BACK TO BASICS: LAMENESS AND PODIATRY Diagnosis Figure 3.
congestion, these findings have not been confirmed histologically.18 The changes categorized as “degenerative” are ac-
tually more representative of osteoarthritis and degenerative joint disease.10,11,13,14,29 Repetitive biomechanical forces exerted on the navicular appa- ratus lead to damage of the fibrocartilage on the flexor surface of the navicular bone, which subse- quently traumatizes the subchondral bone. This can lead to the aforementioned increases in pressure within the medullary cavity, edema, cyst formation, and osteonecrosis of the bone itself.1,10,12 In addi- tion to changes to the navicular bone, advanced im- aging has helped us identify other potential sources of lameness involving the soft tissue structures of the foot. Horses can experience desmitis of the as- sociated ligaments, synovitis of the DIP joint or na- vicular bursa, and tendonitis/tears of the DDFT, which can cause pain.10,15 Pathologies such as ad- hesions, avulsion fractures of the navicular bone, and development of enthesophytes and mineralized foci in the components of the navicular apparatus have also been implicated as sources of lameness. The clinical significance of some of these findings is still under investigation.1,10,27
Most commonly, horses with palmar foot pain “block-out” to a palmar digital (PD) nerve block. Unfortunately, localizing the lameness is often the easiest part of the process. A PD nerve block can desensitize most of the structures within the hoof capsule and, therefore, leave a multitude of possibil- ities responsible for the lameness.1,10,26 For exam- ple, pedal osteitis of the coffin bone wings may present as heel pain and block to a PD nerve block. This is often seen in racehorses and may resolve with rest and therapeutic shoeing.27 Further local- ization of the lameness can be done by injection of local anesthetic into the navicular bursa or the coffin joint. Once the lameness is localized, the next step is to obtain radiographs. Radiographs may often show evidence of chronic inflammation in the form of osseous changes. However, radiographs are not al- ways conclusive because they do not give diagnostic information about soft tissue structures and osseous changes cannot be identified until there is at least a 40% change in bone density.35 This is a common problem when attempting to diagnose a lesion sus- pected to be involving the navicular apparatus.26 The author recommends taking six standard views of the digit in order to completely assess the foot.
Radiographic Views
● Lateromedial: Evaluate sole depth and the boney column alignment and its relationship with the navicular bone and DDFT. Also evaluate the navicular bone for sclerosis, dam- age to the flexor cortex, and mineralization of associated soft tissue structures.
● Dorsopalmar: Evaluate the mediolateral bal- ance of the foot, joints, and the margins of the navicular bone in a normal weight-bearing position.
● 60° downward dorsopalmar view: Evaluate the margins of P3 and the navicular bone.
● 60° downward dorsolateral-palmaromedial and dorsomedial-palmarolateral: Evaluate the distal border and the corresponding wing of the navicular bone.
● Skyline: Evaluate the medullary cavity for sclerosis, cysts, enlargement of synovial in- vaginations, and the integrity of the flexor sur- face of the bone.35
Figure 4. 376 2018 Vol. 64 AAEP PROCEEDINGS
In recent years, magnetic resonance imaging (MRI) has become an indispensable tool for diagnosing lameness causing lesions within the equine foot.1,15 MRI uses strong magnetic fields and radiofrequency pulses to image with great detail to both bone and soft tissue structures deep within the digit. With the use of MRI, we can tailor our treatment plan according to the pathologic findings.10,17–19 Some horses may benefit from coffin joint or navicular bursa injections, whereas other horses may just need rest. In many cases, therapeutic shoeing may
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