ultrasound is difficult
• Penetrating injuries need urgent attention
• Injury assessment where general anaesthesia is inadvisable
• Acute onset of lameness during exercise
• Racehorses at risk of fetlock fractures of palmar osteochondral disease through repetitive fast work
• Monitoring the progress of treatment and to assess readiness for competition

The referral clinic will need to know the case history and any previous diagnostic results.  After the MRI scan, they will provide an interpretation and radiological report.  Other options may also be available by arrangement:

• suggestions regarding treatment and prognosis
• an explanation to the client in appropriate language
• further case management or treatment

Fast work at high speeds during both training and racing places the fetlock, particularly the back of the joint, under extreme forces. This repetitive, extreme shock absorption by the joint can lead to traumatic lesions affecting the subchondral bone and/or cartilage. Whilst the lesions can be severe, horses can appear only mildly lame, ‘work out’ of the lameness or even appear sound but not performing as well as expectations. If left unchecked, these lesions can lead to catastrophic breakdown of the fetlock joint ending the racing career of an individual or resulting in catastrophic fracture and emergency euthanasia.

Although some of the more severe lesions can be seen on x-ray, often by the time they are visible, the disease has progressed. Detection with MRI, due to the superior ability to identify subtle changes earlier in the progression of the disease process, enables rest periods and training modifications to be made before clinical disease terminally affects the individual’s racing career. MRI can also be used to monitor a return to health of the joint before training for racing performance is resumed.