You didn't specify where the "red band" was located on the hoof, but you may be referring to the outside rim of the bottom of the horse's hoof. If this is the case, you are probably seeing bruising and inflammation along the "white line" of the hoof, where the sole meets the hoof wall, above which lies the sensitive lamina of the hoof. This symptom is consistent with laminitis, so this horse needs an appointment with the vet to diagnose what may be happening in addition to the previously diagnosed arthritis. This red band is, in fact, much easier to detect in a white foot versus a dark one, so it is likely that other feet (at least the other front foot) may be affected as well.

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