The proverbial expression "it depends" aptly applies to this topic. Perhaps by addressing the various ways to back up a toe, and the "why" thought process behind it, the "when" follows by logical deduction. You can back up a toe topside (removing horn wall) or from the sole view (removing excess sole thickness ahead of tip of P3). You can physically accomplish "backing up a toe" by mechanically raising the hoof angle (example wedging heels. The raised hoof angle will reduce toe leverage compared to where it was prior, but the question remains: have you balanced the foot, reduced the long toe, made changes that support the internal bony structure oreintation, or did you simply manipulate the distorted hoof capsule?
"Backing up a toe" from the top: This method is generally used when you want to remove flare/lamellar wedge in the toe-pillar regions by rasping away the horn at the bottom 2/3rds of the dished wall near the ground bearing surface. Removing flared wall in the toe region will result in bringing the dorsal wall back into alignment with the new horn growth originating at the top of the hoof(hairline, coronary band). Running your finger down the rasped front wall from the hairline to the ground, it should feel smooth, straight, no dishing, no over-thinning of horn at the bottom. Removal of this excess dished toe will reduce toe leverage and "back up your toe". But you will still require a breakover to the toe in addition to the flare removal. You really need to understand where the internal structures are in relation to your distorted hoof capsule.
You need to know what the rest of the foot is doing. What is the bony column alignment in that foot? What is your sole depth? Frog health? Digital cushion health? Heel location? Abscess address? Lamellar stability in the rest of the foot? What are you doing to address and support these findings? What if you choose to BLUNT the toe in a NON-DISHED, NON-FLARED, hoof in an attempt to "back up" a long toe? True, you can blunt the toe and physically reduce the toe leverage, but you have not addressed the root source of the forward migrated hoof capsule, or better stated, you have not made changes to correct or improve the pathological orientation of the internal bony structures. Simply, you have thinned or bullnosed the distal horn lamellar zone (HLZ) without addressing bony column alignnment and the pathological changes continue.
OVER-REACHING/FORGING: Often backing up th toes on hinds is done by "SQUARING OFF" the hind toes of blunting the hind toes in an attempt to reduce over-reaching injuries. The real problem infact, is looking to address the fores- aligning the bony column will reduce toe leverage and facilitate ease of breakover so that the foot flight will be in phase and "get out of the way" of the hinds. Then you will want to look at the hind hoof bony column alignnment and sole depth uniformity before you decide on what you will do with the toes.
"BACKING UP A TOE" FROM THE BOTTOM: Backing up a toe can be achieved by reducing the excess sole depth under the toe in cases where sole exfoliation is required to achieve uniform healthy sole thickness across the hoof. You need to develop a discerning eye to visualize and/or use techniques and measurements to confirm that you are dealing with this type foot (or take balance rads) otherwise you can damage a thin soled hoof using this method. If excess sole is allowed to build up and compress under the toe, you continue to drop the palmar angle, encouraging a continual worsening of a broken-back bony column alignment, with accumulative pathological changes occuring in the foot over time. By restoring uniform sole thickness with the removal of excess toe sole, the hoof angle will increase (also referred to as standing up the foot) which "brings the toe back" so to speak, reducing dorso-palmar imbalance, getting closer to a healthier 50/50 toe - heel ratio from COA and moving toward bony column alignment.
The hoof capsule distortions, horn tubule directions, grooves, growth ring spacing, periople condition, pigmentation changes, commissure depth readings, and other key tools will help alert you to suspect internal changes and bony orientation. Balance radiographs take the guess work out of the equation. If you are taking over a new case, remember that some/all of these visual landmarks that you rely on, may have been rasped off, sculpted and moulded which may disguise the hoof type you are dealing with. Balance radiographs are an excellent baseline to ensure you are trimming to the internal structures of the foot, rather than hoof capsule distortions alone. Ultimately you want to understand what the bones are doing internally, before you decide on a plan to "back up the toe". Reference: "Best Foot Forward" Vet/farrier/horseowner handbook to a hollistic approach to total horse health; mind, body, sould and hoof.
I will usually find it necessary to back up the toe with the trim or mechanically when there is stretching or distortion of the white line in the toe region.
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