The rotators, or cranial rotation cases, will have rotation of greater than 15 degrees palmar angle and often 20 to 30 degrees
palmar angle. The horn lamellar zone (HLZ) will be 25 to 30 mm. The extensor process-coronary band (EP/CB) measurement will
be increased as the extensor process of the P3 tips downward with the rotation. The sole depth at the tip of P3 (SDT) will
be greatly decreased, with as little as 0 mm once the solar corium penetrates the mature sole. The sole depth at the wing
of P3 (SDW) will be greatly increased.1 Soft tissue changes will include triangular lucencies or lamellar wedges in the dorsal structures that are composed of gas
pockets and serum or purulent material.
Figure 4: A Grade IV laminitic event with reduced blood flow to the coronal cascade and the dorsal and circumflex circulation.
No solar fimbria are present, and the twisting of the circumflex circulation is indicative of the distal margin of P3 falling
below the circulation.
Venography views of the lateral hoof will show marked congestion or total absence of the coronal cascade circulation. The
dorsal lamellar and circumflex circulation will be absent, congested or markedly twisted at the tip of P3 as the margin of
the P3 slips out of the circulation (Figure 4). No fimbria will be present. The terminal arch is usually normal in appearance,
as is the bulbar circulation.2
The sinkers, or horses with FSS, usually start as cranial rotation cases but continue to sink into the hoof capsule as all
soft tissue loses its ability to support the bone column (Figure 5). Horses with FSS can have all the characteristics of a
cranial rotation case (because they have already experienced that part of the disease process) and are so structurally unsound
that the P3 then starts descending in the hoof capsule parallel to the ground.
Figure 5: A case of FSS in the healing phase. Note the cornified sensitive laminae and the twisted and stretched distal margin
that would be evident on a venogram.
Quittor type lesions or rupture of the lateral cartilages and rupture of the bulbs of the heels are not an uncommon sequela
to FSS. The sole becomes flat and very thin, and the entire hoof capsule may be freely twisted by hand.
Any horse with laminitis that has swelling within the pastern region that extends downward to the coronary band is an immediate
medical emergency that requires venography to determine the extent of congestion within the area and the degree of loss of
circulation distal to the swelling. Should all or almost all of the circulation be impeded distal to the pastern, a hoof wall
ablation and casting with a fixation cast should be performed immediately.
Note: A depression at the coronary band or hair above the coronary band that sticks out or up is not a good prognostic indicator
of FSS unless the depression runs around the entirety of the coronary band. Horses with depressions or changes in hair direction
that run from quarter to quarter are usually just cases of high-grade palmar angle or cranial rotation.
The palmar angle of an FSS case can be 1 to 2 degrees or 0 degrees. The HLZ is usually greater than 30 mm at both measurements,
which indicates that the P3 is sliding down and backward in the hoof capsule. The EP/CB will be greatly increased but must
be interpreted in conjunction with the other measurements such as the increased HLZ and decreased SDT or SDW.1 There will be a marked lucency of the dorsal laminae, but unlike the more triangular pattern of a case of cranial rotation,
the FSS case will have a lucency that runs the entirety of the hoof wall and may be more rectangular than triangular.
The venographic findings will show compression of the circumflex or total lack of the circumflex circulation. The dorsal and
circumflex circulation will be greatly reduced or absent. The terminal arch will be markedly reduced or absent. The bulbar
circulation, which in cranial rotation cases always appears full and healthy, will be markedly congested or absent.