Following on from part one as part of our diagnosing this issue, we talked about having radiographs or other scans and imaging done to help us diagnose the underlying problem.
Suppose the radiographs confirm there are no changes to the distal bone structures of the limb, especially to the navicular bone articulation surfaces, nor rupture of the internal blood vessels of the bone itself (as indicated by dark lines within the center of the bone or the attachment sites for the collateral and impar ligaments). In that case, we must investigate the alignment of the navicular bone in relation to the distal interphalangeal joint and the supporting apparatus, such as the distal Sesamoidean impar ligament, the collateral Sesamoidean ligaments, and the deep digital flexor tendon, as all these supporting structures can cause of palmar foot pain when they are forced to cope with the stresses of foot or upper body imbalance.
When viewing the radiographs of these cases, first look at the alignment of the navicular bone in relation to the middle phalanx, and then the alignment of the deep digital flexor tendon in relation to the flexor surface of the navicular bone. When the navicular bone is set more proximal on the distal end of the middle phalanx, then the deep digital flexor tendon will not be congruent with the navicular bone's flexor surface. It is possible to examine this on quality, digital radiographs. Other soft internal structures, such as the digital cushion, can also be scrutinized for their alignment and depth, as a dysfunctional finding will alter the solar border alignment of the distal phalanx within the hoof capsule, and affect the relationship of the distal collateral cartilages to the distal and middle phalanx bones.
Suppose the radiographs confirm that the navicular bone is positioned higher on the middle phalanx than is biologically correct. This would mean that the navicular bone would be more upright, and its distal end would shift towards the palmar aspect of the foot. This will affect the deep digital flexor tendon’s action as it slides back and forth over the navicular bone, increasing tension in the deep digital flexor tendon and the soft tissue structures that attach the navicular bone to the distal interphalangeal joint, for example, the distal impar ligament and the distal collateral ligaments.
In some cases, radiographs of a horse displaying a group of symptoms related to palmar foot pain or navicular syndrome, and we may conclude we are dealing with a disease of the distal sesamoid bone and not a syndrome. Nevertheless, the condition would have started as palmar foot pain and, if left undiagnosed or misdiagnosed, it progresses to navicular disease as time passes and the dysfunctional imbalance in the distal interphalangeal joint and upper body increase.
The high number of horses dealing with palmar foot pain has promoted a great deal of research into the anatomy of the palmar section of the equine foot in recent years and has advanced our understanding of the vascular system of the area. However, to quantify the clinical conclusions we need more research on the function of the supporting ligaments of the distal limb, and the role the navicular bone plays in the equine orthopaedic stance.
In our next blog will look at the location and role of the navicular bone and the physical forces impacting on the foot.