MRI of the Fetlock Region

Introduction
The metacarpo(tarso)phalangeal joint (MPJ) or fetlock joint is often a source of lameness in sport horses of many different disciplines. However, lameness referable to the fetlock region can sometimes be difficult to accurately diagnose. A careful and complete examination of the distal limb of the horse will often indicate the fetlock as a clinical problem. A positive distal limb flexion, frequently thought to be associated with a MPJ problem may actually be due to lameness from elsewhere on the distal limb. In addition the clinical examination may not conclusively identify the MPJ as the source of lameness. Confirming the MPJ region as the source of pain is critical to direct imaging of the appropriate area of this region. Placement of local analgesia directly into the MPJ (intra-articular or intra-synovial) often results in a rapid and significant resolution of the lameness associated with the fetlock joint. In contrast some conditions that affect this joint (particularly in young racehorses in race training) do not respond to intra-articular analgesia but require perineural (regional) diagnostic analgesia to resolve the lameness. Regional diagnostic analgesia should be performed in a distal to proximal sequence with the clinician being cognisant that properly placed nerve blocks can anaesthetise structures proximal to the site of injection. If the fetlock is considered the source of lameness then intra-articular analgesia of the MPJ should be performed first. If this is negative then analgesia of the digital nerves at the level of the base of the sesamoid nerve block should then be performed eliminating the distal limb as a source of pain. If this is negative then a low palmar/plantar nerve block to isolate the MC region as the source of the lameness should be performed. [...]