When faced with a distal Sesamoidean bone that is situated more proximal on the palmar aspect of the middle phalanx (as is seen in many equine radiographs), you have to understand its normal articulation is misplaced in relation to its opposing structures, the distal phalanx and, especially, the middle phalanx. Therefore, when the limb is loaded and the distal metacarpophalangeal joint or fetlock joint travels downwards, the middle phalanx is forced into early contact on its distal palmar aspect at the proximal end level of the bone’s condyle surface. This early and dysfunctional contact of the middle phalanx articulating on the proximal articular surface of the distal Sesamoidean bone results in two distinct dysfunctional issues being stress on the articular and flexor surfaces of the distal Sesamoidean bone, and the distal Sesamoidean bone being forced distally and cranially towards the center of the distal phalanx. This forcing of the distal Sesamoidean bone down and back towards the solar surface of the distal phalanx at the end of the loading phase of the distal limb applies untold stress on the distal Sesamoidean impar ligament and the distal Sesamoidean collateral ligaments that are there to fix and hold the distal Sesamoidean bone in correct biomechanical alignment within the distal interphalangeal joint.
The biomechanical changes do not stop there, as the fulcrum effect of the distal Sesamoidean bone with the deep digital flexor tendon will be altered, as is the deep digital flexor tendon’s insertion with the distal phalanx. I do believe that this dysfunctional anatomical misalignment is what produces the pathology changes noted in the autopsies. As mentioned in my earlier blogs, the pathology changes are evident in the texture and color of the deep digital flexor tendon at its distal insertion to the distal phalanx, and a yellowing of the body of the tendon as it passes over the flexion surface of the navicular bone, and the following signs of fibrous breakdown in the body of the deep digital flexor tendon, the distal Sesamoidean impar ligament, and the onset of arthritic changes in the joint between the distal phalanx and the middle phalanx, which include changes to the extensor processor of the distal phalanx and the extensor tendon itself.
When we turn our thoughts to the pathology changes of the dysfunctional palmar section of the equine foot, it appears that the distal Sesamoidean impar ligament is the first supporting structure to manifest problems due to accommodating the imbalance in the distal interphalangeal joint, and the dysfunctional arc of the distal Sesamoidean bone at the end of the limb’s loading cycle. In the early stage, when there was a group of symptoms of palmar foot pain, the impar ligament revealed signs of bruising at its proximal insertion point to the distal Sesamoidean bone, and this followed through to its distal insertion point, being the distal phalanx and the deep digital flexor tendon. Interestingly, the autopsy conclusions showed that the associated bruising of the structures involved was directly correlated to the orthopaedic balance of the corresponding limb. I predicted bruising would be associated with the corresponding orthopaedic balance upon examination of these cases before the autopsy, where the horse's structural balance and center of mass were established.
In palmar foot pain cases where the weight is loaded to one side of the distal Sesamoidean bone more than the other, due to the animal’s orthopaedic stance, the soft tissue involved presents with bruising on the corresponding weight-bearing side of the equine foot, that being the deep digital flexor tendon, distal Sesamoidean bone and distal Sesamoidean impar ligament. When the weight is carried more dorsally on the distal interphalangeal joint or towards the toe, pathological changes move from the sides to the center of the distal phalanx and insertion of the deep digital flexor tendon, and forward to the extensor processor of the distal phalanx and its connection with the extensor tendon running down the dorsal surface of the middle phalanx.
Next time we look at the interrelationship of the distal Sesamoidean or navicular bone and the rest of the foot and limb.